Crystal structure of a novel cubic pyrophosphate WP2O7

Journal of Alloys and Compounds 309 (2000) 83–87

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Crystal structure of a novel cubic pyrophosphate WP2 O 7 V.V. Lisnyak*, N.V. Stus, N.S. Slobodyanik, N.M. Belyavina, V.Ya. Markiv Taras Shevchenko University, 64 Volodymyrska, 01033 Kyiv, Ukraine Received 10 March 2000; accepted 26 April 2000

Abstract The crystal structure of a novel pyrophosphate WP2 O 7 has been studied by X-ray powder diffraction. It was found that the crystal ˚ The tetravalent state of tungsten in the structure of WP2 O 7 belongs to the well-known MoP2 O 7 structure type (a57.9502 A). pyrophosphate has been confirmed by magnetic susceptibility measurements.  2000 Elsevier Science S.A. All rights reserved. Keywords: Tungsten pyrophosphate; Crystal structure; X-ray powder diffraction; Magnetic susceptibility

1. Introduction Recent investigations of phase formation in the WO 3 – W–P2 O 5 system were devoted to the phases related to different groups of tungsten phosphate bronzes such as MPTB P and DPTB P (mono- and diphosphate tungsten bronzes with pentagonal tunnels), which are formed by ReO 3 type slabs [1,2]. Their high conductivity, as well as other electrical and physical properties were of great importance in the investigation of novel compounds in these series. Series of tungsten phosphate bronzes with the general formula (WO 3 ) 2m (PO 2 ) 4 , m52–8 have been obtained in the last two decades, according to Ref. [3]. It should be noticed that tungsten atoms in various structures can adopt different valence states, namely 15 and 16. The main properties of these bronzes are the electron transport and the charge instability. Some representatives of these series have specific magnetic properties and metallic conductivity at 258C. So the investigation of a novel compound of the series is of great importance for the attainment of novel functional materials for technology and industry. The first representative of the (WO 3 ) 2m (PO 2 ) 4 series, the tungsten (IV) pyrophosphate with (WO 3 ) 2 (PO 2 ) 4 composition, has not been reported yet. The tungsten pyrophosphate has been isolated during a phase equilibrium study in the NH 3 –H 2 O–P2 O 5 –WO 3 system. The crystal structure of WP2 O 7 is related to the well-known structural type of cubic MoP2 O 7 rather than to *Corresponding author. Fax: 1380-44-276-7542. E-mail address: [email protected] (V.V. Lisnyak).

compounds of the (WO 3 ) 2m (PO 2 ) 4 series. So, the crystal structure refinement of WP2 O 7 is reported in the present publication.

2. Experimental details Polycrystals of the WP2 O 7 composition were prepared by heating, in a first stage, of appropriate mixtures of NH 4 H 2 PO 4 and WO 3 in an alumna crucible in air at 723 K to decompose the phosphate. Then the product obtained was added to a 25 wt.% of 85% H 3 PO 4 solution before final heating in hydrothermal conditions up to 673 K. After cooling, the fused mass was leached with boiling distilled water, dried at 473 K to remove traces of water. The obtained polycrystalline powder consists of small black crystals with cubic habit. The analysis for phosphorus and tungsten in the specimens, carried out by X-ray fluorescence on VRA-10, indicated that the W content was 51.32 wt.% while the P content was 17.00 wt.%. The theoretically predicted amounts were 51.40 and 17.32 wt.%, respectively. The water contents (H 2 O wt.%) determined by thermogravimetry were 19.4 wt.% at 295 K, 12 wt.% at 395 K, 1 wt.% at 673 K, and 0.0 wt.% at 1273 K. Before the X-ray study, the isolated product was calcined at 1273 K in an argon atmosphere. WP2 O 7 was found to be stable in a temperature range up to 1373 K. X-ray powder diffraction data (Cu Ka radiation) were collected using a DRON-3 diffractometer controlled by an IBM computer [4]. The diffractometer was equipped with incident soller slits, theta-compensating slits, 0.1-mm

0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00922-1

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receiving slit and scintillation detector. An external standard (silicon) was used for instrumental calibration. The scanning method parameters are the following: observation range 2u 510–858, step scan 0.028, counting time per step 8–10 s, specimen rotation. The diffraction lines consist of Cu Ka 1 , and Cu Ka 2 components, because only the incident beam was filtered. The peak positions and the integral intensities of the observed reflections were determined using full profile analyses. After removal of the Ka 2 component the profiles were fitted using Lorentz functions. Unit cell refinements using the corrected powder diffraction data were carried out with a least-squares refinement program. The crystal structure determination was performed using an original software program package with a special bank for structure types of inorganic compounds (more than 7000 information units) and X-ray diffraction images for this structure type [4]. The testing of the structure models and the structure parameter refinements were carried out with original software [4]. The magnetic susceptibility was measured using a Faraday balance in the temperature range between 77 and 500 K in applied fields of 5.6, 7.8 and 10.3 kOe. The susceptibility of the phosphate displayed a weak field dependence. The field-dependent part of the susceptibility was considered to be due to a small amount of impurities. The reciprocal specific susceptibility values x 21 plotted g vs. the temperature showed a linear relation over a wide

temperature range. The field independent part of the susceptibility was obtained by linear extrapolation of susceptibility to H 21 5 0 (see Ref. [5]).

