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

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 $\Delta /k_{\mathrm{B}}\simeq 2000$ 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 $T_{\mathrm{s}}\simeq 330$ 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 $T_{\mathrm{s}}$, its magnetic susceptibility obeys the Curie-Weiss law with decreasing temperature even below $T_{\mathrm{s}}$ and does not exhibit anomalies at $T_{\mathrm{s}}$. This implies that in the heated crystal, the orbital state of the high-temperature phase remains unchanged below $T_{\mathrm{s}}$, and thus, this orbital state is the metastable state. The structural phase transition at $T_{\mathrm{s}}$ is characterized as an order-disorder transition of Cu${}^{2+}$ orbitals.

6pages. 6figures, to appear in J. Phys. Soc. Jpn. Vol.76 No.1