Journal of Physics and Chemistry of Solids 73 (2012) 136–138

Contents lists available at SciVerse ScienceDirect

Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jpcs

Dipotassium sodium niobium dioxide tetrafluoride, K2NaNbO2F4,crystal structure and characterization

Junjie Li a,b, Shilie Pan a,n, Xuelin Tian a, Fangfang Zhang a, Wenwu Zhao a,b

a Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences,

40-1 South Beijing Road, Urumqi 830011, Chinab Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e i n f o

Article history:

Received 28 March 2011

Received in revised form

27 September 2011

Accepted 16 October 2011Available online 20 October 2011

Keywords:

B. Crystal growth

C. X-ray diffraction

B. Chemical synthesis

C. Infrared spectroscopy

97/$ - see front matter & 2011 Elsevier Ltd. A

016/j.jpcs.2011.10.024

esponding author. Tel.: þ86 991 3674558; fa

ail address: slpan@ms.xjb.ac.cn (S. Pan).

a b s t r a c t

The paper presents a new data on the crystal structure of K2NaNbO2F4. Single crystals of K2NaNbO2F4,

were obtained from the system of KF–NaF–Nb2O5. The compound is cubic, space group Fm3m (No. 225),

and the unit cell parameters are a¼8.4726(4) A, Z¼4, Volume¼608.21(5) A3. The structure was solved

by the direct methods and refined to the reliability factors R1¼0.0116 (wR2¼0.0302). Its structure can

be described as three-dimensional frameworks formed of (0 1 0) [Na2Nb2O4F8]N layers stacked along b

axis. In the framework, niobium, oxygen, fluorine atoms form a rare [NbO2F4]3� anion group in an

anhydrous solid state environment. The IR spectrum further confirmed the presence of NbO2 and NbF4

groups in the structure. The thermal analysis suggests that K2NaNbO2F4 is an incongruently melting

compound.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Niobate fluoride compounds have attracted much attentionbecause of the versatile structures induced from various chemicalsubstitutions at the cation or anion sites. Among the existed niobatefluoride compounds, the most frequently observed anion group was[Nb2O6F]3� , which was originated from a mineral compound,(Na, Ca)(Nb, Ta)2O6(OH/F) [1]. Until now, many compounds with[Nb2O6F]3� anion were synthesized such as ASrNb2O6F (A¼Li, Naand Rb) [2], (Li, Ca)Nb2O6F [3] and KANb2O6F (A¼Ca and Sr) [4]. Inthese compounds, the substitutions occurred to the cation sites. It isnoticed that an anionic substitution can also occur, producing variousanion groups such as [NbOF5]2� [5,6] and [NbO3F]2� [7,8]. Althoughthe [NbO2F4]3� anion has been found in a hydrous compound ofCuNb(OH, F)7 �3H2O [9], to the best of our knowledge, it rarely bereported in an anhydrous solid state environment. In this study, wereinvestigated this [MOxF6�x]

n� system in an anhydrous solid stateenvironment. A cubic compound, K2NaNbO2F4, was obtained andstructurally characterized. Interestingly, the compound has beeninvestigated by Pausewang et al. in 1969, and they only reportedthe crystal system and space group of the title compound by powderX-ray diffraction [10]. There is no discussion of its structure. In thiswork, the structure of K2NaNbO2F4 was determined by single-crystalX-ray diffraction, and the crystal structure was discussed in detail.

ll rights reserved.

x: þ86 991 3838957.

Meanwhile, the thermal stability and infrared spectrum of thiscompound were studied. In this compound, oxygen, fluorine, nio-bium atoms form a rare [NbO2F4]3� anion group in an anhydroussolid state environment.

2. Experimental section

2.1. Crystal growth

All reagents used in the synthesis were of analytical grade.Single crystals of K2NaNbO2F4 were grown from the solution ofKF �2H2O: NaF: Nb2O5: NH4F with molar ratio of 2: 2: 1: 10.According to the ratio, the raw materials of 60 g were put into aplatinum crucible. The growth furnace was quickly heated to850 1C, kept at that temperature for 4 h, and then quickly cooledto 780 1C. Meanwhile, the platinum wire was introduced intoliquid surface. Then, the temperature of the furnace was held at780 1C for 6 h. When the growth of crystal ended, it was lifted outof the liquid surface. The furnace was then cooled to roomtemperature at a rate of 20 1C/h. As a result, transparent platecrystals were obtained for the structure determination.

2.2. Crystal structure

The structure of the compound was determined by the SMARTAPEX II Single-Crystal Diffractometer with a Bruker SMART APEXII 4K CCD diffractometer, and all calculations were performed

Table 1Crystal data and structure refinement for K2NaNbO2F4.

