Europium precipitates in monocrystalline NaCl. Part 1. General features of as-grown state

As-grown and well aged at room temperature NaCl crystals doped with divalent europium were systematically studied by using optical, dielectrical and mechanical methods. It has been found that at room temperature the optical absorption spectrum of these crystals is strongly concentration-dependent. The role of the dopant concentration has been also evidenced by photoluminescence (taken at liquid nitrogen temperature) thermally stimulated depolarization current techniques and the mechanical response of the as-grown crystals. The experimental results obtained have been explained by assuming that the increase of the dopant concentration is accompanied by structural changes of the Eu super(2+)-related precipitates from impurity-vacancy dipoles and small aggregates to orthorhombic EuCl sub(2) of PbCl sub(2)-type.