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CsMgCl3: A promising cross luminescence material
Journal of Solid State Chemistry 227 (2015) 110–116
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
CsMgCl3: A promising cross luminescence material G. Shwetha a, V. Kanchana a,n, G. Vaitheeswaran b a
Department of Physics, Indian Institute of Technology Hyderabad, Ordnance Factory Estate, Yeddumailaram 502 205, Telangana, India Advanced Center of Research in High Energy Materials (ACRHEM), University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad 500 046, Telangana, India
b
art ic l e i nf o
a b s t r a c t
Article history: Received 24 January 2015 Received in revised form 23 March 2015 Accepted 26 March 2015 Available online 2 April 2015
Full-potential linearized augmented plane wave method has been used to study the cross luminescence of halide scintillators. The electronic structure and optical properties of alkali halides such as CsMgCl3, CsCaCl3, and CsSrCl3 are presented. One of the major criteria for the cross luminescence to happen is the energy difference between valence band and next deeper core valence band being lesser when compared to energy gap of the compound, so that radiative electronic transition may occur between core valence band and valence band which might lead to fast scintillation. We found this criterion to be satisfied in these compounds leading to cross luminescence. The presence of high energy peaks in the absorption spectra indicates the creation of holes in the core valence band, which is an essential criterion for the occurrence of cross luminescence. The electronic structure, and optical properties studies clearly indicate CsMgCl3, CsCaCl3, and CsSrCl3 to be cross luminescence materials comparable to CsCl which is one of the well known fast scintillators. In addition, CsMgCl3 is found to be better among the studied compounds with optical isotropy though the compound is structurally anisotropic. & 2015 Elsevier Inc. All rights reserved.
Keywords: Band structure Cross luminescence Electronic structure Optical properties
1. Introduction Cross luminescence is one of the important criteria for fast scintillation. The compounds which exhibit cross luminescence or core valence luminescence (CVL) are very interesting because of their fast decay and high thermal stability. In the past few decades there were continued experimental and theoretical search progressing in various directions searching for scintillators with desired properties, but there is no unique scintillator available, with high efficiency, high yield, fast scintillation, good energy resolution, which provoke further research in exploring fast scintillators which are having very less decay time. Cross luminescence is one of the phenomena for fast scintillation with less decay time of the order of nanoseconds (ns) or less, where radiative transition is observed between halogen derived valence band and upper most cation core valence band resulting in Auger free luminescence. With the incident radiation, electrons from core valence band will be excited to conduction band leaving a hole in the core valence band, and these holes combine radiatively with electrons from the valence band, while the hole created in the valence band might combine with electrons from the conduction band. The main participant of this process is the core holes. The
n
Corresponding author. E-mail address: [email protected] (V. Kanchana).
http://dx.doi.org/10.1016/j.jssc.2015.03.024 0022-4596/& 2015 Elsevier Inc. All rights reserved.
radiative transition of electrons from valence band to core valence band will lead to the cross luminescence (also called Auger free luminescence) with short wavelength [1–12]. The general requirement for CVL is that energy difference between top of the valence band and top of core valence band (EVC ) should be less than the band gap (Eg ) of the compound. If EVC 4 Eg , then Auger transitions are dominated because of their higher probability. The incident energy should be such a way that it will excite the core level electrons, leaving a hole in core valence band. The decay time of CVL compounds is very small with the emission in the short wavelength region. The fast scintillation observed in BaF2 opened up the origin for CVL in scintillators [5,13], which is also observed in AX (A ¼alkali, X¼ halide), and ABX3 (A ¼alkali, B ¼alkali-earth, and X¼halide) [2] compounds. Moving from AB to ABX3 type of compounds, decay time and light output are reported to increase because of the increase in the distance between A–X, thereby reducing the overlap of wave functions which may result in smaller probability of CVL [2]. Though the decay time may be higher in ABX3, it might be advantageous to get a better light yield which further provokes us to explore this series. Here we are mainly interested in studying the cross luminescence in ABX3 (A¼ alkali, B ¼alkali-earth, and X-halide) type, CsMgCl3, CsCaCl3, and CsSrCl3, compounds and would like to explore the type of transition involved through computed electronic structure and optical properties calculations. These are preferred over well known binary compound CsCl because of higher yield of cross
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luminescence reported experimentally [2,8]. Cross luminescence is observed in some of the ABX3 type compounds such as RbCaF3, CsCaCl3, CsSrCl3, CsMgCl3, CsMgF3 [8,9,14–17], barium based fluorides [18], and KF, KMgF3, and KCaF3 [19]. Cross luminescence is also observed in impure systems like Rb1 x Csx Br (x o 0:2) [2], and A1 x Csx CaCl3 (A¼K, Rb) crystals [20,21] (where addition of Cs leads to CVL in these compounds) implying the importance of Cs in the context of cross luminescence. Recently, theoretical studies have been carried out on CsCaCl3 in which the authors reported the elastic constants, and optical properties of these compound [22]. The scintillation properties of doped CsCaCl3 and CsCaI3 are investigated experimentally as well as theoretically. The temperature dependence of photo-luminescence excitation, emission, and decay time was measured (experimentally), the band structure and the efficiency of the cross luminescence of these materials are also explained (theoretically) [23]. The structural, elastic, chemical bonding and optoelectronic properties of CsSrM3 (M¼F, Cl) are studied through the density functional calculations [24]. Our main focus in the present work is to address the cross luminescence of these compounds, the type of transition involved, and also to compare the scintillation characteristics of these three scintillators through the calculated electronic structure, and optical property calculations. The organization of the paper is as follows, in Section 2 we describe the computational details of our calculations, in Section 3 we discuss the results and discussion part followed by conclusions.
