High-pressure synthesis, crystal structure, and electromagnetic properties of CdRh ₂O ₄: An analogous oxide of the postspinel mineral MgAl ₂O ₄

The postspinel mineral MgAl ₂O ₄ exists only under the severe pressure conditions in the subducted oceanic lithosphere in the Earths deep interior. Here we report that its analogous oxide CdRh ₂O ₄ exhibits a structural transition to a quenchable postspinel phase under a high pressure of 6 GPa at 1400 °C, which is within the general pressure range of a conventional single-stage multianvil system. In addition, the complex magnetic contributions to the lattice and metal nonstoichiometry that often complicate investigations of other analogues of MgAl ₂O ₄ are absent in CdRh ₂O ₄. X-ray crystallography revealed that this postspinel phase has an orthorhombic CaFe ₂O ₄ structure, thus making it a practical analogue for investigations into the geophysical role of postspinel MgAl ₂O ₄. Replacement of Mg ²⁺ with Cd ²⁺ appears to be effective in lowering the pressure required for transition, as was suggested for CdGeO ₃. In addition, Rh ³⁺ could also contribute to this reduction, as many analogous Rh oxides of aluminous and silicic minerals have been quenched from lower-pressure conditions.