synthesis, structure, and properties of mixed niobium(iv,v) oxides

Inorganic Materials, Vol. 36, No. 3, 2000, pp. 247-259. Translated from Neorganicheskie Materialy, Vol. 36, No. 3, 2000, pp. 315-329. Original Russian Text Copyright �9 2000 ~, D'yachenko, Istomin, Abakumov, Antipov.

Synthesis, Structure, and Properties of Mixed Niobium(IV, V) Oxides

O. G. D'yachenko, S. Ya. Istomin, A. M. Abakumov, and E. V. Antipov Moscow State University, Moscow, 119899 Russia

Received July 15, 1999

Abstract--This review paper summarizes data on mixed oxides containing niobium in an intermediate valence state, between 4+ and 5+. The major preparation procedures are considered, and the effects of different process parameters----composition of the starting reagents, temperature, pressure, duration, and environment--are analyzed from the viewpoint of the preparation of bulk, single-phase materials with niobium in a controlled formal oxidation state. The optimal synthesis conditions are established for reduced niobates with different structures. The oxidation behavior of the niobates(IV, V) is examined as a function of the structure type. The structural and electrical properties of the reduced niobates are shown to correlate with the carrier concentration (formal oxidation state of niobium). The major factors determining the carrier concentration in the niobates are identified. The results are used to establish the conditions under which a semiconductor-metal transition is possible in reduced niobates.

INTRODUCTION

Mixed oxides containing a transition metal in different oxidation states exhibit a broad spectrum of inter- esting physical properties, including superconductivity and giant magnetoresistance. In studies of such materials, particular attention is given to correlations between composition, structure, and physical properties. Analy- sis of the effect that the chemical composition of these compounds and preparation procedure have on their structure helps in optimizing the search for new materials with the desired properties.

After the discovery of high-Tc "electronic" superconductors containing copper in a d9-d l~ configuration, a large number of works have been centered on the search for new superconductors and investigation of the transport and magnetic properties of mixed oxides containing transition metals in do-d ~ configurations. The reason is that, in the "hole formalism," the d ~ and d 9 configurations are equivalent, since d 9 can be thought of as d l~ + hole (missing electron). Moreover, since the electronic structure of transition-metal cations in a d o_ d ~ state is relatively simple, such compounds are convenient model systems for assessing the structural effects on the electronic spectrum and physical properties.

Among the compounds in question are mixed niobium oxides, niobates(IV, V), in which niobium is in an oxidation state intermediate between 4+ and 5+. The interest in the study of reduced niobates is engendered by the observation of a superconducting transition in the systems MxNbO2 (M = alkali metal) [1-3], Srl _xLaxNb206_ 8 [4, 5], M2Nb ! +xOy (M = Ca, Sr) [6], Srr_xLnxNbl0030 [7], and BaxNbO2_~ [8].

Characteristically, niobates(IV, V) have a framework structure. Despite the wide diversity of structure types engendered by various arrangements of NbO6 octahedra, there are two prototype structures--perovskite, ABO 3, and pyrochlore, A2B2OT--and the structures of the mixed niobates can, to a first approxima- tion, be thought of as derived from distorted perovskite or pyrochlore. The structures of the mixed niobium oxides which will be examined in this work are shown in Fig. 1.

The first group includes the following compounds:

(1) Niobates with the ideal perovskite structure and stoichiometry ANbO 3, A = Sr, Ba (Fig. la).

(2) Mixed niobium oxides crystallizing in various distorted perovskite structures (Fig. lc).

Mixed perovskite-like oxides typically have distorted structures. The character of distortions in niobates(IV, V) is determined mainly by geometric factors (mismatch between the A-O and B-O interatomic distances). Despite the Jahn-Teller nature of Nb(IV), its octahedral surrounding is virtually undistorted. Nio- bates(IV) may have both the ideal perovskite structure (ANbO 3, A = Sr, Ba [9-15]) and various distorted perovskite structures, including GdFeO3-type orthorhombic structures (CaNbO 3 [16]). In the latter case, the symmetry reduction results not from distortions of NbO 6 octahedra but from their cooperative tilting about [110]p (subscript p refers to the perovskite structure) and rotations around [001]p, ensuring a better match between the Ca-O and Nb--O bond lengths.

(3) Mixed niobium oxides with vacancy ordering on the A site, Al +xNb309 (AI/3+/NbO3), A = rare earth (Fig. lb) [17-19].

0020-1685/00/3603-0247525.00 �9 2000 MAIK "Nauka/Interperiodica"

248 D ' Y A C H E N K O et al.

Fig. 1. Structures of mixed niobium oxides: (a) niobates with the ideal perovskite structure; (b) mixed niobium oxides with vacancy ordering on theA site (A-site occupancy, 2/3 + x); (c) mixed niobium oxides crystallizing in different distorted perovskite structures; (d) niobates with the calcium metatantalate (CaTa206) structure; (e) compounds with the tetragonal tungsten bronze structure; (f) mixed niobium oxides with the pyrochlore structure (one oxygen atom is not shown).

INORGANIC MATERIALS Vol. 36 No. 3 2000

SYNTHESIS, STRUCTURE, AND PROPERTIES OF MIXED NIOBIUM(IV, V) OXIDES 249

Fig. 2. Relationships between the perovskite structure and the structure types typical of mixed niobium oxides.

The structure of these perovskite-like oxides can be thought of as an ordered sequence of layers stacked along the c axis:

... NbO2-A 2/3 [] l/30-NbO2-[]O-A 2/3O l/30-NbO2 ....

where [] is a cation vacancy.

Additional A cations fill vacant sites in A2/31--11/30 layers, whereas the DO layers remain unchanged.

(4) Niobates with the calcium metatantalate (CaTa206) structure, ANb206 (A0.sNbO3), A = Sr, Ba, Eu(II) (Fig. ld) [20-27].

This structure can also be regarded as derived from the perovskite structure via a [110]p crystallographic shift followed by a distortion of the metal-oxygen framework. The structure is made up of Nb2O10 units composed of two edge-sharing octahedra. These units are connected at the vertices so as to form an infinite three-dimensional framework. The A cations reside in structural channels, whose cross section depends sig- nificantly on the ionic radius of A.

(5) Compounds with the tetragonal tungsten bronze (TTB) structure of stoichiometry A6Nbl0030 (A0.6NbO3), A = Sr, Ba, Eu (Fig. le) [28-32].

The frequent occurrence of this structure type among mixed niobium oxides is often believed to be associated with the high thermodynamic stability of the TTB-structure compounds [33]. The structural basis of these niobates comprises comer-sharing NbO6 octahedra, with three types of interstices in between, differing in size and the coordination number of interstitial A cations.

