| Summary: | Hydroxyapatite (HAp) is a natural hard tissue constituent widely used for bone and tooth replacement engineering. In the present work, synthetic HAp was obtained from calcium nitrate tetrahydrate (Ca(NO<sub>3</sub>)<sub>2</sub>·4H<sub>2</sub>O) and ammonium phosphate dibasic (NH<sub>4</sub>)<sub>2</sub>HPO<sub>4</sub> following an optimized microwave assisted hydrothermal method. The effect of pH was evaluated by the addition of ammonium hydroxide (NH<sub>4</sub>OH). Hence, different characterization techniques were used to determine its influence on the resulted HAp powders’ size, shape, and crystallinity. By Transmission Electron Microscopy (TEM), it was observed that the reaction pH environment modifies the morphology of HAp, and a shape evolution, from sub-hedral particles at pH = 7 to rod-like nanosized HAp at pH = 10, was confirmed. Using the X-ray Diffraction (XRD) technique, the characteristic diffraction peaks of the monoclinic phase were identified. Even if the performed Rietveld analysis indicated the presence of both phases (hexagonal and monoclinic), monoclinic HAp prevails in 95% with an average crystallite size of about 23 nm. The infrared spectra (FTIR) showed absorption bands at 3468 cm<sup>−1</sup> and 630 cm<sup>−1</sup> associated with OH<sup>−</sup> of hydroxyapatite, and bands at 584 cm<sup>−1</sup>, 960 cm<sup>−1</sup>, and 1090 cm<sup>−1</sup> that correspond to the PO<sub>4</sub><sup>3−</sup> and CO<sub>3</sub><sup>2−</sup> characteristic groups. In summary, this work contributes to obtaining nanosized rod-like monoclinic HAp by a simple and soft method that has not been previously reported.
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