Orbital Configurations and Magnetic Properties of Double-Layered Antiferromagnet Cs$_3$Cu$_2$Cl$_4$Br$_3$

TL;DR

This study investigates the structure and magnetic properties of a new double-layered antiferromagnetic perovskite, revealing a large excitation gap, a structural phase transition, and orbital state changes related to magnetic behavior.

Contribution

It provides the first detailed analysis of the orbital configurations and magnetic properties of Cs$_3$Cu$_2$Cl$_4$Br$_3$, highlighting its structural phase transition and metastable orbital state.

Findings

Large excitation gap of ~2000 K indicating strong antiferromagnetic interactions

Structural phase transition at ~330 K involving orbital reconfiguration

Orbital state remains metastable and unchanged below transition temperature

Abstract

We report the single-crystal X-ray analysis and magnetic properties of a new double-layered perovskite antiferromagnet, Cs$_3$Cu$_2$Cl$_4$Br$_3$. This structure is composed of Cu$_2$Cl$_4$Br$_3$ double layers with elongated CuCl$_4$Br$_2$ octahedra and is closely related to the Sr$_3$Ti$_2$O$_7$ structure. An as-grown crystal has a singlet ground state with a large excitation gap of $\Delta/k_{\rm B}\simeq 2000$ K, due to the strong antiferromagnetic interaction between the two layers. Cs$_3$Cu$_2$Cl$_4$Br$_3$ undergoes a structural phase transition at $T_{\rm s}\simeq330$ K accompanied by changes in the orbital configurations of Cu$^{2+}$ ions. Once a Cs$_3$Cu$_2$Cl$_4$Br$_3$ crystal is heated above $T_{\rm s}$, its magnetic susceptibility obeys the Curie-Weiss law with decreasing temperature even below $T_{\rm s}$ and does not exhibit anomalies at $T_{\rm s}$. This implies that in the heated crystal, the orbital state of the high-temperature phase remains unchanged below $T_{\rm s}$, and thus, this orbital state is the metastable state. The structural phase transition at $T_{\rm s}$ is characterized as an order-disorder transition of Cu$^{2+}$ orbitals.