The structural chemistry of Bi14MO24 (M = Cr, Mo, W) phases:: bismuth oxides containing discrete MO4 tetrahedra

The phases Bi14MO24 (M = Cr, Mo, W) have been studied using differential scanning calorimetry, variable temperature X-ray powder diffraction and neutron powder diffraction. All three compounds were found to undergo a phase change, on cooling, from the previously reported tetragonal symmetry (I4/m) to monoclinic symmetry (C2/m). Transition temperatures were determined to be similar to 306 K (M = W) and similar to 295 K (M = Mo), whereas a gradual transition between 275 and 200 K was observed for M = Cr. The high and low temperature structures are very similar, as indicated by the relationship between the monoclinic and tetragonal unit cell parameters: a(m) = root2a(t), b(m) = c(t), c(m) = a(t), beta similar to 135degrees. High-resolution neutron powder diffraction data, collected at 400 and 4 K, were used to establish the nature of the transition, which was found to involve a reduction in the statistical possibilities for orientation of the MO4 tetrahedra. However, in both tetragonal and monoclinic variants, a degree of orientational disorder of the tetrahedra occurs to give partially occupied sites in the average unit cell. (C) 2003 Elsevier Inc. All rights reserved.