Crystal structures and ionic conductivity of Na5MP2O8F2 (MAl, Ga)

SOLID STATE ELSEVIER

Solid State Ionics 73 (1994) 75-80

IONICS

Crystal structures and ionic conductivity of NasMP2OsF2 ( M - A1, Ga) Damodara M. Poojary a, Abraham Clearfield a,1, V.A. Timofeeva b, S.E. Sigaryov b * Department of Chemistry, TexasA & M University, College Station, TX 77843-3255, USA b Institute of Crystallography Academy of Sciences of USSR, Leninsky prospect 59, Moscow 117333, Russia Received 28 February 1994; accepted for publication 28 March 1994

Abstract

Single crystals of Na5MP2OsF2 (M = A1, Ga) were prepared by the flux technique. Both the compounds crystallize in the space group P3 with a=b= 10.468(3), c=6.599(2) • for the A1 compound and a=b= 10.560(2), c=6.660(2) A for the Ga compound. Low ionic conductivity in these solids is attributed to a highly ordered distribution of the sodium ions in the lattice. Keywords: Ionic conductivity - sodium, X-ray diffraction, Aluminum fluorophosphate

1. Introduction

Recently a number of papers have been published describing a new family ofsuperionic conductors [ I 7 ]. The general formula of this family of compounds is Naa_x+y_~M~P209_xFx.y, where M = C a , AI, Ga, Fe, Ti, etc. These compounds exhibit a glass-like (Vogel-Fulcher-Tammann) behavior in their conductivities and dielectric relaxation times at about the same temperature at which they undergo the phase transition to the superionic state. The crystal structures of several of the compounds have been determined by either powder methods [ 1,3,5,8 ] or single crystal techniques [4,9-15 ]. An analysis of the structural data indicates that the immobile sublattice is built up from MO6 octahedra and PO4 tetrahedra. The connectivity of the polyhedra differs for different members of the family. When the trans vertices of the octahedra are connected to each other and the cis vertices are connected to the PO4 tetrahedra, infinite

chains with repeat unit M 2 ( P O 4 ) 4 ( O , F) are formed. In contrast, layers containing isolated M O 6 octahedra and PO4 tetrahedra are obtained when both the cis and trans-vertices of the octahedra are connected to the vertices of the PO4 group [ 9,11 ]. Experimental evidence indicates that the transition from the isolated chains to the layer structure is associated with the amount of alkaline ions ( - x + y - z ) present. The layer structure is formed when this ionic concentration is a maximum. A detailed structural analysis of this family is available for the Ti compound with x--0, y = 0 [12,15] a n d x - - 1 , y = 0 [9] as well as for the Fe compound with x = 0 . 4 - 0 . 5 , y = 0 [4,13,14]. In this paper we report the crystal structures of NasA1P2OsF2 and NasGaP2OsF2 for which x = y = 1, z = 3. The structure of the aluminum compound has been studied earlier [ 11 ]. Since the samples were prepared by a different technique we have decided to investigate its structure also.

Corresponding author. 0167-2738/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved SSDI0167-2738(94)00112-6

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2. Experimental

Both the crystals were prepared by flux methods at temperatures of about 1000 K. Arlt et al. [ 11 ], however, used a relatively higher temperature ( ~ 1300 K) for the crystallization of the A1 compound. Ionic conductivity measurements were carried out in the frequency range 5-15 × 105 Hz. The conductivity value, a was determined from an analysis of the impedance plots of the C/Na5MP2OsF2/C and Ag/ NasMP2OsF2/Ag electrochemical cell. Diffraction data for the A1 compound were collected on a Rigaku AFC5R diffractometer with graphite monochromated Mo K~t radiation (2=0.71069/~) and a 12 KW rotating anode generator. Cell parameters were obtained from leastsquares refinement of 22 carefully selected reflections chosen from the 41.7-49.4 ° 20 shell immediately preceding data collection. Intensity data were collected up to a 20 limit of 50° using the o 9 - 2 0 method. Of the 475 reflections which were collected 465 were unique (Rint=0.058). The data were corrected for Lorenz and polarization effects. Based on intensity statistics the space group P3 was chosen. The data collection and processing for the Ga compound is similar to that of the AI compound except that Cu Ka ( 1.54178 ~,) radiation was used in the case of the Ga compound. The unit cell parameters were obtained from 24 reflections in the 20 range of 44.848.7 °. A total of 858 reflections were measured up to

