Experimental evidence and structural modeling of nonstoichiometric (0Â 1Â 0) surfaces coexisting in hydroxyapatite nano-crystals

► HRTEM crystal simulation reveal the coexistence of two HA (0 1 0) terminations. ► A Ca-rich (B layer) and PO4-rich (A layer) were identified. ► The HA nano-crystal along [1 0 0] is formed by integer numbers of the BAA sequence. ► Water molecules are differently adsorbed on each crystal surface.