Effect of ROF impurities on electrical, mechanical, and optical characteristics of CaF₂ single crystals

Abstract

Earlier solid solutions of ROF in CaF, with a content of ROF up to 0.2 wt % were investigated by the spectroscopical (FEOFILOV 1954; KAPLYANSKY) and ESR methods (SIWRO).The investigations of more concentrated solid solutions were kept back by difficulties in producing single crystals. As a result of the investigation of phase diagrams ( GORBULEV et al. 1981GORBULEV et al. , 1981 we have ascertained that on the basis of CaF, the ROF components form vast areas of solid solutions of the general formula Ca, -,R,F2 -xO, containing up to 45 mol % LaOF, 40 mol% GdOF, 25 mol% YOF. However, with a decrease in temperature the areas of homogeneity in the solid solutions sharply get narrow.The objective of the present paper was to grow CaF, single crystals with a great content of ROF and investigate some of their physical properties. Methods and resultsThe growing of single crystals was carried out by the Bridgman method in graphite crucibles in an atmosphere of helium of high purity. The pulling rate made up 10 or 25 mm/h. As starting components CaF, in the form of single crystals and ROF oxyfluorides were used. ROF had been synthesized beforehand by sintering oxides, Grade "OOO", and fluorides of high purity, i. e. "chemically pure". When the crystals were slowly cooled to room temperature an opacity of the crystals was observed due to the second phase precipitation already at a contem of 1 mol% ROF. With a view to prevent the solid solution decomposition we have used a quick cooling of crystals (at a rate of 1 -5"/sec) from the temperature exceeding the solid solution decomposition curve (800-1200 "C for different compositions).Thus, colourless transparent single crystals containing up to 5 mol% ROF were produced. The crystals were retained in the metastable state when stored at room temperature during one and a half year without becoming opaque. 1 -CaF,; 2 -98% CaF, -2% LaOF; 3 -99% CaF, -I % GdOF; 4 -95% CaF, -5 % GdOF; 5 -95%CaF2 -5 % LaOF; 6 -98%CaF, -2% YOF; (mol %) The IR spectra of,the single crystals produced within a range of 2-10 pm were recorded by a UR-20-type spectrometer.It has been found that the spectra ofcrystals containing ROF practically do not differ from the pure fluorite spectrum. Thus, crystals Ca, -.R,F, -xO, retain transparency in the IR region irresptive of the ROF content.Knoop Microhardness has been measured by a PMT-3-type hardometer at a load of 100 g. The results are given in Figure I. A considerable increase in microhardness takes place in the solid solution with an increase in the ROF content.Electrical conductivity of samples has been measured at a vacuum of lo-' torr in the temperature interval of 293-973 K using a Va-J-51-type, D.C. electrometer. In addition, measurements have been carried out

Transparent oxyfluoride glass ceramics

“…MF 2 phases can dissolve up to dozens percents rare earth fluorides as well as alkali metal fluorides and oxygen (dissolving of the latter results in formation of oxyfluorides). Heterovalent replacement of fluorine by oxygen is known to cause formation of the relatively broad areas of homogeneity for the fluorite-type solid solutions based on Ca, Sr, Ba, Pb difluorides; and this is especially common for the systems that also contain rare earth cations (MF 2 -ROF systems) [309][310][311]231] and/or NaRF 4 [312]. Substitution of fluorine of oxygen in the crystal lattices of these phases results in essential variations of the unit cell parameters.…”