Repairing Structure In Whitloctite-type Phosphor To Tune The Luminescence Properties Of Eu²⁺

Due to the wide composition variability and structure tolerance,Eu²⁺-doped whitlockiteβ-Ca₃（PO₄）₂（TCP）type phosphors with different visible emissions have been realized by manipulating the chemical compositions.However,the site preference of dopant Eu²⁺on multi-cation sites appears a dispute recently.Based on the different luminescence properties of Eu²⁺in different sites,this paper takes Ca_10.5-0.5xM_x（PO₄）₇:Eu²⁺（M=Li,Na,K）（0≤x≤1）,Ca₇Sr_3.5-0.5xM_x（PO₄）₇:Eu²⁺（M=Na,K）（0≤x≤1）and Ca₉AlNa_x（PO₄）_7-x（SiO₄）_x:Eu²⁺（0≤x≤1）as the research objects,investigating the regulation luminescent properties of Eu²⁺by structural repair in phosphate phosphors.1.A series ofβ-Ca_10.5-0.5xM_x（PO₄）₇:0.6%Eu²⁺（M=Li,Na,K,TCPLi/Na/K_x-Eu;0≤x≤1）phosphors were synthesized through traditional high-temperature solid-state reaction.In this work,a combination of experiments and theoretical calculations is applied to identify the preferential occupancy and variable optical behavior of Eu²⁺through the evolution of Ca_10.5-0.5xK_x（PO₄）₇（0≤x≤1）isostructures.Spectral analysis and theoretical calculation reveal the Eu²⁺prefer to occupy the loose Ca（4）sites and produce the narrow band emission around420 nm.Moreover,the continuous increase of K content compels the redistribution of dopant from Ca（4）to Ca（3）site,resulting in an abrupt spectral shift.Moreover,a vacancy defect repair effect is proposed for the enhancement in luminescence of Ca_10.5-0.5xK_x（PO₄）₇:Eu²⁺,in which the incorporation of K⁺repairs the half-unoccupied Ca（4）sites and benefits the reduction of Eu³⁺to Eu²⁺.To further prove the significant vacancy repair and dopant redistribution effects,Na⁺and Li⁺doped systems are investigated as well.In addition,together with reported Sr²⁺or Gd³⁺doped systems,the preferential occupancy and saltatory spectral variations of Eu²⁺in whitlockite-type phosphors are uniformly clarified by cationic modification events.2.A series ofβ-Ca_10.5-0.5xM_x（PO₄）₇:0.6%Eu²⁺（M=Li,Na,K,TCPLi/Na/K_x-Eu;0≤x≤1）phosphors were synthesized through traditional high-temperature solid-state reaction.Cationic substitution is a prevalent strategy to tune the luminescence spectra of phosphors.In this work,the substitution by alkali metal cations for both half occupied/vacant M（4）site synchronously was supposed to repair the crystal structural vacancy defects.The structural repair effect by introducing Na⁺was verified via Raman spectra,Reitveld refinement and HR-TEM,whereas a similar accommodation of K⁺into the Ca₂Sr（PO₄）₂（CSP）host cannot be realized due to the significant size mismatch.A continuous increase of Na⁺contents led to the progressively structural contraction in CSPN series.As a multiple cations lattice host,CSPN promoted the migration of Eu²⁺activator from M（4）to M（2,3）and M（1）sites,and further regulated the luminescence behaviors.Consequently,the gradual red-shift in spectra terminated at a novel yellow emitting phosphor Ca₇Sr₃Na（PO₄）₇:0.04Eu²⁺.The CSPN phosphor possesses a broad emission band covering 400-750nm with a maximum at about 530 nm under 365 nm excitation.The structural repair in this work can not only migrate Eu²⁺activator among different cation sites towards a new yellow phosphor with great potential application for white LEDs,but also serves as a new strategy for tuning the luminescence properties of phosphor.3.A series of Ca₉AlNa_x（PO₄）_7-x（SiO₄）_x:Eu²⁺（CANPS;0≤x≤1）phosphors were synthesized through traditional high-temperature solid-state reaction.A new host is obtained by substitution of[Si⁴⁺+Na⁺]→P⁵⁺.With the increasing of x,the emission spectra move from 430 nm to 480nm.By fitting and calculating the energy of Eu²⁺in different lattice sites,it is concluded that Eu²⁺occupies the M（4）site preferentially,and then gradually migrate to other sites.When x increases to 1.0,Sr²⁺and Mg²⁺are substituted for Ca²⁺,the spectra are shifted and the purpose of spectral regulation is achieved.All three systems adopt the method of vacancy defect repair.Alkali metal ions are introduced into the host to repair vacancy defects in M（4）site,and then tune the luminescence properties of Eu²⁺.This method provides a new idea for the spectral regulation of Eu²⁺doped Whitloctite-type phosphors.