a 20 limit of 130° among which 726 were unique (Rint = 0.1 1 ). Both structures were solved by the Patterson method using the TEXSAN program [ 16 ]. The positions of the P atom and those of AI/Ga were obtained by deconvolution of the 3-dimensional Patterson function. Other atoms were found by difference Fourier methods. All the atoms were refined anisotropically. The data were corrected for absorption and secondary extinction effects. The weighting scheme was based on counting statistics and included a factor ( p = 0.03) to downweight the intense reflections. The maximum and minimum peaks on the final difference Fourier map were 0.34 and - 0 . 3 0 e - / / k 3 for the A1 compound and the respective values for the Ga compound were 0.65 and - 0.51 e - / / k 3. Neutral atom scattering factors were taken from Cromer and Waber [ 17 ].

3. Results and discussion

The crystallographic data for both the compounds are given in Table 1. Final positional and isotropic thermal parameters are given in Tables 2 and 3, important bond parameters are listed in Tables 4-7. Packing of the A1 structure down the c-axis along with the atom numbering scheme is shown in Fig. 1, polyhedral representation of the structure down the b-axis is shown in Fig. 2. The A1 and Ga compounds are isostructural. In

Table 1 Crystallographicdata for the AI and Ga compounds. Compound Formula weight a (/~) c (/~) V (~3) Z Space group d~c (g/em3) Crystal size ( r a m ) 2 (~) Temperature (*C) Abs. Coeff (cm-~ ) R(Fo) Rw(Fo) GOF

NasA1P2OsF2 369.87 10.468(3) 6.599(2) 626.2(3) 3 P3(~147) 2.942 0.20x0.08×0.12 0.71069 23 9.30 0.032 0.040 1.53

NasGaP2OsF2 412.61 10.560(2) 6.660(2) 643.1 (2) 3 P3(~147) 3.196 0.15×0.10×0.05 1.54178 23 113.24 0.037 0.047 1.85

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Table 2 Positional parameters and B(eq) for NasAI(PO4)2F2. Atom

x

y

z

B(eq )

P(1) AI Na(1) Na(2) Na(3) Na(4) Na(5) Na(6) F(1) O(1) 0(2) 0(3) 0(4)

0.8157(1) I/2 0 1/2 0 0.8525(2) 0.3330 0.3330 0.6147(3) 0.9009(4) 0.6505(4) 0.4310(4) 0.8253(4)

0.6297(1) I/2 0 1/2 0 0.6836(2) 0.6667 0.6667 0.4778(3) 0.7974(4) 0.5705(4) 0.3048(4) 0.5699(4)

0.7492(2) 1/2 1/2 0 0 0.2666(3) -0.0834(5) -0.5270(5) 0.3101(4) 0.7434(5) 0,6991(5) 0.5806(5) 0.9547(5)

0.75(5) 0.51(7) 1.4(2) 1.6(1) 1.4(2) 1.53(8) 1.4(1) 1.4(1) 1.07(6) 1.44(7) 1.15(7) 1.19(7) 1.18(7)

Table 3 Positional parameters and B (eq) for NasGa(PO4)2F2. Atom

x

y

z

B(eq )

Ga P(1) Na(1) Na(2) Na(3) Na(4) Na(5) Na(6) F(1) O(1) 0(2) 0(3) 0(4)

1/2 0.8140(1) 0 1/2 0 0.8288(2) 0.3330 0.3330 0.5190(3) 0.8934(4) 0.9225(3) 0.4319(4) 0.7419(4)

1/2 0.6305(1) 0 1/2 0 0.6837(2) 0.6667 0.6667 0.3784(3) 0.7966(4) 0.5743(4) 0.1338(4) 0.5704(4)

1/2 0.7497(2) 1/2 0 0 0.2610(3) -0.0734(5) -0.5139(5) 0.2998(4) 0.7435(5) 0.7094(4) 0.5755(4) 0.9503(4)

