The effect of Li⁺ co-doping on the photoluminescence properties of YPO₄:Eu is discussed. Interesting behaviours, such as the presence of intermediate bands, shifting of the Eu–O charge transfer band (Eu–O CTB) to a lower wavelength, variation in intensities of magnetic (⁵D₀ → ⁷F₁) and electric dipole (⁵D₀ → ⁷F₂) transitions of Eu³⁺ and shift of ⁵D₀ → ⁷F₀ to higher energy with increasing excitation wavelengths are observed. The Eu³⁺ ion does not have an absorption band in the range 340–350 nm, but after excitation at these wavelengths, a broad emission band (370–570 nm), as well as sharp peaks of Eu³⁺, could be observed. This is due to strong energy transfer from the intermediate band of the host to the Eu³⁺ ion. X-ray photoelectron spectroscopy (XPS) study also confirms that intermediate band emission is not due to Eu²⁺ ion emission. The blue shifting of Eu–O CTB is because of the increase in the optical electronegativity of the Eu³⁺ ion on Li⁺ co-doping. The variation in intensities of the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ dipole transitions is related to (i) overlapping interaction parameters within the ground and excited states, (ii) exchange interaction among atoms/ions and (iii) density of the incoming photons. Shift of ⁵D₀ → ⁷F₀ to a higher energy with increasing excitation wavelengths is because of change in the second order crystal field parameter B₂₀ with excitation wavelength. The significant enhancement of luminescence intensity is found with Li⁺ co-doping due to the increase in crystallinity.

Tb³⁺-doped CaMoO₄ (Tb³⁺ = 1, 3, 5, 7, 10, 15 and 20 atom%) core and core–shell nanoparticles have been prepared by urea hydrolysis in ethylene glycol (EG) as capping agent as well as reaction medium at low temperature ∼150 °C. As-prepared samples were annealed at 500 and 900 °C for 4 h to eliminate unwanted hydrocarbons and/or H₂O present in the sample and to improve crystallinity. The synthesised nanophosphors show tetragonal phase structure. The crystallite size of as-prepared sample is found to be ∼18 nm. The luminescence intensity of the ⁵D₄ → ⁷F₅ transition at 547 nm of Tb³⁺ is much higher than that of the ⁵D₄ → ⁷F₆ transition at 492 nm. 900 °C annealed samples show the highest luminescence intensity. The intensity ratio R (I[⁵D₄ → ⁷F₆]/I[⁵D₄ → ⁷F₅]) lies between 0.3–0.6 for as-prepared, 500 and 900 °C annealed samples. The luminescence decay of ⁵D₄ level under 355 nm excitation shows biexponential behaviour indicating availability of Tb³⁺ ions on surface and core regions of particle; whereas, contribution of Mo–O charge transfer to lifetime is obtained under 250 nm excitation. The CIE coordinates of as-prepared, 500 and 900 °C annealed 5 atom% Tb³⁺-doped CaMoO₄ samples under 250 nm excitation are (0.28, 0.32), (0.22, 0.28) and (0.25, 0.52), respectively. The dispersed particles in polar medium and its polymer film show green light emission. The luminescence intensity is improved significantly after core–shell formation due to extent of decrease of non-radiative rates arising from surface dangling bonds and capping agent. Quantum yields of as-prepared samples of 1, 5 and 7 atom% Tb³⁺-doped CaMoO₄ samples are found to be 10, 3 and 2, respectively.

Graphical representation of Li⁺ co-doped Y₂Ti₂O₇:Er³⁺/Yb³⁺ phosphor showing up-conversion of green and red bands and probing the fluorescence intensity ratio (FIR) for temperature sensing applications.

When Eu³⁺ ions occupy Ca²⁺ sites of CaMoO₄, which has a body centered tetragonal structure with inversion symmetry, only the magnetic dipole transition (⁵D₀→⁷F₁) should be allowed according to Judd–Ofelt theory. Even if there are a few distortions in the Eu³⁺ environment, its intensity should be more than that of the electric dipole transition (⁵D₀→⁷F₂). We report here the opposite effect experimentally and ascribe this to the polarizability effect of the MoO₄ tetrahedron, which is neighboring to EuO₈ (symmetric environment). The contribution of the energy transfer process from the Mo–O charge transfer band to Eu³⁺ and the role of Eu³⁺ over the surface of the particle could be distinguished when luminescence decay processes were measured at two different excitations (250 and 398 nm). Further, the luminescence intensities and lifetimes increase significantly with increasing heat-treatment temperature of the doped samples. This is attributed to the reduction of H₂O from the surface of the particles and a non-radiative process after heat treatment.

The nanoparticles of CaMoO₄ : Eu³⁺ (Eu³⁺ = 0, 1, 3, 5, 7, 10 at. %) are prepared at low temperature (150 °C for 3 h) using urea hydrolysis in ethylene glycol. These are characterized by X-ray diffraction (XRD), infrared spectroscopy (IR) and transmission electron microscopy (TEM). From XRD study, it was found that the solubility limit of Eu³⁺ ions at the Ca²⁺ sites is up to 3 at. % and above this, phase segregation occurs. In combination with Rietveld analysis, its crystal structure was found to be tetragonal phase (space groupI4₁/a (88) and Z = 4 (number of CaMoO₄ formula units per unit cell). Unit cell parameters and bond distances are calculated. The average crystallite sizes of as-prepared, 500 and 900 °C heated samples are found to be 20, 35 and 70 nm, respectively. The lattice strain is found to be 0.003–0.005. From IR study, the bands at 820 and 441 cm⁻¹ are assigned to asymmetric stretching and bending vibrations of the MoO₄ ^{2 −} tetrahedron, respectively. From TEM study, the shape of particle was found to be spherical. The high resolution TEM suggests a change in orientation of the crystal on annealing up to 900 °C.

We have studied the luminescence property of CaMoO₄:Eu³⁺. The emission peaks at 590 (⁵D₀→⁷F₁) and 613 nm (⁵D₀→⁷F₂) for Eu³⁺ are observed after excitation at 266 nm (i.e. Mo–O charge transfer band). The peak intensity of the latter dominates over the former indicating an asymmetric environment of Eu³⁺ in EuO₈ polyhedron or parity mixing. Luminescence intensity increases significantly with co-doping of Gd³⁺. This is ascribed to energy transfer from Mo–O/Gd³⁺ to Eu³⁺. Luminescence intensity increases with annealing up to 900 °C due to the extent of decrease of non-radiative rates. Very high asymmetric values (A₂₁) of 12–16 are found indicating a red emitter. As-prepared samples are dispersible in polar solvents like water, ethanol, methanol, dimethyl sulfoxide (DMSO) and ethylene glycol (EG); and among them, optimum luminescence is found in methanol. Polymer film shows red emission. The quantum yields of as-prepared 2 and 10 at% Gd³⁺ co-doped CaMoO₄:Eu³⁺ under 277 nm (UV excitation) are 21 and 80%, respectively.