The relationships between the perovskite structure and the structures considered above are illustrated in Fig. 2.

The second group comprises mixed niobium oxides with the A2Nb207_~, stoichiometry and pyrochlore structure (Fig. If) [34-43]. This structure type also fea- tures an infinite three-dimensional Nb-O framework built up of comer-shared NbO 6 polyhedra (trigonal antiprisms which can be regarded as heavily distorted octahedra). At the same time, the Nb-O-Nb bond angle in the pyrochlore structure (= 135 ~ differs substantially from that in the perovskite structure (180~ One of the oxygens is coordinated by A cations only and is not incorporated in the octahedral framework. Its position can be partially or fully vacant, resulting in A2NbzO 7_y anion-deficient pyrochlores.

The purpose of this review article is to summarize our earlier data on the niobates containing niobium in a mixed-valent state and analyze the effect of carrier concentration on the structure and resistivity of these materials.

SYNTHESIS OF NIOBATES(IV, V)

Starting Reagents and Reaction Environment

The ability to prepare bulk, single-phase niobate(IV, V) samples with niobium in a controlled formal oxidation state is crucial to the physical study of mixed niobium oxides. The most convenient way of preparing reduced niobates is by solid-state reaction, in which the oxidation state of niobium can be tailored by adjusting the Nb(IV) : Nb(V) ratio in the starting mixture. However, if the formal oxidation state of niobium


250 D'YACHENKO et al.

is determined solely by the Nb(IV) : Nb(V) ratio in the starting reagents, the possibility of side redox processes during synthesis must be ruled out. To this end, particular attention should be paid not only to maintaining a sufficiently low oxygen partial pressure but also to pre- cluding the presence of other reagents capable of oxi- dizing Nb(IV) to Nb(V). If the starting mixture contains alkaline-earth carbonates, as proposed by Akim- itsu et al. [4, 5], NbO2 reacts with CO2 at temperatures above 900~ to give Nb205,

2NbO2 + CO2 ) Nb205 + CO,

which leads to deviations from the nominal Nb(IV) : Nb(V) ratio. Gasparov et al. [8] proposed to prepare Ba-Nb-O compounds by pressing Nb metal and BaO2 into pellets and firing the powder compacts at 500~ in air. In this way, they obtained a new superconducting oxide with the assumed composition Ba~NbO2_8. Later, more in-depth studies of this system [44] showed, however, that, under the conditions in question, metallic Nb reacts with atmospheric nitrogen, and it is the resulting phase which is responsible for the observed superconducting transition. Therefore, the above techniques are unsuitable for the preparation of reduced niobates with a controlled formal oxidation state of niobium.

Good results can be achieved in evacuated ampules if use is made of noncarbonate precursors, such as M2Nb207 or MsNb4OI~ (M = alkaline-earth metal), in combination with rare-earth oxides, NbO2, and Nb2Os. Heating a stoichiometric mixture of starting reagents at 1100-1250~ for 10-48 h in quartz ampules sealed off under vacuum was found to yield single-phase Srl_xLnxNb206, Lnl+xNb309, and Bar_xLnxNbl0030 samples [19, 27, 32]. This method, however, also suf- fers from serious drawbacks. At high temperature, precursors containing alkaline-earth metals may react with quartz, giving rise to the formation of stable silicates and altering the initial cation stoichiometry. This diffi- culty can be obviated by using additional crucibles from inert materials (alumina or zirconia) to preclude contact between the sample and ampule material. Besides, the reaction temperature is limited by the soft- ening temperature of quartz, 1200-1250~ However, this temperature is not high enough for the synthesis of mixed niobium oxides with the pyrochlore structure, Ca 1 _xSrxNbO3, or K 1 _xAxNbO3 (A = Sr, Ba). Heating to higher temperatures leads to destruction of the quartz ampule and, hence, sample oxidation. Therefore, to run the reaction at higher temperatures, other ampule materials are necessary. In our preparations, we used argon- filled, sealed niobium ampules, which allowed us to run reactions at temperatures from 1500 to 1600~ In this way, we obtained single-phase samples of mixed niobium oxides with the pyrochlore structure and also Ca 1 _xSrxNbO3 with 0 < x < 1 [45].

KI _~BaxNbO3 (0.2 _< x _< 0.5) with the ideal perovskite structure could only be prepared at high pressure

(8GPa, 1280-1320~ [46]. Synthesis in quartz ampules yielded mixed-phase materials consisting mainly of Bar_xKAqbl0030 and BasNb4015. An important advantage of high-pressure synthesis is that high- density structures can be stabilized. In contrast to reactions in quartz ampules, which result in high concentrations of cation vacancies (Ba6_xKxNbl0030 and BasNb4015), high pressure contributes to stabilization of K1 _xBaxNbO3 with the ideal perovskite structure.

Effects of Reaction Temperature and Duration

In the synthesis of reduced niobates, the reaction temperature and duration are important process vari- ables. Their effects on the composition of the resulting niobate(IV, V) depend on the nature of the compound. The composition ranges of Sr l_xLnxNb2Or, Lnl+~Nb3Og, Bar_~n~'bl0030, Kl_~BaxNbO3, and Cal_xSr~lbO3 broaden with increasing temperature [19, 27, 32, 45, 46].

In the last two solid-solution systems, the firing time is critical for the synthesis of Ba- or Ca-rich materials. For example, the synthesis of Ko.sBao.zNbO 3 reaches completion in just 0.5 h, whereas synthesis of single- phase Ko.sBa0.sNbO 3 material takes 2 h. More Ba-rich materials remain mixed-phase even after heat treatment for 2 h. Unfortunately, with our facilities, we could not increase the firing time or raise the temperature. For this reason, we obtained only solid solutions containing no more than 50% Ba [45].

Similar trends were revealed for Cal_xSrANbO 3 solid solution. Single-phase CaNbO 3 material was obtained by heat treatment at 1500~ for 20 h, whereas heat treatment at the same temperature for 8 h yielded mixed-phase samples. According to electron-probe x- ray microanalysis (EPXMA) data, the majority phase in that material was the Ca-enriched compound Ca(Ca,Nb)O 3, in which Ca substituted on some Nb sites. Similar phases, (Sr, Ca)(Ca, Nb)O 3, resulted when attempts were made to prepare Ca-rich Ca1 _xSrxNbO3 solid solutions in evacuated quartz ampules [46].

