Ferroelectric phase transitions and domain relaxation in KTa_1-xNb_xO₃

Low-temperature phase transformations in weakly doped quantum paraelectrics: novel features and quantum reentrant dipolar glass state in KTa0.982Nb0.018O3

“…Nb 5þ offcentre ions already far above T C is often used when interpreting experiments, see e.g. [14,30,31,41,42], providing clear indications of polar microregions and glass-like behaviour features in Raman scattering and other related manifestations. This model had been suggested in [43] and supported by X-ray absorption fine structure experiments [44] and theoretical calculations [45 -49].…”

“…The doping of KTO with isovalent Nb impurities leads to a ferroelectric PT emerging at a critical concentration y cr ¼ 0:008; at which T C ¼ 0 K and characteristic properties appear, which are inherent to systems being at the quantum-mechanical displacive limit [8 -12]. At y mc < 0:04; three cubic-tetragonal-orthorhombic-rhombohedral PTs characteristic for KNbO 3 merge at a multicritical point T mc , 40 K [13] (according to [14] T mc , 33 K at y mc , 0:02Þ and then transition temperatures are concentration dependent up to , 700 K for KNbO 3 [15,16]. For y , y mc the transition line T c ¼ f ðyÞ is proportional to ðy -y cr Þ 1=2 and for KTN1.8 the low temperature phase appeared to be ferroelectric with a soft-mode driven displacive type cubic -rhombohedral PT at T C ¼ 27 K (Fig.…”

“…So, in [23,28,29] the incomplete softening of the TO 1 mode and the centralpeak features in weakly doped KTN were taken as indications of a dipole-glass properties. The dielectric permittivity (10 2 -10 7 Hz) for KTN with 0:006 , y # 0:03 was treated in [14] as a superposition of two dispersion mechanisms. One of them dominates in the paraelectric phase, leads to a Nb-concentration dependent freezing temperature T f and was attributed to reorientation motion of polar clouds surrounding isolated or small group of the Nb 5þ off-centre ions.…”

Phase transitions investigation by Raman spectroscopy in highly diluted KTN crystals

“…According to Refs. [1][2][3], solid solutions of KTN x for x ≥ 0.02 display the same sequence of phase transitions as pure KNbO 3 (KNO). On heating KNO exhibits the following sequence of phase transitions: rhombohedral (R3m(C 3v 5 )) → orthorhombic (Amm2(C 2v 14 )) → tetragonal (P4mm(C 4v 1 )) → cubic (Pm3m(O h 1 )) [4].…”

Phase transition in dilute KTN crystal investigated by Raman scattering measurements

“…7, a significant increase in the TO 1 wavenumber is due to the application of a large field. Thus a least-squares fit to the experimental data at 10 K using Eqn (2) gives the TO 1 mode wavenumber as 16.6 š 0.2 cm 1 at ZFC and 17.5 š 0.2 cm 1 with E D 5 kV cm 1 . This result can be clearly related to the shift of T C .…”