Total-scattering descriptions of local and cooperative distortions in the oxide spinel Mg_1-x Cu_x Cr₂ O₄ with dilute Jahn-Teller ions

The normal spinel oxide MgCr₂ O₄ is cubic at room temperature while the normal spinel CuCr₂ O₄ is tetragonal as a consequence of the Jahn-Teller activity of Cu2⊃+ on the tetrahedral sites. Despite different end-member structures, a complete solid solution of Mg_1-x Cu_x Cr₂ O₄ can be prepared with compounds of composition x=0.43 displaying a first-order phase transition at room temperature. Reverse Monte Carlo analysis of total neutron scattering on data acquired between 300 and 15 K on samples with x=0.10, 0.20, and 0.43 provides unbiased local and average structure descriptions of the samples, including an understanding of the transition from local Jahn-Teller distortions in the cubic phase to cooperative distortions that result in a tetragonal structure. Distributions of continuous symmetry measures help to understand and distinguish distorted and undistorted coordination around the tetrahedral site in the solid solutions. Magnetic exchange bias is observed in field-cooled hysteresis loops of samples with dilute Cu2⊃+ concentration and in samples with tetragonal-cubic phase coexistence around 300 K.