The composition of CaMNb207 (M = LaNd, Sm, Gd-Lu, Y) varies nonmonotonically with synthesis temperature. Raising the heat-treatment temperature from 1200 to 1350-1500~ leads to the formation of single-phase materials, whereas higher temperatures and longer firing durations give rise to partial substitution of Ca and/or M for Nb [42, 43]. This is caused by the concurrent processes occurring at higher temperatures, as evidenced by the formation of Nb metal and NbO. These phases can be formed by two mechanisms: disproportionation of NbO2 at high temperatures by the reaction NbO2 ,. Nb(NbO) + Nb205 [40, 41] or oxygen absorption by the Nb ampule, acting as a getter. An additional, higher temperature heat treatment of pyrochlore materials containing no NbO2 increases the concentration of metallic Nb and NbO. This finding pro-


SYNTHESIS, STRUCTURE, AND PROPERTIES OF MIXED NIOBIUM(IV, V) OXIDES

Table 1. Optimal synthesis conditions for reduced niobates with different structures

251

Niobate

Sq _ xLnxNb206

Ba6_ xLnxNbloO30

Lnl + xNb309

Cal _ xSrxNbO3

KI _ xBaxNbO3

CaLnNb207

Synthesis conditions

t, ~

1150*

1200-1250"

1200"

1500"*

1280-1320"**

1350-1500"*

~,h

48

10-48

20

20

0.5-2

6-10

Note

x < 0.4 for La; x < 0.3 for Nd

x < 2.0 for La; x < 1.5 for Ce and Nd

x <_ 0.15 for La; x < 0.10 for Ce; x < 0.05 for Nd

0 < x < 1.0

0.2 <x < 0.5 forx= 0.2 x = 0.5 h; forx= 0.5 x = 2 h

1350~ for Sm, Gd, Dy, Ho, Er, Yb

1400~ for Y, Nd, Pr, Tb, Tm, Lu

1500~ for La, Ce

* Quartz ampules sealed off under vacuum. ** Argon-filled, sealed Nb ampules.

*** High-pressure synthesis (8 GPa).

vides possible evidence that the Nb ampule, absorbing oxygen from the sample, plays a key role in the high- temperature processes. Based on x-ray diffraction (XRD), EPXMA, and thermogravimetry (TG) data for the Er compound, the following reaction scheme can be proposed for these processes:

CaErNb207 + Nb(ampule) ,0.91CaEr(Nbl.8Ca0.1Er0.1)O 7 + 0.36Nb + NbOx(ampule).

Thus, the following process parameters are critical for the preparation of single-phase niobate materials with a controlled oxidation state of niobium:

(1) starting reagents and process environment, (2) reaction temperature, and

(3) heat-treatment time.

Table 1 summarizes the optimal synthesis conditions for the mixed niobium oxides under consideration.

OXIDATION BEHAVIOR OF NIOBATES(IV, V)

In the synthesis of reduced niobates, both the Nb(IV) : Nb(V) and A : Nb ratios must be controlled. The latter may differ from the nominal ratio as a result of the formation of cation vacancies. To ascertain whether or not vacancies can be formed on the A site, we investigated oxidation of mixed niobium(IV, V) oxides.

According to XRD studies, oxidation of Sro.65Lao.35Nb20 6 yields a material in which the major phase has the calcium metatantalate structure but differs in lattice parameters from the unoxidized material. The oxidation product also contains LaNbO4 and Sr2Nb207 as additional phases. Moreover, EPXMA demonstrates that oxidation reduces the La content.

Thus, oxidation of Sr 1 _xLnxNb206 gives rise to the formation of cation vacancies on the A site, without changing the structure type of the material. The concentration of cation vacancies corresponds to the oxidation of Nb to Nb 5+. The general formula of such cation-

�9 [ ] -L , 5 + deficient niobates is Sr I -~-,n~2/3)x (ua)~a'~o2 O6 [27].

As shown later [32, 45], the formation of vacancies on the A site without changes in structure type is characteristic of the oxidation of mixed niobium(IV, V) oxides (Table 2).

Oxidation of the pyrochlore-structure niobate CaNdNb207 leads to the formation of two cation-deficient phases differing in cation composition. Note that the cation-deficient phases produced by oxidation are metastable and are missing in the phase diagrams of the corresponding niobate(V) systems. For example, long- term air annealing of CaNdNb207 yields a mixture of the niobates(V) Ca2Nb207 and NdNbO4.

XRD, TG, and EPXMA data indicate that the oxidation of reduced niobates follows the reaction schemes

Sr 1 _xLnxNb206 02, Sr I _xLn(2/3)x[--](l/3)xNb~+ 06

+ LnNbO4 + Sr2Nb20 7 02,. LnNbO4 + Sr2Nb207 '

Ba6_xLnxNbloO30

02,. Ba 6_ x- ~Lnx- zI-ly § z Nb~o 030

+ LnNbO4 + BasNb4Ol 5,

Ba6NbloO30 02, BaNb206 + BasNbOls.

In the case of TTB-structure materials, oxidation produces vacancies only in Ln-substituted compounds. This behavior can be explained as follows: If the oxidation of Ba6NbloO30 occurred by the scheme proposed for Ba6_xLnxNbloO30, the oxidation state 5+ would be



Table 2. Oxidation behavior of niobates(IV, V)

Structure

CMT

TTB

Pyrochlore

Temperature range, o C

160-350"

300-700*

280-520**

Major phase as identified by EPXMA

before oxidation

Sro.65(4)Lao.35(4)Nb206

Ba4.3(1)Lal.l(3)NbloO3o

Ca].oo)Ndl.o(1)Nb207

after oxidation

Sro.69(7)Lao.18(5)[]0.13Nb206

B a3.6(2)Lao.7(l )1--]1.7NbloO3o

Cao.8(E)Ndo.7(1)[]o.sNb207

Cao.5( 1)NdLo (D[]o.SNb207

Note: CMT -- calcium metatantalate. * Oxygen. ** Air.

attained at the composition Ba)-qNbloO30 (Ba : Nb = 1 : 2). But at this stoichiometry, BaNb20 6 is more stable, and this phase was indeed present in the oxidized material.

