Unusual 180° P-O-P Bond Angles in ZrP 2 O 7 N. Khosrovani, V. Korthuis, and A. W. Sleight* Department of Chemistry, Oregon State University, Corvallis, Oregon 97331-4003 T. Vogt Physics Department, Brookhaven National Laboratory, Upton, Long Island, New York 11973 ReceiVed July 7, 1995 X The structure of cubic ZrP 2 O 7 at room temperature has been solved and refined using a combination of modeling and high-resolution neutron powder diffraction data. The cell edge is 24.74 Å, the space group is Pa3 h, and Z is 108. For those P 2 O 7 units not on a 3-fold axis, the P-O-P angles range from 134° to 162°. Two crystallographically distinct P 2 O 7 groups are on three fold axes with P-O-P angles thus constrained to be 180° on average. The structure of cubic ZrP 2 O 7 was also refined from data taken at 227, 290, 371, 435, and 610 °C. The 3 × 3 × 3 superstructure present at room temperature disappears at about 290 °C, and all P-O-P angles of P 2 O 7 are then constrained by symmetry to be 180° on average. The exceptionally low thermal expansion shown by ZrP 2 O 7 above 290 °C is likely related to the unusual P-O-P angle. Introduction A cubic structure is found for phosphates of the type A 4+ P 2 O 7 , where A 4+ may be Si, Ge, Sn, Pb, Ti, Zr, Hf, Mo, W, Re, Ce, Th, U, or Pu. 1-13 Double substitution on the A site leads to a series of the type A 3+ 0.5 A 5+ 5.5 P 2 O 7 , where A 3+ may be Bi, Sb, or a rare earth and A 5+ may be Nb, Ta, or Sb. 14 One form of Sb 3+ Sb 5+ (P 2 O 7 ) 2 also has a closely related structure. 15 Arsen- ates are reported with this structure for A 4+ ) Zr or Th, 16,17 and vanadates are reported for A 4+ ) Zr or Hf. 18-21 This cubic AM 2 O 7 structure where M may be P, As, or V can be viewed as related to the NaCl structure. The cation is A 4+ , and the anion is (M 2 O 7 ) 4- . The ordered orientation of the (M 2 O 7 ) 4- group necessarily lowers the symmetry; the highest symmetry possible for this framework (Figure 1) is Pa3 h. In this ideal structure with Z ) 4, the M 2 O 7 group is on a 3-fold axis with the bridging oxygen on an inversion center. The M-O-M bond angle is, therefore, constrained to be 180° on average. At high temperature, it appears that all compounds in this structure type can be described in this Pa3 h space group with Z ) 4. However, some, and perhaps all, of these cubic AM 2 O 7 compounds undergo a phase transition with decreasing temperature. It appears that the low-temperature structure remains cubic with the same space group but with a 3 × 3 × 3 superstructure. Only in the case of SiP 2 O 7 has a solution to this superstructure been reported. 22 The thermal expansion of cubic AM 2 O 7 phases can be very low and even negative in the temperature range where there is no superstructure. In the case of ZrP 2 O 7 , the thermal expansion might be considered normal from room temperature to about 290 °C. The superstructure disappears at this temperature, and the thermal expansion from 290 to 610 °C is about 3.5 × 10 -6 °C -1 , which is very low. 21,23,24 The thermal expansion actually becomes negative in the cases of ThP 2 O 7 , UP 2 O 7 , ZrV 2 O 7 , and HfV 2 O 7 . 19-21,23 X Abstract published in AdVance ACS Abstracts, December 15, 1995. (1) Laud, K. R.; Hummel, R. A. J. Am. Ceram. Soc. 1971, 54, 296. (2) Harrison, D. E.; Hummel, R. A. J. Am. Ceram. Soc. 1959, 42, 487. (3) Merz, K. M.; Smyth, H. T.; Kirchner, H. P.; Beal, J. L. Report No. PI-1273-M-8; Cornell Aeronautical Laboratory, 1960. (4) Harrison, D. E.; McKinstry, H. A.; Hummel, F. A. J. Am. Ceram. Soc. 1954, 37, 277. (5) Burdese, A.; Lucco Borlera, M. Ann Chim. (Rome) 1963, 53, 333. (6) Burdese, A.; Lucco Borlera, M. Atti. Accad. Sci. Torino, Cl. Sci. Fis., Mater. Nat. 1959, 94, 107. (7) Huang, C.-H.; Knop, O.; Othen, D. A. Can. J. Chem. 1975, 53, 79. (8) Vo ¨llenkle, H.; Wittman, A.; Nowotny, H. Monatsh. Chem. 1963, 94, 956. (9) Hagman, L.-O.; Kierkegaard, P. Acta Chem. Scand. 1969, 23, 327. (10) Liebau, F.; Bissert, G.; Koppen, N. Z. Anorg. Allg. Chem. 1968, 359, 113. (11) Kinomura, N.; Hirose, M.; Kumada, N.; Muto, F. Mater. Res. Bull. 1985, 20, 379. (12) Bjorklund, C. W. J. Am. Chem. Soc. 1957, 79, 6347. (13) Teweldemedhin, Z. S.; Ramanujachary, K. V.; Greenblatt, M. Mater. Res. Bull. 1993, 28, 427. (14) Oyetola, S.; Verbaere, A.; Guyomard, D.; Crosnier, M. P.; Piffard, Y.; Tournoux, M. Eur. J. Solid State Inorg. Chem. 1991, 28, 23. (15) Verbaere, A.; Oyetola, S.; Guyomard, D.; Piffard, Y. J. Solid State Chem. 1988, 75, 217. (16) Hajo, O. Naturwissenschaften 1965, 52, 344. (17) Le Flem, G.; Lamic, J.; Hagenmulller, P. Me ´ moires Pre ´ sente ´ s A La Socie ´ te ´ Chimique, June 1966; p 1880. (18) Peyronel, G. Gazz. Chim. Ital. 1942, 72, 77. (19) Craig, D. F.; Hummel, F. A. J. Am. Ceram. Soc. 1972, 55, 532. (20) Buchanan, R. C.; Wolter, G. W. J. Electrochem. Soc. 1983, 130, 1905. (21) Korthuis, V.; Khosrovani, N.; Sleight, A. W. Chem. Mater. 1995, 7, 412. (22) Tillmanns, E.; Gebert, W.; Baur, W. H. J. Solid State Chem. 1973, 7, 69. (23) Taylor, D. Br. Ceram. Trans. J. 1984, 83, 129. Figure 1. Ideal structure for cubic AM2O7 compounds shown as corner sharing AO6 octahedra and MO4 tetrahedra. 485 Inorg. Chem. 1996, 35, 485-489 0020-1669/96/1335-0485$12.00/0 © 1996 American Chemical Society