KTb(MoO₄)₂ Green Phosphor with K⁺-Ion Conductivity: Derived from Different Synthesis Routes

The influence of different synthesis routes on the structure and luminescent properties of KTb(MoO₄)₂ (KTMO) was studied. KTMO samples were prepared by solid-state, hydrothermal, and Czochralski techniques. These methods lead to the following different crystal structures: a triclinic scheelite-type α-phase is the result for the solid-state method, and an orthorhombic KY(MoO₄)₂-type γ-phase is the result for the hydrothermal and Czochralski techniques. The triclinic α-KTMO phase transforms into the orthorhombic γ-phase when heated at 1273 K above the melting point, while KTMO prepared by the hydrothermal method does not show phase transitions. The influence of treatment conditions on the average crystallite size of orthorhombic KTMO was revealed by X-ray diffraction line broadening measurements. The electrical conductivity was measured on KTMO single crystals. The orthorhombic structure of KTMO that was prepared by the hydrothermal method was refined using synchrotron powder X-ray diffraction data. K⁺ cations are located in extensive two-dimensional channels along the c-axis and the a-axis. The possibility of K⁺ migration inside these channels was confirmed by electrical conductivity measurements, where strong anisotropy was observed in different crystallographic directions. The evolution of luminescent properties as a result of synthesis routes and heating and cooling conditions was studied and compared with data for the average crystallite size calculation and the grain size determination. All samples' emission spectra exhibit a strong green emission at 545 nm due to the ⁵D₄ → ⁷F₅ Tb³⁺ transition. The maximum of the integral intensity emission for the ⁵D₄ → ⁷F₅ emission under λ_ex = 380 nm excitation was found for the KTMO crashed single crystal.