Lattice thermal expansion behaviour of spinel HgCr₂Se₄

An accurate value of the lattice parameter of the spinel ferromagnetic semiconductor HgCr₂Se₄ has been determined at different temperatures, ranging from 298 to 770 K, by X-ray powder diffraction. The data have been used to evaluate the coefficient of thermal expansion, α, at various temperatures. The temperature dependence of the coefficient of expansion can be represented by the equation α = 10.059 × 10⁻⁶ − 1.862 × 10⁻⁸ T + 3.017 × 10⁻¹¹ T², where T is the temperature in K. The diffraction pattern of HgCr₂Se₄, obtained at 785(5) K, was completely different from that at room temperature. The high-temperature phase persisted up to 900 K, and above this the compound decomposed. Furthermore, the new phase was retained when the sample was cooled from 900 K to room temperature. These results suggest an irreversible structural phase transformation in HgCr₂Se₄ at 785 K. An attempt is made to explain this interesting high-temperature thermal behaviour of HgCr₂Se₄ in relation to its structure and other physical properties.