Mat. Res. Bul l . , Vol. 22, pp. 413-417, 1987. Printed in the USA. 0025-5408/87 $3.00 + .00 Copyright (e) 1987 Pergamon Journals Ltd.

AILaNb207 : A NEW SERIES OF LAYERED PEROVSKITES EXHIBITING ION EXCHANGE AND INTERCALATION BEHAVIOUR

J. Gopalakrishnan and Vasudeva Bhat Solid State and Structural Chemistry Unit

Indian Institute of Science, Bangalore 560012, India

B. Raveau Laboratoire de Cristallographie, Chimie et Physique des Solides, U.A. 251

ISMRa-Universit~, 14032 Caen Cedex, France

(Received November 24, 1986; Communicated by C .N .R . Rao)

ABSTRACT I

A new series of layered perovskite oxides, A LaNb207 (A = Li, Na, K, Rb, Cs, NH 4) constituting n = 2 members of the family A ~_iBnO3n+l, has been prepared. Their structure consists of double perovskite slabs interleaved by A atoms. Hydrated HLaNb207 is formed by topotactic pro- ton exchange of the A atoms in ALaNb207 (A = K, Rb, Cs). The hydrate readily loses water to give anhydrous HLaNb207 which is isostructural with RbLaNb207. HLaNb207 exhibits Bronsted acidity forming intercala- tion compounds with bases such as n-octylamine and pyridine. MATERIALS INDEX: nlobates, perovskites, lanthanium

Several titanates and titanoniobates built up of transition metal-oxygen octahedra possess a lamellar structure wherein the interlayer positions are occupied by large alkali metal atoms (see for review ref. 1-2). Such oxides exhibit ion-exchange and intercalation properties (1,2,3). The rigidity of the thin octahedral layers of such oxides is due to the fact that the octahedra share their edges. Few lamellar oxides, in which the layers are only built of corner-sharing octahedra are known at the present time. Recently, Dion et al (4) synthesized a new family of lamellar oxides of the formula ACa2Nb3OIo which consist of three-octahedra thick perovskite slabs Ca2Nb3Olo interleaved by layers of A atoms. In those latter compounds the perovskite slab which may be regarded as formed by slicing the perovskite structure along one of the three cubic directions is structurally the same as that found in the Aurivil- lius phase Bi4Ti3Ol2 (5) and the Ruddlesden-Popper oxides Sr4Ti3010 (6). Re- cently Jacobson et al. have synthesized similar oxides with thicker perovskite slabs and investigated their ion exchange and intercalation behaviour (7). At the end of this work it has come to our knowledge that M. Tournoux et al. (8) had discovered new members of the family A I (A'n_iNbnO3n+l) with n = 2, 3 and 4. We report in this paper the synthesis and structural characterization of a related series of oxides, AILaNb207 (A = K, Pb, Cs) which may be regarded as n = 2 members of the general family A ~_iBnO3n+l of layered perovskites. We also describe the ion exchange and intercalation behaviour of the new series

of oxides. 413

414 J. GOPALAKRISHNAN, et 8/ . Vol . 22, No. 3

EXPERIMENTAL

A£aNb207 (A = K, Rb, Cs) were prepared by heating in air appropriate mixtures of A2CO 3, La203 and Nb205 at llOO°C for 2 days with one grinding in between.20-25 mole % excess of the carbonate was added to compensate for the loss due to volatilization. After the reaction, the solid product was washed with distilled water and dried in air oven at IIo°C. ALaNb207 with A = Li, Na and NH 4 were prepared from RbLaNb207 by ion exchange with the corresponding molten nitrates. Proton exchange of AlaNb207 (A = K, Rb, Cs) was carried out in aqueous HNO 3 (2-6 N) at 60°C for 24 h. to prepare HLaNb207.xH20. The ex- change was complete as revealed by quantitative determination of the alkali metal in the filtrate by flame photometry. Intercalation of pyridine and n-

octylamine in HLaNb207 was carried out by refluxing the solid with the base (pyridine) or with a ~-heptane solution of the base (~-octylamine). Interca- lation of ~-octylamine was complete in about 2 days while pyridine intercala- tion required much longer time (20 days).

