Crystal structure and magnetic properties of a new layered sodium nickel hydroxide phosphate, Na2Ni3(OH)2(PO4)2

Abstract

Mixed sodium nickel hydroxide phosphate, Na2Ni3(OH)2(PO4)2, has been synthesized hydrothermally from the system NiCO3-Na4P2O7-NaCl-H2O. Its monoclinic crystal structure has been determined by single crystal X-ray diffraction: a = 14.259(5), b = 5.695(2), c = 4.933(1) Å, β = 104.28(3)°, space group C2/m, Z = 2, ρc = 3.816 g cm(-3), R = 0.026. The underlying spin model has been characterized in terms of first-principles electronic structure calculations. The compound is formed by alternating layers of [NiO6] octahedra and [NaO7] polyhedra, combined in the [100] direction with tetrahedral [PO4] oxocomplexes and hydrogen bonds. The novel phase is treated as an isostructural variant of the two-dimensional potassium manganese hydroxide vanadate, K2Mn3(OH)2(VO4)2, which can be formally obtained by morphotropic substitutions of all positions in the cationic sublattice. The stripe arrangement of Ni(2+) ions (S = 1) within [NiO4(OH)2] layers of Na2Ni3(OH)2(PO4)2 is unique in the sense that its magnetic topology places it in between widely discussed honeycomb and kagomé lattices. The Na2Ni3(OH)2(PO4)2 is a low-dimensional magnet, which reaches the short-range correlation regime at Tmax = 38.4 K and orders antiferromagnetically at TN = 33.4 K.