The d-f luminescence of Eu²⁺, Ce³⁺ and Yb²⁺ ions in Cs₂MP₂O₇ (M = Ca²⁺, Sr²⁺)

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• 作者：Tim Senden^a ; ^{t.senden@uu.nl" class="auth_mail" title="E-mail the corresponding author} ; Andries Meijerink^a
• 关键词：Cs2MP2O7 (M = Ca2+ ; Sr2+) ; Lanthanide d&ndash ; f luminescence ; Stokes shift ; Jahn-Teller deformation
• 刊名：Journal of Luminescence
• 出版年：2016
• 出版时间：September 2016
• 年：2016
• 卷：177
• 期：Complete
• 页码：254-260
• 全文大小：467 K

文摘

The efficient narrow band emission of Eu²⁺ in Cs₂MP₂O₇ (M = Ca²⁺, Sr²⁺) is characterized by a large Stokes shift and a high quenching temperature which makes the material promising for application in warm white LEDs. The unusual Eu²⁺ luminescence properties were reported recently but an explanation for the peculiar behavior is lacking. In this paper we aim at providing new insights in the luminescence of the Eu²⁺ emission in Cs₂MP₂O₇ through measurements at cryogenic temperatures (down to 4 K) and by comparison with the d–f luminescence of Ce³⁺ and Yb²⁺ in the same host. The results reveal a sharp onset of the Eu²⁺ emission and excitation bands at 4 K. Usually the sharp onset for narrow excitation and emission bands coincide at an energy corresponding to the zero-phonon (purely electronic) transition, but for Eu²⁺ in Cs₂MP₂O₇ there is a large shift of 3500 cm⁻¹ between the onsets, consistent with the large Stokes shift observed. The onset shift can be explained by emission from a lower energy distorted excited 4f⁶5d¹ state. For Ce³⁺, the f–d absorption bands are at energies expected based on the relation between the absorption energies for Eu²⁺ and Ce³⁺ reported by Dorenbos. Contrary to Eu²⁺, the emission for Ce³⁺ shows a normal Stokes shift and therefore the emission bands are at much higher energies than predicted from the energy of the Eu²⁺ emission and the Dorenbos relations. Based on the present results the unusually large Stokes shift for the Eu²⁺ emission in Cs₂MP₂O₇ is assigned to a Jahn–Teller like deformation in the excited 4f⁶5d¹ state of Eu²⁺ that is not present in the 5d state of Ce³⁺.