3. Results and discussion The structural analysis of the WP2 O 7 compound has been carried out only on the basis of powder diffraction data because our efforts to obtain a well-formed single crystal failed. The first stage of the structural analysis provided information on the space group and the crystallographic system. The indexing of powder diffraction patterns led to cubic symmetry. A least-square refinement of the unit cell led to the data listed in Table 1. Comparison of the X-ray powder diffraction data for WP2 O 7 with those for numerous cubic pyrophosphates of tetravalent metals from the structure type bank led to only one structure model. Fig. 1 shows the experimental diffraction pattern for the tungsten pyrophosphate. The initial position parameters of each atom in WP2 O 7 were estimated from those in the MoP2 O 7 cubic phase [6], which is the nearest neighbor in the series of tetravalent phosphates. Comparison of the observed and calculated intensities shows a good agreement for the proposed structure model. The result of refinement procedure (lattice

Table 1 Powder diffraction data for WP2 O 7 a ˚ d cal (A)

˚ d obs (A)

Ical

Iobs

hkl

˚ d cal (A)

˚ d obs (A)

Ical

Iobs

hkl

4.590 3.975 3.555 3.246 2.811 3.246 2.811 2.650 2.397 2.295 2.205 2.125 2.125 1.988 1.928 1.928 1.874 1.824 1.778 1.778 1.735 1.735 1.695 1.623 1.590 1.559 1.559 1.530

4.592 3.977 3.556 3.245 2.811 3.245 2.811 2.651 2.397 2.295 2.204 – – 1.987 – 1.927 1.874 1.824 – 1.778 – 1.735 1.701 1.623 – – – –

833 1000 130 150 445 150 445 0 532 141 17 1 0 49 14 5 9 147 90 140 11 1 4 174 2 0 0 36

837 982 111 120 442 120 442 – 563 143 10 1 1 58 – 13 10 189 – 227 – 6 2 164 1 1 1 –

111 002 021 112 022 112 022 221 113 222 023 132 123 004 041 223 114 331 042 024 142 124 332 224 043 143 134 333

1.595 1.530 1.530 1.476 1.476 1.452 1.452 1.405 1.384 1.384 1.363 1.344 1.344 1.325 1.325 1.307 1.290 1.290 1.290 1.257 1.257 1.242 1.257 1.242 1.242 1.242 1.242 1.212

– – 1.530 – 1.477 – 1.451 1.406 – – – – 1.344 – 1.325 – – – 1.290 – 1.257 – 1.257 – – – 1.242 1.212

0 36 156 5 0 4 1 66 1 1 0 51 38 8 68 1 2 1 5 25 20 0 20 0 0 1 0 42

1 – 195 – – – 4 53 1 1 1 – 89 – 67 1 – 7 – – 34 – 34 – – – – 41

134 333 115 234 025 125 152 044 225 441 334 135 153 006 442 061 116 235 253 026 062 443 062 443 045 162 126 335

a

Cu Ka radiation.

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Fig. 1. Part of the X-ray diffraction powder patterns (points) and calculated pattern (solid line) for WP2 O 7 (Cu Ka radiation).

constant, atomic coordinates, total isotropic temperature factor, etc.) are listed in Table 2. The high value of the B parameter for the cubic tungsten pyrophosphate cannot be fully explained from chemical bond considerations. It may be caused either by a very weak hexagonal or orthorhombic distortion or by defects. All interatomic distances are in very good agreement with previous data obtained on molybdenum (IV) pyrophosphate, as shown in Table 3. It should be noticed that the calculated lattice constant of WP2 O 7 is in good agreement with the cell volume dependence on ionic radii of the tetravalent element. The unit cell parameters of various M IV P2 O 7 compounds are listed in Table 4. Tungsten pyrophosphate WP2 O 7 is isostructural with all representatives of the cubic pyrophosphate series and is related more closely with MoP2 O 7 . Its structure can be described as a network of WO 6 octahedra sharing corners with P2 O 7 groups thus leading to a three-dimensional framework which is closely related to that of the molybdenum diphosphate MoP2 O 7 [6]. This latter structure is NaCl-like and can be described as made up of two interpenetrating face centered cubic lattices, one with MoO 6 octahedron at each lattice point and the other with

Table 3 Selected interatomic distances and angles in M IV P2 O 7 (M IV5Mo, W) Atom–atom

WP2 O 7

M IV –O(2) P–O(1) P–O(2) O(2)–M IV –O(2) O(1)–P–O(1) O(1)–P–O(2)

MoP2 O 7 [6]

