Spin-induced symmetry breaking in orbitally ordered NiCr_2O_4 and CuCr_2O_4

TL;DR

This study investigates the structural and magnetic phase transitions in NiCr_2O_4 and CuCr_2O_4 spinels, revealing symmetry breaking induced by magnetic ordering and Jahn-Teller effects through high-resolution synchrotron X-ray diffraction.

Contribution

The paper provides detailed insights into the temperature-dependent structural transitions and symmetry breaking in NiCr_2O_4 and CuCr_2O_4, highlighting the role of magnetic ordering and Jahn-Teller distortions.

Findings

Both compounds transition from tetragonal to orthorhombic structures at magnetic ordering temperatures.

NiCr_2O_4 exhibits an additional low-temperature distortion around 30 K.

Structural changes mainly involve distortions in NiO_4 and CuO_4 tetrahedra.

Abstract

At room temperature, the normal oxide spinels NiCr_2O_4 and CuCr_2O_4 are tetragonally distorted and crystallize in the I4_1/amd space group due to cooperative Jahn-Teller ordering driven by the orbital degeneracy of tetrahedral Ni$^{2+}$ ($t_2^4$) and Cu$^{2+}$ ($t_2^5$). Upon cooling, these compounds undergo magnetic ordering transitions; interactions being somewhat frustrated for NiCr_2O_4 but not for CuCr_2O_4. We employ variable-temperature high-resolution synchrotron X-ray powder diffraction to establish that at the magnetic ordering temperatures there are further structural changes, which result in both compounds distorting to an orthorhombic structure consistent with the Fddd space group. NiCr_2O_4 exhibits additional distortion, likely within the same space group, at a yet-lower transition temperature of $T$ = 30 K. The tetragonal to orthorhombic structural transition in these compounds appears to primarily involve changes in NiO_4 and CuO_4 tetrahedra.