Melting behavior and growth of colquiriite laser crystals

Abstract

Colquiriite-type crystals like LiCaAlF 6 , LiSrAlF 6 , LiCaGaF 6 , and LiSrGaF 6 are interesting host crystals for Cr 3+ or Ce 3+ laser ion doping. Although the Ca compounds show interesting optical properties, the growth of these crystals is difficult due to incongruent melting. LiCaGaF 6 crystallizes with CaF 2 deficiency and melts incongruently at 710-720°C. Hydrolysis with traces of moisture is critical for all fluorides. AlF 3 and GaF 3 have the highest sensitivity among the main components of the colquiriites. Hydrolysis can only be slightly suppressed by hydrogen fluoride. Elementary fluorine or reactive gases as carbon tetrafluoride are more effective by several orders of magnitude.

Growth and characterization of LiCaGaF6

“…In order to avoid as much as possible the hydrolysis of the fluorides, the weighing operation and the filling of the platinum crucible were performed in a glove box under 5N nitrogen atmosphere. Recent DTA results [10] indicated, that LiCaGaF 6 melts incongruently. Accordingly, LiF and GaF 3 were used in excess and the molar composition was LiF : CaF 2 : GaF 3 = 0.35:0.3:0.35 instead of ⅓ : ⅓ : ⅓ for the stoichiometric crystal.…”

“…Before starting the measurements the DSC apparatus was evacuated to 7×10 -4 mbar and refilled with argon to remove adsorbed water. DTA measurements that were performed with powder mixtures in the system LiF-CaF 2 -GaF 3 gave recently evidence to suggest incongruent melting of LiCaGaF 6 [10]. The solid line in Figure 2 shows the melting peak of an optically clear crystal piece of this substance.…”

“…Cr-lasers with LiSAF as active medium are commercially available. Even if their optical characteristics are interesting, colquiriites are unfortunately difficult to grow for different reasons: incongruent melting behavior for the case of LiCAF [6], strong thermal anisotropy more pronounced in LiSAF [7,8], unavoidable evaporation of some components like LiF and AlF 3 during the growth from the melt and very high sensitivity to water or oxygen contamination [9,10].…”

Growth of solid solutions with colquiriite structure LiCa0,2Sr0,8AlF6: Ce3+

“…The advantages of these compounds over oxide crystals -the melting point is significantly lower (810 o C) and the band gap is much wider (11 eV). Significant inhomogeneous broadening of the vibrational laser transitions of these ions causes a broad amplification band which gives ability to wavelength tuning and ultrashort pulses generation [4,5]. Ce 3+ activators in Ce:LiCAF are supposed to substitute sites occupied by Ca 2+ , which has an ionic radius similar to that of Ce 3+ (1Å and 1,03 Å respectively) [1].…”

“…However, it is difficult to increase the Ce 3+ concentration in Ce:LiCAF crystals because of the difference between the valences of Ce 3+ and Ca 2+ ions, which can give rise to defects nearby to preserve the electroneutrality of the crystal. Consequently, the segregation coefficient of Ce 3+ ions in Ce:LiCAF crystals is quite small [4]. It is known, that varying the chemical structure of the compound, namely a set of crystal lattice cations, one can improve the optical quality by increasing the isomorphic capacity of solid solution [6].…”