Crystal structure, electronic structure, temperature-dependent optical and scintillation properties of CsCe₂Br₇

Abstract

CsCe 2 Br 7 is a self-activated inorganic scintillator that shows promising performance, but the understanding of the important structure-property relationships is lacking. In this work, we conduct a comprehensive study on CsCe 2 Br 7 . The crystal structure of CsCe 2 Br 7 is refined using single crystal X-ray study for the first time. It crystallizes into the orthorhombic crystal system with Pmnb space group. Its electronic structure is revealed by density functional theory (DFT) calculations. Two cerium emission centers are identified and the energy barriers related to the thermal quenching to 4f ground states of Ce 3+ for these two Ce centers are evaluated. CsCe 2 Br 7 single crystal has better light yield and energy resolution than CsCe 2 Cl 7 , but with an additional slow decay component of 1.7 ms. The existence of a deep trap with a depth of 0.9 eV in CsCe 2 Cl 7 contributes to its higher afterglow level in comparison to that of CsCe 2 Br 7 . The most possible point defects in CsCe 2 Cl 7 and CsCe 2 Br 7 are proposed by considering the vapour pressure in the growth atmosphere upon melting point.ionic activators (in particular Bi 3+ , Ce 3+ and Eu 2+ ), is a constituent of the crystal, for example CaWO 4 and Self-activated single crystals of CsCe 2 X 7 (X = Cl, Br) were developed in 2010. 9-11 The CsCe 2 Cl 7 with a density of 3.6 g cm À3 can reach a gamma-ray excited light yield of 26 000 photons per MeV, but a 50% higher light yield is achieved in CsCe 2 Br 7 with a higher density of 4.0 g cm À3 . The phase diagrams of CsCe 2 Cl 7 12 and CsCe 2 Br 7 13,14 indicate that they both melt congruently. Recently, the crystal structure refinement revealed that CsCe 2 Cl 7 crystallizes in the monoclinic space group P112 1 /b. 15 However, despite the fact that the CsCe 2 Br 7 single crystal showed more promising scintillation properties, several fundamental questions, including structureproperty relationship, are still not understood: (i) the crystal structure and electronic structure; (ii) temperature dependent luminescent behaviors; (iii) the scintillation property comparisons between CsCe 2 Cl 7 and CsCe 2 Br 7 , including non-proportionality and afterglow; (iv) the trapping effect of point defects and their possible origins. All these questions lead us to carry out an in-depth study of CsCe 2 Br 7 . The paper is organized as follows. Firstly, the crystal

Exploration of Nontoxic Cs3CeBr6 for Violet Light-Emitting Diodes

“…The characteristic double peak emission originates from transitions from the 5d excited states to the 2 F 5/2 and 2 F 7/2 levels of the spin−orbit splitted 4f ground state of Ce 3+ . 25 The energy separation of the doublet emission is calculated to be 1822 cm −1 , which agrees well with the value of usual spin−orbit splitting between the 2 F 5/2 and 2 F 7/2 levels (≈2000 cm −1 ) 22 (Figure S3). The unchanged wavelength-dependent PL and PLE spectra further indicate the bright violet emission is contributed to the same Ce-5d → Ce-4f transition.…”

“…Excited-state carriers mainly recombine radiatively through Ce-5d → Ce-4f transition due to its relatively short lifetime. 22 The result indicates the optical transitions mainly occur on Ce 3+ luminescent center, which permits a high overlap between electron and hole wave functions. 24 Both CBM and VBM show small band dispersion, which is attributed to the localized f-orbitals and 0D crystal structure of Cs 3 CeBr 6 .…”

“…However, the Ce-4f → Ce-4f transition is parity-forbidden with a long excited-state lifetime, while the Ce-5d → Ce-4f transition is parity-allowed with a short excited-state lifetime. Excited-state carriers mainly recombine radiatively through Ce-5d → Ce-4f transition due to its relatively short lifetime . The result indicates the optical transitions mainly occur on Ce 3+ luminescent center, which permits a high overlap between electron and hole wave functions .…”

“…21 As shown in Figure 1a, three main equilibrium phases, namely, Cs 3 CeBr 6 , Cs 2 CeBr 5 , and CsCe 2 Br 7 , are observed; however, only Cs 3 CeBr 6 and CsCe 2 Br 7 are thermodynamically stable phases that can be synthesized by the solid-state reaction based on the different molar ratios of CsBr/CeBr 3 , while the Cs 2 CeBr 5 phase shows the noncongruent melting property which decomposes into Cs 3 CeBr 6 and CsCe 2 Br 7 at 685 K. CsCe 2 Br 7 possesses a sheet-like crystal structure with edge-and facesharing CeBr 7 polyhedra interlayered by Cs atoms, showing two Ce 3+ emission centers with different coordination environments. These two emission centers simultaneously contribute to the emission spectrum, leading to a wide emission bandwidth, 22 which is not ideal for LED applications. In contrast, Cs 3 CeBr 6 possesses two Ce 3+ ions with identical coordination environments, which implies only one emission band of Ce 3+ ion in Cs 3 CeBr 6 .…”

Preparing and testing the reliability of long-afterglow SrAl2O4:Eu2+, Dy3+ phosphor flexible films for temperature sensing

“…PL intensity of the phosphors decreases with increasing temperature due to an increased probability of non-radiative transitions. [36][37][38][39] The temperature-depended PL intensity may be tted with the Arrhenius equation:…”