0.77(4) 0.67(4) 1.3(1) 1.4(1) 1.8(2) 1.42(7) 1.3(1) 1.4(I) 0.99(9) 1.5(1) 1.I (1) 1.2(1) 1.2(1)

these structures the metal atoms are located on centers of symmetry at the center of the ac (and bc) faces as well as at the body center. Each metal atom is surrounded by four phosphate groups which in turn are used in bridging two different metal atoms. The metal atoms (A1, Ga) are octahedrally coordinated by four oxygen atoms (pairs of O2 and O3) of the phosphate group and two fluorine atoms. The A1-O bond lengths are in the range of 1.872 (3)-1.895 (3) A. In the case of the gallium compound, the M - O distances are slightly longer 1.945 ( 3 ) - 1.968 ( 3 ) than those in the A1 compound. The M - F bond distances are 1.829 (3) and 1.929(2) in the A1 and Ga compounds, respectively. Among the four phosphate oxygens, O I and 0 4 are involved only in sodium binding while the

other two bind both to the metal and sodium atoms. The P-O2 and P-O3 bond distances are longer (average value= 1.55 A) than the P-O1 and P-O4 distances (average value= 1.52 A). The oxygens with shorter P - O distances are involved only in sodium binding. The asymmetric unit contains six independent sodium positions. Nal and Na3 are located on the 3axis, Na5 and Na6 on the 3-axis, Na2 on a center of symmetry and Na4 on a general position. Pairs of sodiums, (Na l, Na3 and Na5, Na6) alternate along the c-axis with a separation of about 1/2 the c-dimension. Similarly the sodium atom Na2 alternates with the metal atom in the same direction. All the sodium atoms are six-coordinate and display a somewhat

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Table 4 Interatomic distances (]~) for the A1 compound. Estimated standard deviations in the least significant figure are given in parentheses. AI-FI AI-O2 AI-O3 PI-O1 P1-O2 P1-O3 P1-O4 Nal-O1 Na2-F 1 Na2-O2 Na2-O4 Na3-O1 Na4-F1 Na4-OI Na4-O1

1.829(3) 1.895 (3) 1.872(3 ) 1.521 (4) 1.553(4) 1.549(4) 1.519(4) 2.440(3) 2.442 ( 3 ) 2.410(3) 2.303(3) 2.498 (3) 2.358(3) 2.567(4) 2.470(4)

2× 2)< 2)<

6X 2 )< 2X 2× 6×

Na4-O2 Na4-O3 Na4-O4 Na5-FI Na5-FI Na5-F1 Na5-O4 Na5-O4 Na5-O4 Na6-F1 Na6-F1 Na6-F1 Na6-O3 Na6-O3 Na6-O3

2.582(4) 2.459 (4) 2.323 (4) 2.373(4) 2.376(4) 2.378(4) 2.342(4) 2.348(4) 2.345 (4) 2.333(3) 2.336(3) 2.338 (3) 2.357(4) 2.356(4) 2.362(4)

Na4-O2 Na4-O3 Na4-O4 Na5-FI Na5-F1 Na5-FI Na5-O4 Na5-O4 Na5-O4 Na6-F1 Na6-F1 Na6-F1 Na6-O3 Na6-O3 Na6-O3

2.544(4) 2.457(4) 2.336(4) 2.381 (3) 2.382(3) 2.386(3) 2.360(4) 2.365(4) 2.364(4) 2.329(3) 2.330(3) 2.334(3) 2.354(3) 2.348(3) 2.354 (3)

Table 5 Interatomic distances (]~) for the Ga compound. Ga-F1 Ga-O2 Ga-O3 PI-OI P1-O2 P 1-O 3 P 1-O4 NaI-OI Na2-F 1 Na2-O2 Na2-O4 Na3-O1 Na4-F 1 Na4-O 1 Na4-O 1

1.929(2) 1.968(3) 1.945(3) 1.521(4) 1.555(3) 1.549(3) 1.511(3) 2.468(3) 2.436(3) 2.382(3) 2.299(3) 2.526(3) 2.333(3) 2.505(4) 2.578(4)

