Ba_(1-x)R_xF_(2+x) Phase (R = Gd-Lu) with Distorted Fluorite-type Structure-Products of Crystallization of Incongruent Melts in the BaF_2-RF_3 Systems. I. Ba_(0.75)R_(0.25)F_(2.25) Crystals (Synthesis and Some Characteristics)

摘要

The transparent monolithic crystal rods grown by crystallization of the incongruent Ba_(0.75)R_(0.25)F_(2.25) melts (R = Gd-Yb) are shown to be heterogeneous: the larger part of the crystal volume has a distorted fluorite-type cubic lattice, while the smaller part retains an undistorted cubic structure. The distortions were recorded by the methods of X-ray powder and electron diffraction and crystal optics. Two types of chemical heterogeneity of Ba_(0.75)R_(0.25)F_(2.25) crystals are considered as possible sources of distortions: (1) a macroheterogeneous (at a level of 1 mm) distribution of RF_3 over the crystal-rod diameter (a cellular substructure) and (2) a microheterogeneous (cluster) structure with the nanometer dispersion typical of nanostructured materials.