Spin--orbit-enhanced correlation sensitivity and anomalous magnetic response in non-dimerized $4d^4$ ilmenite CdRuO$_3$

RESUMO

We report the synthesis and physical properties of CdRuO$_3$, a nominal Ru$^{4+}$ $4d^4$ ilmenite with edge-sharing RuO$_6$ honeycomb layers. Powder X-ray diffraction establishes a crystallographically non-dimerized $R\bar{3}$ structure with equivalent Ru-Ru bonds and a strongly distorted RuO$_6$ environment. The compacted-pellet resistivity is nonmetallic but non-Arrhenius, while the heat capacity contains a finite residual linear term. Matched nonmagnetic calculations show that PBE+$U$ without spin-orbit coupling remains metallic or semimetallic up to $U_{\mathrm{eff}} = 3$ eV, whereas PBE+SOC+$U$ exhibits a strong $U_{\mathrm{eff}}$ dependence and opens a direct gap of approximately 55 meV at $\Gamma$ for $U_{\mathrm{eff}} = 2.5$ eV. Spin-orbit coupling therefore markedly enhances the correlation sensitivity of the non-cubic Ru $t_{2g}$ manifold. After subtraction of a dilute Curie-Weiss defect contribution, the susceptibility remains weakly nonmonotonic and is inconsistent with both an ordinary Pauli response and independent spin-only $S = 1$ moments. CdRuO$_3$ thus realizes the non-dimerized structural branch predicted for ruthenium ilmenites, but not a simple robust multiorbital metal.