2. Computational details Full potential linearized augmented plane wave method (FPLAPW) has been used to perform the first principles total energy calculations as implemented in wien2K code [25,26]. FP-LAPW is an accurate method for calculating the electronic structure and optical properties of the compounds. The wave function is expanded in spherical harmonics within the atomic sphere, while outside the sphere it is expanded using plane-wave basis. The potential is given by 8P V ðrÞY lm ðr^ Þ inside sphere > < lm lm VðrÞ ¼ P ð1Þ iKr > outside sphere : VKe K
The exchange-correlation potential is calculated within the generalized gradient approximation (GGA) [27], and Tran and Blaha modified Becke–Johnson potential (TB-mBJ) [28,29]. The separation between the core states and valence states is set to 6.0 Ry. The wave functions in the interstitial region were expanded using plane waves with a cutoff of RMT K max ¼ 9 in order to achieve energy eigenvalue convergence, where K max is the plane wave cut-off, and RMT is the smallest muffin tin sphere radii. Convergence tests were carried out using higher RMT K max values, giving no significant changes in the calculated properties. The compounds of present study are insulators, and as local density approximation (LDA) and generalized gradient approximation underestimate the band gap, we have used the Tran and Blaha modified Becke–Johnson potential (TB-mBJ), which gives the band gap values close to the experiments. Throughout our study we have computed the electronic structure, optical properties using the TB-mBJ functional at the experimental lattice parameters with optimized atomic position. We have used 10 10 10 k-mesh with 84 k-points in IBZ (irreducible Brillouine zone) for k-space integration for the electronic structure calculation and 17 17 17 k-mesh for the optical property calculations. The optical properties of the materials can be well explained by using the dielectric function ϵ(ω) which consists of real and imaginary parts ϵ1(ω) and ϵ2(ω) respectively. The ϵ2(ω) can be calculated from electronic band structure, and ϵ1(ω) can be extracted from the ϵ2(ω) by using
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the Kramers–Kroning relation. From the ϵ1(ω) and ϵ2(ω) values one can calculate other optical parameters like refractive index and extinction coefficient [30]: Z ℏ2 e2 X 3 ϵ2 ðωÞ ¼ 2 2 d k〈ck =pα =vk 〉〈vk =pβ =ck 〉 π m ω c;v δðϵck ϵvk ωÞ
ð2Þ
where p is the momentum matrix element between α and β states, ck and vk are the crystal wave functions corresponding to the conduction band and valence bands with crystal wave vector k. Z 1 2 ωϵ2 ðωÞ ϵ1 ðωÞ ¼ 1 þ P dω ð3Þ π 0 ðωÞ2 ω2 where P is the principle value of integration. 2qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 31=2 ϵ21 ðωÞ þ ϵ22 ðωÞ þ ϵ1 ðωÞ 5 nðωÞ ¼ 4 2 2qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 31=2 ϵ21 ðωÞ þ ϵ22 ðωÞ ϵ1 ðωÞ 4 5 kðωÞ ¼ 2
ð4Þ
ð5Þ
Moreover to the best of our knowledge, there are no theoretical calculations available on CsMgCl3, though the other two compounds have some earlier theoretical calculations on optical properties [22– 24]. In the present work, main emphasis is laid on studying the trends in the optical properties of ABX3 compounds. Among the studied compounds CsMgCl3 [31] crystallize in hexagonal crystal structure with space group P63 =mmc with lattice parameters a¼7.269, b¼ 6.187 Å, with Cs atom at (1/3, 2/3, 0.75), Mg at (0 0 0) and Cl at (0.1556, 0.1556, 0.25). CsCaCl3 and CsSrCl3 crystallize in the cubic perovskite structure with space group Pm3m (space group number 221) with Cs atom located at (0, 0, 0), M (Ca, Sr) at (0.5, 0.5, 0.5) and Cl at (0, 0.5, 0.5) with lattice parameters 5.390 and 5.615 Å for CsCaCl3 [22,32] and CsSrCl3 [24,33] respectively. 3. Results and discussions 3.1. Electronic properties The computed energy band structure along the high symmetry points in the first Brillouine zone and total density of states of chloroperovskites are shown in Fig. 1 using the TB-mBJ functional along with CsCl band structure which is a well known cross luminescence material. These compounds are insulators with wide band gap because of large electro-negativity difference between halogen derived valence band and cation derived conduction band. From these band structure plots, we can clearly see that the energy difference between the top of the valence band and the top of the core valence band is less compared to energy gap (Eg ) of the compound, which is an essential criterion to observe cross luminescence. The width of valence band is more for CsMCl3 (M¼Mg, Ca, and Sr), when compared with CsCl as seen from Fig. 1. But for a better cross luminescence, it is desirable that the width of the upper valence band (VB) and core valence band (CVB) are appreciably represented as A and B in the band structure plots respectively. But in ABX3 the VB width is more, while in CsCl, the CVB width is little more than that in CsMgCl3. The higher value of CVB width compared to CsMCl3 (M¼Mg, Ca, and Sr) might be one of the reasons for increased CVL probability in case of AB (CsCl) compared to ABX3 which was discussed earlier. The calculated band gaps, width of the valence band (δEV ), energy difference between top valence band and top core valence band (EVC ), and EVC δEV ¼ Eg2 are given in Table 1, along with available experimental and theoretical values. The schematic diagram corresponding to these representations is shown in
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Fig. 1. Band structure of CsCl, CsMgCl3, CsCaCl3, and CsSrCl3 compounds, along with total density of states using the TB-mBJ functional, where A and B represent the width of the valence band and core valence band respectively. From the band structure plots it is clear that A increases from CsCl to CsMgCl3 and decreases in CsSrCl3, and B decreases from CsCl to CsSrCl3.
Fig. 2(a), and the cross luminescence, which is the transition of electrons from valence band to core valence band, is depicted in Fig. 2(b). From Table 1, we can clearly see that EVC o Eg , δEV 4 Eg2 in case of CsMgCl3 and, δEV o Eg2 for other compounds, and calculated band gap, δEV , EVC and Eg2 values are comparable to the available experimental and other theoretical values. We observed energy gap (Eg ) to decrease from CsMgCl3 to CsCaCl3 and then increase to CsSrCl3, and also the energy difference between the bottom of the valence band and the top of the core valence band (Eg2 ) is found to increase from CsMgCl3 to CsSrCl3 because of the decrease in the width of the valence band. The direct Γ –Γ band gap is predicted for CsMgCl3, whereas an indirect band gap along M–Γ direction is observed for CsCaCl3, and CsSrCl3 compounds. Scintillation phenomena can be observed in various ways depending on the type of transition involved, which are classified as A-type, L-type, and ALtype. In the case of A-type EVC 4 Eg , where Auger luminescence is dominating, in the case of L-type EVC o Eg , where Auger-free luminescence is observed between valence and core valence bands. ALtype of compounds are in between A-type and L-type. The compounds of present study are of L-type (EVC o Eg ), where the cross luminescence can be observed between valence band and next deeper core valence band. Density of states of these compounds are studied for further understanding and the corresponding figures are shown in Fig. 3. Conduction band is due to Mg-s,p and Cs-d states in case of CsMgCl3, whereas these states are due to M-d (M¼ Ca, Sr) and Cs-d with minor contribution from Cl-d in case of CsCaCl3 and CsSrCl3. The valence band is of Cl-p states hybridized with the Mg-s,p and Cs-p states in case of CsMgCl3, Ca-d,p and Cs-p states in the case of CsCaCl3, and Sr-d,p states in case of CsSrCl3 compounds. Valence band spread around the energy
Table 1 Calculated band gaps, in eV, of CsMgCl3, CsCaCl3, and CsSrCl3 compounds using GGA and TB-mBJ functionals along with the width of the valence band (δEV ), the energy difference between the top of valence band and the top of core valence band (EVC ), and Eg2 (EVC δEV ) in eV, along with the available theoretical and experimental values. Method
CsMgCl3
CsCaCl3
CsSrCl3
GGA TB-mBJ Exp δEV EVC Eg2
5.31 7.76 8.5d,e 2.88 (2.9d,e) 5.48 (5.6e), 2.6 (2.6d, 2.7e)
5.50 (5.35a, 5.45b) 6.89 (6.93a) 8.5d, 8.1e 2.24 (2.4d, 2.8e) 5.57 (5.9e) 3.24 (3.1d,e, 2.95b)
5.16 (5.61c) 7.52 (7.65c) 7.7e 1.74 (2.5e) 5.9 (5.8e) 4.16 (3.3e)
a
Ref. [22]. Ref. [23]. c Ref. [24]. d Ref. [9]. e Ref. [17]. b
region 0 to 2.88 eV for CsMgCl3, 0 to 2.24 eV for CsCaCl3, and 0 to 1.74 eV for CsSrCl3 compounds. The width of the VB of CsMgCl3 is little higher because of the presence of Mg-s,p states, and also Cs-p states, whereas these Cs-p states are relatively lesser in CsCaCl3 and they are absent in case of CsSrCl3. VB width of CsCl is less compared to CsMCl3 because of missing states M-d or Mg-s,p states in the valence band. The VB width being more is an essential property and it is discussed in detail while explaining optical properties in the later section. The core valence band is situated around 5.48 to 5.63 eV for CsMgCl3, 5.57 to 5.67 in case of CsCaCl3, and 5.9 to 5.95 eV for CsSrCl3, and these states are mainly due to Cs-p states for all the
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Fig. 2. Schematic diagram showing (a) Eg , EVC , δEV , Eg2 , and δECV representations and (b) cross luminescence.