CaLnNb207 o2, Cal_~Lnl _ y['-]x + y N b 2 0 7 (X < y)

+ Cal_~nl _y[]x + yNb207 (x > y)

02, Ca2Nb207 + LnNbO4"

The oxidation of Cal _xSrxNbO3 with the perovskite structure (cubic or distorted) occurs in roughly the same temperature range, 300-600~ as that of the other reduced niobates (Table 2) but is not accompanied by the formation of vacancies on the A site. Oxidation to Nb s§ would yield compositions

A02.~ [-10.sNb~§ 3. As reported in the literature, such compounds have structures other than cubic perovskite, e.g., calcium metatantalate (SrNb206) or fersmite (CaNb206) structures [47]. Oxidation yields two

N ~ [21 x [4] (~* O~-

P . . . .

s [2] x [2] t~*

e 8 [2] x [3] n" EF

t2g

Pn

[21 x [6] n S P~

[21 x [6] c [ /

big. 3. Model for the band structure of MNbO 3 (M = Sr, Ba).

(Ca, Sr)2Nb20 7 phases, one Sr-rich and the other Ca- rich. The process follows the reaction scheme

Ca l_xSrxNbO 3 02. Ca2_ySryNb20 7 + Sr2_yCa~Jqb2OT.

STRUCTURE AND RESISTIVITY OF REDUCED NIOBATES

The electrical properties of mixed niobium oxides with three-dimensional framework structures can be understood in terms of the one-electron model for band structure proposed by Goodenough (Fig. 3) [48]. In this model, the filled bonding states (mainly O 2s and 2p levels) and empty antibonding states (mainly cation 5s and 5p levels) are separated by a broad gap resulting from the difference in electronegativity between the constituent atoms. The cation 4d states, antibonding with respect to O 2s and 2p, lie within the gap. The nature of the d states--localized or not--is critical to the electronic properties of the materials in question. In the case of SrxNbO 3 and BaNbO 3, having the cubic perovskite structure, the t2g and eg orbitals are nonlocalized because of the large covalent mixing parameters of the Nb and O orbitals. As a result, the n* conduction band, derived mainly from the Nb t2g and O P,t orbitals, is filled only partially, and these compounds feature metallic conductivity. In KNbO3 (Nb 5§ d o configuration), the conduction band is unfilled, and this compound is an insulator [11, 12, 15, 49].

The covalent mixing parameter depends on the B-O-B bond angle. The cation-anion--cation interac- tion is maximal when this angle is 180 ~ because the n- overlap of the Nb t2g and O P,t orbitals is then also maximal. The contribution of the A orbitals to the conduction band can be ignored. At the same time, account should be taken of the indirect influence of the A cation on the B-O covalency: acidic A cations reduce it, thereby promoting the localization of d states [50].

Thus, there are at least two factors critical to the resistivity of reduced niobates---carrier concentration and Nb-O-Nb bond angle, determining the degree of n-overlapping of the Nb t2g and O p~ orbitals.



The formal oxidation state of niobium and, hence, carrier concentration can be controlled by varying the cation or anion composition. The most effective way of tuning the oxidation state of niobium is by substituting heterovalent cations or producing cation vacancies in the A site. Such compositional changes would be expected to have an insignificant effect on the band structure of the material since the A orbitals are not involved in the formation of the conduction band. How- ever, the presence of two cations, differing in crystal- chemical behavior, on the A site, as well as the presence of cation vacancies, may result in undesirable local structural distortions, giving rise to electron localization. A similar effect would be expected upon heterovalent substitution on the B site.

As follows from detailed XRD, neutron diffraction, and TG data [27, 32, 42], anion nonstoichiometry is atypical in niobates(IV, V) in view of the stable octahedral surrounding of Nb. To reduce niobium, one can resort to heterovalent substitution on the anion site, replacing a fraction of the oxygen atoms by, e.g., fluorine, since 02- and F- differ in formal charge state but are close in crystal-chemical behavior.

Thus, carrier concentration can be controlled by (1) partial heterovalent substitution on the A site, (2) generation of cation vacancies in the A site, (3) partial substitution of A cations for Nb, and (4) partial substitution of fluorine for oxygen. Consider in detail the effects of these factors on car-

rier concentration. Heterovalent substitution on the A site is the best

studied and often employed approach to controlling the formal oxidation state of transition metals in mixed oxides. In the structures of the mixed niobium oxides considered above, the following substitutions were accomplished:

(1) niobates with the ideal perovskite structure-- K l _ j3afl'qbO3, 0.2 < x < 0.5; Ko.sSr0.sNbO 3 [46];

(2) mixed niobium oxides with the calcium metatantalate structure--Sr 1 _J-,nxNb206 (Ln = La, Nd), 0 < x < 0.4 [27];

(3) compounds with the TI'B structure-- Ba6_xLnxNbl0030 (Ln = La, Ce, Nd), 0 < x < 2 [32];

(4) niobates with the pyrochlore structure-- Ca 2_~LnxNb207 (Ln = Nd, Y), 0.8 < x < 1.2 [42].

Heterovalent substitutions on the A site enabled the carrier concentration to be varied over a fairly wide range, from 0 to 0.5 electrons per niobium atom, depending on the structure type. In addition to changes in the formal oxidation state of niobium, heterovalent substitution on the A site gives rise to structural changes. At least two distinct types of changes were revealed:

Partial substitution of a larger sized, alkaline-earth cation for a smaller sized, rare-earth cation, as in the solid-solution systems Sr I _xLnxNb206 (Ln = La, Nd)

V 1/3,/~

8.60 r-

8.5t

8.56

8.54

8.52

La

Ce

- N d

8.50 , , J , , , i , , , 0 0.5 1.0 1,5 2.0 x

Fig. 4. Composition dependences of the unit-cell volume in the Ba6_xLnxNbl0030 (Ln = La, Ce, Nd) solid-solution systems; solid solutions exist for x -< 2.0 in the La system and for x < 1.5 in the Ce and Nd systems.

and Ba6_~Ln~lbloO30 (Ln = La, Nd, Ce), must reduce the effective radius of the cations residing in structural channels and, hence, the unit-cell volume.

On the other hand, heterovalent substitution increases the effective ionic radius of Nb because of the partial reduction to Nb 4§ As a result, the unit-cell volume increases with substituent concentration.

The result of this competition depends mainly on the relationship between the ionic radii of the substituent and host. For example, the former effect prevails if the difference in ionic radii is sufficiently large, as in the Ba6_xLnflXlbloO30 (Ln = La, Nd, Ce) systems: rRa2§ = 1.42 A , r t3+ = 1 . 1 8 / ~ , rcd+= 1 .14 /~ , and

rNa3+ = 1 .12 /~ at C N = 8 [51] . Indeed, the un i t -ce l l vo l -

ume o f Ba 6 _xLnxNbloO30 (Ln = La, Nd , Ce) was found to increase with x (Fig. 4).

