Models of Ni- and Co-ion occupation in LiNi_0.5Co_0.5PO₄ orthophosphate and its magnetic structure

Abstract

The neutron diffraction, magnetic and heat capacity measurements have been carried out to study the polycrystalline sample LiNi0.5Co0.5PO4 prepared by the glycerol-nitrate synthesis method. Models of Ni- and Co-ion occupation the 4с octahedral position in a crystal structure LiNi0.5Co0.5PO4 are calculated for a paramagnetic state. The best model is the Ni- and Co-ions occupy the 4с site in Pnma patent space-group in sequence Ni-Co-Ni-Co or Co-Ni-Co-Ni. It is shown that nickel ions form ab planes alternating with the planes of cobalt ions in the direction of the c crystallographic axis. At 7 K, An average magnetic moment of 3d-ions is equal to 1.90 (9) μB. The moments are ordering antiferromagnetically and parrallel to the bc plane 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 parameter β = 0.198 and Néel temperature T N = 14.1 (1) K, obtained from heat capacity data. This temperature agrees well with T cr = 14.3 (2) K, determined with magnetic measurement. Maybe the short-range magnetic order exists in LiNi0.5Co0.5PO4 over temperatre region (14–16) K, that is confirmed by the maximum on a temperature dependence of the magnetization at 16.1 (5) K.