Fig. 3. Density of states of CsMgCl3, CsCaCl3, and CsSrCl3 using TB-mBJ functional.
compounds. From the calculated electronic structure and density of states, we observe the width of the valence band and width of the core valence band to decrease from CsMgCl3 to CsSrCl3, and also the core valence band shifts to higher energy region as we move from Mg to Sr i.e., energy difference between the bottom of valence band and the top of core valence band increases from CsMgCl3 to CsSrCl3 which is also confirmed from optical properties calculations, discussed in the subsequent sections.
indicates the ionic bonding in these compounds with very less covalency M (Mg, Ca, and Sr) to Cl. It is to be noted that our results are in agreement with the available theoretical calculation [22,24]. As discussed earlier on moving from the AB to ABX3 the distance between the ions (A–X) increases (distance between Cs to Cl is 3.57, 3.64, 3.81, and 3.97 Å for CsCl, CsMgCl3, CsCaCl3, and CsSrCl3 respectively) resulting in reduced overlapping of wave functions indicating more ionicity in the case of ABX3 than AB (CsCl) which is also discussed in the electronic structure calculations.
3.2. Chemical bonding 3.3. Optical properties The accurate bonding nature of the material can be explained on the basis of electronic charge density plots, where charge transfer between ions indicates the ionic nature and sharing of charges between ions indicates the covalent nature. The difference electron density plots of these compounds along with CsCl are shown in Fig. 4 along (1 1 0) crystallographic plane in order to visualize the chemical bonding nature of these compounds. The distribution of electronic densities of all the atoms shows isolated spheres indicating ionic nature of these compounds. There are only very few iso-lines present from M (Mg, Ca, Sr) to Cl indicating weak covalent bond between M to Cl (which is absent in case of CsCl). The nearly isolated spherical charge distribution around the atoms is evident from the plots which
Optical properties are quite important for materials, without which there is no possibility to improve the properties of scintillators, photonic devices, etc. which substantiates its importance. The important factor for calculation of optical property is frequency dependent dielectric function. The calculated dielectric functions of all the compounds are shown in Fig. 5, which is the sum of all transitions from valence band to conduction band. From the figure, we can see that the optical response of the system shifts to lower energies moving from CsMgCl3 to CsCaCl3 and to higher energy levels to CsSrCl3 in agreement with electronic structure calculations where band gap decreases from Mg to Ca then increases to Sr. The imaginary
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Fig. 4. Difference valence electron-density plots of (a) CsCl, (b) CsMgCl3, (c) CsCaCl3, and (d) CsSrCl3 using TB-mBJ functional.
Fig. 5. Real and imaginary parts of dielectric function (ϵ1(ω), ϵ2(ω)), refractive index (n(ω)), and absorption coefficient (α(ω)) of (a) CsMgCl3, (b) CsCaCl3, and (c) CsSrCl3 using TB-mBJ functional.
part of dielectric function starts at a particular energy called threshold energy, and these threshold energies for these compounds are 7.8, 6.7, and 7.1 eV respectively for CsMgCl3, CsCaCl3, and CsSrCl3 indicating the transition of electrons from valence band to the bottom of conduction band i.e. from Cl-p states to Cs-d, M-d (M¼Ca, Sr, and Mg) states. The higher energy spectra are due to transition of electrons from core valence band i.e. Cs-p states to conduction band.
The peak in the higher energy region of dielectric function indicates the transition of electrons from core valence band to conduction band creating hole in the core valence band which plays a major role in CVL, and this hole recombines with the electrons in the valence band giving rise to the cross luminescence with fast decay. In the spectra of the imaginary part of dielectric function a peak present near to the band gap indicates the transition of electrons from valence band to
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these compounds similar to CsCl. From the calculated electronic structure and optical properties of these compounds, we can observe the CVL in these compounds which is also confirmed experimentally [17]. We predict that among CsMCl3 (M¼Mg, Ca, and Sr), CsMgCl3 is a good scintillator followed by CsCaCl3, and CsSrCl3 because of the decrease in the width of the valence band and width of the core valence band from CsMgCl3 to CsSrCl3, and also spectra are similar to CsCl, which is a well known fast scintillator.
4. Conclusions
Fig. 6. Absorption coefficient of CsMgCl3, CsCaCl3, and CsSrCl3 in comparison with CsCl.
conduction band. It is to be mentioned that one cannot see the transition of electrons from valence band to core band which might be the order of 3 eV because these are the emission characteristics which are absent in the dielectric function plots or absorption spectra plots. The energy difference between bottom of valence band and top of core valence band (Eg2 ) increase from CsMgCl3 to CsSrCl3, because of which one might expect the emission (which is due to transition of electron from valence band to core valence band) to shift to high energy regions. The width of lower energy region of the imaginary part of the dielectric function decreases from Mg to Sr because of the decrease in the width of the valence band. Similarly higher energy region of imaginary part of dielectric function shifts to higher energies as we move from the Mg to Sr, which might be due to shifting of core valence band towards higher energies as we move from Mg to Sr. The decrease in the width of valence band, width of the core valence band, and dielectric function will lead to decrease in the number of carriers available for recombination from Mg to Sr compound, and also core band shifted to higher energy region resulting in higher carrier recombination in CsMgCl3 followed by CsCaCl3 and CsSrCl3. The refractive index of all the compounds is shown in Fig. 5. The static dielectric constant values for CsMgCl3 are 1.586 and 1.584 along the x and z directions, respectively indicating the optical isotropy of these compounds though the structure is anisotropic which is a key criterion for ceramic scintillators, and these values are 1.58 and 1.53 for CsCaCl3 and CsSrCl3 compounds respectively. The absorption spectra of the compounds are shown in Fig. 5, which is having the same characteristics as that of dielectric function. We have also compared the absorption spectra of all the studied compounds with that of CsCl compound which is a fast scintillator, and is shown in Fig. 6. From the figure, we can clearly see that the absorption characteristics from 8 to 20 eV is almost similar for all compounds except for CsMgCl3, where the dip found around 12 eV for all the compounds including that CsCl is absent and we find a continuous increase in the absorption spectra which may be due to the following reasons: (i) First it is vividly seen that the width of the VB is significantly higher than other ABX3 and CsCl, which certainly would facilitate continuous absorption, as it is clear that absorption coefficient is directly proportional to the width of the band. (ii) Mg-s states present in the conduction band, and Cs-p states present in the valence band of CsMgCl3 will increase the probability of transition from valence band to conduction band compared to CsCaCl3 and CsSrCl3 where these states are absent (or almost negligible). (iii) Other possible reason might be that the energy difference between VB and CVB increases from CsMgCl3 to CsSrCl3 leading to continuous absorption in CsMgCl3. Macdonald et al. [17] reported that the presence of additional cation M (M¼Mg, Ca, and Sr) weakly influences the core excitation formation indicating the presence of cross luminescence in
Ab-initio calculations were carried out to study the electronic structure and optical properties of CsMgCl3, CsCaCl3, and CsSrCl3 which are predicted to be fast scintillators similar to their binary halides. The calculated band structure clearly reveals the investigated compounds to be insulators with the calculated band gap values in reasonable agreement with the experiments. In addition, we find these compounds to be L-type insulators, with EVC o Eg , where EVC is the energy difference between the top of the valence band and the top of the core valence band, and Eg is the band gap of the compounds, and this is an essential criterion for the CVL to be observed. The DOS plots show that the valence band contribution is mainly from Cl-p states for all the compounds, and the conduction bands are dominated by Mg-s, p and Cs-d states in the case of CsMgCl3, and M(Ca, Sr)-d and Cs-d states in other compounds. Our calculated charge density plots clearly indicate the ionic nature of these compounds. From the optical properties calculations, it is observed that the absorption spectra shifts to lower energies moving from CsMgCl3 to CsCaCl3 and to higher energy levels in CsSrCl3 because of the decrease in band gap values from Mg to Ca, followed by an increase in Sr. The peak in the higher energy region of the absorption spectra indicates the creation of holes in the core valence band which plays a major role in the CVL. Optical isotropy is observed in CsMgCl3 though the compound is structurally anisotropic. From the calculated electronic structure, and optical properties we conclude these compounds to be good scintillators, and CsMgCl3 is found to be better among the studied compounds.
