Magnetic evolution of the antiferromagnetic Co2−xCux(OH)PO4 (0 ≤ x ≤ 2) solid solution. A neutron diffraction study

Abstract

The Co_2−xCu_x(OH)PO₄ (0 ≤ x ≤ 2) solid solution was prepared from hydrothermal synthesis. Neutron powder diffraction patterns show that the Co²⁺ and Cu²⁺ ions are simultaneously present in both the [MO₄(OH)₂] octahedra and the [MO₄(OH)] distorted trigonal bipyramid topologies. The evolution of the lattice parameters follows Vegard’s law in the whole range of substitution. This study allowed us to determine correctly the a and b crystallographic parameters of the Cu₂(OH)PO₄ phase which were interchanged in the literature. The magnetic behaviour in the cobalt–copper compounds indicates the existence of overall antiferromagnetic interactions as predominant. Three-dimensional magnetic ordering with critical temperatures of 69, 64, 60 and 47 K for x = 0.1, 0.3, 0.5 and 1 respectively is observed. The magnetic study of Co_1.9Cu_0.1(OH)PO₄ suggests a spin-glass like state below 10 K. AC measurements obtained at different frequencies and applied fields confirm the freezing process in Co_1.9Cu_0.1 and the long range interactions in the Co_2−xCu_x(OH)PO₄ (x ≤ 1) phases. For x > 1, the magnetic dimensionality decreases with the increase of Cu(II) amount being of short range for x = 2. These results are attributed to the presence of the unpaired electron in the d_x²−y² orbital and the absence of overlap between neighbour ions. Specific-heat measurements confirm the evolution to a short range magnetic system with the Cu(II) amount. From low-temperature neutron diffraction data, it can be observed that the existence of antiferromagnetic order for x ≤ 1 is originated by the antiparallel ordering of both ferromagnetic linear octahedral chains and trigonal bipyramidal dimers. The propagation vector is k = [0,0,0] and the magnetic moments are aligned in the z direction. The values of the main magnetic exchange pathways [M–O–M] are characteristic of ferro- and antiferromagnetic couplings with a superexchange ferromagnetic angle, M(1)–O(3)–M(2) of 107–109°, which plays an important role in the competition of the freezing process. These results are explained on the basis of both the electronic configuration and the correlations between structural and magnetic properties.