Investigation on the Luminescence Properties of InMO₄ (M = V⁵⁺, Nb⁵⁺, Ta⁵⁺) Crystals Doped with Tb³⁺ or Yb³⁺ Rare Earth Ions

We explore the potential of Tb- and Yb-doped InVO₄, InTaO₄, and InNbO₄ for applications as phosphors for light-emitting sources. Doping below 0.2% barely change the crystal structure and Raman spectrum but provide optical excitation and emission properties in the visible and near-infrared (NIR) spectral regions. From optical measurements, the energy of the first/second direct band gaps was determined to be 3.7/4.1 eV in InVO₄, 4.7/5.3 in InNbO₄, and 5.6/6.1 eV in InTaO₄. In the last two cases, these band gaps are larger than the fundamental band gap (being indirect gap materials), while for InVO₄, a direct band gap semiconductor, the fundamental band gap is at 3.7 eV. As a consequence, this material shows a strong self-activated photoluminescence centered at 2.2 eV. The other two materials have a weak self-activated signal at 2.2 and 2.9 eV. We provide an explanation for the origin of these signals taking into account the analysis of the polyhedral coordination around the pentavalent cations (V, Nb, and Ta). Finally, the characteristic green (⁵D₄ → ⁷F_J) and NIR (²F_5/2 → ²F_7/2) emissions of Tb³⁺ and Yb³⁺ have been analyzed and explained.