Empirical formula K2NaNbO2F4

Formula weight 302.1

Temperature 293(2) K

Wavelength 0.71073 A

Crystal system Cubic

space group Fm3m

Unit cell dimensions a¼8.4726(4) A

Volume 608.21(5) A 3

Z 4

Calculated density 3.299 g/cm3

absorption coefficient 3.431 mm�1

F(0 0 0) 568

Crystal size 0.16�0.10�0.05 mm3

Theta range for data collection 4.17 to 27.35 deg.

Limiting indices �10rhr10, �10rkr7, �10rhr10

Reflections collected/unique 774/56 [R(int)¼0.0289]

Completeness to theta¼27.39 100%

Absorption correction Numerical

Max. and min. transmission 0.7455 and 0.6242

Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 56/0/9

Goodness-of-fit on F2 1.232

FinAl R indices [I42s(I)]a R1¼0.0116, wR2¼0.0302

R indices (All data)a R1¼0.0116, wR2¼0.0302

Extinction coefficient 0.0096(11)

Largest diff. peak and hole 0.123 and �0.251 e A�3

a R1 ¼S Foj j� Fcj jj j=S Foj j and wR2 ¼ ½SwðF02�Fc

2Þ2=SwF0

4�1=2 for F0

2 42sF0

2� �

.

a

c

b

a

c

b

J. Li et al. / Journal of Physics and Chemistry of Solids 73 (2012) 136–138 137

with programs from the SHELXTL-97 crystallographic softwarepackage [11]. The structure was checked for missing symmetryelements with PLATON [12]. The structure was solved by directmethods. Equivalent reflections were then averaged. Final least-squares refinement is on F0

2 with data having F02Z2s(F0

2). Thereliability factors converged to R1¼0.0116, wR2¼0.0302. Accord-ing to the result of single-crystal X-ray diffraction, K2NaNbO2F4

crystallizes in the cubic system, space group Fm3m (No. 225) withunit-cell parameters a¼8.4726(4) A, Z¼4, Volume¼608.21(5) A3

(see Table 1).

2.3. Powder X-ray diffraction

Powder X-ray diffraction analysis of K2NaNbO2F4 was per-formed at room temperature with a scan step width of 0.021 and afixed counting time of 1 s/step using an automated Rigaku X-raydiffractometer equipped with a diffracted-beam monochromatorset for Cu Ka (l¼1.5418 A) radiation. The experimental powderX-ray diffraction pattern of K2NaNbO2F4 is in agreement with thecalculated one based on its single-crystal data, suggesting that theobtained powder phase is pure (see Figure S1 in the SupportingInformation).

Fig. 1. Coordination polyhedra in K2NaNbO2F4 (a), [Na2Nb2O4F8]6� building units

(b) and cations coordinate environments (c). The red octahedra are [NbO2F4]3�

anions and the blue octahedra are Na-centered. The green spheres are F or O

atoms; the pink spheres are K; the blue spheres are Na, the red spheres are

Nb atoms.

3. Results and discussion

As shown in Fig. 1(a), the K2NaNbO2F4 structure can bedescribed as three-dimensional frameworks formed of (010)[Na2Nb2O4F8]N layers stacked along the b axis. The layers areinterconnected via the sharing of O or F atoms. The layers arecomposed of small [Na2Nb2O4F8]6� building units that delineatefour-sided windows, which stack to form channels. The Kþ

cations are located in these channels (see Fig. 1(b)). The buildingunits are composed of Na(1) and Nb(1) polyhedra. The cationcoordinate environments are illustrated in Fig. 1(c). In thiscompound, oxygen, fluorine, niobium form a rare [NbO2F4]3�

anion in an anhydrous compound. The Nb(1) atom is located out

of the plane formed by the equatorial fluorides or oxides, whoseNb(1)-F(1) and Nb(1)–O(1) distances are 1.9508(17) A (see TableS1 in the Supporting Information). The coordination environ-ments of Na(1) atoms are similar to that of Nb(1) atoms. The Katoms are twelve-coordinate with O(1) and F(1) atoms.

To further verify the coordination environment of niobiumatom, we recorded Fourier transform infrared spectrometer inthe range 400 to 4000 cm�1 using the Bruker Equinox 55 Fourier

J. Li et al. / Journal of Physics and Chemistry of Solids 73 (2012) 136–138138

transform infrared spectrometer. Absorption peaks were assignedfrom literature precedents [13,14]. The bands at 816.1 and887.5 cm�1 are attributed to the NbO2 asymmetric stretch-ing mode of the NbO2 groups, while the bands at about499.7 cm�1 are attributed to the stretch vibrations of the NbF4

group in this complex. The result of its IR spectrum analysis isconsistent with the one obtained from its single-crystal X-raydiffraction analysis.

