Raman scattering and the evolution of polar order in Li-doped and Nb-doped KTaO3

Abstract

A comparative study of first-order Raman scattering in K 0.984 Li 0.016 TaO 3 and KTa 0.976 Nb 0.024 O 3 single crystals has been performed. The soft-phonon line shows marked differences in the two systems, revealing a distinctive behavior with regard to the scale of polar order. Analysis of the low-frequency spectrum allows for an estimate of the polar correlation length in the Li-doped sample; this is limited to the nanometric scale, in agreement with the view of a dipole-glass transition. No evidence of a similar situation is found in the Nb-doped sample, where the general trend of soft-mode frequencies and intensities suggests a transition to a state of long-range order, for the quoted concentration. The intensities of the hard-phonon spectra and the TO 4 linewidth are shown to confirm the different evolution of the polar transition in the two systems.

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

“…This model had been suggested in [43] and supported by X-ray absorption fine structure experiments [44] and theoretical calculations [45 -49]. At the same time, this disagrees with experimental findings [9,21,36,50], where, for example, no simple relationship between first order Raman intensity and Nb concentration was found. Moreover, according to NMR study on 93 Nb [20,51], Nb 5þ impurities are on-centre at high temperatures, but freeze out in off-centre position below T C due to interaction with the soft mode.…”

“…The spectra were recorded with a resolution of 2 cm 21 at cooling run and after cooling the sample at a constant rate of 1 K/min from 100 K to the lowest attainable temperature (11 K) and subsequent heating to the temperature of measurement. The spectral regions inspected were 8 -120 and 520 -600 cm 21 , where the soft TO 1 and hard TO 4 modes can be observed, respectively.…”

“…The specimens, fabricated as the thin polished plates with Pt -Au electrodes evaporated onto the {100} faces, were suspended in a He-flow cryostat allowing temperature cycling within 10 -300 K at a rate^(10 -100) mKs 21 .…”

Onset of spontaneous electrostrictive strain below 520 K in Pr-dopedSrTiO3

“…Studies in the past, on the two well-known quantum paraelectric systems, ST ͑Refs. 2-8, 17, and 18͒ and KTaO 3 , [19][20][21][22][23] have shown that ferroelectric correlations develop only at temperatures well below room temperature ͑300 K͒. In this scenario, unless something dramatic takes place, a ferroelectric state by chemical substitutions in quantum paraelectric systems is not expected to occur at high temperatures ͑near 300 K or above͒.…”

Phase transition in dilute KTN crystal investigated by Raman scattering measurements

“…27 In contrast, no electric field effect was reported in the TO 1 spectrum of KTN with x D 0.024. 28 This shows that the long-range order is achieved in this crystal, even in the spontaneous case. 28 As the cluster number is directly related to the Nb content, this means that the long-range order is more rapidly reached in KTN2.4% than in KTN1.2%.…”