Abstract
Pb{sub 1.83}Mg{sub 0.29}Nb{sub 1.71}O{sub 6.39} crystallizing in a cubic pyrochlore structure exhibits a typical feature of quantum paraelectrics--its permittivity continuously increases on cooling and levels off below {approx}30 K without any signature of a structural phase transition. Broadband dielectric spectra do not show any dielectric dispersion in the real part of permittivity up to 8.8 GHz. Terahertz and infrared spectra reveal a soft polar optic mode, which is responsible for the temperature dependence of the permittivity. The leveling off of the permittivity at low temperatures obeys the Barrett formula, and the fitted vibrational zero-point energy (1/2)k{sub B}T{sub 1} corresponds to the measured soft-mode frequency. The number of observed infrared phonons exceeds that predicted from the factor-group analysis, which indicates that the structure is at least locally noncubic.
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