Correlation between the structural and antiferromagnetic phase transitions in ZnCr 2 Se 4

Structural properties of normal‐type spinel ZnCr 2 Se 4 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 7 O h ‐Fd3m and 24 D 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 Cr 3+ ions in CrSe 4 chains along [110]. The antiferromagnetic magnetic structure below T N is made by a spiral magnetic coupling between the ferromagnetic CrSe 4 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.