668 CRYSTALLINE H3PO4 CROMER, D. T. (1974). International Tables for X-ray Crystallography, Vol. IV, pp. 148-151. Birmingham: Kynoch Press. (Present distributor Kluwer Academic Publishers, Dordrecht.) The tabulated f ' values need to be adjusted by wavelength-independentcorrections given by KISSEL, L. & PRATT, R. H. (1990). Acta Cryst. A46, 170- 175. CRUICKSHANK,D. W. J. (1949). Acta Cryst. 2, 65-82. DETITI'A, G. T. (1985). J. Appl. Cryst. 18, 438-440. FRENCH, S. & WILSON, K. (1978). Acta Cryst. A34, 517-525. HANSEN, N. K. & COPPENS,P. (1978). Acta Cryst. A34, 909-921. HERMANSSON, K. (1984). Acta Universitatis Upsaliensis, p. 744. Doctoral Dissertation, Univ. of Uppsala. Moss, G. R., SOUHASSOU, M., ESPINOSA, E., LECOMTE, C. & BLESSING, R. H. (19.95). Acta Cryst. B51, 650-660. NELMES, R. J. (1975). Acta Cryst. A31, 273-279. O'KEEFE, M., DOMENG~S, B. & GIBBS,G. V. (1985). J. Phys. Chem. 89, 2304-2309. STEWART,R. F., DAVIDSON,E. R. & SIMPSON,W. T. (1965). J. Chem. Phys. 42, 3175-3187. ZACHARIASEN, W. H. (1967). Acta Cryst. 23, 558-564. Acta Cryst. (1995). B51, 668-673 Crystal Structure of the Antiferroelectric Perovskite Pb2MgWO6 BY G. BALDINOZZIAND PH. SCIAU* Laboratoire de Chimie-Physique du Solide, URA CNRS 453, Ecole Centrale Paris, 92295 Chdtenay-Malabry CEDEX, France M. PINOT Laboratoire Ldon Brillouin, CEA-CNRS, CEN Saclay, 91191 Gif-sur-Yvette CEDEX, France AND D. GREBILLE Laboratoire CRISMAT, ISMRA, Bd du Mardchal Juin, 14050 Caen CEDEX, France (Received 25 July 1994; accepted 1 December 1994) Abstract Lead magnesium tungstate, Pb2MgWO6, Mr = 718.54. Phase I: cubic, Z = 4, Fm3m, a = 8.0058 (4)A, V = 513.1 (2) A 3, Dx = 9.30 Mg m -3 at 350 K, final Rwp = 4.5 and 7.7%, RBragg = 2.9 and 5.7% for neutron and X- ray powder data, respectively. Phase II: orthorhombic, Pmcn (Pnma), Z = 4, a = 7.9440(4) and 7.9041 (3), b = 5.6866(3) and 5.7035(2), c = 11.4059(5) and 11.4442 (4) A, V= 515.3 (1) and 515.9 (1) A 3 at 294 and 80 K, respectively, Dx = 9.26 Mg m -3 at 294 K. Final Rwp = 4.0 and 8.5%, RBragg = 4.0 and 9.2% at 294 K and Rwp = 4.0 and 7.4%, RBragg = 2.9 and 8.4% at 80 K for neutron and X-ray powder data, respectively. To achieve the determination of the structures, X-ray and neutron powder diffraction data were refined together using the Rietveld profile method. The Pb main displacement in the orthorhombic phase from the ideal cubic positions is almost along the [0121o direction. The O displacements correspond to a weak distortion of the octahedra. Introduction The ideal structure of perovskite-type oxide compounds (ABO3, space group Pm3m) is well known and very simple. Its prototype is CaTiO3. It consists of tetra- valent B cations at the center of corner-sharing oxygen * Author to whom correspondence should be addressed. © 1995 International Union of Crystallography Printed in Great Britain - all fights reserved octahedra, at the cubic cell origin for example, and divalent A cations at the cell center. In the general family of lead-based perovskites, various compounds have been synthesized by occupying the B site by two species of cation (Galasso, 1990). This leads to the general formula Pb2B'xB" I _ IO 6. Depending on ionic radii and charges, ordering of cations may occur, giving rise to different structural characteristics (different types of superstructure cells, for example) and to various very selective physical properties. The prototype structure of the totally ordered materials consists of a cubic cell with a doubled parameter around 8 A, and of a centered space group Fm3m resulting from the alternation of cations B' and B". These perovskite oxides often have very large dielectric permittivities. The degree of long-range order between the species B' and B" induces different behavior as a function of temperature. Partially or fully disordered compounds exhibit diffuse transitions and relaxor phenomena which are adequate for technological applications (capacitors, actuators, etc.). Ordered com- pounds present sharper transitions and their sequence of phase transitions is also strongly dependent on the cations involved. The ordered complex perovskite Pb2MgWO6 (PMW) is quite interesting. It undergoes a first-order phase tran- sition from the cubic phase (Fm3m) to an orthorhombic antiferroelectric phase at 312K (Smolenskii, Krainik & Agranovskaya, 1961). The low-temperature phase is Acta Crystallographica Section B ISSN 0108-7681 © 1995