Crystal Structure Tuning And Properties Of β-Ca₃（PO₄）₂ And A₂LnNbO₆ Based Luminescent Materials

The fourth generation lighting device represented by white light emitting diode（WLED）puts forward higher requirements for the phosphors.However,the luminous efficiency and thermal stability of the traditional phosphors cannot meet the actual requirements of WLED.In this paper,novelβ-Ca3（PO4）2 and A2Ln Nb O6 based phosphors were designed and synthesized to meet the urgent need of high color rendering index,high efficiency and good thermal stability.The local environment around activator was adjusted via cation substitution,activator pair doping and energy transfer,which was benefited for tuning the emission peak position,emission intensity and other optical properties of phosphors;The designed concept of full spectrum emission was put forward forβ-Ca3（PO4）2-based phosphor via the solid solution composition control.Moreover,the influence mechanism of crystal structure tuning on the luminescent property was discussed.Finally,several new kinds of WLEDs with high color rendering index（CRI）and low correlated color temperature（CCT）were packaged by the synthesized full spectrum emission phosphors.The specific research contents are as follows:（1）Novel Ca9Al1-x Yx（PO4）7:Eu²⁺solid solution phosphors were synthesized with tunable emission colors from blue to cyan.The redistribution process of Eu²⁺in multiple cation sites caused by the structural evolution was elucidated by Gaussian fitting,fluorescence lifetime and bond energy theory,which caused the change of CIE coordinate from（0.1828,0.1192）to（0.1723,0.3305）.Based on the crystal structure evolution of Ca9Al1-x Yx（PO4）7,the relationship between the bond length of cationic site and the blue or red shift of emission peak was established.Besides,the thermal stability behaviors of Ca9Al1-x Yx（PO4）7:Eu²⁺exhibited that the decline rate of low energy band was faster than that of high energy band due to the thermally active phonon-assisted effect.（2）Inspired by the multiple crystallographic sites inβ-Ca3（PO4）2 structures and facile crystal-site engineering method,two kinds of Ca9Y（PO4）7-Ca10Li（PO4）7:Eu²⁺and Ca₉Y（PO₄）₇-Ca₁₀Na（PO₄）₇:Eu²⁺,Mn²⁺phosphors were obtained.The structure characteristics and photoluminescence properties were investigated by the Rietveld refinement on the basis of X-ray diffraction data,density functional theory,and low/room/high temperature spectra.The nonlinear phase evolution introduced by the disappearances of vacancy,decreases of polyhedron number and changes of occupying atom in the different five cationic sites caused the redistribution of Eu²⁺among the cationic sites,which achieved full visible spectra emission.The WLED fabricated by the Ca9Y（PO4）7-Ca10Na（PO4）7:Eu²⁺exhibited good CRI（81.5）and low CCT（4087 K）under 365 nm excitation.Moreover,the R9 value of WLED was further enhanced from24.6 to 93.1 with the help of Eu²⁺-Mn²⁺energy transfer in Ca₉Y（PO₄）₇-Ca₁₀Na（PO₄）₇:Eu²⁺,Mn²⁺,accompanied by higher CRI（95.1）and lower CCT（3213 K）.（3）Novel Eu²⁺and Mn²⁺co-doped（1-x）β-Ca₃（PO₄）₂-x Ca₉La（PO₄）₇ phosphors were designed to realize white light emission.The effects of variation x on lattice structure,color-tunable emission,thermal stability,and energy transfer efficiency from Eu²⁺to Mn²⁺were systematically investigated.Tunable color emissions were achieved by manipulating the redistribution of Eu²⁺ions among different cationic sites under the influence of empty site effect in the M（4）site.Meanwhile,the changes of critical distances among the Eu²⁺and Mn²⁺caused by the variation x resulted in the changes of energy-transfer efficiency from different Eu²⁺luminescent centers to Mn²⁺due to the existence of structural confinement effect.The calculated results indicated that Eu1-Mn and Eu2-Mn possessed higher energy-transfer efficiencies than other Eu-Mn pairs.Under the combined influence of the two effects,full visible white emission covering from 400 to 750 nm had been realized via the adjustment of solid solution composition,which made the fabricated WLED possessed high CCT（86.9）and R9（87.2）,as well as low CCT（3947 K）.