Bridgman-grown Cd1-xCaxF2 crystals were doped with SmF3, YbF3, and YF3• X-Ray powder diffraction photographs showed complete solid solution between CaF2 and CdF2. The crystals were fired in a Cd atmosphere at 500°. Electrical resistivity and Hall effect studies showed that crystals doped with 0.1 mole % REF3 and fired in Cd were n-type semiconductors with a room-temperature resistivity, carrier concentration, and mobility independent of crystal composition between 0 and 1% CaF2. Between 1 and 10% CaF2 the resistivity increased from about 1 to about 107 ohm-cm. The intense infrared absorption, which is associated with the conductivity and which causes the blue color of the crystals, followed the same pattern. Up to 1% CaF2 the crystals were blue. With 10% CaF2 added, the crystals were colorless. Similar sharp changes in resistivity and infrared absorption were observed wi h REF3 dopant concentration variations. Optical absorption studies indicated Yb2+ ions in CdF2-7% CaF2¯0.1% YbF3 crystals after Cd firing, but no Sm2 + or Y2 + ions were observed until the CaF2 content was very high (greater than 99.5%). The lack of correspondence between RE2 + formation and the cessation of conductivity are considered on the basis of chemical oxidation potentials. A model is proposed for the conductivity in which the Cd2+ ions nearest the RE3+-substituted ion share an electron thus creating an “impurity” donor level. This model is consistent with the variations of the conductivity and of the RE2+ concentrations with changing CdF2/ CaF2 ratios. © 1966, American Chemical Society. All rights reserved.