Pressure Dependence of Superconducting Behavior of 4d and 5d Transition Metal Compounds CaRh₂and CaIr₂

Herein, the pressure dependence of the electrical resistance R of superconducting 4d and 5d transition metal compounds, CaRh2 and CaIr2, with superconducting transition temperatures, Tc's, as high as 6.09 and 6.04 K at ambient pressure, respectively, is investigated to depict their Tc-pressure (p) phase diagrams. The superconductivity of these samples is investigated based on plots of R vs temperature (T) over a wide pressure range. It is demonstrated that the Tc of both compounds decreases slowly as pressure increases but saturates in a high pressure range. This behavior is similar to that of SrIr2 reported previously, suggesting that it may be a unique behavior of the 4d and 5d transition metal compounds investigated. The crystal structures of CaRh2 and CaIr2 are determined based on powder X-ray diffraction patterns generated via synchrotron radiation at high pressures, and no structural phase transitions are observed up to ∼20 GPa. The magnetic field dependence of R-T plots recorded at 12.2 and 7.98 GPa for CaRh2 and CaIr2, respectively, is analyzed using three pairing models. Consequently, the superconductivity of CaRh2 and CaIr2 could not be explained merely based on s-wave dirty limit and s-wave clean limit models but rather the p-wave polar model. We fully analyzed the magnetic behavior of the superconducting phases of CaRh2 and CaIr2 at ambient and high pressures using three different methods: the Werthamer-Helfand-Hohenberg/Maki, empirical, and Ginzburg-Landau models. This report provides a systematic study pertaining to the pressure dependence of superconductivity in binary-element compounds comprising alkali-earth and 4d or 5d transition metal atoms.