ANOMALOUS DISPERSION AND THERMAL EXPANSION IN LIGHTLY-DOPED KTa1-xNbxO3

The temperature dependences of the lattice constant and TA phonon dispersion in dilute single crystals of KTa1-xNbxO3 with 0.012 <= x <= 0.09 have been studied using x-ray diffraction and neutron scattering techniques, respectively. For x <= 0.06 an anomalous low-temperature expansion of the cubic lattice parameter is observed at a temperature T-min that increases monotonically with Nb concentration, with no measurable change in symmetry down to 10 K. Concurrent with T-min is a homogeneous broadening of the x-ray Bragg peaks which persists down to 10 K, as well as a strong relief of extinction seen by neutron diffraction. Inelastic neutron scattering measurements reveal that the coupling between TA and TO modes, first, observed in pure KTaO3, is both wavevector and concentration dependent. The q-dependence of the TO-TA mode coupling exhibits a peak at small 9 which appears to shift to higher q with increasing x. In addition, a characteristic temperature T-o is observed above and below which both TA and TO branches harden, and which coincides with T-min. In no crystal was the TO zone center mode observed to condense fully. Instead, the minimum energy attained by the q = 0 TO mode at T-o for x = 0.09 is significantly higher than that measured for either x = 0.00 or 0.012.