Relaxation of the⁴S_3/2Level of Er³⁺in Cs₂NaErCl₆

Abstract

Octahedral (Oh ) symmetry of the RE 3+ sites in the Cs2NaREC16 crystals removes all of the electric dipole intensity from the 4f-4f transitions. Thus the decay of luminescence originating in the excited states of RE ions is governed by phonon assisted (vibronic) transitions, multiphonon relaxation and phonon assisted energy transfer (cross relaxation and energy migration to impurities). Contribution of these processes to the decay of the 4S3 /2 state of erbium in Cs2 ΝaΕrCl 6 has been studied in the 4.2-300 K temperature region. It has been concluded that at low temperatures the 4S3 /2 state decays exclusively by vibronic transitions. At about 100 K the cross relaxation via the 4I9/2 state becomes operative. Efficiency of this process grows strongly with increasing temperature, reducing the 4S3 /2 lifetime by almost three orders of magnitude. No evidence has been found that multiphonon relaxations contribute to the decay of the 4S3 /2 state. PACS numbers: 78.55.ΗxIn our previous work [1] we reported for the first time the absorption properties of Cs 2 ΝaErC16 and we proposed the assignments of vibronic transitions of Er3+. More extensive study of the compound has been next reported by Hasan and Richardson [2]. Both the studies are very useful for the interpretation of luminescence spectra and lifetimes since they provide the energies of excited states of Er3+ as well as the vibrational modes of ErC1 3 6 -moiety. Preliminary data on Er3+ luminescence obtained with broadband excitation into the 4 F7/2, 4F5/2, 2G9/2 and 4G1 1 / 2 group of levels indicated that the 4F9 /2, 4I9/ 2 and 4Ι11/2 luminescent states are populated by direct vibronic transitions from initially excited levels or by ion-ion interaction [3].In this work the processes contributing to the relaxation of the 4S3/2 state of Er3+ are examined. As the Er3+ ions in Cs 2 ΝaErCl6 are situated in the sites of octahedral (Oh) symmetry, the electric dipole transition are forbidden. Therefore the excited 4S3 /2 level may decay by phonon assisted (vibronic) transitions, multiphonon relaxation to the next lower lying level and by ion-ion interaction. In Fig. 1 we present the lifetimes of the 4S3 /2 level of Er3+ in Cs2 ΝaErCl6 and in Cs2ΝaEr0.2Y0 4Yb0.4C16 recorded in the 100-300 Κ temperature region. Temperature influences more strongly the lifetimes measured for diluted sample indicating (941)