Optical and thermal characterization of microcrystalline Na₃RE(PO₄)₂:Yb orthophosphates synthesized by Pechini method (RE = Y, La, Gd)

Highlights: • The Na₃RE(PO₄)₂ compounds were obtained in orthorhombic or monoclinic modifications. • The phosphates show structural transformations at high temperatures. • The vibrational studies confirmed new monoclinic structure of yttrium phosphate. • The decay time of Yb³⁺ emission was detected for Na₃Y(PO₄)₂. - Abstract: Na₃RE(PO₄)₂ orthophosphates (RE = Y, La, Gd) doped with Yb³⁺ ions were synthesized by Pechini method and characterized by differential scanning calorimetry/thermogravimetric analysis, X-ray diffraction, IR and Raman spectroscopies as well as optical techniques (emission spectra and fluorescence decay time). All measurements, apart from DSC, were performed at room temperature. The spectroscopic properties were discussed in terms of the structure of the obtained materials. They show that lanthanum and gadolinium phosphates synthesized by Pechini method crystallize in the orthorhombic structure, similar to that of Na₃Ce(PO₄)₂. For the first time double sodium yttrium orthophosphate has been obtained in monoclinic modification, that could be compared to one of the Na₃Gd(PO₄)₂ polymorph. Annealing of the obtained orthophosphates at 700 °C leads to grain of ∼200 nm. The microstructure of the powders annealed at 700 °C strongly depends on RE³⁺ cations