The solids were characterized by X-ray powder diffraction and thermogravi- metry. X-ray patterns were recorded with a Jeol JDX-BP powder diffractometer using Ni-filtered CuK~ radiation. Thermogravimetric curves were recorded using a Sartorius microbalance at a heating rate of I°C per minute.

RESULTS AND DISCUSSION

Powder X-ray diffraction data reveal that single-phase soiids of composi- tion ALaNb207 are formed by the reaction of A2CO 3 (A = K, Rb, Cs), La 2 03 and Nb205. The diffraction patterns are related to ACa2Nb3OIo (4) and could be indexed on tetragonal or orthorhombic systems ; the a and b parameters are around 3.9 ~ and the c-parameter is around 10.5 ~ or--their--multiple (Table i). As compared to ACa2Nb~OIo, the ~-parameters of ALaNb207 is shorter by-~3.9 or ,~ 2x3.9 ~ (the height of NbO 6 octahedron is -~,3.9 ~). The unit cell para- meters of ALaNb207 are therefore consistent with a layered perovskite struc- ture consisting of two-octahedra-thick LaNb207 layers interleaved by alkali me- tal atoms (Fig.l). The Rb and Cs compounds are similar to RbNdNb207 (8) probably

0 0 0 0 • • •

b -

[o) [b) (c)

FIG. i

Idealized structures of (a) RbLaNb207 (b) KLaNb207 and (c) NaLaNb207. The large circles between perovskite slabs denote alkali metal atoms and small circles within the perovskite slabs, lanthanum atoms.

V o l . 22 , N o . 3 L A Y E R E D P E R O V S K I T E S 415

Table i. Crystallographic data of ALaNb207

Compound Lattice parameters Volume per (~) formula unit (~3) Z

KLaNb207 Or thorhombic : a = 7.806(14) -- 161 8 b = 7.668(14)

c = 21.54 (4)

RbLaNb207 T e t r a g o n a l : a = 3.885(2) 166 I x

c = 10.989(3)

CsLaNb207 T e t r a g o n a l : a = 3.905(2) 171 i xx

c = 11.185(6)

LiLaNb207 T e t r a g o n a l : a = 3.877(4) 153 2

c = 20.31 (2)

NaLaNb207.2H20 T e t r a g o n a l : a = 3 .899(2) 195 2 c = 2 5 . 7 1 (1)

NaLaNb207 Tetragonal : a = 3.904(4) 160 2

c = 20.99 (2)

NH4LaNb207 T e t r a g o n a l : a = 3.879(7) 165 1

c = 10.95(2)

HLaNb207.xH20 T e t r a g o n a l : a = 3.891(2) 185 i

c = 12.213(6)

HLaNb207 T e t r a g o n a l : a = 3.894(3) 159 i

c = 10.459(7)

~-C8HI7NH 3 [LaNb207] Tetragonal : a = 3.886(7) 459 I

c = 30.38 (5)

C5H5NHFLaNb207] Tetragonal : a = 3.90(2) 221 1

c = 14.55(4)

x p(experimental) = 5.11 g/cm 3 ; p(X-ray) = 5.23 g/cm 3 .

Mx p(experimental) 5.45 g/cm 3 ; ~(X-ray) 5 54 g/cm 3.

belonain~ to the space group P4/m or P4/mmm(no systematic absences) ; accordin-

gly the adjacent perovskite slabs are not likely to be dispaced with respect

to one another. On the other hand, the doubling of the ~ parameter in the case

of KLaNb207 which is isostructural with KNdNb207 (8) may indicate a displacement

of the adjacent perovskite slabs by b/4 (Fig.l). The probable space croups for

this compound are P2aa or Pmaa, the systematic absence being hkO : h=2n, hOl : h=2n.

