Spectroscopy evaluation of crystalline and amorphous Cd₂V₂O₇ as blue phosphors

Cd₂V₂O₇ compounds in crystalline and amorphous phases were fabricated by the melt-quenching process. Characterizations such as X-ray diffraction, Raman spectroscopy and photoluminescence were performed. X-ray diffraction patterns of the crystalline sample showed peaks associated with a pure Cd₂V₂O₇ monoclinic structure, in agreement with the Raman vibrational spectrum. In the case of the amorphous sample, X-ray diffraction patterns only exhibited a broad band, typical of a glassy structure, whereas its Raman spectrum displayed two broad vibrational modes centered at 350 and 850 cm⁻¹, attributed to stretching vibrations of VO₃ groups. In spite of the structural differences, both samples presented similar photoluminescence features, consisting of a wide band in the 375–525 nm range with two peaks at 411 and 432 nm, associated respectively with the ³T₂ → ¹A₁ and ³T₁ → ¹A₁ electronic transitions in the VO₄ tetrahedron, under 340 nm excitation. Thus, blue light emission with CIE1931 chromaticity coordinates x~ 0.200 and y~ 0.145, and color purity of 62–63%, is achieved from the crystalline and amorphous Cd₂V₂O₇ compounds. The emission decay time profiles were well fitted to a bi-exponential function from which the calculated average lifetime values resulted to be 112 ± 13 and 99 ± 4 ns for the crystalline and amorphous Cd₂V₂O₇ samples, respectively. Theoretical calculations based on the density of electronic states revealed that the photoluminescence arises through charge transference processes from 3d orbitals of four-fold coordinated vanadiums to 2p orbitals of three-fold coordinated oxygens in the VO₄ tetrahedron, being the basic unit of Cd₂V₂O₇ in crystalline and amorphous phases.