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, ρ_c = 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 Ni²⁺ 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 T_max = 38.4 K and orders antiferromagnetically at T_N = 33.4 K.

Single crystals of the RbCuAl(PO ₄ ) ₂ compound with an open 3D framework structure were obtained by a hydrothermal route. The compound orders antiferromagnetically at T _N = 10.5 K and exhibits spontaneous magnetization at lower temperature.

The crystal structure of a new modification of the niahite mineral NH₄MnPO₄ · H₂O, obtained in the form of single crystals under hydrothermal conditions in the Mn₂O₃-(NH₄)₃PO₄-H₂O system, is determined by X-ray diffraction (Xcalibur-S-CCD diffractometer, R = 0.0259): a = 17.582, b = 4.909, and c = 5.731 Å; space group D _2h ¹⁶ = Pnam, Z = 4, D _x = 2.497 g/cm³. It is shown that the new compound is a polymorphic modification of niahite; it has a layered structure, which differs from the structure of the mineral by the presence of the center of inversion and the doubled a unit cell parameter. It is proposed to consider the minerals and synthetic analogues of the niahite and triphylite groups as members of the common polysomatic series.

The novel phosphate hydroxide Na_2−xCo₆(OH)₃[HPO₄][H_x/3PO₄]₃with the mineral ellenbergerite topology and alkaline cations in the framework channels shows strong antiferromagnetic interaction and magnetic transition atT_N= 44 K.