Upconversion in Er3+-dimer systems: Trends within the series Cs3Er2X9 (X=Cl,Br,I)

Abstract

The dimer compounds ${\mathrm{Cs}}_3$ ${\mathrm{Er}}_2$ ${\mathit{X}}_9$ (X=Cl,Br,I) were synthesized and grown as crystals. Their f-f absorption spectra were measured and their upconversion luminescence behavior was studied as a function of temperature. Both ${}_{}{}^4{}_{}{}^{}$ ${\mathit{I}}_{15/2}$ ${\to }^4$ ${\mathit{I}}_{11/2}$ and ${}_{}{}^4{}_{}{}^{}$ ${\mathit{I}}_{15/2}$ ${\to }^4$ ${\mathit{I}}_{9/2}$ excitations were used. The highest-energy phonons were found by Raman spectroscopy at 285, 190, and 160 ${\mathrm{cm}}^{\mathrm{-}1}$ in ${\mathrm{Cs}}_3$ ${\mathrm{Er}}_2$ ${\mathit{X}}_9$ (X=Cl,Br,I), respectively. The upconversion and cross-relaxation behavior is essentially determined by two factors: (i) As a result of the lower phonon energies multiphonon-relaxation processes are orders of magnitude less competitive than in fluorides; (ii) as a result of their dimeric character with very short intradimer ${\mathrm{Er}}^{3+}$-${\mathrm{Er}}^{3+}$ distances and the relatively short interdimer separations all types of nonradiative energy-transfer processes are very competitive. The following trends were observed: a redshift of the f-f multiplet baricenters of up to 1% between chloride and iodide, a reduction of crystal-field splittings up to 50% between chloride and iodide, enhanced near-infrared to visible upconversion efficiency between chloride and bromide or iodide. All these trends are rationalized with simple models. For some selected crystal-field transitions energy splittings resulting from exchange interactions within the dimers were observed.