Total scattering descriptions of local and cooperative distortions in the oxide spinel (Mg,Cu)Cr2O4 with dilute Jahn-Teller ions

TL;DR

This study uses total scattering and reverse Monte Carlo analysis to investigate local and cooperative distortions in Mg_{1-x}Cu_xCr2O4 spinels, revealing how Jahn-Teller effects influence structural transitions and magnetic properties.

Contribution

It provides a detailed local and average structure analysis of Mg_{1-x}Cu_xCr2O4 solid solutions, highlighting the transition from local Jahn-Teller distortions to cooperative tetragonal distortions.

Findings

Identification of local Jahn-Teller distortions in cubic phase

Observation of a first-order structural transition at x=0.43

Detection of magnetic exchange bias in dilute Cu2+ samples

Abstract

The normal spinel oxide MgCr2O4 is cubic at room temperature while the normal spinel CuCr2O4 is tetragonal as a consequence of the Jahn-Teller nature of Cu2+ on the tetrahedral sites. Despite different end-member structures, complete solid solutions of Mg_{1-x}Cu_xCr2O4 can be prepared that display a first-order structural transition with composition x = 0.43 at room temperature. Reverse Monte Carlo analysis of total neutron scattering on data acquired between 300 K 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.