Comparative study of Eu³⁺ doped calcium borate glasses and crystalline calcium hexaborate phosphors

Calcium borate glasses and crystalline calcium hexaborate phosphors were prepared by doping with varying concentration of Eu₂O₃. Crystalline samples of composition: (100−x)CaB₆O₁₀-xEu₂O₃ (x = 0 to 5 mol%) and their analogous glass samples of composition: (25−0.25x)CaO-(75−0.75x)B₂O₃-xEu₂O₃ (x = 0 to 5 mol %) were prepared by solid-state reaction and melt-quenching techniques respectively. The samples were characterized by X-ray diffraction (XRD), differential scanning calorimetry, Raman, Fourier transform infrared, photoluminescence (PL) and the ¹¹B Magic Angle Spinning (MAS) and Nuclear Magnetic Resonance (NMR) spectroscopy. A comparison was made between the light emission properties of crystalline and glassy borate phosphors. Rietveld analysis of XRD patterns confirm the monoclinic structure of calcium hexaborates. The density of glasses increases from 2.42 to 3.04 g cm⁻³ and the glass transition temperature increases from 727 to 751 °C with an increase in concentration of Eu₂O₃ from 0 to 5-mol%. Raman and FTIR studies show vibration modes in tri, hexa and pentaborate anions containing [BO₄] and [BO₃] structural units in crystalline and glass samples. The concentration of tetrahedral borons in the undoped 25CaO–75B₂O₃ glass and crystalline CaB₆O₁₀ was found to be 37 and 35% respectively. Eu₂O₃ doped calcium borate glasses emit intense red color while the crystalline calcium hexaborates emit whitish-blue color at an excitation wavelength of 392 nm. The emission of bluish light in Eu doped crystalline calcium hexaborates reveals that partial reduction of Eu³⁺ into Eu²⁺ takes place in the crystalline sample. In both glass and crystalline samples, the PL intensity increases with an increase in concentration of Eu₂O₃ from 1 to 4 mol%, while at 5 mol% Eu₂O₃ concentration, quenching of luminescence is observed. The luminescence intensity of glasses is found to be significantly higher than that of crystalline samples.