Potassium titanyl phosphate, KTiOPO₄. II. Structural interpretation of twinning, ion exchange and domain inversion

In the KTiOPO₄ (KTP) structure, it is shown that there are hole sites related by pseudosymmetry to the inequivalent potassium sites K(1) and K(2). The formation of twin domains is through a twofold rotation with the principal feature of the twinning being the displacements of the K atoms onto their respective hole sites thus reversing the polarity of the crystal. Ionic diffusion paths for K⁺ along [001] involving both potassium and hole sites are described and are used to explain the enhanced ionic conductivity seen along this direction in KTP crystals. With this structural model, an explanation is also given for the reported phenomenon of domain inversion which is known to occur in the modified layer when Rb⁺ and Ba²⁺ are substituted into one of the {001} surfaces of KTP. It is shown from structural principles that ion exchange should be more efficient at the -Z end of the crystal, as is indeed observed in practice.