Energy transfer in Cs2NaHo0.99Er0.01Cl6 and assignment of the 5I6 and 5I4 levels of HoCl 3–6

Abstract

Under excitation into the (²H_11/2)Γ₆+S₆ vibronic level of ErCl^3–₆ at or above liquid-nitrogen temperature, the ⁵I₄→⁵I₈ luminescence transition of HoCl^3–₆ is observed in Cs₂NaHo_0.99Er_0.01Cl₆. This transition is not observed at these temperatures under excitation into vibronic levels of the ⁵F₃, ⁵F₅ terms of HoCl^3–₆ in neat Cs₂NaHoCl₆. The three-ion process responsible for the population of ⁵I₄ involves transfer from ²H_11/2 or (⁴S_3/2) ErCl^3–₆ to (⁵S₂) HoCl^3–₆ followed by a cross-relaxation between Ho³⁺ neighbours. The ⁵I₆→⁵I₈ infrared luminescence transition of HoCl^3–₆ is not observed in neat Cs₂NaHoCl₆ under excitation into vibronic levels of ⁵F₃, but is observed in Cs₂NaHo_0.99Er_0.01Cl₆ since luminescence from ⁵F₅ is quenched by energy transfer to ErCl^3–₆. Together with detailed analyses of the ⁵F₃, ⁵S₂, ⁵F₅→⁵I₆ luminescence transitions of HoCl^3–₆ in several cubic elpasolite hosts, these results permit the following energy level assignments (in cm^–1) for HoCl^3–₆ in octahedral symmetry: ⁵I₆ levels: Γ₃(8620), aΓ₅(8628), Γ₂(8667), bΓ₅(8705), Γ₄(8726) and Γ₁(8744). ⁵I₄ levels: Γ₁(13 232), Γ₄(13 241), Γ₃(13 247) and Γ₅(13 449). The liquid-helium-temperature ⁵F₃â†�⁵I₈ absorption spectrum of Cs₂NaHoCl₆ has been reanalysed.