Condensed Matter Seminars

ics Condensed Matter
Thursday, February 1, 2018
11:00 AM
Physics Building, Room 313
Sergey Vakhrushev [Host: Despina Louca]
Ioffe Institute, St. Petersburg, Russia
"Phase transitions and mode coupling in PbZrO3 and Zr-rich PbZr1-xTixO3 solid solutions"
ABSTRACT:

(PbZrO3)1-x-(PbTiO3)x (PZT) solid solutions probably represent the most studied group of functional dielectrics materials. For many years the main efforts were devote to the study of the PZT compounds at the morphotropic boundary region (x≈0.5). However recently Zr-rich (x<0.06) compounds attracted new attention due to their potential for the electric energy storage and electricaloric application. Beside possible application related interest these crystals demonstrate extremely reach phase diagram including antiferroelectric, ferroelectric and incommensurate phases.

In my presentation I would like to concentrate on the dynamical features related to the phase transitions in the Zr-rich PZT (including PbZrO3 itself) and in the PbHfO3. In the papers [1,2] we demonstrated that the antiferroelectric phase transition In PbZrO3 with order parameter described by the wavevector qAFE=(¼ ¼ 0) can be considered as a missed incommensurate transition with some arbitrary wavevector, corresponding to the flat part of the dispersion curve of the TA mode. Later in PbHfO3 and PbZrO3 at high pressure the minima at the TA dispersion curves were found [3,4], resulting in the realization of the incommensurate phases.

Extremely complicated diffraction pattern is observed in the PZr1-xTixO3 crystals with x<0.06. In the intermediate phase between the paraelectric and antiferroelectric phases incommensurate phase is sometime observed similar to that in the PbZrO3 under high presuure. And in addition complicated system of the satellite peaks in the vicinity of the qM=(½ ½ 0) including first order and second order satellites exists. In addition to the satellite peaks near the M-points we found second order satellites near the main Bragg peaks.

Observed diffraction pattern can be fully described by the incommensurate structure determined by 2 wavevectors from the same star: q1=(0.5+δ 0.5-δ -δ)  and q2 =(0.5-δ δ 0.5+ δ). Combination of the q1 and q2 describes all observed superstructure peaks.

Creation of the true incommensurate phase can be attributed to the mode softening not at qM, but at a position shifted from the zone boundary. Such unusual soft mode can be described in terms of the coupling of 2 modes in the vicinity of M-point, namely TA mode and oxygen tilt mode. Such coupling is forbidden at qM but became allowed aside of it. Proposed model provides qualitative agreement with the results of the inelastic and diffuse X-ray scattering measurements 

 

References
[1] A. K. Tagantsev et al., Nat. Commun., 4, 2229 (2013) [2]R. G. Burkovsky, et al. Phys. Rev. B 90, 144301 (2014) [3] R.G. Burkovsky, et al.. J. Phys.: Condens. Matter,  27, 335901 (2015) [4] R.G. Burkovsky, et al.., Sci. Reports, 7, 41512 (2017)

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