Abstract
In situ Raman spectroscopic measurements have been carried out at high pressure up to 33 GPa using a diamond anvil cell to investigate the structural transitions in relaxor ferroelectric 0.85Pb(Zn1/3Nb2/3)O3-0.15PbTiO3. Raman modes are found to be broad due to substitutional disorder at the B-site of the perovskite. Evolution of spectra with pressure gives evidence for structural instabilities around 2.2, 6.3, and 14.6 GPa. New modes at 343 and 376 cm−1 appear across the transition at 6.3 GPa, characteristic of the high pressure antiferrodistortive rhombohedral phase (PII). The pressure dependence of mode frequency, width of the Raman bands, and integrated intensity of structurally sensitive A1(TO) mode at 272 cm−1 are obtained; their effect on polar ordering and structural transitions are discussed. The disappearance of the mode around 200 cm−1 and the appearance of a new one around 120 cm−1 are evident around 14.6 GPa, and these are attributed to a possible new phase PIII. The reported pressure-induced suppression of diffuse x-ray scattering on Pb-based relaxors is consistent with the observed Raman features.
Highlights
Perovskite-type (ABO3) relaxor ferroelectric materials have attracted the scientific community from the point of view of fundamental physics as well as due to their outstanding dielectric, electro-elastic, and electro-optic properties.[1,2] Due to their excellent performance, they play an important role in a number of potential technological applications.[3,4,5] Relaxor ferroelectrics exhibit slim hysteresis loops, and a broad frequency and temperature dependent dielectric maximum indicative of multiple scales of relaxation
It is widely accepted that substitutional disorder on the A and/or B site by cations of different atomic radii and valencies can lead to the formation of nanometer-size polar nanoregions (PNRs) and chemically ordered regions (CORs), which are dispersed in a non-polar paraelectric matrix.[9,10,11,12,13]
The driving mechanism for the existence of PNRs is yet to be understood. Many investigations such as neutron scattering, dielectric spectroscopy, Raman, Brillouin and infrared spectroscopy have been employed to understand the dynamical aspects of PNRs and their influence on macroscopic properties such as ferroelectric phase transition.[10,11,14,15,16,17,18,19]
Summary
Solutions of (1-x)Pb(Zn1/3Nb2/3)-x(PbTiO3) ((1-x)PZN-xPT) relaxor ferroelectrics belong to the family of perovskites. For x ≤ 0.08, the symmetry of the ferroelectric phase is rhomohedral, whereas compounds with x = 0.11-0.15, are found to be in tetragonal phase.[14] x = 0.15 compound located away from the MPB region shows a tetragonal–cubic phase transition at high temperature and exhibits relaxor ferroelectric behavior.[14,21] Recently, we have investigated the temperature evolution of PNRs and structural phase transition on this system using dielectric, polarized Raman spectroscopy[20] and Brillouin spectroscopy.[21] Upon cooling to ambient temperature, the correlation among the PNRs increases (below TB∼650 K),[20,21] and the PNRs merge to form larger PNRs and exhibit a long-range order of a normal ferroelectric material This ferroelectric ordering is expected to be incomplete since a fraction of PNRs are reported to persist till ambient temperature.[14] the PNRs in the Pb-relaxor are understood to be pinned onto CORs and driven by its chemical ordering.[12,13,25,26,27,28] On the other hand, in contrast to these findings, several reports indicating non-substantial role of CORs on PNRs dynamics .[29,30,31]. Low temperature Raman spectra have been measured to ascertain that there is no signature of phase transition up to 90 K and to identify the group theoretically predicted phonons
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