Electron–vibrational interaction in 5d state of Eu2+ ion in LiMgBF6, Li2NaBF6 and Li3BF6:Eu2+ phosphors

Journal of Luminescence 139 (2013) 22–27

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Electron–vibrational interaction in 5d state of Eu2 þ ion in LiMgBF6, Li2NaBF6 and Li3BF6:Eu2 þ phosphors Priyanka Bhoyar, S.J. Dhoble n Department of Physics, R.T.M. Nagpur University, Nagpur 440033, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 July 2012 Received in revised form 12 January 2013 Accepted 31 January 2013 Available online 17 February 2013

Recently reported photoluminescence in Eu2 þ ions activated complex hexafluorides phosphors LiMgBF6, Li2NaBF6 and Li3BF6 was studied to estimate electron–vibrational interaction (EVI) parameters such as Huang–Rhys factor, effective phonon energy, Stokes shift and zero phonon line position. Validity of results was established by modeling the emission line which was found to be in good agreement with experimental photoluminescence spectra. & 2013 Elsevier B.V. All rights reserved.

Keywords: Electron–vibrational interaction parameters Complex hexafluoride phosphors Computer modeling and simulation Photoluminescence

1. Introduction Rare earth ion can exist in various valance states among which the trivalent state is the most prevalent, but Sm and Eu are known to exist in divalent as well as trivalent states. Luminescence of Eu has been studied extensively for both divalent as well as trivalent states and a number of applications have been reported. Blue-emitting BaMgAl10O17:Eu2 þ phosphor popularly known as BAM is used in plasma display panels (PDP) and light-emitting diodes (LEDs) [1]. SrBaSiO4:Eu2þ phosphor is a good choice as green component in white LEDs [2]. Red emitting LiCaBO3:Eu3þ phosphor is used for white light emitting diodes (WLEDs) [3]. ZnMoO4:Eu3þ is an efficient red emitting phosphor for LED application [4]. CaZrSi2O7: Eu3þ is potential red component for WLEDs [5]. Valence state of Eu ion can be identified from photoluminescence (PL) spectra depending upon their photoluminescence characteristic. Eu3 þ emission corresponds to 5D0-7Fj transition where j ¼(0–4) and Eu2 þ emission corresponds to 4f65d1-4f7 allowed electric dipole transition. Some of the compounds like CaF2:Eu phosphor for RPL dosimetry show Eu2 þ emission in blue region (428 nm) and Eu3 þ emission in red region (613 nm) of the visible spectrum [6]. Similarly Sr2B5O9Cl:Eu shows coexistence of Eu2 þ and Eu3 þ emission [7]. The lowest energy state of Eu2 þ ion is 4f7 which is half filled. The Eu2 þ absorption spectra are attributed to transition from 4f7

n

Corresponding author. E-mail address: [email protected] (S.J. Dhoble).

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.01.038

(8S7/2) ground state to various states in 4f65d1 configuration of Eu2 þ ion. 4f65d1 configuration splits into a number of levels depending upon crystal environment in host lattice. The emission spectrum is expected to occur due to transition from the lowest 5d level to ground state 4f7(8S7/2). The position of 5d level depends strongly upon crystalline environment. Thus 4f-5d absorption and 5d-4f emission depends upon the host and therefore it is important to analyze EVI and crystal field splitting parameters for 5d state of Eu2 þ in different hosts. EVI studies have been carried out for many materials in the past. Brik et al. have reported EVI in the 5d state of Ce3 þ ion in halosulphate phosphor [8]. Similarly, Bartwal et al. have reported electron–vibrational interaction in the Eu2 þ ion in 5d states of MgxSr1  xAl2O4 doped with Eu2 þ and Nd3 þ [9]. Bartwal et al. have reported EVI in 5d state of Eu2 þ in CaAl2O4 co-doped with Eu2 þ and Er3 þ [10]. Not much literature is available on hexafluoride doped with Eu. To quote a few, Shiran et al. have studied absorption, photoluminescence and radioluminescence of LiCaAlF6:Eu crystal [11]. Puppalwar et al. have recently reported photoluminescence of LiMgBF6:Eu and Li2NaBF6:Eu phosphors [12] and Li3BF6:Eu2 þ [13]. In the present work we have used experimental data on photoluminescence of LiMgBF6:Eu2 þ , Li2NaBF6: Eu2þ and Li3BF6:Eu2 þ which were recently reported by Puppalwar et al. [12,13], to estimate EVI parameters namely Huang–Rhys factor, effective phonon energy, stokes shift and zero phonon line position. LiMgBF6 has colquiriite structure, where each cation occupies a deformed octahedral site in a distorted hexagonal nearly closed packed fluorine environment. Eu2 þ occupies Mg position and thus sees a six-fold octahedral coordination of fluorine [12].

