Antiferroelectric modulations in Sb2WO6 and Sb2MoO6

The structure of Sb2WO6 [M(r) = 523.4, triclinic, <F(1)over bar>, a = 11.132 (1), b = 9.896 (4), c = 18.482 (7) Angstrom, alpha = 90.20 (4), beta = 96.87 (8), gamma = 90.21 (5)degrees, D-x = 6.88 g cm(-3), Z = 16, Mo K alpha, lambda = 0.7107 Angstrom, mu = 338.5 cm(-1), F(000) = 3536] has been refined as an enlarged 2a x 2b x 2c F-centred superstructure of the previously reported structure [Castro, Millan, Enjalbert, Snoeck & Galy (1994). Mat. Res. Bull. 29, 871-879] refined in the space group P1. The re-refinement follows the observation, initially by TEM, of satellite reflections at G +/- 1/2(111)*, where G represents a reflection of the P1 reciprocal lattice. A final value of 0.040 for R(1) = Sigma(h) parallel to F-obs(h)\-\F-calc(h)parallel to/Sigma h parallel to F-obs(h)\ was obtained for 3316 merged reflections with I(h) > 3 sigma[I(h)], compared with R(1) = 0.12 for the previous refinement. The refined structure is described in terms of an antiferroelectric modulation of a P12(1)/a1 underlying parent structure in the original setting. Twinning of the crystal was successfully modelled in the refinement. Synthesis of the previously unknown phase Sb2MoO6 [M(r) = 435.5, triclinic, <F(1)over bar>, a = 10.758 (1), b = 9.673 (2), c = 17.57 (1) Angstrom, alpha = 90.00 (5), beta = 96.98 (3), gamma = 90.05 (2)degrees, Z = 16, D-x = 4.97 g cm(-3)] is also reported, along with evidence for its isostructuralism with Sb2WO6.