Dielectric relaxation studies of alkali-metal-doped strontium and barium fluoride

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Abstract
The audio frequency dielectric constant has been determined over the temperature range 5.5-400K for various samples of alkali-doped strontium and barium fluoride. For strontium fluoride, strong relaxations are found for sodium, potassium, and rubidium dopants and the activation energy is largest for the potassium dopant. For barium fluoride, strong relaxations are observed for potassium and rubidium and the activation energy is larger for rubidium. A physical argument based on ion size and activation energy is given supporting the assignment of the principal peaks to substitutional alkali-metal-nearest neighbour-vacancy pairs. The activation energy for the motion of the bound vacancy in rubidium-doped barium fluoride is found to be about the same as that reported for the motion of 'free' vacancies and consequently the current interpretation is consistent with empirical observation that in all materials studied to data the energy for reorientation of a bound interstitial or vacancy around a substitutional ion is less than or equal to the energy for motion of the free species.