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 ∼306 K (M=W) and ∼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=√2a_t, b_m=c_t, c_m=a_t, β∼135°. 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.