Low temperature behaviour of quantum paraelectric SrTiO3 weakly doped withCa2+ impurities

The influence of point defects on the dielectric susceptibility as observed on Sr1-xCaxTiO3 with x less than or equal to 0.007 is described within the framework of two microscopic models. While the 'soft impurity oscillator m odel' readily explains the induced lattice softening at low concentrations, an adequate description of the ferroelectric instability and nonlinear die lectric behaviour requires dominance of a 'mode coupling model' at higher c oncentrations. It involves the soft-mode and tunnelling ions, whose appeara nce is explained within a percolation approach.