Effect of cation vacancies on the crystal structure and luminescent properties of Ca 0.85−1.5x Gd x Eu 0.1 □ 0.05+0.5x WO 4 (0 ≤ x  ≤ 0.567) scheelite-based red phosphors

Abstract

The Ca0.85-1.5xGdxEu0.10.05+0.5xWO4 (0 ≤ x ≤ 0.567) series of cation-deficient scheelites is investigated to unveil the influence of the cation vacancies on the crystal structure and luminescent properties. The concentration of the vacancies is varied by the heterovalent substitution of Gd 3+ for Ca 2+ , keeping the concentration of the Eu 3+ luminescent centers constant in all compounds of the series. The crystal structure of the materials is studied using a combination of transmission electron microscopy and synchrotron X-ray powder diffraction. However, the difference in the local coordination environment of the A cation species noticeably affects the line width and the multiplet splitting of the 4f 6 -4f 6 transitions.

New cation deficient scheelites (Sr,Ce)nWO4 (n<1): Structural and optical properties

“…For 0.33 concentration, the modulations changed to a monoclinic 1 D symmetry. Our results agree with those of [21] for high structural vacancy concentration of 0.15 and 0.2, but contrary to [21] the modulations (3+2)D appears in strontium cerium tungstate at very low vacancy concentrations (0.05).…”

“…3c), ED patterns show incommensurate spots with increasing intensities. A variation of the incommensurate modulations with (3+2)D tetragonal symmetry was already observed in Ca(Gd,Eu)WO4 compounds [21], with no incommensurate modulations for cations vacancy concentrations less than 0.15 and for higher vacancies concentrations (0.2) incommensurate (3+2)D modulations were observed. For 0.33 concentration, the modulations changed to a monoclinic 1 D symmetry.…”

“…The scheelite structure can accommodate various cations substitutions on the A site, allowing a tuning of the properties. The substitution by rare earth cations RE on the A sites modifies several properties of the scheelite compounds, such as promoting ionic conduction, so reducing the width of the forbidden band andchanging the spectral range of the visible light absorption and of the photoluminescence emissions [10,[20][21].…”

“…The appearance of incommensurate modulations is directly linked to the amount of structural vacancies. Batuk et al [21] showed that for Ca0.85-1.5xGdxEu 0.1 0.05-0.5xWO4 (0<x<0.567), at low vacancies concentrations (x≤0.15) cations and cations vacancies are randomly distributed, as for higher vacancy concentrations (0.2≤x<0.33), the scheelite structure evolves from a incommensurate structure (3+2)D tetragonal symmetry to a monoclinic distortion with a (3+1)D superstructure and a monoclinic phase A2(BO4)3.…”

Controllable hydrothermal synthesis and photoluminescence properties of CaGd2(WO4)4:Eu3+ red‐emitting phosphor

“…In the last few years, scheelite‐like type compounds have attracted much research interests in many areas, such as fluorescent lamps, plant growth lighting, and white‐light‐emitting diodes (W‐LEDs). Typically, (A′, A″) m (WO 4 ) n type scheelite structure tungstates (A′, A″ = alkali metal ions, alkaline‐earth ions, or rare‐earth ions) have been widely studied as ideal hosts for Eu 3+ ‐activated red phosphors in W‐LEDs 1,2 . In this compound, the Eu 3+ ions occupy the positions of A′/A″ ions without inverting symmetry, resulting in a strong red emission ascribed to 5 D 0 → 7 F 2 electric‐dipole transition.…”