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

Crystal growth and numerical simulation of CaF 2 -rich-based solid solution with TmF 3 have been performed by the Bridgman-Stockbarger technique with initial Tm content 0.1 mol%, 1 mol% and 5 mol%. 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 CaF 2 -rich- TmF 3 solid solution with an axial thermal gradient about G=12 K.cm −1 and a pulling rate of v=4 mm.h −1 . Optical absorption spectroscopy evidenced the presence of both Tm 2+ and Tm 3+ cations within the matrix. Their respective effective partition coefficients k eff (Tm 2+ ) and k eff (Tm 3+ ), close to unity, have been calculated. For low TmF 3 initial content, with 0.1 mol.% and 1 mol.%, k eff (Tm 2+ ) >1 and k eff (Tm 3+ ) >1 whereas k eff (Tm 2+ ) <1 and k eff (Tm 3+ ) <1 for the attempt with 5 mol% TmF 3 initial content. For this latter, a global effective partition coefficient, including both Tm 3+ and Tm 2+ contents, is logically deduced to be k eff ([Tm])<1. Chemical analysis carried out by ICP-AES and LIBS confirmed this trend with an effective partition coefficient of the global [Tm] content in the crystal that can be lower than 1. The global estimated partition coefficient of [Tm] in CaF 2 matrix is thus ranging in between 0.906 and below 1 for 5 mol.% TmF 3 molar fraction. The expected congruent melting point in the CaF 2 -rich-region of the CaF 2 -TmF 3 phase diagram is evidenced and is higher than 1 mol.% and below 5 mol.% TmF 3 molar fraction. Finally, carbon environment and TmF 3 synproportionation reaction are pointed out as the main likely causes of Tm 3+ cation reduction into Tm 2+ during the growth process. • Single crystals of CaF 2 doped homogeneously with TmF 3 were successfully grown by the Bridgman-Stockbarger technique. • The crystal habit in the vicinity of the preferential crystallographic directions (110) and (210) is highlighted. • The presence of Tm 2+ is due to the carbon crucible and partial synproportionation reaction of TmF 3 . • Effective partition coefficient 0.906 ≤ k eff < 1 is estimated at 5% initial TmF 3 molar fraction in CaF 2 -TmF 3 system. • The congruent point in the CaF 2 -rich region of the CaF 2 -TmF 3 system lies in between 1% and 5% molar fraction of TmF 3.