Crystallographic and Electronic Structures of Nb:YTaO4 and Tb:YTaO4 Single-Crystal Scintillators

Single crystals of YTaO4, partially substituted with Nb on the Ta site or Tb on the Y site, are potential alternatives to toxic CdWO4 single-crystal scintillators. Compared with ultraviolet (UV)-emitting YTO, these isovalently substituted Nb:YTO and Tb:YTO compounds emit in the visible region, matching Si-photodiode detectors. Upon Nb or Tb incorporation, a bandgap narrowing takes place. The present study elucidates its origin in detail based on single-crystal diffraction measurements and subsequent density functional theory calculations and high-resolution X-ray photoelectron spectroscopy. Nb 4d band overlaps with the conduction band bottom of YTO, causing a red-shifted emission. On the contrary, the Tb ground levels overlap with the top of the valence band, favoring intraatomic Tb green emission.