Spin Crossover of Yb²⁺ in CsCaX₃ and CsSrX₃ (X = Cl, Br, I) – A Guideline to Novel Halide‐Based Scintillators

Abstract

In this paper, the temperature dependence of the photoluminescence of Yb2+ doped into the halidoperovskites CsMX3 (M = Ca, Sr; X = Cl, Br, I) is presented. Yb2+ shows spin‐forbidden high‐spin (HS) and spin‐allowed low‐spin (LS) emission bands in all compounds. Upon excitation of the higher energetic LS transition, very different behaviors of the interplay of the two emissive transitions are observed. In the chlorides, the HS‐based emission becomes already dominant at higher temperatures than 50 K; in the bromides, a temperature‐dependent population of the HS state is observed at higher temperatures than 200 K, whereas in the iodides, the LS emission remains dominant even above 300 K. A vibrational relaxation model is attempted for the explanation of this behavior that reveals a very delicate dependence on the mode energies of the [YbX6]4− octahedra. FIR spectra of the pure Yb2+‐based compounds CsYbX3 are also presented to experimentally verify the estimated average mode energies from the model. Finally, the relevance for novel Yb2+‐based scintillators is discussed.

A Theoretical Framework for Ratiometric Single Ion Luminescent Thermometers—Thermodynamic and Kinetic Guidelines for Optimized Performance

“…[174][175][176][177][178] Also anomalous thermal quenching effects often found in nanocrystals was demonstrated to be a successful strategy to achieve high relative sensitivities. [118,179,180] For the sake of completion, it is also noteworthy that divalent lanthanides such as Sm 2+ , [181][182][183] Eu 2+ , [184] Tm 2+ [185][186][187][188] and especially Yb 2+ [189][190][191][192] show potential for luminescence thermometry, although their thermal behavior is often not governed by simple Boltzmann statistics anymore, but more complex temperaturedependent excited state dynamics involving a crossover to the 4f n-1 5d 1 configuration. [193,194] Unlike the chemically more challenging examples of divalent lanthanides, Pr 3+ has also recently been introduced as an alternative, more stable luminescent thermometer with the same underlying principle of a 4f 2 -4f 1 5d 1 crossover (with eventual incorporation of thermal ionization into the conduction band).…”

Evaluation of photoluminescence and scintillation properties of Yb²⁺-doped SrCl₂₋<i>_x</i>Br<i>_x</i> crystals

“…It is reported that spin-allowed transition is often observed in hosts with heavy anions or high local site symmetries. 30), 31) In CsMX 3 :Yb 2+ (M = Ca, Sr; X = Cl, Br), spin-allowed transition has been observed for CsCaBr 3 :Yb 2+ with local C 4v symmetry, whereas no spinallowed transition has been observed for CsCaCl 3 :Yb 2+ with local O h symmetry. The occurrence of spin-allowed transition has been related to the variation of the anions rather than local site symmetry for CsMX 3 :Yb 2+ (M = Ca, Sr; X = Cl, Br).…”

Optical Absorption Spectra of Divalent Neodymium (Nd2+) in Bromide and Iodide Hosts

“…In recent years, spectroscopic properties of divalent lanthanide elements (Ln 2+ ) have been intensively studied, both experimentally [1][2][3][4][5][6] and theoretically. [7][8][9][10][11][12] However, electronic spectroscopy of divalent neodymium still remains practically an unexplored area.…”