Synthesis, spectroscopic properties and applications of divalent lanthanides apart from Eu2+

Abstract In this review, modern insights into the synthesis and fundamental optical properties as well as applications of divalent lanthanides as spectral energy converters are discussed, which have especially evolved within the last two decades. An emphasis is put on the state-of-the-art knowledge about the luminescence of other divalent lanthanides than the most commonly applied representative Eu 2+ and what special features in their luminescence arises due to the different number of 4f n electrons. In particular, the luminescence of Sm 2+ , Tm 2+ , Yb 2+ and the less stable ions Nd 2+ and Dy 2+ will be regarded. A broad overview over the synthetic approaches to inorganic compounds based on divalent lanthanide ions will be conveyed as well and shall demonstrate the fascinating developments in the field of inorganic solid-state chemistry of the lanthanides. Besides their fundamental optical properties, also applications of the presented divalent lanthanide activators are discussed. Sm 2+ is a well-known temperature and pressure sensor due to its interconvertible 4f‐4f and 5d-4f transitions. Moreover, nanocrystalline BaFCl:Sm 2+ has an appreciable potential as novel X-ray storage phosphor. Tm 2+ is interesting for upconversion in photovoltaic and lasing applications, whereas Yb 2+ in halides may be a promising alternative to Eu 2+ for scintillation materials and phosphors for light-emitting diodes (LEDs). Nd 2+ and Dy 2+ are currently emerging as novel near infrared (NIR)-absorbing and emitting activators that might become interesting as lasing ions. The spectroscopy of these unusual lanthanide ions provides many non-trivial features due to the excited two-open shell 4f n-1 5d 1 configuration overlapping with the excited 4f n levels. Finally, a perspective for future developments will be given.