Correlation between the structural and antiferromagnetic phase transitions in ZnCr₂Se₄

Structural properties of normal-type spinel ZnCr₂Se₄ have been studied as a function of temperature by means of neutron diffraction. It is found that the structural phase transition (T_C) from a cubic to orthorhombic symmetry simultaneously occurs at the antiferromagnetic phase transition temperature (T_N ∼ 21 K), where the cubic and orthorhombic symmetry is ⁷O_h-Fd3m and ²⁴D_2h-Fddd, respectively. The structural phase transition is mainly characterized by the cooperative displacements of Se^2- ions, which are induced by a magnetostriction resulting from the magnetic interaction of Cr³⁺ ions in CrSe₄ chains along [110]. The antiferromagnetic magnetic structure below T_N is made by a spiral magnetic coupling between the ferromagnetic CrSe₄ chains around crystallographic screw axes, which are parallel to an orthorhombic C axis. The spiral long-range order of the spins of Cr^3- is incommensurate and its periodicity of q along a reciprocal lattice axis C* shows temperature dependence from about 0.47 (2.5 K) to 0.43 (18 K). The spiral periodicity of the spin order along the C axis shows a first order transition at about 21 K.