Electrical and Optical Properties of Rare Earth Doped Cadmium Fluoride Single Crystals

Abstract

Single crystals of CdF2 have been doped with all rare earth elements and with Y, Sc, and U. Firing these crystals in a Cd atmosphere or under vacuum at 500° produced samples having electronic, n-type conductivity for all the dopants except La, Ce, Pr, Eu, and U. The temperature dependences of the resistivity, Hall coefficient, and mobility were similar for all crystals containing conductivity-producing dopants and were similar to the temperature dependences expected for impurity conduction. Optical absorption measurements indicated that no divalent rare earth ions (except Eu+2) and no U+3 ions were present in the crystals either before or after heat treatment. Absorption studies on all the conducting crystals showed essentially identical infrared absorption that increased in intensity with wave length and that decreased in intensity between 300 and 77 °K. but not between 77 and 6°K. Yellow to yellow-green phosphorescent and thermoluminescent radiation was observed and was very similar for all the unfired crystals containing conductivity-producing dopants. Chemical oxidation potentials were used to explain the existence of M+3 and U+4 ions and the absence of M+8 (except Eu+3) and U+s ions both in the untreated and in the Cd treated samples. The presence of ions in either the +2 or the +4 oxidation state was used to explain the absence of conductivity in some of the doped crystals. The following model for the conductivity and the associated infrared absorption is proposed. The impurity states giving rise to conduction are formed by the sharing of one electron among the twelve Cd nearest neighbors to the substituted M+3 dopant. The associated optical absorption is caused by charge transfer between these cadmium "impurity" states. © 1965, American Chemical Society. All rights reserved.