˚ Distance (A) 1.860(13)36 1.540(7) 1.503(14)33 Angles (8) 85.736 94.336 113.533 105.133

1.92536 1.514(0) 1.420(0)33 89.936 90.136 112.933 105.733

P2 O 7 groups. In both WP2 O 7 and MoP2 O 7 each octahedron is surrounded by 12 octahedrons at a distance of about ] ˚ 8 /Œ2 A. The WO 6 octahedron is connected to each of them in the same manner by two of its vertices via one edge of a PO 4 tetrahedron for one vertex and two edges from the P2 O 7 group for the other. The projection of the structure of W IV P2 O 7 along the z direction is represented in Fig. 2. The P–O distances are rather irregular. The ˚ are those in which the longest ones (1.539, 1.510 A) oxygen bridging atoms are involved and the shortest ones ˚ correspond to oxygen atoms that are also (1.503, 1.42 A)

Table 2 Crystallographic data for WP2 O 7 (MoP2 O 7 type structure) Atom

Site

x /a

4.00 W(1) 8.00 P(1) 4.00 O(1) 24.00 O(2) Space group ˚ Lattice constant (A) Calculated density for Z54 Independent reflections ˚ 2) Total isotropic B factor (A Reliability factors Specimen thickness (mm) Linear absorption coefficient

4(a) 0.000(0) 8(c) 0.3882(9) 4(b) 0.5000(0) 24(d) 0.2121(13) Pa3 a57.9502(3), b57.9502(3), c57.9502(3) d calc 54.729 g cm 23 , d obs 54.03 g cm 23 102 1.22(7) R W 50.0457, R IO 50.0660 100.00 495.65 cm 21

y /b

z /c

Occupancy

0.000(0) 0.3882(9) 0.5000(0) 0.0858(26)

0.0000(0) 0.3882(9) 0.5000(0) 0.9528(27)

1.000(0) 1.000(0) 1.000(0) 1.000(0)

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Table 4 The unit cell parameters of typical MP2 O 7 cubic pyrophosphates Compound

Space group

˚ a (A)

References

GeP2 O 7 TiP2 O 7 SnP2 O 7 ReP2 O 7 MoP2 O 7 WP2 O 7 PbP2 O 7 NbP2 O 7 ZrP2 O 7

Pa3 Pa3 Pa3 Pa3 Pa3 Pa3 Pa3 Pa3 Pa3

7.62 7.80 7.89 7.94 7.94 7.95 8.01 8.06 8.24

[7] [7] [7] [7] [6] [ a] [7] [7] [7]

a

Presented publication.

bonded to Mo IV and W IV atoms, respectively. Such peculiarities seem to be quite usual in the pyrophosphates’ crystal chemistry. The temperature dependence of the reciprocal susceptibility for WP2 O 7 is shown in Fig. 3. The effective magnetic moment and the paramagnetic Curie temperature were calculated to be 1.6 mB per formula unit and 218 K, respectively. The magnetic moment of W 41 , considering a Russell–Saunders coupling scheme for the tungsten ion is 1.63 mB [8]. Assuming that all tungsten atoms are tetravalent (6d 2 configuration), as expected from the chemical formula, the effective magnetic moment is calculated to be 1.61 mB , which fairly well coincides with the observed value.

4. Conclusion The novel pyrophosphate WP2 O 7 is characterized by X-ray powder diffraction. It was found that the crystal structure of WP2 O 7 belongs to the well-known cubic M IV P2 O 7 structure type. The tetravalent state of tungsten pyrophosphate was detected by magnetic susceptibility measurement.

References

Fig. 2. Projection of the structure of W IV P2 O 7 along [001].

[1] A. Leclair, M.-M. Borel, Eur. J. Solid State Inorg. Chem. 65 (1987) 45. [2] M. Greenblat, E. Wang, in: Proceedings of the 197th ACS National Meeting ’89, Dallas, 9–14 April, 1989, p. 520. [3] Z.S. Teweldemedhin, K.V. Ramanujachary, J. Solid State Chem. 95 (1991) 21.

Fig. 3. Temperature dependence of reciprocal susceptibility for WP2 O 7 .

V.V. Lisnyak et al. / Journal of Alloys and Compounds 309 (2000) 83 – 87 [4] V. Markiv, N. Belyavina, in: Proceedings of the Second International Scientific Conference of Engineering and Functional Materials, EFM ’97, Lviv, 14–16 October, 1997, p. 260. [5] W.S. Glaunsinger, H.S. Horowitz, J. Solid State Chem. 29 (1979) 117.

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[6] A. Leclair, M.-M. Borel, A. Grandin, B. Raveua, Eur. J. Solid State Inorg. Chem. 25 (1988) 323. [7] D.E.C. Corbridge, in: 4th Edition, The Structural Chemistry of Phosphorus, Elsevier, Amsterdam, 1974, p. 542. [8] J.P. Girolt, M. Goreaud, Ph. Labbe, Mater. Res. Bull. 16 (1981) 811.