Measurements of elastic constants in super-ionic Ba1-xLaxF2+x

Abstract
Measurements of Brillouin frequency shifts as a function of temperature have been performed on BaF2 and Ba1-xLaxF2+x (x=0.393). In BaF2 the associated elastic constant C11 shows a pronounced decrease at Tc which continues up to the highest temperature measured, whereas C44 is unaffected. In Ba1-xLaxF2+x both C11 and C44 show decreases at a lower value of Tc, these decreases tending to saturate at higher temperatures. The results are examined in the light of models of highly defective fluorites.