Polymorphs of NASICON-Type Na3Sc2(PO4)3/Eu2+ Phosphors Analyzed by Single Crystal Structure Determination and Molecular Dynamics Simulations

This study aimed to determine the site assignment of Eu2+ in the Na3Sc2(PO4)3/Eu2+ (NSP/Eu2+) host lattice for phosphors with NASICON-type frameworks. For this purpose, molecular dynamics (MD) simulations, in which an adiabatic shell method based on crystal structure refinement data for polymorphs of NSP, was employed and verified to be effective. Precise crystal structure analysis of good-quality single crystals indicated the presence of three types of phases: a gamma phase assigned to the R3c space group [gamma(trig)-NSP] reported previously, a monoclinic phase assigned to the I2/a space group [alpha(mono)-NSP], and another monoclinic phase assigned to the C2/c space group [gamma(mono)-NSP]. In the MD simulations of alpha(mono)-NSP with two crystallographically independent Na sites, Na+ ion hopping between the sites frequently occurred. However, the MD simulations of the cells with one type of Na+ ion partially replaced by an Eu2+ ion and vacancy showed that the Eu2+ ions were preferentially located at a distorted octahedral site, and Na+ ion hopping did not occur. The alpha(mono)-NSP-phase Eu2+-doped phosphors obtained via a conventional solid-state reaction method exhibited intense blue luminescence, which was assigned to the Eu2+ d-f transition, under irradiation at 370 nm, whereas the intensity of the light emitted by the (trig)-phase phosphors was lower. The luminescence and thermal quenching of the alpha(mono)-NSP phase phosphors was improved when K+ ions were substituted at Na+ ion sites. The quantum yields were significantly improved compared to those of NSP/Eu2+, being almost comparable with those of a commercial BaMgAl10O17/Eu2+ (BAM) phosphor. The luminescence properties of NSP/Eu2+ are discussed based on the crystal structure refinement and MD simulation results.