416 J . GOPALAKRISHNAN, et 8]. Vol. 22 , No. 3

We could prepare the lithium, sodium and ammonium analogues of ALaS~207 by ion exchange starting from RbLaNb207. The powder diffraction data show that the ammonium compound is isostructural with the ~Rb and C~ analogues, while the Li and Na compounds possess a slightly different structure. It is reaso- nable to expect the structure of Li and Na compounds to be similar to ~ - NaCa2Nb3Oio (4) where the alkali metal atoms are statistically distributed at the interlayer positions of a Srn+lTinO3n+1-type structure (Fig. ic). The unit cells of all the members of ALaNb207 may involve a doubling of ~ and ~ axes due to tilting of NbO 6 octahedra in the perovskite slabs, although powder X-ray diffraction does not reveal such a doubling. Electron diffraction shows such a doubling in the case RbLaNb207. ~le have been able to isolate a hydrated form of the sodium co~pound, NaLaNb207.H20 ; the larger ~ parameter (25.62 ~) of this compound reveals that the water molecules are located at interlayer posi- tions probably solvating the alkali metal ion.

Proton exchange of ALaNb207 (A = K, Rb, Cs) in dilute nitric acid readily yields HLaNb207.xH20. The number of water molecules in the hydrated phase as determined by thermogravimetry is 3.4. The hydrated phase, which crystallizes in a tetragonal structure with a = 3.891 and c = 12.213 ~, readily loses water around 90°C to give the anhydrous phase which is stable up to 3OO°C. HLaNb207 is very different from HNdNb207 (8) : it is indeed isostructural with RbLaNb207 and CsLaNb207 crystallizing in a tetragonal structure with ~ = 3.894 ~ and c = 10.459 ~, whereas HNdNb207 belongs to the same structural type as Sr3Ti207. The cohesion of perovskite layers in HLaNb207 is obviously due to hydrogen bonding as in other protonic layered oxides. An estimate of the thickness of the LaNb207 layer in ALaNb207 can be made by assuming that the Rb-O and Cs-O polyhedra are similar to those in RbCa2Nb3Olo. The values given for these po- lyhedra are 3.42 and 3.57 ~ respectively (9). The thickness of the LaNb207 layer is likely to be 7.57-7.62 ~. Thus the O-H...O distance in HLaNb2OTis expected to be 2.84-2.88 ~. This value is slightly larger than the O-H...O distance found in other lamellar oxides such as HTiNbO 5 (2.51-2.63 ~) and HCrO 2 (2.49 ~) (Ii), but is close to the value reported for HCa2Nb3010 (2.88 ~) (9).

HLaNb207 shows Bronsted acidity similar to other protonated oxides such as HTiNbO 5 and HCa2Nb3010. Thus reaction with ~-octylamine in ~-heptane yields the intercalation compound, n-C8HI7NH3[LaNb207J which crystallizes in a tetragonal structure with a = 3.886--~ and c = 30.38 A. The increase in c- parameter on intercalation--(~ 19.9 ~) indicates that the organoammonium c~tion probably exists in a bilayer configuration with kinks (12). HLaNb207 also reacts with pyridine to form C5H5NH[LaNb20 ~ with a tetragonal cell, ~ = 3.90 and _c = 14.55 A.° The fact that HLaNb207 intercalates pyridine shows that it is a stronger Bronsted acid than HTiNbO 5 and similar layered oxides. The in- crease in c--parameter due to pyridine intercalation (~4.0 ~) is smaller than the increases observed in other pyridine intercalation compounds such as those of MoO 3 (13) and V205 gel (14). The presence of an infrared absorption band at 1530 cm -I reveals that, as expected, pyridine is present as pyridinium ion in the intercalation compound. The absence of c-axis doubling in the inter- calation compounds probably indicates that adjacent perovskite slabs are not displaced with respect to one another.

ACKNOWLEDGEMENT

One of us (J.G.) thanks the Indian National Science Academy, New Delhi and the French Academy of Sciences, Paris, which enabled him to visit Profes- sor B. Raveau's laboratory at the University of Caen.

Vol. 22, No. 3 LAYERED PEROVSKITES 417

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