Synthesis, structure, ionic conductivity, and phase transformation of new double chloride spinel, Li2CrCl4

A new double chloride spinel Li2CrCl4 was prepared and characterized by Rietveld X-ray structure refinement, differential thermal analysis, high-temperature X-ray diffraction analysis, and electrical conductivity measurement. The new phase has C2c symmetry with lattice parameters a = 7.446(1), b = 10.162(2), c = 10.172(1) Å, and γ = 132.11(1)°; its monoclinic (m) unit cell is related to that of the parent cubic spinel (c) according to am ∼ 1212 ac, bm ∼ ac, and cm ∼ ac. The structure is a 1 : 1 ordered type with Cr2+ and Li2− ions occupying the octahedral sites. The ordered CrCl6 octahedra elongation caused by the strong cooperative Jahn-Teller effect of d4 state leads to a lattice deformation from orthorhombic to monoclinic symmetry. The monoclinic phase turned reversibly into the cubic spinel through the two-phase region of 222–305°C. Li2CrCl4 showed a high ionic conductivity of 6.3 × 10−2S cm−1 at 400°C, and its ionic conduction is discussed in comparison with other chloride spinels, Li2MCl4 (M = Mg, Mn, Fe, Co, Cd).