Phase Equilibria and Crystal Growth for LiREF 4 Scheelite Laser Crystals
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1111-D01-07
Phase Equilibria and Crystal Growth for LiREF4 Scheelite Crystals Detlef Klimm1, Ivanildo A. dos Santos2, Izilda M. Ranieri2, and Sonia L. Baldochi2 1
Leibniz Institute for Crystal Growth, Max-Born-Str. 2, 12489 Berlin, Germany
2
Center for Lasers and Applications, IPEN, Butantã 05422-970, São Paulo, SP, Brazil
ABSTRACT The scheelite type laser crystals LiREF4 melt congruently only for RE being one of the elements Er, Tm, Yb, Lu, or possibly Y, respectively. For RE = Eu, Gd, Tb, Dy, or Ho the corresponding scheelites undergo a peritectic melting under the formation of the corresponding rare earth fluoride. The melting behavior of LiREF4 mixed crystals with two or more RE is not yet known well. If RE is a mixture of Gd and Lu, Gd rich solid solutions melt peritectically under formation of (Gd,Lu)F3 and Lu rich solid solutions melt directly without formation of other solid phases. INTRODUCTION Most of the binary systems LiF–REF3, where RE is one of the rare earth elements from lanthanum to lutetium, or yttrium, respectively, were described by Thoma [1]. For the smaller rare earth ions, starting from RE = Eu, the systems contain one intermediate phase LiREF4 that crystallizes at ambient conditions in the scheelite structure. At high pressure (10.7 GPa in the case of LiLuF4 [2]) a reversible phase transformation to a monoclinic phase is observed for the smaller RE, contrary the scheelites decompose under formation of RE rich phases and LiF for the larger RE (beyond 11 GPa in the case of LiGdF4 [3]). Solid state synthesis and lattice parameters for these compounds were reported by Keller and Schmutz [4]. The thermal stability of the scheelites under ambient pressure is larger for the smaller RE3+. LiEuF4 is stable up to 690°C where it decomposes peritectically under the formation of β-EuF3. Peritectic decomposition to a melt and the low-T (β-) phase of the REF3 is also observed for RE = Gd, Tb, Dy, Ho, and Y; but the peritectic point shifts in this sequence closer to the solidus of the LiREF4 phase. LiErF4 is controversially reported to melt incongruently [5] or congruently [1]. If one assumes a smooth variation of the melting temperatures Tf for neighboring RE, congruent melting seems more realistic, since LiTmF4, LiYbF4, and LiLuF4 melt congruently [1]. Recently from a DFT evaluation structural parameters a0, c0, elastic stiffness coefficients cij, and thermodynamic parameters were derived for most of the LiREF4 [6]. It was concluded that the scheelite structure should be stable also for the large RE from Pm to Ce, but the energies for the LiREF4 from Gd to Lu are more favorable, which is in reasonable agreement with the experimental phase diagrams [1]. The LiREF4 scheelites attract not only academic interest as some of them are interesting laser host materials where the host RE3+ can be replaced by another rare earth laser ion RE′3+. LiYF4 (YLF), often doped with Nd, is a good example that is offered by several suppliers commercially. YLF crystals can be grown from melts with slight LiF excess (molar
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