Point defect activation volumes in the alkaline-earth fluorides

Abstract

The effect of pressure on complex impedance has been measured for sodium-, potassium-, rubidium-, and praseodymium-doped strontium fluoride and potassium-, rubidium-, and erbium-doped barium fluoride in the region of each principal relaxation. The activation volume is determined for each relaxation and is compared with a theoretical prediction. From the results, it is concluded that the dominant mode responsible for vacancy motion can be modelled by the zone-centre Raman mode and that the principal mode involved in interstitial motion can be modelled by the zone-centre transverse optic mode. In addition, an estimate of the activation volume for the motion of 'free' defects is made. Finally, it is found that in general the thermal expansion coefficient for the migration volume of a defect is negative. Also, the compressibility of the migration volume for vacancies is found to be larger than for interstitials.