Phosphates A I E II PO 4 , where A I and E II are ele ments in oxidation states +1 and +2, include repre sentatives of different structure types: β tridymite, arcanite, glaserite, olivine, maricite, etc. [1][2][3][4].Compounds with the structure of the mineral mar icite, NaFe 0.90 Mn 0.06 Mg 0.03 Co 0.01 PO 4 , attract the attention of experts by the possibility of using them as electrodes of sodium ion cells antiferromagnets, and materials with nonlinear optical activity (caused by the similarity of the structures of maricite and potassium titanyl phosphate) [5][6][7][8]. A synthetic analogue of mar icite is NaFePO 4 (space group Pnma, Z = 4) [4]. In the polyhedral form, the crystal structure can be represented by columns of edge sharing FeO 6 octahedra extended along the b axis and linked with each other through PO 4 tetrahedra. The cavities in the structures accommodate Na + ions. The crystal chemical formula of maricite type compounds is (M1) [6 + 4] {(M2) [6] (TO 4 )} 3∞ , where {(M2) [6] (TO 4 )} 3∞ is the octahedral-tetrahedral frame work of the structure, M1 is an extraframework posi tion. In maricite, cobalt and manganese ions are iso morphously substituted for iron [7][8][9][10][11][12]. Lithium con taining LiE II PO 4 phosphates (E II = Mg, Ni, Co, Fe, Mn, Cd) have an olivine structure related to maricite, and potassium containing KE II PO 4 compounds (E II = Be, Ni, Mg, Cu, Co, Zn) have a β tridymite structure [13][14][15][16][17].Among the studied NaE II PO 4 phosphates, nickel containing compounds with a maricite structure are absent. At the same time, contradictory data have been reported for the NaNiPO 4 and Na 4 Ni 7 (PO 4 ) 6 (space group Cm) phases [18,19]. The latter has the X ray diffraction pattern similar to that of NaNiPO 4 . Because of the close ionic radii of nickel and cobalt, the possible polymorphism of the nickel containing phosphate is of interest, since three polymorphs, in addition to maricite, are known for isostoichiometric NaCoPO 4 [8][9][10][11].The aim of this study is to synthesize sodium nickel phosphate NaNiPO 4 , refine its structure, and study its thermal behavior and thermal expansion. EXPERIMENTALThe initial reagents for the synthesis of NaNiPO 4 were chemically pure NaNO 3 , Ni(CH 3 COO) 2 ⋅ 4H 2 O, and H 3 PO 4 . Stoichiometric amounts of 1 M aqueous solutions of the metal salts were poured together under continuous stirring at room temperature, and then a solution of orthophosphoric acid containing a stoichi ometric amount of phosphate ion was added. The reaction mixture was dried at 80°С, the powder was ground in an agate mortar, and heat treated in air at 600°С for 24 h. To study the thermal behavior of the sample, it was subjected to stepwise annealing at tem peratures 700-1000°С. After each stage of heating, the sample was ground.The chemical composition and homogeneity of the sodium nickel phosphate sample was monitored by a JEOL JSM 7600F scanning electron microscope with a thermal field emission gun (Schottky cathode) equipped with an OXFORD X Max 80 (Premiu...