Symmetry-mode analysis of the ferroelectric phase in K2SeO4

Publisher Website
Google Scholar
Abstract
The recently determined ferroelectric phase of K2SeO4 is analysed in terms of symmetry modes of the room-temperature structure. The distortion relating both phases is shown to be a superposition of 63 modes transforming according to A1g, B2u, Sigma 2 and Sigma 3 symmetries. The amplitudes for each of them have been determined and indicate that the primary mode Sigma 2 is not predominant in the distortion. In contrast, the preceding incommensurate phase is shown to be described by a A1g+ Sigma 2 distortion which can be compared term by term with the symmetry-mode decomposition performed for the ferroelectric phase. The Sigma 2 distortion is shown to be essentially identical in both phases. Only its global amplitude turns out to be slightly higher in the lock-in phase. In this way the 'constant amplitude' ansatz usually considered in theoretical analysis of incommensurate phases is experimentally confirmed. Similar results are obtained for the A1g distortion. It is also shown that the displacement pattern for the Sigma 2 soft mode proposed by Haque and Hardy (1980) from a simple lattice dynamical rigid-ion model is not far apart from the frozen Sigma 2 distortion pattern observed in the ferroelectric phase, proving that such a simple model is essentially correct. Finally, as a consequence of the present analysis, a new approach to the problem of determining incommensurate structure is suggested.