High-Pressure X-ray Diffraction Study of Scheelite-Type Perrhenates

The effects of pressure on the crystal structure of scheelite-type perrhenates were studied using synchrotron powder X-ray diffraction and density-functional theory. At ambient conditions, the studied materials AgReO₄, KReO₄, and RbReO₄, exhibit a tetragonal scheelite-type crystal structure described by space group <i>I</i>4₁/<i>a</i>. Under compression, a transition from scheelite-to-M'-fergusonite (space group <i>P</i>2₁/<i>c</i>) was observed at 1.6 and 7.4 GPa for RbReO₄ and KReO₄, respectively. The transition involves a relative volume decrease. On the other hand, AgReO₄ underwent a phase transition to the M-fergusonite structure (space group <i>I</i>2/<i>a</i>) at 13.6 GPa. In this case there is no appreciable volume discontinuity. The room-temperature pressure-volume equation of state for the three studied perrhenates was estimated using a second-order Birch-Murnaghan equation of state. The results for the low-pressure phase are confirmed by density-functional theory calculations. The analysis of the bulk modulus shows that the compressibility of the compounds decreases following the sequence RbReO₄ > KReO₄ > AgReO₄, which is related to the compressibility of the RbO₈, KO₈, and AgO₈ bidisphenoid units. Density-functional theory also offers valuable insights into the elastic constants. Despite giving a good description for the low-pressure phase in the three compounds, density-functional theory cannot catch the structural phase transition observed in experiments. Reasons for it are discussed in the manuscript.