The latter effect prevails in Kl _ xBafl'qbO3 (0.2 < x < 0.5) reduced niobates. Since barium and potassium are very close in 12-fold-coordinated ionic radius (raa2+ ~-

rK+ = 1.60/~), the observed increase in the lattice

parameter of the Kl_xBaxNbO3 solid solution (Fig. 5) with increasing Ba content testifies to an increase in the effective ionic radius of Nb (rNb,§ = 0.69 ~, rNbs+ =

0.64/~) and, hence, its partial reduction to Nb 4§

In the Srl _xLaxNb206 solid solutions, the unit-cell volume varies nonmonotonicaUy with composition



a,A

4.08

4.04 O

O

I I I Io I 4.0 0 0.4 0 8 x

Fig. 5. Composition dependence of the lattice parameter for K 1 _ xBaxNbO3 (0.2 < x < 0.5).

V 1/3,/~

7.79

7.78

La

7.77 Nd

I I I I I I I I I

0 0.1 0.2 0.3 0.4 x

Fig. 6. Composition dependences of V 113 in the Sq _ xLnxNb206 (Ln = La, Nd) solid-solution systems.

because of the interplay between the above effects. The ionic radii of Sr 2§ and La 3§ differ not very much (rsr2. =

1.25/~ and rLa3§ = 1.18 /~ at CN = 8); as a result, the

lattice parameter varies in a complex manner across the solid-solution series (Fig. 6). The difference in the ionic radii of Sr 2+ and Nd 3+ is more essential (rs?. = 1.25/~

and rNd3§ = 1.12/~ at CN = 8), and the unit-cell volume

of Srl _~Nd~Nb206 decreases systematically with x.

If an Nb(V) compound contains vacancies in A sites, as in LnNb309, the formal oxidation state of nio-

bium can be varied by reducing vacancy concentration. Such solid solutions, with the compositions Lnl + xNb309, were prepared for Ln = La (0 <x < 0.15), Ce (0 < x < 0.1), and Nd (0 < x < 0.05). In these systems, the unit-cell volume also increases with increasing rare-earth concentration and decreasing niobium valence [19].

As pointed out above, heterovalent substitution on the A site allows the formal oxidation state of nio- b ium-and, hence, carder concentration--to be varied over a fairly wide range, from 5+ to 4.5+. However, the formation of cation vacancies on the A site narrows this range, as first suggested on the basis of data on the oxidation behavior of Sq _xLnxNb206 solid solutions (see above).

Later structural data for Ba6_xLnxNbl0030, obtained by both powder and single-crystal x-ray diffraction techniques, provided conclusive evidence for the formation of vacancies on the A site. In mixed niobium oxides with the TrB structure, the concentration of such vacancies is higher in the narrower, tetragonal channels in comparison with the broader, pentagonal channels. The presence of vacancies depends on synthesis conditions (temperature, Po2, heat treatment

time) and, as mentioned above, increases the formal oxidation state of niobium in comparison with that expected for the nominal composition [32].

Yet another factor which may increase the formal oxidation state of niobium in the compounds under consideration is partial substitution of the A cation for Nb. For example, in the case of Ca-containing mixed niobium oxides, such as (Ca,Sr)NbO3 and CaLnNb207, phases with partial substitution of Ca for the Nb site may be formed [42, 43, 45]. This can be explained by the more pronounced tendency for Ca to be in octahedral coordination in comparison with the other alkaline-earth and rare-earth metals. This effect can be sup- pressed almost fully by adjusting the synthesis temperature and duration (Table 1).

Heterovalent substitution (M 3+ for M 2§ or M 2§ for M § raises the concentration of conduction-band carri- ers. As the content of the heterovalent A-site substituent--and, hence, carrier concentration--rises, the increase in resistance ratio R(T)/R(273) with decreasing temperature tends to be notably slower, as exempli- fied by the resistivity data for the Ba6_~LaxNbl0030 (0 < x < 2.0) solid-solution system. In the temperature range shown in Fig. 7, the resistivity of Ba6Nbl0030 rises by more than two orders of magnitude, whereas that of Ba4La2Nb~0030 increases by only about a factor of 3 [32].

The Ba6_ ~l-,nxNbl0030 (Ln = La, Ce, Nd) solid solutions exhibit semiconducting behavior, presumably because of the low carder concentration, <0.4 electrons per Nb atom. Account should also be taken of the vacancies in the A site, which also reduce carder concentration. Note that the carrier concentration in



R(T)/R(273)

O Ba6NbloO3o

O 100 O 0 t~asLatNDl0U30

O

0 zx Ba4La2NbloO3o

<>

<>

<> <> <>

50

0 100 200 T, K

Fig. 7. Resistance ratio as a function of temperature for Ba 6 _ xLaxNbloO3o.

255

Na4.sW10030, possessing metallic conductivity, is notably higher, 0.45 electrons per W atom. Another possibility is that the semiconducting behavior of these compounds is associated with electron localization near the Fermi level because of the disordered distribution of Ba 2+, Ln 3+, and vacancies on the A site of the TTB structure.

A similar temperature variation of resistivity was observed for Srl_xLnxNb206 (0 < x < 0.4) and Kl _xBaxNbO3 (0.2 < x < 0.5) [27, 46]. These materials also show semiconducting behavior, which can be attributed to the low carrier concentration (0.2 electrons per Nb atom) in the former solid solution, whereas the latter, having the ideal perovskite structure, would be expected, according to the Goodenough's one-electron model (Fig. 3), to exhibit metallic behavior. The observed semiconducting behavior of Kt_xBa~NbO 3 (0.2 < x < 0.5) and K0.sSr0.sNbO 3 seems to be due to Anderson's electron localization near the Fermi level, resulting from K + and Ba 2+ disordering. A similar localization mechanism was proposed by Ellis et al. [52] to account for the semiconducting properties of Nal_xSrxNbO3. In going from K-substituted compounds to BaNbO 3, where Nb is in the average formal oxidation state 4+, the electrical behavior changes from semiconducting to metallic, which can be accounted for by both the higher carrier concentration in BaNbO 3

and the absence of substituents on the A site, occupied by Ba z+ only (Fig. 8).

Metallic behavior of conductivity was also observed in Ca I _xSrxNbO3 solid solutions, where Nb is also in the formal oxidation state 4+, while CaNbO 3, featuring the same oxidation state of Nb, exhibits semiconducting behavior [45]. The latter finding is attributable to the small Nb-O-Nb bond angle (153 ~ + 1 ~ in the structure of CaNbO 3 (GdFeO 3 type) compared to that in the ideal perovskite structure (180~ The decrease in this angle is accompanied by a reduction in the n-overlap of the Nb tEg and O p~ orbitals, resulting in electron localization and semiconducting behavior. Partial substitution of the larger sized cation Sr z+ for calcium reduces the orthorhombic distortion of the structure, resulting in metallic behavior of conductivity even at a fairly low Sr concentration (Ca0.9Sr0.tNbO3).

