Spin-induced symmetry breaking in orbitally ordered NiCr_2O_4 and CuCr_2O_4

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.

References in corpus (3)

• Structural, orbital, and magnetic order in vanadium spinels
• Spin-phonon coupling in antiferromagnetic chromium spinels
• Magnetic phase evolution in the spinel compounds Zn${}_{1-x}$Co${}_x$Cr${}_2$O${}_4$

Cited by in corpus (7)

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• Quantifying Confidence in Density Functional Theory Predicted Magnetic Ground States
• Magnetic and Structural Phase Transitions in the Spinel Compound Fe1+xCr2-xO4
• Transverse acoustic phonon anomalies at intermediate wavevectors in MgV${}_2$O${}_4$
• Resolving structural changes and symmetry lowering in spinel FeCr2S4
• Quantum spin liquids by geometric lattice design