Abstract
The present work has been carried out to clarify how Cr-ionic species such as Cr^<2+>, Cr^<3+>, and Cr^<4+> ions are incorporated into the Mg_2SiO_4 crystal during the crystal growth, and also how their concentration in the crystal are varied with the oxygen-gas partial pressure in the growth atmosphere. The Cr-doped forsterite (Cr: Mg_2SiO_4) crystal having a dimension of about 35mm in diameter and about 185mm in length was grown by the Czochralski method (called as the Cz one) and by discontinuously changing the oxygen-gas partial pressure in order of 0.03atm, 0atm, and 0.03atm. Furthermore, the Cr: Mg_2SiO_4 crystal having a dimension of about 5mm in diameter and about 35mm in length was grown by the floating zone method (called as the FZ one) under the oxygen-gas pressure of 1atm, to know the concentration distribution of the Cr ionic species in the crystal grown in the un-mixed gas atmosphere of oxygen. The crystal texture observation, the Cr concentration analysis and the optical absorption spectrum measurement were performed on the crystal grown by Cz method and on the one by FZ one (called as the Cz crystal and the FZ one, respectively), and the following results were obtained. As to the Cz crystal, the change in the oxygen-gas partial pressure brought about the contrast change in the crystal texture and the corresponding change in the Cr-ionic species concentration. From the change direction of crystal texture contrast, which is perpendicular to the growth striations, it was made clear that each of Cr-ionic species already existing at its own concentration in the melt is directly incorporated into the growing crystal through the solid-liquid interface. In the case of the FZ crystal, the concentration of the Cr^<4+> ionic species was observed to be remarkably high, in a sharp contrast to those of the Cr^<2+> and the Cr^<3+> ionic species, where the existence of the Cr^<2+> ionic species was not actually recognized, and the Cr^<3+> ionic species was slight in the concentration.
