Direct observation of strong $t_{2g}\text{−}{e}_g$ orbital hybridization and effects of $f$ orbitals in a molecular analogue of chromium perovskite

Chromium-based perovskites have drawn plenty of attention due to their intriguing magnetic properties and potential technique applications. While a complete understanding of the microscopic magnetic mechanisms underlying their macroscopic properties presents great challenges, especially when involving $t_{2g}\text{−}{e}_g$ ($t\text{−}e$) hybridization and rare earth (RE) $f$ orbitals. Here, with the recently discovered molecular analogue of perovskite $\left[{\mathrm{Ce}}_2^{\mathrm{III}}{\mathrm{Ce}}^{\mathrm{IV}}{\mathrm{Cr}}_8^{\mathrm{III}}{O}_8{\left(O_2\mathrm{CPh}\right)}_{18}\left({\mathrm{HO}}_2\mathrm{CPh}\right)\right]$, abbreviated as ${\mathrm{Ce}}_3{\mathrm{Cr}}_8$, we combine first-principles calculations and a superexchange model to successfully identify an anomalous ferromagnetism (FM)-dominated superexchange interaction between Cr ions originated from the $t\text{−}e$ hybridization, which does not follow Goodenough-Kanamori rules. The great sensitivity of the $t\text{−}e$ hybridization with respect to the angle of Cr-O-Cr can lead to a significant change in the magnetic interaction of ${\mathrm{Ce}}_3{\mathrm{Cr}}_8$ with the angle of Cr-O-Cr varying within only a few degrees, e.g., a ground-state transition from FM to antiferromagnetism. Additionally, the Ce $f$ orbitals near the Fermi level can largely reduce this sensitivity by interacting with the Cr $d$ orbitals via the virtual charge transfer process. Our results are strongly supported by an extended superexchange model developed with the inclusion of $f$ orbitals within the $t\text{−}e$ hybridization framework. These findings complete the theory of superexchange magnetism in chromium-based perovskites including RE $f$ orbitals and in the meanwhile introduce a new avenue for fine-tuning the magnetic characteristics via modifying the transition metal $d$/RE $f$ interactions at the nanoscale.