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High-temperature powder sintering is an integral part of the dense ceramic manufacturing process. In order to find the optimal conditions for producing a ceramic product, the information about its behavior at high temperatures is required. However, the data available in the literature are very contradictory. In this work, the thermal stability of hydroxyapatite prepared by a solid-state mechanochemical method and structural changes occurring during sintering were studied. Stoichiometric hydroxyapatite was found to remain as a single-phase apatite structure with the space group P63/m up to 1300 °C inclusively. A further increase in the sintering temperature leads to its partial decomposition, a decrease in the crystallite size of the apatite phase, and the appearance of significant structural strains. It was shown that small deviations from stoichiometry in the Ca/P ratio upward or downward during the hydroxyapatite synthesis lead to a significant decrease in the thermal stability of hydroxyapatite. An apatite containing almost no hydroxyl groups, which is close to the composition of oxyapatite, was prepared. It was shown that the congruent melting of stoichiometric hydroxyapatite upon slow heating in a high-temperature furnace does not occur. At the same time, the fast heating of hydroxyapatite by laser radiation allows, under certain conditions, its congruent melting with the formation of a recrystallized monolayer of oxyhydroxyapatite. The data obtained in this study can be used when choosing sintering conditions to produce hydroxyapatite-based ceramics.

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For the first time, silicate‐substituted hydroxyapatites have been prepared from mixtures containing different amounts of silicon (0.2–2 mol per mol of apatite unit cell) by dry mechanochemical synthesis at room temperature in a planetary ball mill. The XRD, FTIR, TEM, and NMR spectroscopic data show that the product of the mechanochemical synthesis is a single‐phase nanocrystalline apatite containing different amounts of carbonate and silicate ions and adsorbed water. In the annealed samples, three silicon concentration subranges can be distinguished, each of which is characterized by specific evolution of the lattice parameters. The formation mechanism of the silicate‐substituted hydroxyapatite obtainable by this method is discussed. The studies indicate that the silicon substitution limit in the silicate‐substituted lattice achievable by the dry mechanochemical synthesis followed by heat treatment is 1.2 mol per mol of apatite unit cell.

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