Cubic ZrW1.75Mo0.25O8 from a Rietveld refinement based on neutron powder diffraction data

Abstract

The solid solution in the system Zr–Mo–W–O with composition ZrW1.75Mo0.25O8 (zirconium tungsten molybdenum octaoxide) was prepared by solid-state reactions as a polycrystalline material. Its structure has cubic symmetry (space group P213) at room temperature. The structure contains a network of corner-sharing ZrO6 octahedra (.3. symmetry) and MO4 (M = W, Mo) tetrahedra (.3. symmetry). Along the main threefold axis of the cubic unit cell, the MO4 tetrahedra are arranged in pairs forming M2O8 units in which the M1O4 tetrahedra have larger distortions in terms of bond distances and angles than the M2O4 tetrahedra. These units are disordered over two possible orientations, with the M—Oterminal vectors pointing to the [111] or [overline{1}overline{1}overline{1}] directions. The reversal of the orientations of the M2O8 units results from the concerted flips of these units. The time-averaged proportions of flipped and unflipped M2O8 units were determined and the fraction of unflipped M2O8 units is about 0.95. The order degree of the M2O8 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.