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The oriented hierarchical architecture of hydroxyapatite (HA) shows excellent performance in multiple functions in human hard tissues such as bone and teeth. It is a challenge to mimic the architecture of biomineralization products. This study introduces a simple method for the hierarchical architecture of pure stoichiometric HA by a one-pot hydrothermal process without adding any organic molecules. The reaction process is researched by XRD, FT-IR, SEM, and TEM, and the HA crystal growth mechanism is discussed. The short rod-like HA nanocrystals are orderly assembled into micrometer-sized platy particles with the DCPD (CaHPO4·2H2O, brushite) shape retained. The ordered HA hierarchical architectures are achieved through topotactic transformation due to the crystal structural similarity from DCPD through DCPA (CaHPO4, monetite) to HA. It is found for the first time that the growth of HA crystals on DCPA is accompanied by orientation relationship of HA [110] // DCPA [110] and HA (112) // DCPA (112). The re...