2× 2)< 2)<

6× 2× 2x 2)< 6)<

Table 6 Intramolecular angles (°) for the A1 compound. F1-AI-F1 F1-A 1-O2 F1-A1-O2 F I-AI-O3 F1-A1-O3 O2-A1-O2 O2-AI-O3 O2-A 1-O3 O3-AI-O3

180.0 93.1 ( 1 ) 86.9( 1 ) 88.6( 1) 91.4(1 ) 180.0 91.1 ( 1 ) 88.9( 1) 180.0

d i s t o r t e d o c t a h e d r a l g e o m e t r y . N a 1 a n d N a 3 are surr o u n d e d by o n l y O 1 a t o m s a n d N a 2 by pairs o f F, 0 2 a n d 0 4 . N a 4 is b o n d e d by all the p h o s p h a t e oxygens

2)< 2)< 2 )< 2)<

O 1-P1-O2 O1-P1-O3 O I-P 1-O4 O2-P1-O3 O2-P1-O4 O3-PI-O4

110.4(2) 109.4(2) 112.1 (2) 105.7(2) 108.4(2) 110.6(2)

2)< 2 )<

a n d a f l u o r i n e a t o m . T h e three b i n d i n g sites o f N a 5 a n d N a 6 o c t a h e d r a are o c c u p i e d by F a t o m s while

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Table 7 Intramolecular angles (*) for the Ga compound. F1-Ga-F1 F1-Ga-O2 F1-Ga-O2 F1-Ga-O3 F l-Ga-O3 O2-Ga-O2 O2-Ga-O3 O2-Ga-O3 O3-Ga-O3

180.0 85.4( 1) 94.6( 1 ) 92.0( 1) 88.0( 1) 180.0 88.0( I ) 92.0( 1) 180.0

2X 2X 2X 2×

OI-P1-O2 O1-P1-O3 O1-P1-O4 O2-P1-O3 O2-P1-O4 O3-PI-O4

110.4(2) 109.5(2) 112.3(2) 105.1 (2) 107.8(2) 111.5 (2)

2× 2X

Fig. 2. Polyhedral representation of the AI structure down the baxis. The a-axis is horizontal and c-axis is vertical.

Fig. 1. Projection of the AI structure down the c-axis showing the atom numbering scheme. the other three are o c c u p i e d b y 0 4 a n d 0 3 a t o m s respectively. T h e structure p r e s e n t e d here for the A1 c o m p o u n d is very s i m i l a r to that r e p o r t e d by Arlt et al. [ 11 ] in t e r m s o f the u n i t cell p a r a m e t e r s a n d space group. Alt h o u g h the fractional c o o r d i n a t e s for all the a t o m s in these two studies are closely s i m i l a r i n their value, the sign o f their z-parameters are reversed. Since these crystals are c e n t r o s y m m e t r i c , the o p p o s i t e signs are n o t due to different e n a n t i o m e r s . T h e r e f i n e m e n t o f

Fig. 3. Same as Fig. 2 but the signs of the z-coordinates of the atoms are reversed.

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D.M. Poojary et al. / Solid State lonics 73 (1994) 75-80

our structure with opposite signs for the z-coordinates resulted in very high R-factors ( > 30%) indicating that these two structures are indeed different. The negative signs for the z parameter would result in a different orientation of polyhedra in the present study as compared to that reported by Arlt et al. [ 1 1 ] which is demonstrated in Figs. 2 and 3. The ionic conductivity values of the title compounds are less than 10 -7 s cm -1 at 293 K. The low a can be understood by taking into account the ordered distribution o f the sodium ions over the available crystallographic sites. As described above, there are a total of fifteen sodium atoms in the unit cells of these compounds. The a value, however, increases with rising temperature reaching a value of ~ 10 -4 s c m - ~ at 600 K [ 18 ]. In order to achieve this high a value the sodium sublattice of the c o m p o u n d s have to undergo a rearrangement in the u n i t cell. Structural studies of these ionic conductors at elevated temperatures are being investigated.

Supplementary material available Additional crystallographic data are available with the Publisher a n d are provided on request.

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