Electron and hole trapping in Li2MoO4 cryogenic scintillator
Introduction
Lithium molybdate (Li2MoO4, LMO) is a perspective cryogenic scintillating material, which can be used for the registration of rare events [[1], [2], [3], [4]]. Easy crystal growth and low self-radiation due to the absence of radioactive lithium isotopes are apparent advantages of LMO. However, a serious disadvantage of the crystal is its low scintillation light yield amounting only as much as 0.4–0.97 keV/MeV (~200–450 ph/MeV) at low temperatures [[3], [4], [5]]. The obtained values are very low in comparison to those in calcium or lead molybdates [[6], [7], [8]], though the emission centers in all three molybdates have the same origin, namely these are excitons self-trapped at MoO4 complexes (STE) [9]. The drop of the scintillation yield at low temperatures may be caused by the presence of trapping centers in the crystal. The traps capture charge carriers and prevent their migration through the crystal lattice as well as their binding to excitons. Even shallow traps will adversely influence the energy migration at cryogenic temperatures needed for the operation of cryogenic scintillating bolometers. Therefore, the knowledge on the origin of defect states is of crucial importance for improvement of scintillation performance of LMO.
The analysis of thermostimulated luminescence (TSL) curves allows to obtain data on the crystal structure defects. TLS curves for X-ray irradiated Li2MoO4 were presented previously in Refs. [4,10,11]. An intensive low temperature TSL peaks were observed at ~20 and ~40 K [4,11]. The possibility of the co-existence of self-trapped electrons and holes was supposed in Ref. [11]. At higher temperatures the observation of TSL is complicated due to strong thermal quenching of LMO luminescence. Previously a set of overlapping TSL peaks with the most intensive one at 170 K were detected in Ref. [11], while a strong peak at ~210 K has been reported in Ref. [4]. However, only general considerations were earlier given on the relation of TSL peaks to the traps connected with crystal structure defects. Method of electron paramagnetic resonance (EPR) allows to determine the origin of paramagnetic defect centers in the crystals. Therefore, correlated studies using EPR and TSL methods allow to connect TSL peaks to particular types of defects, if these defects are paramagnetic. Previously the study of trapping centers in polycrystalline samples of Li2MoO4 using EPR and TSL has been performed in Ref. [12]. Two hole trapping centers, which are thermally released at 180 and 340 K, have been detected and attributed to the MoO4− and MoO56− centers, respectively. The thermal release of the former center is also accompanied by TSL peak at 144 K. The presence of the latter center has been ascribed to the presence of additional non-phenacite phase in the sample that indicates rather moderate quality of the studied samples.
In the present paper, we study the origin and thermal stability of trapping centers in Li2MoO4 single crystals and powders by EPR and TSL methods. Several electron and hole trapping centers were detected and their origin was determined. The influence of charge carrier trapping on energy transfer processes is discussed.
Section snippets
Experimental details
Single crystals of Li2MoO4 of high optical quality were grown from a stoichiometric melt using the low temperature gradient Czochralski technique [13]. XRD analysis have shown that the crystals are single phase and isostructural to phenacite Be2SiO4 crystal structure with a space group R3 [14].
The obtained crystals were very fragile which made their orientation for the EPR analysis problematic. However, there was an attempt to cut the samples from the bulk specifically in a parallelepiped shape
EPR studies of single crystal Li2MoO4
No remarkable signals occurred in EPR spectra prior to the irradiation with X-rays. However, a number of resonance lines appeared upon the irradiation of a sample at 77 K (Fig. 1). The lines around 3375 G marked as “quartz” originate from the quartz sample holder.
Conclusions
The origin of hole and electron trapping centers in Li2MoO4 single crystals has been determined using correlated studies by EPR and TSL methods in the temperature region 80–300 K. In total, eight hole- and electron-type paramagnetic centers were detected by EPR technique, which differ by the g-tensor value and thermal stability. Three hole centers were ascribed to the O− type centers perturbed by different types of defects, in particular, by the lithium and molybdenum vacancies nearby. The
CRediT authorship contribution statement
M. Buryi: Investigation, Writing - original draft, Methodology, Formal analysis. V. Babin: Investigation, Writing - original draft, Formal analysis. V. Laguta: Writing - review & editing, Validation. D.A. Spassky: Conceptualization, Investigation, Writing - original draft. V. Nagirnyi: Investigation, Writing - original draft. V.N. Shlegel: Resources, Methodology.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The financial support from the Czech Science Foundation (project No. 20-12885S) as well as from the Estonian Research Council, project PUT PRG111 and the ERDF funding in Estonia granted to the Centre of Excellence TK141, Project No. 2014-2020.4.01.15–0011 are gratefully acknowledged. V.N. Shlegel was supported by Russian Science Foundation (grant No.18-12-00003).
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