Structural and Thermal Properties of Tetragonal Double Tungstate Crystals Intended for Ytterbium Laser Composites

Crystals of undoped and laser active Yb-doped NaT 1 - x Yb x (WO 4 ) 2, T = Y ( x = 0, 0.1), Lu ( x = 0, 0.1, 0.5), have been grown by the Czochralski (T = Y) or the top seeded solution growth (T = Lu) methods, depending on their melting nature, congruent or incongruent, respectively. From single-crystal X-ray diffraction analysis, the noncentrosymmetric tetragonal space group I 4̄ has been established for all of them. The Na + /T 3+ (Yb 3+ ) occupancy factors for 2 b and 2 d sites show the lowest departure from the random distribution, and thus the largest cationic disorder, in NaLu 0.9 Yb 0.1 (WO 4 ) 2, namely 0.43/0.57 and 0.56/0.44, respectively. The thermal expansion tensors have been determined up to 973 K for the five crystals; NaLu(WO 4 ) 2 with α 1 =8.2(2) × 10 - 6 K - 1 and α 3 = 17.3(4) × 10 - 6 K - 1 coefficients has lower anisotropy than NaY(WO 4 ) 2 with α 1 = 8.4(2) × 10 - 6 K - 1 and α 3 =18.5(4) × 10 - 6 K - 1 coefficients. From the point of view of the thermal steadiness, the evaluation of the Yb-doped laser layer/optically inert substrate lattice mismatch, about 2.5 times higher for NaY 0.9 Yb 0.1 (WO 4 ) 2 /NY(WO 4 ) 2 than for NaLu 0.9 Yb 0.1 (WO 4 ) 2 /NaLu(WO 4 ) 2 at temperatures up to 973 K, points out to this last set as the best simple combination of tetragonal Na-based double tungstates intended for epitaxially grown laser active composites. An approach to minimize the laser layer/substrate lattice mismatch derived of the Yb incorporation in the layer is developed through the tailoring of the composition of its optically inert substrate.