Structure evolution and high-temperature luminescence versus negative thermal expansion of Ho,Tm-doped Yb ₂ Mo ₃ O ₁₂ crystals

The nature of the phenomenon of negative thermal expansion was studied at the atomic level using the in-situ single-crystal and powder high-temperature X-ray diffraction (HTXRD) of Ho,Tm-doped Yb₂Mo₃O₁₂ crystals grown by the flux-melt technique. Phase transitions, structure deformation and luminescence were investigated in the temperature range 303-1273 K in air. Under ambient conditions, Ho,Tm-codoped Yb₂Mo₃O₁₂ has monoclinic crystal structure of Al₂W₃O₁₂ (P2₁/c) structure type: a = 16.554 (2), b = 9.859 (1), c = 16.667 (2) Å, β = 107.88 (1)°, V = 2588.6 (5) ų. A reversible transformation monoclinic ↔ orthorhombic occurs at about 320 K according to the HTXRD data. The orthorhombic structure [Pbcn, a = 13.7388 (2), b = 9.8582 (2), c = 9.9472 (2), V = 1347.25 (4) ų at 373 K] shows remarkable negative volumetric thermal expansion up to about 1173 K (average α_v = -15 × 10^-6 K^-1); above this temperature molybdate starts to evaporate. Monoclinic modification in contrast to the orthorhombic one expands only positively. Careful analysis of the orthorhombic crystal structure from non-ambient single-crystal diffraction data showed that the bond lengths in MoO₄ and YbO₆ polyhedra do not change with temperature; the decrease of structure volume is due to the angular deformation of the framework built from MoO₄ and YbO₆ polyhedra. This study demonstrates the potential of Yb³⁺, Ho³⁺, Tm³⁺-codoped crystalline systems for developing a highly sensitive ratiometric optical thermometer by exploiting efficient energy transfer, a negative thermal expansion-induced emission stabilization at ∼655 nm within 570 to 800 K, and phonon-activated population of the Tm³⁺³F₃ level for ∼700 nm emission under 980 nm excitation.