Electrical conductivity of γ-irradiated crystals of La0.95Ba0.05F2.95 superionic conductor

Abstract

INTRODUCTIONNonstoichiometric R 1-y M y F 3-y single crystals (R = La, Ce, Pr, Nd; M = Ca, Ba; y ≤ 0.15) on the basis of tysonite matrices RF 3 (trigonal system, space group ) are superionic conductors with high anionic conductivity (σ). A nonmonotonic dependence of (σ on the concentration of the impurity component MF 2 is typical of R 1 -y M y F 3 -y [1-3]: the maximum value σ = 5 × 10 -4 S/cm at 20°С is observed for crystals with ~5 mol % MF 2 . Along with electrolytic proper ties, R 1 -y M y F 3 -y crystals have good technological characteristics, such as the absence of phase transi tions, low electron conductivity, and thermal and chemical stability. These fluoride superionics are used in gas and liquid sensors of fluorine and in solid state galvanic batteries [4,5]. For a number of practical applications it is important to know the behavior of the exploitation characteristics (in this case, ionic con ductivity) of superionic crystals under extreme condi tions. One such condition for functioning chemical sensors of fluorine is operation under irradiation.The purpose of this study was to investigate the influence of γ irradiation on the ionic conductivity of La 0.95 Ba 0.05 F 2.95 single crystals. EXPERIMENTAL RESULTS AND DISCUSSIONNonstoichiometric La 0.95 Ba 0.05 F 2.95 single crystals were grown by the Bridgman method from a melt and structurally certified at the Institute of Crystallography of the Russian Academy of Sciences (IC RAS). The crystals were grown in a KRF 1 setup (designed and produced at the Special Design Bureau of IC RAS) with resistive heating; all elements of the thermal block are made of graphite. The axial temperature gra dient between the hot and cold furnace zones was 30 K/cm; the radial gradient in the growth zone was 1 3 P c <5 K/cm. The rate at which the graphite crucible was lowered (which roughly corresponds to the rate of crystal growth) was chosen to be 5.2 ± 0.2 mm/h. To suppress pyrohydrolisis, the crystals were grown in a fluorinating atmosphere of tetrafluoroethylene pyroli sis products.The grown single crystal boules were cylinders 40-50 mm in length and 12 mm in diameter. The sub stance losses during growth were <2.5% of the charge mass. The samples were cut from the boules in the form of 5 mm cubes. The samples were not orien tated, because the conduction anisotropy of La 0.95 Ba 0.05 F 2.95 single crystals is insignificant [2] and can be neglected in the first approximation. La 0.95 Ba 0.05 F 2.95 crystals are transparent in the visible spectral range.La 0.95 Ba 0.05 F 2.95 crystals were γ irradiated at the Institute for High Energy Physics of the Russian Academy of Sciences (Protvino) The irradiation dose was 2 × 10 6 rad (source γ 60 Co). After irradiation the crystals acquired a light brown color.The ionic conductivity of single crystals was found from an analysis of the frequency dependence of the complex impedance in the range of 5-5 × 10 5 Hz (a Tesla BM 507 impedance meter). Dag 580 graphite paste was used to form electrodes. The electrophysical measurements were ...