Ionic conductivity and point defects in pure and doped MnF 2 and MgF 2 single crystals

Abstract

Conductivity, σ, of MnF 2 and MgF 2 single crystals, pure and doped (with Li +, Na +, Y 3+, Gd 3+), has been measured, from room temperature to 500°C. Further, some crystals were contaminated with O 2− as an additional impurity. These tetragonal (rutile structure) crystals both behave like typical ionic conductors. Of particular interest is the existence of a large anisotropy, σ being largest when measured parallel to the c-axis. Study of the conductivity isotherms and anisotropy as functions of impurity concentration allows identification of the conduction mechanism in terms of the migration of two mobile defects: the fiuorine-ion vacancy, V F , and interstitial, F i. A value of 1.44 eV was obtained for the enthalpy of formation of the intrinsic anion Frenkel defect, 0.80 eV for the migration enthalpy of a V F and 0.88 eV for an F 1 in MnF 2 parallel to the c-axis. Similar values were obtained for MgF 2. This work shows that more information about point defects can be obtained from conductivity measurements in non-cubic cyrstals than in cubic ionic crystals, because of the additional information from conductivity anisotropy.