POINT DEFECTS, INTERACTION.DEFECT ASSOCIATION IN SrCl₂ DOPED WITH Na⁺, K⁺, Rb⁺ OR Gd³⁺

Abstract

The extrinsic conductivity of SrCl2 crystals doped with Na+, K+ or Rb+ is interpreted by the Lidiard-Debye-Hückel theory, taking into consideration the presence of monovalent ions in interstitial positions. For SrCl2 : Na+ we calculate the Gibbs free energy of solubility. The study of SrCl2 : Gd3+ crystals gives the value of the mobility enthalpy of the interstitial chlorine

Retarded ionic motion in flourites

“…The ionic conductivity of nominally pure and doped SrC12 has been studied by several authors [24,32,33]. Consistent values for the conductivity activation enthalpies were obtained.…”

“…For SrC12 this frequency has the value 4.44 X 1012 s -1 , and this value should be considered in a further analysis of the pre-exponential factor. The data of Gervais et al [32] lead to the following relation for the chloride ion vacancies /AV~ 1 = (1.4 X 102/T) exp(-0.336 eV/kT) cm2/V s.…”

“…All data for doped materials revealed substantial association of oppositely charged defects at low and moderate temperatures. Gervais et al [32] interpreted all extrinsic conductivities using the Debye-Hiickel-Lidiard theory and arrived at a reasonably consistent set of data for the migration enthalpy and entropy of chloride ion vacancies in SrC12. For the attempt frequency v0, which appears in the pre-exponential factor of the mobility relation, they have used the value 5.22 × 1012 s -1 [34] which appears to be close to the frequency, of a Raman active mode [35].…”

CALCULATIONS OF POINT DEFECT PROPERTIES.Disorder in Pure and Doped Strontium Chloride

“…Doping with monovalent cations leads to an enhancement of ionic conductivity and has led to proposals that alkali cations occupy regular cation sites with charge compensation by anion vacancies. Recently, however, it has been suggested that alkali cations on interstitial sites must also be considered [5].…”

“…However we need to consider equilibria between the various point defects. The simplest equilibrium that we might expect is that between defects formed by modes B and C, Gervais et al [5] (see also [12]) suggest that an equilibrium of the type may occur at high dopant concentrations. We find that for NaCl the formation of an alkali cation interstitial and a strontium vacancy is unfavourable by 2.98 eV.…”