Combined 5d–4f and 4f–4f luminescence of Pr3+ ion under tribo- and X-ray excitation in anhydrous praseodymium(III) sulfate crystals

Abstract

Combined 5d–4f and 4f–4f luminescence of Pr³⁺ ions was discovered in anhydrous praseodymium(III) sulfate crystals under both mechanical and X-ray excitation. The X-ray excited luminescence and triboluminescence spectra exhibit intense broad bands in the UV region (215–350 nm), corresponding to 4f¹5 d¹ → 4f² transitions, along with weaker bands in the visible range (485–725 nm) attributed to intra-4f-shell transitions. The emission spectra are identical to the photoluminescence spectrum of the anhydrous crystals, indicating that the same transitions occur regardless of the excitation mechanism. The measured PL quantum yields were 73 % for 5d–4f transitions and 0.36 % for 4f–4f transitions, with corresponding excited-state lifetimes of 18 ns and 335 ns, respectively. Excitation efficiencies for 5d–4f transitions (⁴f₁5d₁ → ⁴f₂) and 4f–4f transitions (from multiplets ³P_{0, 1} and ¹D₂ to multiplets ³H_4,5,6 and ³F_2,3,4) were compared. It was found that in X-ray excited luminescence and triboluminescence for Pr³⁺ ion the excitation of 4f–4f transitions is noticeably more efficient than the excitation of 5d–4f transitions.