Electronic and structural properties of RbCeX2 (X2: O2, S2, SeS, Se2, TeSe, Te2)

Abstract

Triangular lattice delafossite compounds built from magnetic lanthanide ions are a topic of recent interest due to their frustrated magnetism and realization of quantum disordered magnetic ground states. Here we report the evolution of the structure and electronic ground states of $\mathrm{RbCe}{X}_{2}$ compounds, built from a triangular lattice of ${\mathrm{Ce}}^{3+}$ ions, upon varying their anion character $({X}_{2}={\mathrm{O}}_{2}$, ${\mathrm{S}}_{2}, \mathrm{SeS}, {\mathrm{Se}}_{2}, \mathrm{TeSe}, {\mathrm{Te}}_{2})$. This includes the discovery of a new member of this series, ${\mathrm{RbCeO}}_{2}$, that potentially realizes a quantum disordered ground state analogous to ${\mathrm{NaYbO}}_{2}$. Magnetization and susceptibility measurements reveal that all compounds manifest mean-field antiferromagnetic interactions and, with the exception of the oxide, possess signatures of magnetic correlations onset below 1 K. The crystalline electric field level scheme is explored via neutron scattering and ab initio calculations in order to model the intramultiplet splitting of the $J=5/2$ multiplet. In addition to the two excited doublets expected within the $J=5/2$ manifold, we observe one extra local mode present across the sample series. This added mode shifts downward in energy with increasing anion mass and decreasing crystal field strength, suggesting a long-lived anomalous mode endemic to anion motion about the ${\mathrm{Ce}}^{3+}$ sites.