STRUCTURAL STUDIES OF THE NONLINEAR OPTICAL, FERROELECTRIC, AND IONIC-CONDUCTING PROPERTIES OF NEW MEMBERS OF THE KTiOPO₄ FAMILY. E.L. Belokoneva, B.V. Mill, Moscow State University, Russia. S.Yu. Stephanovich, Karpov Inst. of Physical Chemistry, Russia. K.S. Knight, W.I.F. David, Rutherford Appleton Laboratory, UK.

New compounds with KTiOPO₄ (KTP) structure - AMOXO₄: A- Na,K,Rb,Tl,Ag; M-Sb,Ta; X-Ge,Si - have been prepared that are analogous to the more familiar phosphates and arsenates. Their crystal structures have been investigated using X-ray single crystal and neutron high resolution powder diffraction data at high and low temperatures.

Nonlinear optical properties (SHG measurements on powder samples) indicate that all compounds are, like KTP, ferroelectrics with Curie points between 272K (TlSbOGeO₄) and 1200K. The lower transition temperatures and associated smaller SHG signals of these new compounds compared with the phosphates and arsenates results from a higher degree of pseudosymmetry and regularity of crystal structures. Importantly, substitution of highly polarisable Nb in KSb_1-xNb_xGeO₅ and KTa_1-xNb_xGeO₅ systems increases the SHG signal. Indeed in the latter solid solution, the SHG signal approaches that of KTP.

The lost of polarity (and concomitant loss of ferroelectric and nonlinear optical properties) occurs at a second-order phase transition which exhibits both order-disorder and displacive behaviour. The dominant displacements and disordering are associated with the A-cations and agree with the Abrahams-Jamieson-Kurtz (AGK) criteria.

The most important property of AgSbOSiO₄ is cationic conductivity. Ion exchange channels, marked by Ag-atoms, are described and the anisotropy of conductivity in the KTP structure is explained. The behaviour of A-cations determines the ionic-conducting and ferroelectric properties; M-octahedra together with A-cations make a considerable contribution to the non-linear optical properties.