Acknowledgments V.K. would like to acknowledge IIT Hyderabad for the computational resources. G.S. would like to acknowledge IIT Hyderabad for computational facility and MHRD for the fellowship. G. V. thank Center for Modelling Simulation and Design-University of Hyderabad (CMSD-UoH) for providing computational facility. References [1] P. Lecoq, A. Annenkov, A. Gektin, M. Korzhik, C. Pedrini, Inorganic Scintillators for Detector Systems Physical Principles and Crystal Engineering, SpringerVerlag, Heidelberg, 2006. [2] P.A. Rodnyi, Radiat. Meas. 33 (2001) 605. [3] P.A. Rodnyi, Radiat. Meas. 38 (2004) 343. [4] S. Kubota, J.z. Ruan(Gen), M. Itoh, S. Hashimoto, S. Sakuragi, Nucl. Instrum. Methods Phys. Res. Sect. A 289 (1990) 253. [5] A.N. Belsky, I.A. Kamenskikh, V.V. Mikhailin, A.N. Vasil'ev, J. Electron Spectrosc. Relat. Phenom. 79 (1996) 111. [6] C.W.E. van Eijk, J. Lumin. 60 (1994) 936. [7] S. Zazubovich, Radiat. Meas. 33 (2001) 699. [8] I.A. Kamenskikh, M.A. MacDonald, V.V. Mikhailin, I.H. Munro, M.A. Terekhin, Rev. Sci. Instrum. 63 (1992) 1447. [9] A.P. Shpak, O.A. Glike, A.G. Dmitriev, P.A. Rodnyi, A.S. Voloshinovskil, S.M. Pidzyrailo, J. Electron Spectrosc. Relat. Phenom. 68 (1994) 335. [10] J. Jansons, Z. Rachko, J. Valbis, J. Andriessen, P. Dorenbos, C.W.E. van EijK, N.M. Khaidukov, J. Phys.: Condens. Matter 5 (1993) 1589. [11] G. Shwetha, V. Kanchana, Phy. Rev. B 86 (2012) 115209. [12] A. Aguado, A. Ayuela, J.M. López, J.A. Alonso, J. Phys. Soc. Jpn. 68 (1999) 2829. [13] Yu.M. Aleksandrov, V.N. Makhov, P.A. Rodnyi, T.I. Syreishchikiva, M.N. Yakimenko, Sov. Phys. Solid State 26 (1984) 1734.
116
G. Shwetha et al. / Journal of Solid State Chemistry 227 (2015) 110–116
[14] A.V. Voloshinovskii, P.A. Rodnyi, A.G. Dmitriev, E.N. Melchakov, S.N. Pidzyrailo, J. Appl. Spectrosc. 59 (1993) 560. [15] M. Koshimizu, N. Yahaba, R. Haruki, F. Nishikido, S. Kishimoto, K. Asai, Opt. Mater. 36 (2014) 1930. [16] A.N. Belsky, P. Chevallier, E.N. Mel'chakov, C. Pédrini, P.A. Rodnyi, A.N. Vasil'ev, Chem. Phys. Lett. 278 (1997) 369. [17] M.A. Macdonald, E.N. Mel'chakov, I.H. Munro, P.A. Rodnyi, A.S. Voloshinovsky, J. Lumin. 65 (1995) 19. [18] P.A. Rodnyi, M.A. Terekhin, E.N. Mel'chakov, J. Lumin. 47 (1991) 281. [19] J.L. Jansons, V.J. Krumins, Z.A. Rachko, J.A. Valbis, Solid State Commun. 67 (1988) 183. [20] Y. Chornodolskyy, G. Stryganyuk, S. Syrotyuk, A. Voloshinovskii, P. Rodnyi, J. Phys.: Condens. Matter 19 (2007) 476211. [21] A.S. Voloshinovskii, V.B. Mikhailik, P.A. Rodnyi, Radiat. Meas. 24 (1995) 383. [22] K. Ephraim babu, N. Murali, K. Vijay babu, P.T. Shibeshi, V. Veeraiah, Acta Phys. Pol. A 125 (2014) 1179.
[23] M. Tyagi, M. Zhuravleva, C.L. Melcher, J. Appl. Phys. 113 (2013) 203504. [24] Hayatullah, S. Naeem, G. Murtaza, R. Khenata, M.N. Khalid, Physica B 414 (2013) 91. [25] P. Blaha, K. Schwarz, P. Sorantin, S.B. Tricky, Comput. Phys. Commun. 59 (1990) 399. [26] P. Blaha, K. Schwarz, G.K.H. Madsen, D. Kvasnicka, J. Luitz, WIEN2K, An Augmented Plane Wave Plus Local orbitals Programme for Calculating Crystal Properties, Karlheinz Schwarz, Techn. Universität Wien, Austria, 2001. [27] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [28] D.J. Singh, Phys. Rev. B 82 (2010) 205102. [29] D. Koller, F. Tran, P. Blaha, Phys. Rev. B 83 (2011) 195134. [30] A.A. Mubarak, Comput. Mater.Sci. 81 (2014) 478. [31] G.L. Mcpherson, T.J. Kistenmacher, G.D. Stucky, J. Chem. Phys. 52 (1970) 815. [32] R.L. Moreira, A. Dias, J. Phys. Chem. Solids 68 (2007) 1617. [33] F. Prokert, Phys. Status Solidi B 104 (1981) 261.
