Luminescence properties of TlAlF4 crystal

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Highlights

  • Temperature dependent PL excitation and emission spectra were studied.

  • The doublet structure of Tl+ ion A excitation bands (A1, A2) are observed.

  • The onset point of STE emission band is observed at 150 K.

  • The calculated activation energy for the main TL glow peak at 40 K is ~71 meV.

Abstract

A single crystal of TlAlF4 is grown by a self-seeded vertical Bridgman-method. The measured powder X-ray diffraction pattern is compared with the standard JCPDS pattern and found to be in good agreement. The crystal structure and cell parameters are calculated and refined by Rietveld refinement analysis. The optical absorption spectrum is measured and the characteristic spectral properties of Tl+ bands are studied. The temperature dependent photoluminescence (PL) excitation and emission spectra are measured in the temperature range of 10–300 K. The doublet structure of Tl+ ion A excitation bands (A1, A2) are observed and correlated with absorption bands. The excitation under both bands yields the same broad emission band in the range of 330–500 nm with the maximum peak at around 390 nm in room temperature. At low temperatures, this emission band is well resolved into two emission bands with maximum at 371 and 404 nm for separate excitations of 227 and 236 nm, respectively. The schematic configuration coordinate energy level diagram for emission of Tl+ ion is also proposed. Low temperature thermally stimulated luminescence (TSL) were carried out and analysed the trap depth (E) and frequency factor values. The mechanisms of the processes, responsible for the emission band and formation of defects pairs of the type of Tl0–Vk center have been discussed in detail manner.

Introduction

Effective luminescence materials relies on the localization of excited electrons and holes at certain impurities (activator), which act as luminescence centers. Typically activators or luminescent centers belong to either rare earth materials (Ce3+Eu2+) [[1], [2]] or transition metal ions (Cr3+, Mn4+) [[3], [4], [5]]. Apart from the rare earth and transition metal ions, a number of heavy 6p (Tl, Pb, Bi) and 5p (In, Sn, Sb) ions are also important multivalent ions, which act as luminescent centers in many materials [6]. The history of investigation on ions with ns2 electronic configuration in their ground states such as Ga+, In+, Tl+, Sn2+, Pb2+, etc started from 1927 [7]. It is well known that these ns2 ions will produce several absorption bands on the long-wavelength side of the UV absorption edge generally labelled as A, B, C, D1, D2, in the order of increasing photon energy [8]. Among these ions, Tl+ ion in alkali halides and other host materials were studied as high efficiency luminescence center and reviewed most extensively by several researchers [[9], [10]]. Because of the large optical band gap, the alkali halides are the best choice of host materials where the s2 centers of Tl+ ion can reveal many UV-absorption bands [[11], [12]]. Thallium based compounds have attractive scintillation properties and may be used in a variety of application for radiation detection. Thallium is not considered as alkali metal and it belongs to the family of group 13 elements, however the chemical properties are very close to alkali metals in many ways [[13], [14]]. In addition, Tl+ has very high Z value (81), atomic weight of 204.37 and density (11.83 g/cm3) which will effectively increase the density of materials compare to other alkali based scintillators, [15]. Certainly, materials with high atomic number (Z) and high density are favorable for scintillation application. Tl+ ion is well-known and ideal for the X-rays and γ-rays spectrometry applications.

The investigation on absorption and luminescence spectra of thallium halides has a long history as an interesting subject compared with several active researches in other ionic crystals with Cl, Br and I anions. However, there are very few works on the spectroscopy of fluoride anion crystals activated with s2 ions [[16], [17], [18]]. This in turn involves the difficulty in synthesis of those fluoride compounds. The single crystal growth of fluorides are complicated because of corrosive nature of fluorine and specific experimental conditions due to the strong reactivity of the fluorine with oxygen, which forms complexes such as OH hydroxyl radicals and efficiently substitute the F ions in the crystalline structure.

As early as in 1937, Brosset studied the layered crystal structure of MAlF4 (M = Tl, K, Rb, NH4) ternary compounds [19]. The TlAlF4 compound belongs to crystal system of tetragonal and space group of P4/mmm. A. Bulou and J. Nouet studied the structural and phase transitions analysis of this compound [20]. H. Mizoguchi et al. studied the valence band structure of TlAlF4 and emission spectrum using sintered pellets at 77 K only [21]. The main goal of this paper is to study the luminescence mechanism of Tl+ in the TlAlF4 crystal using photoluminescence emission and excitation spectra in the temperature range of 10–300 K. However, to the best of the authors’ knowledge there are no further reports dealing about the luminescence study of TlAlF4 single crystal. We observed the energy transfer process and Tl+ band separation at 10 K and discussed in detail manner in this manuscript.

Section snippets

Experimental

A TlAlF4 single crystal is grown by using a vertical Bridgman-Stockbarger method from the stoichiometric ratio of equimolar mixer of individual fluorides of Thallium (TlF) and Aluminum (AlF3). The weighted individual fluorides were thoroughly mixed and grounded uniformly by using the mortar and pestle and then loaded into a conically tipped graphite crucible and sealed in a quartz ampoule with vacuum level of ~2 × 10−6 mbar. All the synthesis process are carried out inside of a glove box filled

Powder XRD analysis

The phase identification of the synthesized compound is performed through powder X-ray diffraction (PXRD) pattern analysis using finely crushed powder of the grown crystal piece. The samples are scanned in 2θ range of 10–70° at room temperature. The recorded powder diffraction pattern is in good agreement to that of JCPDS data (#98-020-0637) [22]. No extra peaks are observed in the diffraction pattern, which confirms the purity of the phase formation. The Rietveld refinement analysis is done

Discussion

The observed broadening of the absorption bands are due to the crystal lattice vibrations. In several alkali halide crystals this broadening was observed and the detailed explanations are given based on Jahn–Teller effect and the Frank–Condon principle [27]. The ns2 ions have three excited state electronic vibrational energy levels, such as 3P0 (lowest energy level), and the two splitting components of 3P1 (lower) and 3P2 (upper) [28]. The splitting energy levels of 3P1 are responsible for the

Conclusions

In summary, optical absorption spectra measurements shows bands around 200–250 nm whichacharacteristic spectral properties due to electronic transitions of Tl+ ions. At the same wavelength region, doublet structure of Tl+ ion A excitation bands (A1, A2) are observed in PL excitation spectra and correlated with absorption bands. The observed broad emission spectrum around 390 nm is very similar to that of alkali halides doped with Tl+ ion. It has been found that the complex nature of intrinsic

Declaration of competing interest

There is no conflict of interest.

Acknowledgment

This research was supported by the National Research Foundation of Korea (NRF No-2018R1A6A1A06024970) and IBS-R016-D1 and (No- 2019R1I1A1A01052459) funded by the Ministry of Education, Korea.

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