Models of Ni- and Co-ion occupation in LiNi_0.5Co_0.5PO₄orthophosphate and its magnetic structure

The neutron diffraction, magnetic and heat capacity measurements have been carried out to study the polycrystalline sample LiNi_0.5Co_0.5PO₄prepared by the glycerol-nitrate synthesis method. Models of Ni- and Co-ion occupation the 4coctahedral position in a crystal structure LiNi_0.5Co_0.5PO₄are calculated for a paramagnetic state. The best model is the Ni- and Co-ions occupy the 4csite inPnmapatent space-group in sequence Ni-Co-Ni-Co or Co-Ni-Co-Ni. It is shown that nickel ions formabplanes alternating with the planes of cobalt ions in the direction of theccrystallographic axis. At 7 K, an average magnetic moment of 3d-ions is equal to 1.90 (9)μ_B. The moments are ordering antiferromagnetically and parallel to thebcplane decreasing to zero at 15 K. In the high-spin state a temperature dependence of the Ni²⁺/Co²⁺ion-magnetic moment is well described within the 2D Ising model with order parameterβ= 0.198 and Néel temperatureT_N= 14.1 (1) K, obtained from heat capacity data. This temperature agrees well withT_cr= 14.3 (2) K, determined with magnetic measurement. Maybe the short-range magnetic order exists in LiNi_0.5Co_0.5PO₄over temperature region (14-16) K, that is confirmed by the maximum on a temperature dependence of the magnetization at 16.1 (5) K.