Abstract
Temperature and composition dependences of the I41/amd → [Formula: see text] phase transition in the Mg x Cu1 - x Cr2O4 spinel solid solution, due to the melting of the cooperative Jahn-Teller distortion, have been studied by means of single-crystal X-ray diffraction. Crystals with x = 0, 0.10, 0.18, 0.43, 0.46, 0.53, 1 were grown by flux decomposition methods. All crystals have been refined in the tetragonal I41/amd space group except for the Mg end-member, which has cubic symmetry. In Mg x Cu1 - x Cr2O4 the progressive substitution of the Jahn-Teller, d9 Cu2+ cation with spherical and closed-shell Mg2+ has a substantial effect on the crystal structure, such that there is a gradual reduction of the splitting of a and c unit-cell parameters and flattening of the tetrahedra. Single-crystal diffraction data collected in situ up to T = 1173 K show that the tetragonal-to-cubic transition temperature decreases with increasing Mg content. The strength of the Cu-Cu interaction is, in effect, modulated by varying the Cu/Mg ratio. Structure refinements of diffraction data collected at different temperatures reveal that heating results in a gradual reduction in the tetrahedron compression, which remains significant until near the transition temperature, however, at which point the distortion of the tetrahedra rapidly vanishes. The spontaneous strain arising in the tetragonal phase is large, amounting to 10% shear strain, et, and ∼ 1% volume strain, Vs, in the copper chromite end-member at room temperature. Observed strain relationships are consistent with pseudoproper ferroelastic behaviour ([Formula: see text] ∝ Vs ∝ [Formula: see text], where qJT is the order parameter). The I41/amd → [Formula: see text] phase transition is first order in character for Cu-rich samples and then evolves towards second-order character. Although a third order term is permitted by symmetry in the Landau expansion, this behaviour appears to be more accurately represented by a 246 expansion with a change from negative to positive values of the fourth-order coefficient with progressive dilution of the Jahn-Teller cation.
Highlights
Complex AB2O4 oxides with the spinel structure comprise a family of materials, which exhibit a wide range of electronic, magnetic and optical properties through the variation of cations on tetrahedral (A) and octahedral (B) sites
Single-crystal diffraction data collected in situ up to T = 1173 K show that the tetragonal-to-cubic transition temperature decreases with increasing Mg content
A third order term is permitted by symmetry in the Landau expansion, this behaviour appears to be more accurately represented by a 246 expansion with a change from negative to positive values of the fourth-order coefficient with progressive dilution of the Jahn–Teller cation
Summary
Complex AB2O4 oxides with the spinel structure comprise a family of materials, which exhibit a wide range of electronic, magnetic and optical properties through the variation of cations on tetrahedral (A) and octahedral (B) sites. The archetypical spinel structure belongs to the space group Fd3"m (No 227) and is usually described as a pseudocubic closepacked array of O atoms with the A and B cations occupying one eighth of the tetrahedral sites and one half of the octahedral sites, respectively. The effects on the crystal structure and on the tetragonal-to-cubic phase transition of progressive substitution of the Jahn–Teller and d9 Cu2+ cation with the spherical and closed-shell Mg2+ cation in the MgxCu1 À xCr2O4 solid solution have been studied. Given the relevance of strain in lowering the symmetry, as in copper chromite, CuCr2O4, which determining the structure-electronic properties relation, in is a tetragonally distorted spinel with unit-cell parameters situ high-temperature (HT) single-crystal diffraction data are ratio c/a < 1 (Fig. 1, left panel). The residues were washed with boiling water and the single crystals removed from the solidified flux
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
