A was observed by both dielectric loss and internal friction methods.Earlier investigators'sz reported that 0-and A13+ can each form different impurity vacancy pairs which cause Debye-type losses in CaF2. In this study the addition of sodium fluoride to CaF2 caused a loss peak at higher frequencies, for the Same temperature, than other loss peaks reported.The polycrystalline samples used were prepared from Baker reagent grade CaF? powder which was pressed into disks and rods and fired a t 750' t o 1000°C, depending on the extent of doping.The disks used for the dielectric loss experiments were 38 mm in diameter by 2 to 3 mm thick. Gold was evaporated onto the faces of the disks to form electrodes. This method of electrode application was free from space charge effects which could cause serious errors. The loss peak at each temperature was determined from measurements of the dissipation factor between 50 cps and 300 kcps.Forster's technique3 was used for internal friction measurements on rods 4 mm in diameter and from 100 to 150 mm long.Undoped polycrystalline CaFz specimens gave small loss peaks. This Debye-type loss was investigated by doping specimens with various materials to find which addition enhanced the observed loss peak. Sodium fluoride produced the observed peak, since the addition of 1% S a F to CaF2 before firing caused the dissipation factor (tan u ) to increase from 2 X to 0.3. This corresponds to a dipole concentration of about 1.0% assuming jump distances are about the length of the ( F --F -) interatomic distances in CaR.Doping the CaF? with 0-had no effect on the NaF loss peak. The 0-was added t o the CaF2 by mixing Ca(0H)z with the CaF2 powder or calcining CaFz in air as reported by B~n t i n c k .~ I t has been reported6,6 that NaF added to CaFz resulted in Fvacancies, V+, with Na+ occupying a normal Ca++ site in the fluoride lattice. Thus, it would be expected that the resulting dipoles would be composed of a V+ associated with an Na+ impurity. This Same dipole structure was observed by Wacht-man7 in ThOz doped with CaO.The dependence of the frequency of the loss peaks on temperature is shown in Fig. 1. The activation energy for dipole relaxation is 0.53 i 0.05 ev from dielectric loss data and 0.53 f 0.03 ev from internal friction data. The dipole relaxation time in an electric field is given by Eq. (1); mechanical dipole relaxation time is given by Eq. (2). = (7.25 X sec) exp (0.53/kT) (1) Tmech. = (3.29 x 10-16 sec) exp (0.53/kT)The observed activation energies agree well with that required for VFdiffusion which is rep0rtedl9~~6 as 0.55 f 0.03 ev. The dielectric loss data were plotted on a Cole-Cole8 plot (d vs. e"), which showed that the observed loss peak (corrected for ohmic losses) consisted of more than a single relaxation time, which can occur if more than one kind of jump is possible. The values of TO (the preexponential term in Eqs.(1) and (2)) are a function of the lattice geometry and the type of stress applied to the dipole. The present results on polycrystalline specimens show ...
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