Figure 1.

The orthorhombic HAP supercell model (352 atoms) and its geometric relation to the model of a hexagonal unit cell (44 atoms, marked by black thick lines; the black dashed line shows this unit cell periodically repeating) viewed from the Z-axis. The two types of calcium positions, Ca1 (green circle) and Ca2 (blue circle line), are located around the axis of the OH channel. The corresponding circle radii are r(H–Ca1) and r(O–Ca2). Notation: light blue—Ca, green—Mg, red—O, brown—P, white—H atoms.