Brillouin scattering, DSC, dielectric and X-ray diffraction studies of phase transitions in antiferroelectric PbHfO3:Sn

Abstract

Specific heat, dielectric, powder X-ray diffraction and Brillouin scattering studies of phase transitions in antiferroelectric PbHf0.975Sn0.025O3 crystal were performed. The specific heat data revealed clear anomalies at T1=473.5 and T2=426.3K on cooling, which could be attributed to onset of first order phase transitions from the paraelectric (PE) phase to an intermediate antiferroelectric phase (AFE2) and the AFE2 phase to another antiferroelectric phase (AFE1), respectively. The estimated entropy changes at T1 and T2 pointed to mainly an order–disorder and displacive character of these transitions, respectively. X-ray diffraction data showed a complex superstructure of the intermediate phase with a=11.895(6)Å, b=11.936(4)Å, c=8.223(3)Å at 453K. Brillouin studies revealed pronounced softening of longitudinal acoustic (LA) mode in the PE phase associated with its broadening. The broadening and softening exhibited maximum values at T1. Additional acoustic anomalies, that is, abrupt frequency shifts for LA and transverse acoustic (TA) modes were also observed at T2. Brillouin scattering data also showed presence of a broad central peak (CP) that exhibited highest intensity at T1. The observed temperature dependences of acoustic modes and CP indicate order–disorder character of the FE to AFE2 phase transition and importance of polar precursor clusters in the PE phase. The obtained data also suggest that the intermediate antiferroelectric phases in Sn4+ doped PbHfO3 and PbZrO3 may have very similar structures with incommensurate modulations.