Heat capacity and sources of random fields in K1-xLixTaO3

Heat capacity data are presented for KTaO 3 doped with various amounts of Li. Two anomalies are resolved at low temperatures: a peak of c/T 3 versus T at 10K similar to one found in pure KTaO 3 and in many ferroelectrics; and a kink in c/T 3 versus T at 30K only in samples heavily doped with Li. These findings may be explained in terms of a random internal field coupling non-linearly to lattice modes. Internal fields are produced by off-set charges and when these charges become mobile with rising temperature the fields average to zero and the lattice modes are allowed to vibrate in a bias-free manner. The charges responsible for both anomalies in KTaO 3 are believed to be of extrinsic nature: electrons dissociated from unknown donors are responsible for the 10K anomaly while intentionally doped Li ions substituting for K at an off-centred position induce the 30K anomaly.