P. Bhoyar, S.J. Dhoble / Journal of Luminescence 139 (2013) 22–27

Li2NaBF6 and Li3BF6 belong to family of A2BMX6 compounds (where A and B are monovalent cations and M is a trivalent cation) having elpasolite and cryolite (A¼B) structures, respectively. Elpasolite (A2BMX6 type) and cryolite (A3MX6 type) structures are derived from perovskites by a cationic ordering between monovalent cation B and trivalent cation M in octahedral sites [14]. In simple perovskites (ABX3 type) all [BX6] octahedra are equivalent sharing a corner in three directions of space, whereas in elpasolites there are two types of cations in octahedral coordination, thus two types of octahedra [BX6] and [MX6] are arranged alternately along the three directions of space [15]. The energy level scheme of 4f65d configuration of Eu2 þ in six-fold octahedral coordination is shown in Fig. 1. Here six ligands form an octahedral complex and ligands are positioned along the axes of Cartesian coordinate system with a metal ion at the origin. The orbital lying along the axes ðdx2 y2 , dz2 Þ will be more strongly repelled than the orbitals with lobes directed between the axes (dxy, dzx, dyz). The 4f65d1 levels splits into lower (t2g) and upper (eg) level due to octahedral crystal field splitting. The lower (t2g) and upper (eg) levels are triply degenerate (dxy, dzx, dyz ) orbital and doubly degenerate ðdx2 y2 , dz2 Þ orbital, respectively [16]. Absorption takes place due to transition from ground state to different levels in t2g and eg levels and emission takes place due to transition from lowest level t2g to ground state. Eu2 þ has d1 configuration and dxy, dzx, dyz orbitals are degenerate; that is they have same energy and therefore we have three choices to fill one d1 electron. Thus, Jahn–Teller distortion is expected in d1 configuration. According to the Jahn–Teller theorem, in an electronically degenerate state distortion must occur to lower the symmetry, remove the degeneracy and lower the energy. If octahedron is distorted by the means of tetragonal compression then d1 electron is placed into dxy level which is now at lower energy than the other two dzx and dyz. Thus degeneracy is removed as required by the Jahn–Teller theorem and additional stabilization energy is gained, whereas when octahedron is distorted by elongation then both dzx and dyz levels will have energy lower than dxy and degeneracy is only partially removed and also stabilization energy gained in this case is lesser than that of tetragonal compression. Therefore, this case does not fulfill conditions of the Jahn–Teller theorem. So Jahn–Teller distortion via tetragonal compression is expected in this case [17].

2. Therotical outline The chief parameters of electron–vibrational interaction are stokes shift, Huang–Rhys factor, effective phonon energy and zero phonon line position. Emission properties of rare earth ion and rare earth ion in solid system are largely different. The emission properties largely

23

depend upon the type of host. Rare earth ion in solid system shows emission at different spectral positions than absorption. This shift in energy of emission and absorption is known as Stokes shift and it has a magnitude of (2S  1):o

DES ¼ ð2S1Þ :o

ð1Þ

The constant S is called Huang–Rhys coupling constant (also known as Pekar–Haung–Rhys factor) which indicates the average number of phonons of energy :o. It measures the strength of electron lattice coupling. S o1 indicates weak coupling regime, 1oSo5 indicates intermediate coupling regime and S 45 indicates strong coupling regime [18]. :o is the effective phonon energy. Equation proposed by Henderson which is required to calculate S and :o is sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  ffi pffiffiffiffiffiffiffiffiffiffiffiffi :o ð2Þ !ðTÞ ¼ 8 ln2 :o S coth 2kT where !ðTÞ is the full width of half maxima (FWHM) of emission curve at absolute temperature T. The values of DES and !ðTÞ are obtained from experimentally obtained excitation and emission curves, respectively. The values of S and o are obtained by solving Eqs. (1) and (2). The validity of the results obtained is checked by modeling the emission band shape and comparing it with the experimental curve. The intensity of emission band at energy E is given by the expression   eS Sm e:o=kT I¼ 1 þ S2 ð3Þ m! mþ1 and m¼

DE , :o

DE ¼ E0 E

ð4Þ

where m is the effective number of phonons involved in emission transition and E0 is the energy at zero phonon line position [19]. The point of intersection of emission and excitation curves indicates zero phonon line position. Nazarov et al. have discussed this quantum mechanical description by means of single configurational coordinate model. This model utilizes the assumption that frequency of vibrational mode is the same for both the electronic states (ground state and excited state) as well as ground state parabola and excited state parabola have same curvatures. Also anharmonicity and Jahn–Tellar interaction are neglected which may lead to complicated vibronic bands [20]. This model has been used to estimate EVI parameters in 5d state of Eu2 þ and Ce3 þ [8–10]. It has been found that the model is successful in describing EVI parameters.

3. Results and discussion 3.1. Analysis

Fig. 1. Energy level scheme of Eu2 þ in six-fold octahedral coordination.

