The RE 3+ doped YbNbO 4 and Yb 3 NbO 7 nanophosphors (RE=Ho, Er and Tm) were synthesized via a calcination-assisted hydrothermal procedure. YbNbO 4 intensified almost the whole up-conversion (UC)/down-conversion (DC) emissions of RE 3+ by ∼2 times when compared with Yb 3 NbO 7 components, indicating that a lower symmetry environment in the former case is more favorable for photoluminescence. UC luminescence in each case was dominated by a green band at ∼539 nm for Ho 3+ and a NIR band at ∼803 nm for Tm 3+ , while doping YbNbO 4 and Yb 3 NbO 7 with Er 3+ separately yielded the slightly stronger green 556 nm and red 658 nm emissions. The UC/DC luminescence mechanisms were elaborated via Yb 3+ → RE 3+ forward/backward energy transfer and cross relaxation.
Fluorite structural Yb 3 TaO 7 and Yb 3 NbO 7 with glass-like thermal conductivity are investigated as candidate thermal insulation materials in this work. The relationship between thermo-mechanical properties and crystal structure of Yb 3 TaO 7 and Yb 3 NbO 7 ceramics has been determined. The crystal structure is characterized via X-ray diffraction (XRD), Raman spectrum and X-ray photoelectron spectroscopy (XPS), and the microstructure is observed by scanning electron microscope (SEM). Yb 3 TaO 7 and Yb 3 NbO 7 exhibit the ultralow glass-like thermal conductivity (1.04 W m −1 K −1 ), which derives from the long-range disordered atomic arrangement. The weaker bonding strength leads to higher TECs, lower thermal conductivity and Young' modulus of Yb 3 NbO 7 , compared with Yb 3 TaO 7 . Besides, the outstanding high-temperature phase stability, high hardness and fracture toughness of Yb 3 TaO 7 and Yb 3 NbO 7 imply that they are promising high-temperature thermal insulation ceramics.
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