Figure 3.
Images of supercell models with 2 × 2 × 2 = 8 unit cell of HAP (visualization using Jmol after DFT calculations: (a) top view—along the c axis of the OH channel (a yellow circle highlights the main central part around one OH channel); (b) supercell model in isometric projection; (c) HAP supercell model in lateral projection for the P63 phase (OH groups are parallel oriented (ordered) in OH channel along c axis and therefore can create a summarily polar state); (d) HAP supercell model in lateral projection for the P63/m phase (OH groups are anti-parallel oriented (disordered) in OH channel along c axis and therefore can create a summarily non-polar state).