Synthesis and properties of niobium barium phosphate glasses

Abstract Niobium barium phosphate glasses were prepared by melting different compositions of P 2 O 5 –Nb 2 O 5 –BaO–K 2 O in an induction furnace. Three routes of preparation were adopted: (a) the material was melted at 1300°C, cast in a stainless steel mould and cooled to room temperature with a rod placed in contact with the upper surface of the liquid; (b) poured into the mould without the rod, (c) just removed from the furnace and cooled. Samples were prepared by varying the concentration of Nb 2 O 5 from 0 to 40 mol%, BaO from 10 to 40 mol%, K 2 O from 5 to 30 mol% and maintaining 30 mol% of P 2 O 5 . X-ray diffraction and thermal analyses were performed to determine if these materials were amorphous, the temperature of the crystallization peak (635–780°C) and the crystalline phase melting temperature (825–950°C). The glass transition temperature for the sample P 2 O 5 30 mol%, Nb 2 O 5 30 mol%, BaO 30 mol% and K 2 O 10 mol% is 660 ± 5°C and the thermal expansion coefficient is (11.68 ± 0.5) × 10 −6 C −1 . The addition of BaO and consequently reduction of K 2 O increase the temperature of the crystallization peak from 662°C to 838°C. Most of the studied compositions are amorphous only if the liquid is quenched. The sample of composition previously mentioned did vitrify when cooled in a crucible withdrawn from the furnace. Optical transparent glasses were produced and absorption bands in the range of 400–2500 nm were not detected. Absorption increased monotonically at wavelengths less than 380 nm.