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
CsMgCl3: A promising cross luminescence material G. Shwetha a, V. Kanchana a,n, G. Vaitheeswaran b a
Department of Physics, Indian Institute of Technology Hyderabad, Ordnance Factory Estate, Yeddumailaram 502 205, Telangana, India Advanced Center of Research in High Energy Materials (ACRHEM), University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad 500 046, Telangana, India
b
art ic l e i nf o
a b s t r a c t
Article history: Received 24 January 2015 Received in revised form 23 March 2015 Accepted 26 March 2015 Available online 2 April 2015
Full-potential linearized augmented plane wave method has been used to study the cross luminescence of halide scintillators. The electronic structure and optical properties of alkali halides such as CsMgCl3, CsCaCl3, and CsSrCl3 are presented. One of the major criteria for the cross luminescence to happen is the energy difference between valence band and next deeper core valence band being lesser when compared to energy gap of the compound, so that radiative electronic transition may occur between core valence band and valence band which might lead to fast scintillation. We found this criterion to be satisfied in these compounds leading to cross luminescence. The presence of high energy peaks in the absorption spectra indicates the creation of holes in the core valence band, which is an essential criterion for the occurrence of cross luminescence. The electronic structure, and optical properties studies clearly indicate CsMgCl3, CsCaCl3, and CsSrCl3 to be cross luminescence materials comparable to CsCl which is one of the well known fast scintillators. In addition, CsMgCl3 is found to be better among the studied compounds with optical isotropy though the compound is structurally anisotropic. & 2015 Elsevier Inc. All rights reserved.
Keywords: Band structure Cross luminescence Electronic structure Optical properties
1. Introduction Cross luminescence is one of the important criteria for fast scintillation. The compounds which exhibit cross luminescence or core valence luminescence (CVL) are very interesting because of their fast decay and high thermal stability. In the past few decades there were continued experimental and theoretical search progressing in various directions searching for scintillators with desired properties, but there is no unique scintillator available, with high efficiency, high yield, fast scintillation, good energy resolution, which provoke further research in exploring fast scintillators which are having very less decay time. Cross luminescence is one of the phenomena for fast scintillation with less decay time of the order of nanoseconds (ns) or less, where radiative transition is observed between halogen derived valence band and upper most cation core valence band resulting in Auger free luminescence. With the incident radiation, electrons from core valence band will be excited to conduction band leaving a hole in the core valence band, and these holes combine radiatively with electrons from the valence band, while the hole created in the valence band might combine with electrons from the conduction band. The main participant of this process is the core holes. The
n
Corresponding author. E-mail address: [email protected] (V. Kanchana).
http://dx.doi.org/10.1016/j.jssc.2015.03.024 0022-4596/& 2015 Elsevier Inc. All rights reserved.
radiative transition of electrons from valence band to core valence band will lead to the cross luminescence (also called Auger free luminescence) with short wavelength [1–12]. The general requirement for CVL is that energy difference between top of the valence band and top of core valence band (EVC ) should be less than the band gap (Eg ) of the compound. If EVC 4 Eg , then Auger transitions are dominated because of their higher probability. The incident energy should be such a way that it will excite the core level electrons, leaving a hole in core valence band. The decay time of CVL compounds is very small with the emission in the short wavelength region. The fast scintillation observed in BaF2 opened up the origin for CVL in scintillators [5,13], which is also observed in AX (A ¼alkali, X¼ halide), and ABX3 (A ¼alkali, B ¼alkali-earth, and X¼halide) [2] compounds. Moving from AB to ABX3 type of compounds, decay time and light output are reported to increase because of the increase in the distance between A–X, thereby reducing the overlap of wave functions which may result in smaller probability of CVL [2]. Though the decay time may be higher in ABX3, it might be advantageous to get a better light yield which further provokes us to explore this series. Here we are mainly interested in studying the cross luminescence in ABX3 (A¼ alkali, B ¼alkali-earth, and X-halide) type, CsMgCl3, CsCaCl3, and CsSrCl3, compounds and would like to explore the type of transition involved through computed electronic structure and optical properties calculations. These are preferred over well known binary compound CsCl because of higher yield of cross
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luminescence reported experimentally [2,8]. Cross luminescence is observed in some of the ABX3 type compounds such as RbCaF3, CsCaCl3, CsSrCl3, CsMgCl3, CsMgF3 [8,9,14–17], barium based fluorides [18], and KF, KMgF3, and KCaF3 [19]. Cross luminescence is also observed in impure systems like Rb1 x Csx Br (x o 0:2) [2], and A1 x Csx CaCl3 (A¼K, Rb) crystals [20,21] (where addition of Cs leads to CVL in these compounds) implying the importance of Cs in the context of cross luminescence. Recently, theoretical studies have been carried out on CsCaCl3 in which the authors reported the elastic constants, and optical properties of these compound [22]. The scintillation properties of doped CsCaCl3 and CsCaI3 are investigated experimentally as well as theoretically. The temperature dependence of photo-luminescence excitation, emission, and decay time was measured (experimentally), the band structure and the efficiency of the cross luminescence of these materials are also explained (theoretically) [23]. The structural, elastic, chemical bonding and optoelectronic properties of CsSrM3 (M¼F, Cl) are studied through the density functional calculations [24]. Our main focus in the present work is to address the cross luminescence of these compounds, the type of transition involved, and also to compare the scintillation characteristics of these three scintillators through the calculated electronic structure, and optical property calculations. The organization of the paper is as follows, in Section 2 we describe the computational details of our calculations, in Section 3 we discuss the results and discussion part followed by conclusions.