Similar considerations account for the semiconducting properties of the pyrochlore-structure compounds CaMNb207 (M = L a N d , Sm, Gd-Lu, Y) [42, 43]. As pointed out above, the coupling between the Nb 4d and O pn orbitals is strongest when the Nb-O-Nb bond angle is 180 ~ In the pyrochlore-type structure of CaNdNb207, the Nb-O-Nb angle is as small as 133.5(2) ~ which leads to electron localization and semiconducting behavior. This way of reasoning is



R(T)/R(150)

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.7 0

x=0.2

x=0.4

x=0.3

x = 0.5 (K0.5Sr0.5NbO3)

x= 1.0

I I

50 100

T,K

Fig. 8. Resistance ratio as a function of temperature for K 1 _ xBaxNbO3 and Ko.5Sr0.5NbO3 .

r 150

consistent with the one-electron band-structure model proposed by Subramanian et al. [34] for pyrochlores containing no A cations with a lone s 2 electron pair.

Semiconducting behavior is also exhibited by reduced niobates of composition Lal § (x = 0.05, 0.10, 0.15) [19]. As in the niobates considered above, the resistance ratio of these materials decreases with decreasing Nb valence. Under the assumption that Lal +xNb309 is an n-type semiconductor, the temperature variation of resistance can be correlated with electron concentration and mobility. At a low electron concentration, we observe, as in the case of La 1.05Nb309, a

maximum in resistance, centered around 180 K (Fig. 9). It seems likely that, in this niobate, the energy required to promote electrons from donor levels to the conduction band is sufficiently low, and there are conduction electrons even at low temperature. Below 180 K, the concentration of conduction electrons is temperature-independent, and the rise in resistivity with temperature is only due to a decrease in electron mobility. Above 180 K, the concentration of conduction electrons rises owing to the thermal activation of electrons from donor levels to the conduction band, and resistivity drops. At higher electron concentrations, we observe an increase in resistance ratio with decreasing temperature, characteristic of semiconductors.

The effect which heterovalent substitution on the anion site has on the oxidation state of niobium was

analyzed using the KNbO3_xFx solid-solution system as an example. The only mixed niobium oxyfluorides described earlier in the literature are MNbO2F (M = Li, Na, K), of which only KNbO2F has an undistorted perovskite structure and is an insulator [53].

The composition range of cubic perovskite solid solutions was found to be 0.06 < x < 0.7. Note that

0.25

0.20

0.15

0.10

0.05

/',.. I

!

\ N

0 ~ 1 r

100 200 300

T,K

Fig. 9. Resistance as a function of temperature for an Lal.05Nb30 9 sample.



Table 3. Composition, structure, and electrical behavior of reduced niobates

257

Composition Structure Nb valence Electrical behavior

K 1 _xBaxNbO3, 0.2 < x < 0.5 1 |

Ko.sSr0.sNbO3 BaNbO3 j

CaNbO 3

Ca! _ xSrxNbO3 0.1 <x< 0.3

0.4 <x< 1.0

Lnl + xNb309

La, x< 0.15

Ce, x<0.10

Nd, x < 0.05

KNbO 3 _xFx 0.06 < x < 0.7

Ba6 _ xLnxNbl0030 La, x < 2.0

Nd, x< 1.5 Ce, x< 1.5

Sr I _ xLnxNb206 La, x <_ 0.4

Nd, x <_ 0.3

CaMNb207, M - LaNd, Sm, Gd-Lu, Y

Cubic perovskite

GdFeO 3

Distorted perovskite

Cubic perovskite

Distorted perovskite

Cubic perovskite

TTB

CaTa206

Pyrochlore

4.8-4.5

4.5

4.0

4.0

4.0

4.0

5-4.85

5-4.9

5-4.95

4.94--4.3

4.8-4.6

4.8-4.65 4.8-4.65

5-4.8

5-4.85

4.5

Semiconducting H

Metallic

Semiconducting

Metallic H

Semiconducting H

M

M

unsubstituted KNbO3 has an orthorhombically dis- toned perovskite structure at room temperature and undergoes a transformation into a cubic form at 435~ Partial substitution of fluorine for oxygen, accompanied by a decrease in the formal oxidation state of niobium, increases the perovskite subcell volume and leads to the formation of an undistorted, cubic perovskite structure.

The resistivity of the most F-rich material, KNbO2.3F0. 7, was found to drop with increasing temperature, characteristic of semiconductors. The resistivity of the KNbO3_xF x solid solutions is several orders of magnitude higher than that in the KI _ xBa~NbO3 system, where the oxidation state of niobium varies owing to the heterovalent substitution on the A site. This result is attributable to the higher ionic- ity of the Nb--F bond in comparison with Nb-O.

Table 3 summarizes the structural and electrical properties of the compounds studied.

None of the mixed niobium(IV, V) oxides considered here exhibits superconductivity. The only reduced niobates which have been shown with certainty to pos- ses superconducting properties are MxNbO2, where M = alkali metal. In these compounds, Nb is in an intermediate valence state, between 3+ and 4+, and their structure differs markedly from those of the

niobates(IV, V) examined here. It is built from NbO 6 trigonal prisms stacked along the c axis, like those in MoS2. The prisms share lateral edges to form infinite layers, with the alkali-metal ions residing in between.

The electrical properties of the reduced niobates are determined primarily by the occupation of the conduction band and the n-overlapping of the Nb t2g and O p~ orbitals. Metallic behavior of conductivity is possible at a sufficiently high concentration of conduction electrons (>0.5 electrons per Nb atom). The ideal or weakly distorted perovskite structure, in which the Nb-O-Nb bond angle is close to 180 ~ , is most conducive to metallic behavior.

Reduced niobates with perovskite-related structures in which these conditions are fulfilled might be expected to undergo a semiconductor-metal transition and possess superconducting properties.

ACKNOWLEDGMENTS

We are grateful to the fellow workers of the Labora- tory of Inorganic Crystal Chemistry, Chemical Faculty, Moscow State University, and the Arrhenius Labora- tory, University of Stockholm, for their assistance with this study and many fruitful discussions.



This work was supported by the Russian Foundation for Basic Research, grants no. 97-03-33432a and no. 98-15-96058.

