Re-examination of non-centrosymmetric NaBaPO4 crystal structure revealed by single-crystalline Raman spectroscopy and DFT calculations

NaBaPO 4 is a M I M II PO 4 -type orthophosphate that has attracted renewed interest as a host material for multicolor phosphors exhibiting photochemically tunable-color emissions. To facilitate the development of such unique multicolor phosphors, the photochemical reactivity of NaBaPO 4 should be understood in relation to its crystal structure through electronic-structure calculations and optical property analyses. Although centrosymmetric crystal structures with the space groups of (Z = 2) and (Z = 4) have been reported for NaBaPO 4 , our experimental results on single-crystalline NaBaPO 4 yielded results inconsistent with these structures. In this study, the crystal structure of NaBaPO 4 was re-examined using X-ray diffraction pole-figure analysis, Raman spectroscopy and density functional theory (DFT) calculations. The Raman spectra exhibit extra PO 4 -related bands that are incompatible with the reported centrosymmetric structures, suggesting a less symmetric structure than the reported one. DFT calculations indicate that Na + , located at a centrosymmetric 1 a site in the structure, is energetically stabilized by a slight displacement in the c -axis direction, leading to a loss of the inversion symmetry. Based on these findings, the crystal structure was refined using a structural model with the space group of . The refined structure confirms two crystallographically different PO 4 units, which reasonably explains the observed Raman spectra.