Processes of structural disordering and superionic phase transition in the Li_(0.12)Na_(0.88)Ta_yNb_(1-y)O_3 solid solution

摘要

Structural disordering and phase transitions in the Li _(0.12)Na_(0.88)Ta_yNb_(1-y)O_3 solid solutions were investigated by Raman spectra. Phase transition from ferroelectric to antiferroelectric phase at t=300÷350°C is clearly seen. In this case the line, corresponding to the stretching modes of oxygen atoms disappears. Thermal disordering of the Li_(0.12)Na_(0.88)Ta_yNb_(1-y)O_3 solid solutions structure, introduced by mobile lithium ions, happens gradually. It is facilitated by misorientation and considerable deformation of oxygen octahedra with changing of their symmetry. Static disordering of the structural units in the Nb~(5+) and Ta~(5+) sublattice lowers the point of ferroelectric-antiferroelectric phase transition and can facilitate the transition to the superionic state. However, from the data obtained, it is clear that the phase transition to the superionic state doesn't appear in Raman spectra.