Determination of the lithium Frenkel energy in lithium tantalate

The ferroelectric to paraelectric phase transition is modeled as an order-disorder process with lithium atoms jumping between their normal sites and adjacent interstitial sites. This lithium jump corresponds to lithium Frenkel defect formation. At temperatures significantly away from the Curie temperature it is found that the lithium Frenkel energy is about 0.165 eV. As the temperature approaches T_c, disorder abounds and eventually the lithiums become equally distributed between the two available sites. The previously reported spontaneous polarization, P_s, data for lithium tantalate are analyzed in terms of this order-disorder model. The slope of P²_s with temperature is directly in line with the model's prediction.