Infrared and THz Soft-Mode Spectroscopy of (Ba,Sr)TiO₃ Ceramics

Abstract

We performed a study of the temperature dependence of the dielectric response of (Ba,Sr)TiO ₃ (BST) ceramics for the whole composition range in a wide frequency range by means of Fourier transform infrared, time-domain THz, microwave and low-frequency dielectric spectroscopy. Room-temperature spectra of the paraelectric dielectric response were compared with the results of first-principle simulations within the effective Hamiltonian approach with a good semi-quantitative agreement. It is shown that in the paraelectric phase away from the transition temperature, the complex THz soft mode, consisting of two overlapping heavily damped oscillators, fully accounts for the low-frequency dielectric properties. However, close to T _C additional central-mode type dispersion, apparently caused by the pre-transitional polar clusters, is to be expected in the microwave dielectric spectra.

Keywords:

• Soft mode
• permittivity
• IR reflectivity
• Ba_xSr_{1 - x}TiO₃ ceramics

Acknowledgments

The work was supported by the Grant Agency of the Czech Rep. (project No. 202/06/P219) and Academy of Sciences of the Czech Rep. (project AVOZ10100520). One of the authors (T.O.) acknowledges financial support of L'Oreal Fellowship Program “For Women in Science”. I.P. and L.B. also acknowledge NSF grants DMR-0701558, DMR-0404335 and DMR-0080054 (C-SPIN), ONR grants N00014-04-1-0413 and N00014-08-1-0915, and DOE grant DE-FG02-05ER46188. E.S. and V.L. acknowledge grants N.Sh.-5169.2006.2, N.Sh.-2628.2008.2 and OFN grant.

Notes

**On leave from A.I. Ioffe Physical-Technical Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russian Federation.