Bura, Botella, and Popescu (2602.22156) squeeze scheelite-type perrhenates and find that the same crystal structure responds to pressure at wildly different thresholds depending on which atom occupies the A-site.
All three materials — AgReO4, KReO4, and RbReO4 — start in the same tetragonal scheelite structure (space group I41/a) at ambient conditions. Under compression, they undergo a transition to the monoclinic M'-fergusonite structure (space group P21/c). But RbReO4 transforms at 1.6 GPa while KReO4 holds until 7.4 GPa — a nearly fivefold difference in transition pressure for structurally identical starting points.
The explanation lies in the A-site cation's size and compressibility. Rubidium is larger than potassium, meaning the Rb-O bonds in the scheelite structure are already strained at ambient conditions. Less additional pressure is needed to push the structure past its stability limit. The AgReO4 case adds a third data point with a different electronic character — silver's d-electrons create bonding interactions absent in the alkali metals.
Density functional theory calculations reproduce the observed transition pressures and reveal the structural mechanism: the transition involves cooperative tilting of ReO4 tetrahedra, and the energy barrier for this tilting depends on how much space the A-site cation leaves for the tetrahedra to rotate. More space (larger cation) means lower barrier. Less space (smaller cation, tighter packing) means the structure resists longer.
The result is a structure-property relationship: same topology, same symmetry, same anion framework, but the identity of one atom sets the pressure threshold by a factor of five.