Effect of Cr and Mn doping on ferroelectric and dielectric properties of Li_1.72Na_0.28Ge₄O₉ single crystals

Abstract

Li_1.72Na_0.28Ge₄O₉ single crystals pure and slightly doped with chromium and manganese, obtained by Czochralski growth, were investigated for their ferroelectric and dielectric properties. The change in stoichometry of Li_{2− x} Na _x Ge₄O₉ (x = 0.28) crystals doped with Cr and Mn (0.1 mol.%) had an influence on the value of ferroelectric-to-paraelectric phase transition temperature, T _c. The applied doping affects the T _c value by no more than 10 K. Moreover, we observed strong anisotropy of physical properties of lithium tetragermanate crystals. It has been found that Li_1.72Na_0.28Ge₄O₉ crystal doped with transition metal ions reveals ferroelectric activity in the b-crystallographic direction, (010 in Pca2₁ space group), below T _c = 260 K. This activity, measured by the value of electrical permittivity, is much less for doped crystals than pure ones. Dielectric permittivity ε′, conductivity and loss tangent of the crystals, all as the function of temperature in the range of 113–433 K, at frequency between 1 Hz to 1 MHz were measured. The values of conductivity activation energies, E _a, were estimated. Hysteresis loops as well as coercive field and spontaneous polarization were measured and calculated. Switching current transients were analyzed.

Keywords:

• ferroelectric properties
• dielectric properties
• tetragermanate single crystals
• doping