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Potassium titanyl phosphate KTiOPO₄ (KTP) and its family of structural analogues form a unique class of non-linear optical materials. KTP, with its large non-linear coefficients and birefringence, is an extremely efficient second harmonic generator (SHG) of Nd-YAG laser light and has become a particularly important material for an increasing number of non-linear optic applications in both the commercial and purely scientific fields. At ambient temperature and pressure, the structure of KTP assumes the acentric Pna2₁ spacegroup and we have identified a high-pressure first-order phase transition at 5.8 GPa that retains this space group symmetry. Such isosymmetric transitions have come to recent prominence in mineral physics with the discovery of similar phase changes in orthoenstatite (MgSiO₃), anorthite (CaAl₂Si₂O₈) and amazonite microcline (KAlSi₃O₈). However, for all of these materials the changes have been fairly subtle and the phase transitions have only been identified through very detailed and careful work. As the phase transition in KTP is particularly pronounced and the accompanying structural changes are relatively large, the family of KTP-type structural analogues is an important class of materials for the study of these isosymmetric transitions. We believe these transitions may be a general phenomenon in complex framework-type structures that have sufficient structural degrees of freedom to allow changes in compression mechanism without an alteration of crystal symmetry.

Our previous high-pressure structural studies of KTP indicated that the K atoms may play a key role in the structural phase transition; therefore it is of particular interest to establish what effect substituting the K site with another atomic species may have on the nature of the transition. With this in mind, we have carried out a high-pressure structural study of the sodium doped material Na_0.3K_0.7TiOPO₄ (NaKTP) to determine the structure of its high-pressure phase. The experiment was conducted in two parts: the phase transition pressure was located using angle dispersive powder diffraction techniques at the SRS Daresbury laboratory and the high-pressure structure was determined by single-crystal X-ray diffraction techniques.

The results of the high-pressure powder diffraction study indicated that the phase transition pressure for NaKTP is between 5.2 GPa and 5.9 GPa, and that the structure of the high-pressure phase is very similar to that of KTP (although the powder-diffraction data were of insufficient quality for structural refinement). Subsequently, the single-crystal X-ray diffraction studies were conducted at a pressure of 6.7 GPa, and the results indicated that the structure is very similar to that of KTP. It was found, however, that the main effect of partially substituting the K-sites with Na atoms is to increase the distortions of the high-pressure structure. Indeed, it would require a pressure of 8.9 GPa to induce the same degree of distortion in the KTP structure as that measured at 6.7 GPa in KTP.

It is particularly interesting to note that the phase transition pressure is not altered significantly by partial Na substitution. A similar behaviour has also been observed in the thallium analogue TlTiOPO₄ (TlTP), which has been shown by high-pressure Raman spectroscopy techniques to have a phase transition at 6.0 GPa. As there is a large difference between the atomic masses and atomic volumes of Tl, K, and Na, it is rather remarkable that the phase transition pressures should be so similar, which suggests that the framework may play a more dominant role at the transition. This notion has been given additional support from a single-crystal X-ray diffraction study of KTiOAsO₄ (KTA), where a buckling of the framework in the vicinity of the As2 atom was identified and found to be reminiscent of the changes in the KTP structure at high pressure. This suggests that substitution of an alternative atomic species into the framework has a similar effect to pressure. Clearly, it is important to conduct further high-pressure structural studies on these and other KTP-type structural analogues.