Chemical bond parameters and photoluminescence of a natural-white-light Ca9La(VO4)7:Tm3+,Eu3+ with one O2−→V5+ charge transfer and dual f-f transition emission centers

Abstract

The relationship between the photoluminescence properties and the crystal structure of undoped, Eu3+ or/ and Tm3+ singly or codoped Ca9La(VO4)7 (CLaVO) samples was discussed. Under the excitation of UV light, CLaVO:Tm3+, CLaVO, and CLaVO:Eu3+ exhibit the characteristic emissions of Tm3+ (1G4→3H6, blue), O2−→V5+ charge transfer (CT), and Eu3+ (5D0→7F2, red), respectively. By adjusting the doping concentration of Tm3+ and Eu3+ ions in CLaVO, a natural white emission in a single composition with the color temperature at 6181K was obtained. Based on the dielectric theory of complex crystal, the chemical bond parameters of La-O and V-O bonds were quantitatively calculated. The standard deviation of environmental factor of every bond (EFSD), which can be expressed as σ(hei)=(1/N)∑i=1N(hei−μ)2 (hei=(fciαbi)1/2QBi and μ=(1/N)∑i=1Nhei), was proposed to quantitatively express the distortion degree of VO43− from that of an ideal tetrahedron. The maximum change of EFSD comes from the [VO4]− tetrahedra in CLaVO sample by comparison with that of EFSD of isostructural Ca9Gd(VO4)7. This is possible the key reason that the undoped CLaVO sample has self-activated emission while the self-activated emission of its isostructural Ca9Gd(VO4)7 sample cannot be found. The quantitative calculation also demonstrated that the broad excitation bands at 319nm in CLaVO:Tm and at 335nm in CLaVO:Eu were due to the O-V2 and O-V3 (overlap with O-V2) CT, not the CT energy of O2−-Eu13+ (O2−-Tm13+), O2−-Eu23+ (O2−-Tm23+), and O2−-Eu33+ (O2−-Tm33+). The environmental factors surrounding the atoms V1, V2 and V3 were calculated to be 1.577, 1.6379 and 1.7554, respectively. It can be demonstrated that the excitation spectra at 319nm for CLaVO:Tm and 335nm for CLaVO:Eu came from the O-V2 and O-V3 CT, respectively.