|Abstract or Summary
- When the dc conductivities of various pure KC1 crystals were
compared it was found that crystals grown under an HC1 atmosphere,
from salt purified by ion exchange, exhibited lower knee temperatures
than any of the crystals studied. These knee temperatures
were found to be approximately 350°C. From this the total divalent
cation concentration was estimated to be 1.8 x 10¹⁴cm⁻³. It was also
found that no absorption bands were present, in ion exchange purified
crystals, in the region between 185 mμ and 300 mμ.
The conductivities of colored KC1 crystals were found to be
much greater than for uncolored crystals. Above approximately
400°C the current decreased with time according to the expression
i/i₀ = 1 - c(t-t₀), where t₀ was the time when the current was i₀. c was found to depend on temperature according to the expression c = 17,000 exp (-1.12 ev /kT) sec⁻¹. The decrease in current was associated with a loss of F-centers from the crystals. Optical scanning of partially bleached crystals
showed that, during bleaching, a definite boundary was formed between
a colorless region at the surfaces and a colored region in the
The electronic transport number was determined to be 0.96 at 400°C. Thus the increased conductivity was attributed to ionized F-centers. Plots of log₁₀σ vs. 1000/T were linear with two different
slopes, depending on the history of the sample. If gold contacts were
evaporated onto freshly cleaved crystals rectification was observed.
The rectification could be eliminated by either heating the colored
crystals for five minutes, at temperatures greater than approximately
500°C, after applying the contacts, or by sanding the crystals
prior to applying the contacts. Accompanying the loss of rectification
was a decrease in the conductivity and an increase in the activation
energy for conduction, from 1.13 ev to 1.22 ev.
Plots of σ, at a given temperature, vs. n[subscript F]¹[superscript /]² were found
linear. From this, and the theory of rectifying contacts, a calculation
of the thermal ionization energy for F-centers was made. The
value obtained was 2.04 ev.
Conductivity and electronic transport number measurements
were made on CaC1₂-doped KC1 crystals. From observations that the conductivity was not changed significantly after coloring and that the
electronic transport number was less than 0.15, it was determined
that the calcium remained divalent after coloring.