Effect of chemical and hydrostatic pressure on the cubic pyrochlore Cd2Ru2O7

The cubic pyrochlore $\mathrm{C}{\mathrm{d}}₂\mathrm{R}{\mathrm{u}}₂{\mathrm{O}}₇$ develops a metalliclike state below the antiferromagnetic transition at ${T}_{\mathrm{N}}\ensuremath{\approx}90\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, in contrast to the sharp transition to an insulating state below ${T}_{\mathrm{N}}$ of the $5d$-analog ${\mathrm{Cd}}₂{\mathrm{Os}}₂{\mathrm{O}}₇$. We have synthesized polycrystalline $\mathrm{C}{\mathrm{d}}₂\mathrm{R}{\mathrm{u}}₂{\mathrm{O}}₇$ under high pressure, and investigated the responses of its electronic behavior to hydrostatic pressure and isovalent substitutions of ${\mathrm{Ca}}²⁺$ for ${\mathrm{Cd}}²⁺$. Three characteristic anomalies are identified in the electrical transport and magnetic properties of $\mathrm{C}{\mathrm{d}}₂\mathrm{R}{\mathrm{u}}₂{\mathrm{O}}₇$, signaling an intimate correlation between charge and spin degrees of freedom. Interestingly, we found that the metalliclike state of $\mathrm{C}{\mathrm{d}}₂\mathrm{R}{\mathrm{u}}₂{\mathrm{O}}₇$ below ${T}_{\mathrm{N}}$ is very fragile and can be suppressed by a small hydrostatic pressure of $\ensuremath{\le}1\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ or substitution of 5--10% ${\mathrm{Ca}}²⁺$ for ${\mathrm{Cd}}²⁺$, resulting in a resistivity behavior similar to that of ${\mathrm{Cd}}₂{\mathrm{Os}}₂{\mathrm{O}}₇$. In addition, the resultant insulating state below ${T}_{\mathrm{N}}$ is very robust against pressure, and the resistivity evolves gradually into two distinct activated regions under higher pressures. We constructed a temperature-pressure phase diagram for $\mathrm{C}{\mathrm{d}}₂\mathrm{R}{\mathrm{u}}₂{\mathrm{O}}₇$ and discussed its peculiar metalliclike state in terms of the electronic itinerancy/localization dichotomy via side-by-side comparisons with the related compounds ${A}₂\mathrm{R}{\mathrm{u}}₂{\mathrm{O}}₇(A=\mathrm{Ca},\mathrm{Hg})$ and ${\mathrm{Cd}}₂{\mathrm{Os}}₂{\mathrm{O}}₇$. Our results demonstrate that these cubic $4d/5d$ pyrochlore oxides offer an important paradigm for studying the exotic physics of correlated electrons on the border of (de)localization in the presence of strong geometrical frustration.