Low-temperature phase transformations in weakly doped quantum paraelectrics: novel features and quantum reentrant dipolar glass state in KTa0.982Nb0.018O

An unusual sequence of phase transitions (PT) and reentrant dipole glass-like phase formation at low temperatures was found recently in KTaO3 weakly doped with Li and Nb (K0.9986Li0.0014Ta0.976Nb0.024O3) [Phys. Rev. B 63 (2001) 172]. We report on detailed low frequency (100 Hz-1 MHz) permittivity and Raman light scattering studies of similar composition, but without Li admixture, KTa1-xNbxO3 with x = 0.018 (KTN1.8). The aim of the study is to answer the question if the reentrant dipole glass-like phase exists in KTN1.8 and what is the microscopic origin of this phase. A detailed study of the sharp low-temperature PT observed at T-C similar to 27 K revealed properties inherent to the reentrant glass-type state at lower temperatures. The substitution of Nb for Ta influences the TOI soft lattice mode and leads to PT with the long-range ferroelectric ordering. A crossover to an order-disorder polar microregion dynamics with a non-standard epsilon'(T) behaviour and dipole glass-like formation were found below T-C (at similar to 15 K), which is attributed to the randomness of the Nb distribution. A crossover to the Iona-range order was found under a dc bias field. (C) 2004 Elsevier Ltd. All rights reserved.