REFERENCES

1. Geselbracht, M.J., Richardson, T.J., and Stacy, A.M., Superconductivity in the Layered Compound LixNbO 2, Nature (London), 1990, vol. 345, pp. 324-326.

2. Kellerman, D.G., Gorshkov, V.S., Tyutyunnik, A.P., et al., Synthesis, Superconducting Properties, and Elec- tronic Structure of LixNbO2, Sverkhprovodimost: Fiz., Khim., Tekh., 1992, vol. 2, no. 11, pp. 2146-2153.

3. Kellerman, D.G., Zubkov, V.G., Tyutyunnik, A.P., et al., Superconductivity in NaxNbO2, Sverkhprovodimost: Fiz., Khim., Tekh., 1992, vol. 5, no. 5, pp. 961-963.

4. Akimitsu, J., Amano, J., Sawa, H., et al., A New Non- Cu-Oxide Superconductor: (Srl_xLnx)Nb206_y (Ln: La, Nd, Pr, and Ce), Jpn. J. Appl. Phys., 1991, vol. 30, pp. L1155-L1156.

5. Akimitsu, J., Amano, J., Tomimoto, K., et al., Supercon- ductivity in Sr(Ln)-Nb-O System (Ln: La, Pr, Nd, Ce, Gd, and Ho, Physica C (Amsterdam), 1991, vols. 185- 189, pp. 723-724.

6. Nakamura, A., A2Nbl +xOy (A = Ca, Sr) (0 < x < 0.5): New Reduced Niobate Superconductors with the Dis- torted Defect-Perovskite Structure, Jpn. J. Appl. Phys., 1994, vol. 33, part 2, pp. 583-586.

7. Solodovnikov, S.E, Yudanova, L.I., Gurov, V.V., et al., Search for Superconductors with the Tetragonal Tung- sten Bronze Structure in the Sr-La-Nb-O System, Zh. Neorg. Khim., 1995, vol. 40, pp. 179-183.

8. Gasparov, V.A., Strukova, G.K., and Khasanov, S.S., Superconductivity above 20 K in Barium-Niobium- Oxide Compounds, Pis'ma Zh. Eksp. Teor. Fiz., 1994, vol. 60, no. 6, pp. 425-428.

9. Ridley, D. and Ward, R., The Preparation of a Strontium- Niobium Bronze with Perovskite Structure, J. Am. Chem. Soc., 1955, vol. 77, pp. 6132-6136.

10. Krylov, E.I. and Sharnin, A.A., Preparation and Proper- ties of Niobium Bronzes, Zh. Obshch. Khint, 1955, vol. 25, pp. 1637-1640.

11. Hessen, B., Sunshine, S.A., Siegrist, T., and Jimenez, R., Crystallization of Reduced Strontium and Barium Nio- bate Perovskites from Borate Fluxes, Mater. Res. Bull,, 1991, vol. 26, pp. 85-90.

12. Isawa, K., Sugiyama, J., Matsuura, K., et al., Synthesis and Transport Properties of Srl_xNbO 3 (0.75 < x < 0.90), Phys. Rev. B: Condens. Matter, 1993, vol. 47, pp. 2849-2853.

13. Kreiser, R.R. and Ward, R., The Preparation of a Barium Niobium Bronze, J. Solid State Chem., 1970, vol. 1, pp. 368-371.

14. Casals, M.T., Alonso, J.A., Rasines, I., and Hidalgo, M.A., Preparation, Neutron Structural Study, and Characteriza- tion of BaNbO3: A Pauli-like Metallic Perovskite, Mater. Res. Bull., 1995, vol. 30, pp. 201-208.

15. Svensson, G. and Werner, P.-E., Determination of the Composition of BaNbO 3 Using Profile Refinement and Phase Analysis, Mater. Res. Bull,, 1990, vol. 25, pp. 9-14.

16. Lamure, J. and Colas, J.-L., Formation de roxyde double CaNbO3 du niobium(IV) et d'une varitt6 du niobate Ca2Nb207, C. R. Acad. Sci., 1970, vol. 270, pp. 700-701.

17. Trunov, V.K., Averina, I.M., Evdokimov, A.A., and Frolov, A.M., Crystal Structure of La0.33NbO 3, Kristal- lografiya, 1981, vol. 26, pp. 189-191.

18. Iyer, P.N. and Smith, A.J., Double Oxides Containing Niobium, Tantalum, or Protactinium, III, Acta Crystal- logr., 1967, vol. 23, pp. 740-746.

19. Abakumov, A.M., Shpanchenko, R.V., and Antipov, E.V., Synthesis and Properties of Niobium Bronzes R 1 + xNb309 (R = La, Ce, Nd), Mater. Res. Bull., 1995, vol. 30, no. 1, pp. 97-103.

20. Galasso, E, Layden, G., and Ganung, G., ANb206 and ATa206 Phases, Mater. Res. Bull., 1968, vol. 3, pp. 397--408.

21. Brusset, H., Giller-Pandraud, and Volitis, S.D., 15.tude du polymorphisme du mttaniobate de strontium SrNb206, Mater. Res. Bull,, 1971, vol. 6, pp. 5-14.

22. Marinder, B.-O., Wang, P.-L., and Werner, P.-E., Powder Diffraction Studies of SrNb206 and SrNb6016, Acta Chem. Scand. A, 1986, vol. 40, pp. 467-475.

23. Trunov, V.K., Averina, I.M., and Velikodnyi, Yu.A., Refined Crystal Structure of SrNb206, Kristallografiya, 1981, vol. 26, pp. 390-391.

24. Whiston, C. and Smith, A., Double Oxides Containing Niobium or Tantalum: II. Systems Involving Strontium or Barium, Acta Crystallogr., 1967, vol. 23, pp. 82-84.

25. Sirotinkin, S.P., Sirotinkin, V.P., and Trunov, V.K., Struc- ture of the Low-Temperature Form of BaNb206, Zh. Neorg. Khim. 1990, vol. 35, pp. 1609-1611.

26. Fayolle, J.-P., Studer, E, Desgardin, G., and Raveau, B., Etude de nouveaux oxydes ternaires d'europium diva- lent, de type bronze oxyg~ne de tungst~ne quadratique et de type ptrovskite, J. Solid State Chem., 1975, vol. 13, pp. 57-64.

27. Istomin, S.Ya., D'yachenko, O.G., Antipov, E.V., et al., Synthesis and Characterization of Reduced Niobates [Sr 1 _xLnxNb206, Ln = La, Nd], Mater. Res. Bull., 1994, vol. 29, no. 7, pp. 743-749.

