Dielectric properties and charge transport in the(Sr,La)NbO3.5−xsystem

Abstract

The dielectric response of layered perovskite-related insulating SrNbO 3 . 5 and conducting SrNbO 3 . 4 1 , SrNbO 3 . 4 5 , and La 0 . 2 Sr 0 . 8 NbO 3 . 5 single crystals is investigated. The measurements are performed alongthe c axis, i.e., perpendicular to the layers, in the frequency range from 1 MHz to 1.8 GHz. The intrinsic dielectric properties could be monitored only at such relatively high measuring frequencies, since strong contact contributions at the sample-electrode interface dominate at low frequencies. In addition to the known phase transitions in the SrNbO 3 . 5 compound, a phase transition at T300 K in SrNbO 3 . 4 1 and SrNbO 3 . 4 5 is reported here. The frequency-dependent ac conductivity in all three conducting compounds follows the universal dielectric response behavior. Together with results on the dc conductivity, this finding indicates that hopping of localized charge carriers, most likely of polaronic character, is the dominating charge-transport process. For all SrNbO 3 . 5 - x compounds, relatively high values of the dielectric constant are found.