2. Computational details Full potential linearized augmented plane wave method (FPLAPW) has been used to perform the first principles total energy calculations as implemented in wien2K code [25,26]. FP-LAPW is an accurate method for calculating the electronic structure and optical properties of the compounds. The wave function is expanded in spherical harmonics within the atomic sphere, while outside the sphere it is expanded using plane-wave basis. The potential is given by 8P V ðrÞY lm ðr^ Þ inside sphere > < lm lm VðrÞ ¼ P ð1Þ iKr > outside sphere : VKe K
The exchange-correlation potential is calculated within the generalized gradient approximation (GGA) [27], and Tran and Blaha modified Becke–Johnson potential (TB-mBJ) [28,29]. The separation between the core states and valence states is set to 6.0 Ry. The wave functions in the interstitial region were expanded using plane waves with a cutoff of RMT K max ¼ 9 in order to achieve energy eigenvalue convergence, where K max is the plane wave cut-off, and RMT is the smallest muffin tin sphere radii. Convergence tests were carried out using higher RMT K max values, giving no significant changes in the calculated properties. The compounds of present study are insulators, and as local density approximation (LDA) and generalized gradient approximation underestimate the band gap, we have used the Tran and Blaha modified Becke–Johnson potential (TB-mBJ), which gives the band gap values close to the experiments. Throughout our study we have computed the electronic structure, optical properties using the TB-mBJ functional at the experimental lattice parameters with optimized atomic position. We have used 10 10 10 k-mesh with 84 k-points in IBZ (irreducible Brillouine zone) for k-space integration for the electronic structure calculation and 17 17 17 k-mesh for the optical property calculations. The optical properties of the materials can be well explained by using the dielectric function ϵ(ω) which consists of real and imaginary parts ϵ1(ω) and ϵ2(ω) respectively. The ϵ2(ω) can be calculated from electronic band structure, and ϵ1(ω) can be extracted from the ϵ2(ω) by using
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the Kramers–Kroning relation. From the ϵ1(ω) and ϵ2(ω) values one can calculate other optical parameters like refractive index and extinction coefficient [30]: Z ℏ2 e2 X 3 ϵ2 ðωÞ ¼ 2 2 d k〈ck =pα =vk 〉〈vk =pβ =ck 〉 π m ω c;v δðϵck ϵvk ωÞ
ð2Þ
where p is the momentum matrix element between α and β states, ck and vk are the crystal wave functions corresponding to the conduction band and valence bands with crystal wave vector k. Z 1 2 ωϵ2 ðωÞ ϵ1 ðωÞ ¼ 1 þ P dω ð3Þ π 0 ðωÞ2 ω2 where P is the principle value of integration. 2qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 31=2 ϵ21 ðωÞ þ ϵ22 ðωÞ þ ϵ1 ðωÞ 5 nðωÞ ¼ 4 2 2qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 31=2 ϵ21 ðωÞ þ ϵ22 ðωÞ ϵ1 ðωÞ 4 5 kðωÞ ¼ 2
ð4Þ
ð5Þ
Moreover to the best of our knowledge, there are no theoretical calculations available on CsMgCl3, though the other two compounds have some earlier theoretical calculations on optical properties [22– 24]. In the present work, main emphasis is laid on studying the trends in the optical properties of ABX3 compounds. Among the studied compounds CsMgCl3 [31] crystallize in hexagonal crystal structure with space group P63 =mmc with lattice parameters a¼7.269, b¼ 6.187 Å, with Cs atom at (1/3, 2/3, 0.75), Mg at (0 0 0) and Cl at (0.1556, 0.1556, 0.25). CsCaCl3 and CsSrCl3 crystallize in the cubic perovskite structure with space group Pm3m (space group number 221) with Cs atom located at (0, 0, 0), M (Ca, Sr) at (0.5, 0.5, 0.5) and Cl at (0, 0.5, 0.5) with lattice parameters 5.390 and 5.615 Å for CsCaCl3 [22,32] and CsSrCl3 [24,33] respectively. 3. Results and discussions 3.1. Electronic properties The computed energy band structure along the high symmetry points in the first Brillouine zone and total density of states of chloroperovskites are shown in Fig. 1 using the TB-mBJ functional along with CsCl band structure which is a well known cross luminescence material. These compounds are insulators with wide band gap because of large electro-negativity difference between halogen derived valence band and cation derived conduction band. From these band structure plots, we can clearly see that the energy difference between the top of the valence band and the top of the core valence band is less compared to energy gap (Eg ) of the compound, which is an essential criterion to observe cross luminescence. The width of valence band is more for CsMCl3 (M¼Mg, Ca, and Sr), when compared with CsCl as seen from Fig. 1. But for a better cross luminescence, it is desirable that the width of the upper valence band (VB) and core valence band (CVB) are appreciably represented as A and B in the band structure plots respectively. But in ABX3 the VB width is more, while in CsCl, the CVB width is little more than that in CsMgCl3. The higher value of CVB width compared to CsMCl3 (M¼Mg, Ca, and Sr) might be one of the reasons for increased CVL probability in case of AB (CsCl) compared to ABX3 which was discussed earlier. The calculated band gaps, width of the valence band (δEV ), energy difference between top valence band and top core valence band (EVC ), and EVC δEV ¼ Eg2 are given in Table 1, along with available experimental and theoretical values. The schematic diagram corresponding to these representations is shown in
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Fig. 1. Band structure of CsCl, CsMgCl3, CsCaCl3, and CsSrCl3 compounds, along with total density of states using the TB-mBJ functional, where A and B represent the width of the valence band and core valence band respectively. From the band structure plots it is clear that A increases from CsCl to CsMgCl3 and decreases in CsSrCl3, and B decreases from CsCl to CsSrCl3.
Fig. 2(a), and the cross luminescence, which is the transition of electrons from valence band to core valence band, is depicted in Fig. 2(b). From Table 1, we can clearly see that EVC o Eg , δEV 4 Eg2 in case of CsMgCl3 and, δEV o Eg2 for other compounds, and calculated band gap, δEV , EVC and Eg2 values are comparable to the available experimental and other theoretical values. We observed energy gap (Eg ) to decrease from CsMgCl3 to CsCaCl3 and then increase to CsSrCl3, and also the energy difference between the bottom of the valence band and the top of the core valence band (Eg2 ) is found to increase from CsMgCl3 to CsSrCl3 because of the decrease in the width of the valence band. The direct Γ –Γ band gap is predicted for CsMgCl3, whereas an indirect band gap along M–Γ direction is observed for CsCaCl3, and CsSrCl3 compounds. Scintillation phenomena can be observed in various ways depending on the type of transition involved, which are classified as A-type, L-type, and ALtype. In the case of A-type EVC 4 Eg , where Auger luminescence is dominating, in the case of L-type EVC o Eg , where Auger-free luminescence is observed between valence and core valence bands. ALtype of compounds are in between A-type and L-type. The compounds of present study are of L-type (EVC o Eg ), where the cross luminescence can be observed between valence band and next deeper core valence band. Density of states of these compounds are studied for further understanding and the corresponding figures are shown in Fig. 3. Conduction band is due to Mg-s,p and Cs-d states in case of CsMgCl3, whereas these states are due to M-d (M¼ Ca, Sr) and Cs-d with minor contribution from Cl-d in case of CsCaCl3 and CsSrCl3. The valence band is of Cl-p states hybridized with the Mg-s,p and Cs-p states in case of CsMgCl3, Ca-d,p and Cs-p states in the case of CsCaCl3, and Sr-d,p states in case of CsSrCl3 compounds. Valence band spread around the energy
Table 1 Calculated band gaps, in eV, of CsMgCl3, CsCaCl3, and CsSrCl3 compounds using GGA and TB-mBJ functionals along with the width of the valence band (δEV ), the energy difference between the top of valence band and the top of core valence band (EVC ), and Eg2 (EVC δEV ) in eV, along with the available theoretical and experimental values. Method
CsMgCl3
CsCaCl3
CsSrCl3
GGA TB-mBJ Exp δEV EVC Eg2
5.31 7.76 8.5d,e 2.88 (2.9d,e) 5.48 (5.6e), 2.6 (2.6d, 2.7e)
5.50 (5.35a, 5.45b) 6.89 (6.93a) 8.5d, 8.1e 2.24 (2.4d, 2.8e) 5.57 (5.9e) 3.24 (3.1d,e, 2.95b)
5.16 (5.61c) 7.52 (7.65c) 7.7e 1.74 (2.5e) 5.9 (5.8e) 4.16 (3.3e)
a
Ref. [22]. Ref. [23]. c Ref. [24]. d Ref. [9]. e Ref. [17]. b
region 0 to 2.88 eV for CsMgCl3, 0 to 2.24 eV for CsCaCl3, and 0 to 1.74 eV for CsSrCl3 compounds. The width of the VB of CsMgCl3 is little higher because of the presence of Mg-s,p states, and also Cs-p states, whereas these Cs-p states are relatively lesser in CsCaCl3 and they are absent in case of CsSrCl3. VB width of CsCl is less compared to CsMCl3 because of missing states M-d or Mg-s,p states in the valence band. The VB width being more is an essential property and it is discussed in detail while explaining optical properties in the later section. The core valence band is situated around 5.48 to 5.63 eV for CsMgCl3, 5.57 to 5.67 in case of CsCaCl3, and 5.9 to 5.95 eV for CsSrCl3, and these states are mainly due to Cs-p states for all the
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Fig. 2. Schematic diagram showing (a) Eg , EVC , δEV , Eg2 , and δECV representations and (b) cross luminescence.