28. Nguyen, N., Choisnet, J., and Raveau, B., Sur de nouveaux bronzes oxyg~nes de niobium quadratiques, C. R. Seances Acad. Sci., Ser. C, 1976, vol. 282, pp. 303-306.

29. Studer, E, Allais, G., and Raveau, B., Niobates et tantalates d'europium de type "bronze de tungst~ne quadratique"-I, J. Phys. Chem. Solids, 1980, vol. 41, pp. 1187-1197.

30. Hessen, B., Sunshine, S., Siegrist, T., etal., Structure and Properties of Reduced Barium Niobium Oxide Single Crystals Obtained from Borate Fluxes, Chem. Mater., 1991, vol. 3, pp. 528-534.

31. Feltz, A. and Langbein, H., Ober die Verbindungen

IV v Ba9CullNbV4045 und Untersu- BatNb 2 Nb 8 030 und .IV V chungen zur Mischkristallbildung mit BatTl 2 Nb s 03o,

Z. Anorg. Allg. Chem., 1976, vol. 425, pp. 47-56. 32. D'yachenko, O.G., Istomin, S.Ya., Fedotov, M.M., et al.,

Structure and Properties of Bat_xLnxNbloO3o, Ln = La,



Ce, and Nd Compounds, Mater Res. BulL, 1997, vol. 32, no. 4, pp. 409-419.

33. Bazuev, G.V. and Shveikin, G.P., Slozhnye oksidy ele- mentov s dostraivayushchimisya d- i f-obolochkami (Mixed Oxides of Transition and Rare-Earth Metals), Moscow: Nauka, 1985.

34. Subramanian, M., Aravamudan, G., and Subba Rao, G., Oxide Pyrochlores--A Review, Prog. Solid State Chem., 1981, vol. 15, pp. 55-143.

35. Sleight, A., New Ternary Oxides of Mercury with the Pyrochlore Structure, Inorg. Chem., 1968, vol. 7, pp. 1704-1708.

36. Brisse, E, Stewart, D., Seidl, V., and Knop, O., Pyro- chlores: VII. Studies of Some 2-5 Pyrochlores and Related Compounds and Minerals, Can. J. Chem., 1972, vol. 50, pp. 3648-3666.

37. Beech, E, Jordan, W.M., Catlow, C.R., et al., Neutron Powder Diffraction Structure and Electrical Properties of the Defect Pyrochlores Pbt.sM206.5 (M = Nb, Ta), J. Solid State Chem., 1988, vol. 77, pp. 322-335.

38. Chincholkar, V., New Compounds with Pyrochlore Structure, J. Inorg. Nucl. Chem., 1972, vol. 34, pp. 2973-2974.

39. Lewandowski, J., Pickering, I., and Jacobson, A., Hydro- thermal Synthesis of Calcium-Niobium and Tantalum Oxides with the Pyrochlore Structure, Mater Res. BulL, 1992, vol. 27, pp. 981-988.

40. Bazuev, G.V. and Shveikin, G.P., Phase Relations in the Y-Nb-O System, Zh. Neorg. Khim., 1973, vol. 18, pp. 1930-1934.

41. Safronenko, M.G., Bogatov, Yu.E., and Molodkin, A.K., Reaction of Lanthanum Oxide with Niobium(II) and (IV) Oxides, Zh. Neorg. Khim., 1988, vol. 33, pp. 2650-2653.

42. Istomin, S.Ya., D'yachenko, O.G., Antipov, E.V., and Svensson, G., Synthesis and Characterization of Reduced Niobates CaLnNb207, Ln=Y, Nd with a Pyro- chlore Structure, Mater Res. Bull., 1997, vol. 32, no. 4, pp. 421--430.

43. Istomin, S.Ya., D'yachenko, O.G., Antipov, E.V., et al., Synthesis and Chracterization of Reduced Niobates

CaLnNb207, Ln = La-Pr, Sm, Gd-Lu, with the Pyro- chlore-Type Structure, Mater Res. Bull., 1998, vol. 33, no. 8, pp. 1251-1256.

44. Bacon, EE., Hou, J.G., Sleight, A.W., and Nielsen, R.L., Nitride Formation by Air Ignition, J. Solid State Chem., 1995, vol. 119, pp. 207-209.

45. Istomin, S.Ya., Svensson, G., D'yachenko, O.G., et al., Perovskite-Type Cal _xSrxNbO3 (0 _< x _< l) Phases: A Synthesis, Structure, and Electron Microscopy Study, Z Solid State Chem., 1998, vol. 141, pp. 514-521.

46. Kopnin, E.M., Istomin, S.Ya., D'yachenko, O.G., et al., Synthesis, Structure, and Resistivity Properties of K l _ xBaxNbO3 (0.2 _< x _ 0.5) and K0.sSr0.sNbO 3, Mater. Res. Bull., 1995, vol. 30, no. 11, pp. 1379-1386.

47. Galasso, E, Layde, G., and Ganung, G., ANb206 and ATa206 Phases, Mater Res. Bull., 1968, vol. 3, pp. 397-408.

48. Goodenough, J.B., Metallic Oxides, Prog. Solid State Chem., 197l, vol. 5, pp. 145-399.

49. Isawa, K., Itti, R., Sugiyama, J., et al., Photoelectron Spectroscopic Study of Sr 1 _xNbO3, Phys. Rev. B: Con- dens. Matter, 1994, vol. 49, pp. 3534-3538.

50. Rao, C.N.R. and Gopalakrishnan, J., New Directions in Solid State Chemistry: Structure, Synthesis, Properties, Reactivity, and Materials Design, Cambridge: Cam- bridge Univ. Press, 1986. Translated under the title Novye napravleniya v khimii tverdogo tela, Novosibirsk: Nauka, 1990.

51. Shannon, R.D. and Prewitt, C.T., Effective Ionic Radii in Oxides and Fluorides, Acta Crystallogr, Sect. B: Struct. Crystallogr Cryst. Chem., 1969, vol. 25, no. 5, pp. 925-946.

52. Ellis, B., Doumerc, J.-E, Dordor, E, et al., Conduction Mechanism in Polycrystalline Na I _xSrxNbO3 Niobium Bronzes, Solid State Commun., 1984, vol. 51, pp. 913-917.

53. Abakumov, A.M., Lin'kova, E.B., Shpanchenko, R.V., et al., Synthesis and Properties of Potassium Niobium Oxyfluorides KNbO 3_xFx with Cubic Perovskite Struc- ture, Zh. Neorg. Khim., 1996, vol. 41, no. 6, pp. 985-988.


synthesis, structure, and properties of mixed niobium(iv,v) oxides

Documents