（4）β-Ca3（PO4）2:Eu²⁺-type phosphors with improvement of quantum efficiency and tunable emission color were synthesized through the incorporation of Na⁺or K⁺.The quantum efficiency and emission intensity were increased by 2.81 and 1.99 times due to the reduction of Eu³⁺to Eu²⁺in Ca_10-xNa_1+x（PO₄）₇:0.01Eu²⁺,which was ascribed to the achievement of charge balance and cavity neutralization in Na（4）site after the incorporation of Na⁺in the neighboring Ca（1）,Ca（2）and Ca（3）sites.Tunable emission colors from near white to green were realized owing to the change of occupation ratio of Eu²⁺at Ca（1）,Ca（2）and Ca（3）sites relative to Na（4）site.Moreover,the incorporation of K⁺in Ca10-y K1+y（PO4）7:0.01Eu²⁺led to the enhancement of quantum efficiency by 1.66 time.Furthermore,the incorporation of Na⁺in 0.1Ca9Y（PO4）7–0.9Ca_10-z Na₁₊z（PO₄）₇:0.01Eu²⁺,0.22Mn²⁺system also improved the quantum efficiency1.28 time,along with the decrease of blue light ratio significantly.Additionally,the warm emission WLED was fabricated via the modified single-phase full visible emission 0.1Ca₉Y（PO₄）₇–0.9Ca_9.9Na_1.1（PO₄）₇:0.01Eu²⁺,0.22Mn²⁺phosphor,which exhibited an excellent CRI of 92.9 at CCT of 3887 K.（5）A series of novel Bi³⁺and Mn⁴⁺single-and co-doped double perovskite Ca2La1-x Gdx Nb O6 phosphors were designed.By elaborately tailoring the structure via Gd³⁺substitution for La³⁺,the crystal compressive strain decreased,accompanied by the increased average bond length and decreased polyhedron distortion of（Ca/Ln）O6polyhedron.These changes caused the increase of emission intensity and thermal stability of Ca₂La_1-xGd_xNb O₆:Bi³⁺and Ca₂La_1-xGd_xNb O₆:Mn⁴⁺by 423%and 130%,respectively.The emission colors of Ca2La1-x Gdx Nb O6:Bi³⁺and Ca2La1-x Gdx Nb O6:Mn⁴⁺tuned from（0.2020,0.2472）to（0.1581,0.0992）and（0.7114,0.2885）to（0.7109,0.2890）.Meanwhile,the tunable emission color was also observed in Ca2La Nb O6:Bi³⁺,Mn⁴⁺and Ca₂Gd Nb O₆:Bi³⁺,Mn⁴⁺energy transfer process.The CIE coordinates changed from（0.2020,0.2472）to（0.2989,0.3001）and（0.1581,0.0992）to（0.3060,0.2553）.Furthermore,Gd³⁺was introduced into Ca₂La Nb O₆:Bi³⁺,Mn⁴⁺phosphors,combined with energy transfer and structure tailoring,the emission intensity of Bi³⁺could be maintained and the emission intensity of Mn⁴⁺was improved by 267%.（6）Novel double perovskite(Ca_1-x Srx)₂YNb O₆:Bi³⁺and Sr₂YNb O₆:0.004Bi³⁺,y Mn⁴⁺phosphors were designed and synthesized.The Sr²⁺substitution resulted in the decrease of polyhedron distortion,which caused the emission spectra of（Ca1-x Srx）2Yb NO6:Bi³⁺changed from single peak at 434 nm to double peak at 420 nm and 494 nm.Moreover,Sr²⁺substitution can also significantly improved the thermal stability of（Ca1-x Srx）2YNb O6:Bi³⁺.Furthermore,based on the double emission peaks in Sr₂YNb O₆:Bi³⁺,novel Sr₂YNb O₆:0.004Bi³⁺,y Mn⁴⁺phosphors were synthesized.Under 330 nm excitation,the emission peak could cover the whole visible region from400 nm to 750 nm,realizing full spectra emission.The calculation results showed that the maximum energy transfer efficiency between Bi³⁺and Mn⁴⁺could exceed 80%,and the energy transfer mechanism was dipole-dipole interaction.