Tetrahedral Displacive Disorder in the Scheelite-Type Oxide RbReO₄

Abstract

Oxides exhibiting the scheelite-type structure are an important class of functional materials with notable applications in photocatalysis, luminescence, and ionic conductivity. Like all materials, understanding their atomic structure is fundamental to engineering their physical properties. This study outlines a detailed structural investigation of the scheelite-type oxide RbReO 4 , which exhibits a rare long-range phase transition from I4 1 /a to I4 1 /amd upon heating. Additionally, in the long-range I4 1 /a model, the Re− O tetrahedral distance undergoes significant contraction upon warming. Recent studies of other scheelite oxides have attributed this apparent contraction to incoherent local-scale tetrahedral rotations. In this study, we use X-ray pair distribution function analysis to show that RbReO 4 undergoes a unique symmetry-lowering process on the local scale, which involves incoherent tetrahedral displacements. The rare I4 1 /a to I4 1 /amd long-range phase transition was found to occur via a change from static to dynamic disorder on the local scale, which is due to the combination of the size of the A-site cation and lattice expansion. This demonstrates how careful manipulation of the ionic radius of the A-site in the scheelite structure can be used to induce local-scale disorder, which has valuable implications for tailoring the physical properties of related materials. Article pubs.acs.org/IC

Tetrahedra Rotational and Displacive Disorder in the Scheelite-Type Oxide CsReO4

“…There are two distinct A−O bond distances in the AO 8 dodecahedra of the tetragonal scheelite structure (space group I4 1 /a, #88, see Figure 1a), whereas the BO 4 tetrahedra have a single B−O distance. 7 In this structure, the A-and B-site cations occupy the special Wyckoff sites 4b (0, 1/4, 5/8) and 4a (0, 1/4, 1/8) respectively, while the oxygen anions occupy the general 16f (x, y, z) site. The wide range of potential metal cations with different ionic radii (IR) and charge combinations reflect the diverse range of possible functionalities that are attainable using the scheelite structure�from the magnetic properties observed in the A 1+ B 7+ O 4 combination KOsO 4 8 to the degradation of organic dye pollutants observed in the A 4+ B 4+ O 4 combination ZrGeO 4 .…”

“…Recently, an X-ray pair distribution function (XPDF) study by Marlton et al identified that the I4 1 /a to I4 1 /amd phase transition in RbReO 4 was preceded by incoherent translations or displacive disorder of the ReO 4 tetrahedra on the local scale. 7 It was Variable-temperature SXRD and XPDF were used to understand the orthorhombic Pnma to tetragonal I4 1 /a phase transition. While the long-range average structure (derived from the SXRD data) shows a phase transition upon heating, the short-range local structure (derived from the XPDF data) shows that rotations of the ReO 4 tetrahedra persist above the phase transition accommodated by the flexible Cs polyhedra, highlighting the apparent complexity of this seemingly simple structure.…”

Ab Initio Study of the Electronic Structure and Lattice Dynamics of Scheelite Type AgTcO4

“…The orientation of the tetrahedra in scheelites is temperature dependent, see for example, refs. [35,46]. Thermal effects induce small changes in the orientation of the tetrahedra and greater anisotropic thermal motion of the lighter oxygen atoms leads to an apparent contraction in the experimentally observed Tc–O bond distance.…”

“…Thermal effects induce small changes in the orientation of the tetrahedra and greater anisotropic thermal motion of the lighter oxygen atoms leads to an apparent contraction in the experimentally observed Tc–O bond distance. [ 35 ] For the scheelite structure, the positions of Ag and Tc atoms are fixed by symmetry, only the position of the O atom varies. The optimized atomic coordinates are found to be in good agreement with the reported experimental and theoretical values as shown in Table 2 .…”

“…A detailed study on structural and vibrational properties of was recently reported by Mondal et al [ 31 ] Both and undergo a temperature‐induced phase transition to the tetragonal scheelite structure upon heating. [ 32,33 ] The corresponding compounds are generally isostructural with the pertechnetates, [ 34,35 ] and their reported chemical characteristics are remarkably similar. Multiple characterization approaches have already been utilized for evaluating the environment of in technetium‐bearing glasses, including X‐ray absorption spectroscopy (XAS), X‐ray diffraction (XRD), and Raman spectroscopy.…”