The thermal analysis of K2NaNbO2F4 were carried out on asimultaneous Netzsch STA 449C thermal analyzer instrument,with a heating rate of 2.5 1C/min in an atmosphere of flowingN2 from 25 to 1100 1C. The results of the thermal analysisare represented in Figure S3. From the thermogravimetric analy-sis (TGA) curve, it can be seen that weight loss occurs at 899 1C.At this temperature, there is an endothermic peak on thedifferential thermal analysis (DTA) curve. It tentatively suggeststhat K2NaNbO2F4 melts incongruently. To further verify thatK2NaNbO2F4 melts incongruently, 1 g of K2NaNbO2F4 polycrystal-line powder was heated to 910 1C, and quickly cooled to roomtemperature. Analysis of the powder X-ray diffraction patternof the recovered solid revealed that its diffraction pattern isdifferent from that of the initial K2NaNbO2F4 powder, whichfurther confirmed that K2NaNbO2F4 is an incongruently meltingcompound.

4. Conclusions

In this work we have obtained a compound K2NaNbO2F4 fromKF–NaF–Nb2O5 mixture, and solved its structure from single-crystal diffraction data. It has a three-dimensional frameworkformed of (010) [Na2Nb2O4F8]N layers. In the framework, oxy-gen, fluorine, niobium atoms form a rare [NbO2F4]3� anion in ananhydrous solid state environment. The infrared spectrum of thetitle compound is consistent with the crystallographic study.Finally, the thermal stability was studied, which suggested thatK2NaNbO2F4 is an incongruently melting compound.

Acknowledgment

This work is supported by Main Direction Program of Knowl-edge Innovation of Chinese Academy of Sciences (Grant no.KJCX2-EW-H03-03), the ‘‘National Natural Science Foundation ofChina’’ (Grant nos. 50802110, 21001114), the ‘‘One HundredTalents Project Foundation Program’’ of Chinese Academy ofSciences, the ‘‘Western Light Joint Scholar Foundation’’ Programof Chinese Academy of Sciences, the ‘‘High Technology Researchand Development Program’’ of Xinjiang Uygur AutonomousRegion of China (Grant no. 200816120) and Scientific ResearchProgram of Urumqi of China (Grant no. G09212001).

Appendix A. Supplementary materials

Supplementary data associated with this article can be foundin the online version at doi:10.1016/j.jpcs.2011.10.024.

References

[1] M.A. Subramanian, G. Aravamudan, G.V.Subba Rao, Prog. Solid State Chem.15 (1983) 55.

[2] J.H. Choy, J.Y. Kim, S.J. Kim, J.S. Sohn, O.H. Han, Chem. Mater. 13 (2001) 906.[3] F.L. Berre, M. Crosnier-Lopez, C. Galven, J. Fourquet, C. Legein, M. Body,

J. Buzare, Dalton Trans. 23 (2007) 2457.[4] C.Y. Yoo, S.J. Kim, J. Phys. Chem. Solids 69 (2008) 1475.[5] M.R. Marvel, J. Lesage, J. Baek, P.S. Halasyamani, C.L. Stern, K.R. Poeppelmeier,

J. Am. Chem. Soc. 129 (2007) 13963.[6] A.J. Norquist, C.L. Stern, K.R. Poeppelmeier, Inorg. Chem. 38 (1999) 3448.[7] M. Lundberg, O. Savborg, Chem. Scr. 13 (1979) 197.[8] F. Galasso, W. Darby, J. Phys. Chem. 66 (1962) 1318.[9] M.P. Crosnier-Lopez, H. Duroy, J.L. Fourquet, J. Solid State Chem. 108 (1994)

398.[10] G. Pausewang, W. Ruedorff, Z. Anorg. Allg. Chem. 364 (1969) 69.[11] SAINT-Plus, version 6.02A, Bruker Analytical X-ray Instruments, Inc.,

Madison, WI, 2000.[12] G.M. Sheldrick, SHELXTL, Version 6.14, Bruker Analytical X-ray Instruments,

Inc., Madison, WI, 2003.[13] G. Pausewang, W. Ruedorff, Z. Anorg. Allg. Chem. 366 (1969) 64.[14] K.B. Andersen, E. Christensen, R.W. Berg, N.J. Bjerrum, J.H. von Barner, Inorg.

Chem. 39 (2000) 3449.