Work

Single crystal growth by the Bridgman-Stockbarger technique of CaF2-rich - TmF3 solid solutions: Evidence of mixed valence Tm2+ and Tm3+ cations

Year: 2024

Type: article

Abstract: Crystal growth and numerical simulation of CaF2-rich-based solid solution with TmF3 have been performed by the Bridgman-Stockbarger technique with initial Tm content 0.1mol%, 1mol% and 5mol%. Centimeter-sized single crystals were obtained. Growth in the vicinity of (110) and (210) crystallographic directions have been identified as the natural crystal habit for the CaF2-rich- TmF3 solid solution with an axial thermal gradient about G=12K.cm-1 and a pulling rate of v=4mm.h-1. Optical absorption spectroscopy evidenced the presence of both Tm2+ and Tm3+ cations within the matrix. Their respective effective partition coefficients keff(Tm2+) and keff(Tm3+), close to unity, have been calculated. For low Tm initial content, with 0.1mol.% and 1mol.% Tm, keff(Tm2+) >1 and keff(Tm3+) >1 whereas keff(Tm2+) <1 and keff(Tm3+) <1 for the attempt with 5mol% Tm initial content. For this latter, a global effective partition coefficient, including both Tm3+ and Tm2+ contents, is logically deduced to be keff(Tm)<1. However, chemical analysis carried out by ICP-AES and LIBS highlighted a steady decreasing of Tm content along the crystal length featuring typically the preferential segregation of Tm within the solid phase. The effective partition coefficient of the global Tm content in the crystal was calculated and found higher than 1 with keff(Tm) = 1.018 ± 0.040 for the attempt with 5mol% Tm initial content. Due to inherent inaccuracies of employed techniques, this opposite trend suggest that the unknown but expected congruent melting point in the CaF2-rich-region of the CaF2-TmF3 phase diagram does exist and is strongly suspected to be close to 5mol% TmF3 molar fraction. Finally, after a brief review of possible thermodynamical mechanisms leading to Tm2+ in the CaF2 matrix, TmF3 non-congruent volatilization is pointed out as the main likely cause of Tm3+ cation reduction into Tm2+ during the growth process.

Source: Journal of Alloys and Compounds

DOI: 10.1016/j.jallcom.2024.177784