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 octa-oxide) 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) octa-hedra (.3. symmetry) and MO(4) (M = W, Mo) tetra-hedra (.3. symmetry). Along the main threefold axis of the cubic unit cell, the MO(4) tetra-hedra are arranged in pairs forming M(2)O(8) units in which the M1O(4) tetra-hedra have larger distortions in terms of bond distances and angles than the M2O(4) tetra-hedra. These units are disordered over two possible orientations, with the M-O(terminal) vectors pointing to the [111] or [] 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.