High-Pressure Synthesis, Crystal Structure, and Electromagnetic Properties of CdRh2O4: an Analogous Oxide of the Postspinel Mineral MgAl2O4

The postspinel mineral MgAl2O4 exists only under the severe pressure conditions in the subducted oceanic lithosphere in the Earth’s deep interior. Here we report that its analogous oxide CdRh2O4 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 MgAl2O4 are absent in CdRh2O4. X-ray crystallography revealed that this postspinel phase has an orthorhombic CaFe2O4 structure, thus making it a practical analogue for investigations into the geophysical role of postspinel MgAl2O4. Replacement of Mg2+ with Cd2+ appears to be effective in lowering the pressure required for transition, as was suggested for CdGeO3. In addition, Rh3+ could also contribute to this reduction, as many analogous Rh oxides of aluminous and silicic minerals have been quenched from lower-pressure conditions.