Fig. 3. Density of states of CsMgCl3, CsCaCl3, and CsSrCl3 using TB-mBJ functional.
compounds. From the calculated electronic structure and density of states, we observe the width of the valence band and width of the core valence band to decrease from CsMgCl3 to CsSrCl3, and also the core valence band shifts to higher energy region as we move from Mg to Sr i.e., energy difference between the bottom of valence band and the top of core valence band increases from CsMgCl3 to CsSrCl3 which is also confirmed from optical properties calculations, discussed in the subsequent sections.
indicates the ionic bonding in these compounds with very less covalency M (Mg, Ca, and Sr) to Cl. It is to be noted that our results are in agreement with the available theoretical calculation [22,24]. As discussed earlier on moving from the AB to ABX3 the distance between the ions (A–X) increases (distance between Cs to Cl is 3.57, 3.64, 3.81, and 3.97 Å for CsCl, CsMgCl3, CsCaCl3, and CsSrCl3 respectively) resulting in reduced overlapping of wave functions indicating more ionicity in the case of ABX3 than AB (CsCl) which is also discussed in the electronic structure calculations.
3.2. Chemical bonding 3.3. Optical properties The accurate bonding nature of the material can be explained on the basis of electronic charge density plots, where charge transfer between ions indicates the ionic nature and sharing of charges between ions indicates the covalent nature. The difference electron density plots of these compounds along with CsCl are shown in Fig. 4 along (1 1 0) crystallographic plane in order to visualize the chemical bonding nature of these compounds. The distribution of electronic densities of all the atoms shows isolated spheres indicating ionic nature of these compounds. There are only very few iso-lines present from M (Mg, Ca, Sr) to Cl indicating weak covalent bond between M to Cl (which is absent in case of CsCl). The nearly isolated spherical charge distribution around the atoms is evident from the plots which
Optical properties are quite important for materials, without which there is no possibility to improve the properties of scintillators, photonic devices, etc. which substantiates its importance. The important factor for calculation of optical property is frequency dependent dielectric function. The calculated dielectric functions of all the compounds are shown in Fig. 5, which is the sum of all transitions from valence band to conduction band. From the figure, we can see that the optical response of the system shifts to lower energies moving from CsMgCl3 to CsCaCl3 and to higher energy levels to CsSrCl3 in agreement with electronic structure calculations where band gap decreases from Mg to Ca then increases to Sr. The imaginary
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Fig. 4. Difference valence electron-density plots of (a) CsCl, (b) CsMgCl3, (c) CsCaCl3, and (d) CsSrCl3 using TB-mBJ functional.
Fig. 5. Real and imaginary parts of dielectric function (ϵ1(ω), ϵ2(ω)), refractive index (n(ω)), and absorption coefficient (α(ω)) of (a) CsMgCl3, (b) CsCaCl3, and (c) CsSrCl3 using TB-mBJ functional.
part of dielectric function starts at a particular energy called threshold energy, and these threshold energies for these compounds are 7.8, 6.7, and 7.1 eV respectively for CsMgCl3, CsCaCl3, and CsSrCl3 indicating the transition of electrons from valence band to the bottom of conduction band i.e. from Cl-p states to Cs-d, M-d (M¼Ca, Sr, and Mg) states. The higher energy spectra are due to transition of electrons from core valence band i.e. Cs-p states to conduction band.
The peak in the higher energy region of dielectric function indicates the transition of electrons from core valence band to conduction band creating hole in the core valence band which plays a major role in CVL, and this hole recombines with the electrons in the valence band giving rise to the cross luminescence with fast decay. In the spectra of the imaginary part of dielectric function a peak present near to the band gap indicates the transition of electrons from valence band to
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these compounds similar to CsCl. From the calculated electronic structure and optical properties of these compounds, we can observe the CVL in these compounds which is also confirmed experimentally [17]. We predict that among CsMCl3 (M¼Mg, Ca, and Sr), CsMgCl3 is a good scintillator followed by CsCaCl3, and CsSrCl3 because of the decrease in the width of the valence band and width of the core valence band from CsMgCl3 to CsSrCl3, and also spectra are similar to CsCl, which is a well known fast scintillator.
4. Conclusions
Fig. 6. Absorption coefficient of CsMgCl3, CsCaCl3, and CsSrCl3 in comparison with CsCl.
conduction band. It is to be mentioned that one cannot see the transition of electrons from valence band to core band which might be the order of 3 eV because these are the emission characteristics which are absent in the dielectric function plots or absorption spectra plots. The energy difference between bottom of valence band and top of core valence band (Eg2 ) increase from CsMgCl3 to CsSrCl3, because of which one might expect the emission (which is due to transition of electron from valence band to core valence band) to shift to high energy regions. The width of lower energy region of the imaginary part of the dielectric function decreases from Mg to Sr because of the decrease in the width of the valence band. Similarly higher energy region of imaginary part of dielectric function shifts to higher energies as we move from the Mg to Sr, which might be due to shifting of core valence band towards higher energies as we move from Mg to Sr. The decrease in the width of valence band, width of the core valence band, and dielectric function will lead to decrease in the number of carriers available for recombination from Mg to Sr compound, and also core band shifted to higher energy region resulting in higher carrier recombination in CsMgCl3 followed by CsCaCl3 and CsSrCl3. The refractive index of all the compounds is shown in Fig. 5. The static dielectric constant values for CsMgCl3 are 1.586 and 1.584 along the x and z directions, respectively indicating the optical isotropy of these compounds though the structure is anisotropic which is a key criterion for ceramic scintillators, and these values are 1.58 and 1.53 for CsCaCl3 and CsSrCl3 compounds respectively. The absorption spectra of the compounds are shown in Fig. 5, which is having the same characteristics as that of dielectric function. We have also compared the absorption spectra of all the studied compounds with that of CsCl compound which is a fast scintillator, and is shown in Fig. 6. From the figure, we can clearly see that the absorption characteristics from 8 to 20 eV is almost similar for all compounds except for CsMgCl3, where the dip found around 12 eV for all the compounds including that CsCl is absent and we find a continuous increase in the absorption spectra which may be due to the following reasons: (i) First it is vividly seen that the width of the VB is significantly higher than other ABX3 and CsCl, which certainly would facilitate continuous absorption, as it is clear that absorption coefficient is directly proportional to the width of the band. (ii) Mg-s states present in the conduction band, and Cs-p states present in the valence band of CsMgCl3 will increase the probability of transition from valence band to conduction band compared to CsCaCl3 and CsSrCl3 where these states are absent (or almost negligible). (iii) Other possible reason might be that the energy difference between VB and CVB increases from CsMgCl3 to CsSrCl3 leading to continuous absorption in CsMgCl3. Macdonald et al. [17] reported that the presence of additional cation M (M¼Mg, Ca, and Sr) weakly influences the core excitation formation indicating the presence of cross luminescence in
Ab-initio calculations were carried out to study the electronic structure and optical properties of CsMgCl3, CsCaCl3, and CsSrCl3 which are predicted to be fast scintillators similar to their binary halides. The calculated band structure clearly reveals the investigated compounds to be insulators with the calculated band gap values in reasonable agreement with the experiments. In addition, we find these compounds to be L-type insulators, with EVC o Eg , where EVC is the energy difference between the top of the valence band and the top of the core valence band, and Eg is the band gap of the compounds, and this is an essential criterion for the CVL to be observed. The DOS plots show that the valence band contribution is mainly from Cl-p states for all the compounds, and the conduction bands are dominated by Mg-s, p and Cs-d states in the case of CsMgCl3, and M(Ca, Sr)-d and Cs-d states in other compounds. Our calculated charge density plots clearly indicate the ionic nature of these compounds. From the optical properties calculations, it is observed that the absorption spectra shifts to lower energies moving from CsMgCl3 to CsCaCl3 and to higher energy levels in CsSrCl3 because of the decrease in band gap values from Mg to Ca, followed by an increase in Sr. The peak in the higher energy region of the absorption spectra indicates the creation of holes in the core valence band which plays a major role in the CVL. Optical isotropy is observed in CsMgCl3 though the compound is structurally anisotropic. From the calculated electronic structure, and optical properties we conclude these compounds to be good scintillators, and CsMgCl3 is found to be better among the studied compounds.
