Cubic ZrW(1.75)Mo(0.25)O(8) from a Rietveld refinement based on neutron powder diffraction data

The solid solution in the system Zr–Mo–W–O with composition ZrW(1.75)Mo(0.25)O(8) (zirconium tungsten molybdenum octa­oxide) was prepared by solid-state reactions as a polycrystalline material. Its structure has cubic symmetry (space group P2(1)3) at room temperature. The structure contains a network of corner-sharing ZrO(6) octa­hedra (.3. symmetry) and MO(4) (M = W, Mo) tetra­hedra (.3. symmetry). Along the main threefold axis of the cubic unit cell, the MO(4) tetra­hedra are arranged in pairs forming M (2)O(8) units in which the M1O(4) tetra­hedra have larger distortions in terms of bond distances and angles than the M2O(4) tetra­hedra. These units are disordered over two possible orientations, with the M—O(terminal) vectors pointing to the [111] or [[Image: see text] [Image: see text] [Image: see text]] directions. The reversal of the orientations of the M (2)O(8) units results from the concerted flips of these units. The time-averaged proportions of flipped and unflipped M (2)O(8) units were determined and the fraction of unflipped M (2)O(8) units is about 0.95. The order degree of the M (2)O(8) unit orientation is about 0.9. During the reversal process, the M-atom site has a migration about 0.93 Å, one of the O-atom sites has a 0.25 Å migration distance, whereas two other O-atom sites migrate marginally (≃ 0.08 Å). The results prove the constraint strategy to be a reasonable approach based on the ratcheting mechanism.