Magnetic phase transitions in the LiNi0.9M0.1PO4 (M = Mn, Co) single crystals

The LiNiPO4, LiNi0.9Mn0.1PO4, and LiNi0.9Co0.1PO4 single crystals are studied with heat capacity and neutron diffraction measurements over the temperature interval (10-30) K. Two peaks are observed on the temperature dependence of heat capacity for LiNiPO4, and LiNi0.9Co0.1PO4 samples. One peak indicates the first order phase transition from an antiferromagnetic commensurate (C) structure to an incommensurate (IC) one upon heating. According to neutron diffraction, in LiNiPO4 the IC ordering is described by the propagation vector k = 2 pi/b(0, 0.080, 0) at the Neel temperature T-N = 20.8 K, and k = 2 pi/b(0, 0.098, 0) at T-N = 20.2(1) K for LiNi0.9Co0.1PO4. A further increase in temperature leads to the second order phase transition to a paramagnetic state at critical temperature T-IC = 21.7 K and 21.1 K for LiNiPO4 and LiNi0.9Co0.1PO4, respectively. The C and IC phases coexist over the temperature interval (20.6-20.8) K and (20.2-21.2) K in LiNiPO4 and LiNi0.9Co0.1PO4, respectively. In the LiNi0.9Mn0.1PO4 the magnetic phase transition occurs at T-N = 22.7 K, but a magnetic scattering is observed up to 24.6 K.