Acknowledgments V.K. would like to acknowledge IIT Hyderabad for the computational resources. G.S. would like to acknowledge IIT Hyderabad for computational facility and MHRD for the fellowship. G. V. thank Center for Modelling Simulation and Design-University of Hyderabad (CMSD-UoH) for providing computational facility. References [1] P. Lecoq, A. Annenkov, A. Gektin, M. Korzhik, C. Pedrini, Inorganic Scintillators for Detector Systems Physical Principles and Crystal Engineering, SpringerVerlag, Heidelberg, 2006. [2] P.A. Rodnyi, Radiat. Meas. 33 (2001) 605. [3] P.A. Rodnyi, Radiat. Meas. 38 (2004) 343. [4] S. Kubota, J.z. Ruan(Gen), M. Itoh, S. Hashimoto, S. Sakuragi, Nucl. Instrum. Methods Phys. Res. Sect. A 289 (1990) 253. [5] A.N. Belsky, I.A. Kamenskikh, V.V. Mikhailin, A.N. Vasil'ev, J. Electron Spectrosc. Relat. Phenom. 79 (1996) 111. [6] C.W.E. van Eijk, J. Lumin. 60 (1994) 936. [7] S. Zazubovich, Radiat. Meas. 33 (2001) 699. [8] I.A. Kamenskikh, M.A. MacDonald, V.V. Mikhailin, I.H. Munro, M.A. Terekhin, Rev. Sci. Instrum. 63 (1992) 1447. [9] A.P. Shpak, O.A. Glike, A.G. Dmitriev, P.A. Rodnyi, A.S. Voloshinovskil, S.M. Pidzyrailo, J. Electron Spectrosc. Relat. Phenom. 68 (1994) 335. [10] J. Jansons, Z. Rachko, J. Valbis, J. Andriessen, P. Dorenbos, C.W.E. van EijK, N.M. Khaidukov, J. Phys.: Condens. Matter 5 (1993) 1589. [11] G. Shwetha, V. Kanchana, Phy. Rev. B 86 (2012) 115209. [12] A. Aguado, A. Ayuela, J.M. López, J.A. Alonso, J. Phys. Soc. Jpn. 68 (1999) 2829. [13] Yu.M. Aleksandrov, V.N. Makhov, P.A. Rodnyi, T.I. Syreishchikiva, M.N. Yakimenko, Sov. Phys. Solid State 26 (1984) 1734.
116
G. Shwetha et al. / Journal of Solid State Chemistry 227 (2015) 110–116
[14] A.V. Voloshinovskii, P.A. Rodnyi, A.G. Dmitriev, E.N. Melchakov, S.N. Pidzyrailo, J. Appl. Spectrosc. 59 (1993) 560. [15] M. Koshimizu, N. Yahaba, R. Haruki, F. Nishikido, S. Kishimoto, K. Asai, Opt. Mater. 36 (2014) 1930. [16] A.N. Belsky, P. Chevallier, E.N. Mel'chakov, C. Pédrini, P.A. Rodnyi, A.N. Vasil'ev, Chem. Phys. Lett. 278 (1997) 369. [17] M.A. Macdonald, E.N. Mel'chakov, I.H. Munro, P.A. Rodnyi, A.S. Voloshinovsky, J. Lumin. 65 (1995) 19. [18] P.A. Rodnyi, M.A. Terekhin, E.N. Mel'chakov, J. Lumin. 47 (1991) 281. [19] J.L. Jansons, V.J. Krumins, Z.A. Rachko, J.A. Valbis, Solid State Commun. 67 (1988) 183. [20] Y. Chornodolskyy, G. Stryganyuk, S. Syrotyuk, A. Voloshinovskii, P. Rodnyi, J. Phys.: Condens. Matter 19 (2007) 476211. [21] A.S. Voloshinovskii, V.B. Mikhailik, P.A. Rodnyi, Radiat. Meas. 24 (1995) 383. [22] K. Ephraim babu, N. Murali, K. Vijay babu, P.T. Shibeshi, V. Veeraiah, Acta Phys. Pol. A 125 (2014) 1179.
[23] M. Tyagi, M. Zhuravleva, C.L. Melcher, J. Appl. Phys. 113 (2013) 203504. [24] Hayatullah, S. Naeem, G. Murtaza, R. Khenata, M.N. Khalid, Physica B 414 (2013) 91. [25] P. Blaha, K. Schwarz, P. Sorantin, S.B. Tricky, Comput. Phys. Commun. 59 (1990) 399. [26] P. Blaha, K. Schwarz, G.K.H. Madsen, D. Kvasnicka, J. Luitz, WIEN2K, An Augmented Plane Wave Plus Local orbitals Programme for Calculating Crystal Properties, Karlheinz Schwarz, Techn. Universität Wien, Austria, 2001. [27] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [28] D.J. Singh, Phys. Rev. B 82 (2010) 205102. [29] D. Koller, F. Tran, P. Blaha, Phys. Rev. B 83 (2011) 195134. [30] A.A. Mubarak, Comput. Mater.Sci. 81 (2014) 478. [31] G.L. Mcpherson, T.J. Kistenmacher, G.D. Stucky, J. Chem. Phys. 52 (1970) 815. [32] R.L. Moreira, A. Dias, J. Phys. Chem. Solids 68 (2007) 1617. [33] F. Prokert, Phys. Status Solidi B 104 (1981) 261.