Infrared and broadband dielectric spectroscopy of PZN-PMN-PSN relaxor ferroelectrics: Origin of two-component relaxation

Dielectric spectra of several solid solutions of $Pb{Mg}1∕3{Nb}2∕3{O}₃{{-}}Pb{Sc}1∕2{Nb}1∕2{O}₃{{-}}Pb{Zn}1∕3{Nb}2∕3{O}₃$ (PMN-PSN-PZN) relaxor ferroelectrics were investigated in a broad frequency range from $20{{0.3em}{0ex}}Hz$ up to $100{{0.3em}{0ex}}THz$ by a combination of dielectric spectroscopy $(20{{0.3em}{0ex}}Hz--53{{0.3em}{0ex}}GHz)$, time-domain terahertz spectroscopy $(0.1--0.9{{0.3em}{0ex}}THz)$, and infrared reflectivity ($20--3000{{0.3em}{0ex}}{cm}^{{-}1}$, $0.6--90{{0.3em}{0ex}}THz$). The very strong and broad dielectric relaxation observed below phonon frequencies was analyzed in terms of the distribution of relaxation times, using the Tichonov regularization method. This revealed slowing down of the longest and mean relaxation times obeying the Vogel-Fulcher law. The relaxation splits into two components near Tₘ and the origin of both components is discussed. The formation of polar clusters below the Burns temperature (700--8000.3em0exK) is manifested by the appearance of dielectric relaxation in the terahertz range and by splitting of the polar modes in the infrared spectra. On heating from low temperatures, the A₁ component of the strongly split TO₁ mode softens toward the Burns temperature, but the softening ceases near 4000.3em0exK, which could be a signature of a polar cluster percolation temperature.