Eu³⁺Activated Molybdate and Tungstate Based Red Phosphors with Charge Transfer Band in Blue Region

Abstract

Alkaline earth metal and rare earth tungstate and molybdate compounds are promising candidates as host materials for high efficiency narrow spectral emission red phosphors when activated by trivalent europium (Eu3+). These phosphors emit in the range of 610–620 nm with a full width at half maximum (FWHM) of less than 10 nm, which makes them suitable for use in high color rendering index phosphor-converted white light emitting diodes (pc-WLEDs). However these phosphors suffer from poor excitation using the currently available GaInN based high efficiency blue LEDs in the 400–470 nm range. This paper discusses the role of various crystallographic structures on the excitation characteristics of the phosphor. Using appropriate crystal structures, it has been shown that Mo6+-O2− charge transfer band edge shifts from far-UV (280–350 nm) to near-UV and Blue (380–430 nm) region as the coordination number of the transition metal ion is increased from 4 to 6 making these phosphors suitable for excitation with near-UV or Blue LEDs. Furthermore it has been demonstrated that phosphors crystallized from flux exhibit significantly higher emission intensity compared to powders of the same composition synthesized using solid state reactions.

Inorganic Phosphor Materials for Lighting

“…This can increase the excitation efficiency of the Eu 3? ion, but can be useful only when the electric dipole allowed CT absorption bands are located in spectral ranges where NUV or blue chips emit [152]. As mentioned above, molybdate, tungstate, niobate, and tantalate hosts activated with Eu 3?…”

“…As mentioned above, molybdate, tungstate, niobate, and tantalate hosts activated with Eu 3? have been widely investigated [151][152][153][154][155][156][157][158][159][160][161][162][163][164][165][166][167], also exploting co-doping with Sm 3? , but only in a limited number of cases the two strategies described above (high Eu 3?…”

Optical properties of Eu 3+ activated thorium molybdate and thorium tungstate: Structure, local symmetry and photophysical properties

“…In this process electron is transfer from the lower energy valence band (formed by 2p orbitals of oxygen atom) to higher energy conduction band (formed by 4d and 5d orbitals of Mo and W respectively). Since Pauling electronegativity of Mo ($2.16) is higher than W ($2.36) [25]; it would be easier for Mo 6+ to get oxidized thereby lowering the Mo 6+ À ÀO 2À bond energy compared to W 6+ À ÀO 2À . Thus an excited state in tungstates is at higher energy position relative to molybdates which can be seen from the excitation spectra 3a and 3b.…”

Layer structured Na2Ni(MoO4)2 particles as a visible-light-driven photocatalyst for degradation of methylene blue

“…It has been reported that the optical absorption wavelength positions of charge transfers (CT) in MoO x groups depend on coordination number (CN) x [35]. Dutta et al [36] have reported the summary on CT extension methodologies from molybdates with kinds of crystal space group such as tetragonal, hexagonal, monoclinic, orthorhombic, cubic, and triclinic structures. It is important that CNs of Mo in molybdates can shift CT band to longer wavelengths.…”

“…A cubic crystalline phase with MoO 6 groups could provide the longer CT wavelength. So far the longest CT band edge (480 nm) has been reported in cubic molydate host, e.g., (Gd,Y) 6 MoO 6 , which provides CT-absorbing groups of MoO 6 [36]. Molydates with other structure type show shorter CT band edges, for example, 320 nm (from MoO 4 ) in orthorhombic Gd 2 (MoO 4 ) 3 , 330 nm (from MoO 4 ) in tetragonal CaMoO 4 , 400 nm (from MoO 4 ) in monoclinic Gd 2 MoO 6 , 430 nm (from MoO 6 ) in hexagonal Y 6 MoO 6 .…”