Puppalwar et al. have recently reported the photoluminescence of Eu ion in LiMgBF6 and Li2NaBF6 at room temperature [9]. The excitation spectra of LiMgBF6:Eu consist of broad band around 254 nm (39,370 cm  1) and 394 nm (25,381 cm  1) and few small peaks at 320 nm (31,250 cm  1), 340 nm (29,412 cm  1) and 425 nm (23,529 cm  1). Emission spectra show a peak at 431 nm (23,202 cm  1) which corresponds to Eu2 þ emission and two well resolved peaks at 594 nm (16,835 cm  1) and 619 nm (16,155 cm  1) which correspond to Eu3 þ emission [12]. The excitation spectra of Li2NaBF6:Eu is similar to that of LiMgBF6:Eu. Emission spectra of Li2NaBF6:Eu show a peak at 430 nm (23,256 cm  1) which corresponds to Eu2 þ emission and

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P. Bhoyar, S.J. Dhoble / Journal of Luminescence 139 (2013) 22–27

1.0

0.8 -1

38857 cm 0.6

26053 cm-1

0.4

24163 cm-1

31679 cm-1

Normalised intensity

Normalised intensity

1.0

27537 cm-1

0.2

0.8

38857 cm-1 26053 cm-1

0.6

0.4

31679 cm-1

24163 cm-1

27537 cm-1

0.2

0.0

0.0 25000

30000

35000

25000

40000

wavenumber (cm-1)

30000

35000

40000

wavenumber (cm-1)

1.0

42324 cm-1

Normalised intensity

30377 cm-1 0.8

36023 cm-1

0.6

26746 cm-1 0.4

0.2

25000

30000

35000

40000

45000

wavenumber (cm-1) Fig. 2. Experimental excitation spectra of (a) LiMgBF6:Eu2 þ , (b) Li2NaBF6:Eu2 þ and (c) Li3BF6:Eu2 þ are shown in solid line. The dashed lines show decomposed Gaussian curve whose maxima are indicated.

two well resolved peaks at 594 nm (16,835 cm  1 ) and 617 nm (16,207 cm  1) which correspond to Eu3 þ emission. Thus the emission spectra of both the compounds show characteristic emission of both Eu2 þ and Eu3 þ . In the present work EVI analysis is carried out for Eu2 þ emission [12]. Also, Puppalwar et al. have recently reported the photoluminescence of Eu ion in Li3BF6 [13]. Photoluminescence of Li3BF6:Eu2 þ at room temperature is shown in Fig. 2. The excitation spectra consist of two well resolved broad bands around 251 nm (39,840 cm  1) and 350 nm (28,571 cm  1). Emission spectra show peaks at 391 nm (25,575 cm  1) and 425 nm (23,529 cm  1) which correspond to Eu2 þ emission [13]. Lets say the two sites be named site(1) and site(2) corresponding to emission 425 nm (23,529 cm  1) and 391 nm (25,575 cm  1) respectively. The occurrence of doublet structure of Eu in Li3BF6 can be attributed to Li þ occupying two different sites. Li3BF6 has cryolite structure; similar to Na3AlF6 it can be recognized as double perovskite (ABX3) cell. Then Li þ ion occupies two different positions namely A and B (octahedral position) which are positions with coordination numbers 6 and 12 respectively. Since this is not consistent for ions of same size and structure undergoes some distortion [21]. Considering the ionic radii it is well expected that Eu2 þ will take up the position of Mg2 þ , Na þ and Li þ in LiMgBF6, Li2NaBF6

and Li3BF6 phosphors, respectively. When Eu2 þ substitutes Mg2 þ in LiMgBF6 no charge compensation is required whereas extra charge of Eu2 þ is presumably compensated by Li vacancy and/or interstitial F ion in case of Li2NaBF6 and Li3BF6. The room temperature excitation spectra of LiMgBF6:Eu, Li2NaBF6:Eu and Li3BF6:Eu are shown in Fig. 2. The excitation spectra of Eu2 þ ion in octahedral coordination are characterized by two broad bands. The low energy and high energy bands are associated with 4f7 [8S7/2]-4f65d1[t2g] and 4f7 [8S7/2]-4f65d1[eg] optical transitions, respectively. The same nature is observed in all three compounds. The experimental excitation spectra for all the compounds are decomposed into five individual Gaussians (marked with dashed lines in Fig. 2) and the maxima of each of these Gaussian bands are marked on graphs in Fig. 2. The lowest energy Gaussian band which represents the lowest 5d state is identified for each compound. It is difficult to obtain the lowest 5d level as resolution is strongly decreased at room temperature and higher temperature; however, in case of the systems studied in this work all five peaks are distinctly seen in the excitation spectra and hence it is easier to identify the lowest 5d level. The experimental emission spectra are shown in Fig. 3. Normalized intensity scale for both emission and excitation spectra has been used to carry out comparative analysis of different compounds. Fig. 4 shows emission and excitation spectra simultaneously, and

P. Bhoyar, S.J. Dhoble / Journal of Luminescence 139 (2013) 22–27

22997 cm-1

22709 cm-1

1.0

0.8

Normalised intensity

normalised intensity

1.0

0.6

0.4

0.2

0.0

25

0.8

0.6

0.4

0.2

0.0 16000

18000

20000

wavenumber

22000

24000

18000

19000

(cm-1)

20000

21000

wavenumber

22000

23000

24000

(cm-1)

Fig. 3. The experimental emission spectra of (a) LiMgBF6:Eu2 þ , (b) Li2NaBF6:Eu2 þ and (c) Li3BF6:Eu2 þ . The dashed line indicates calculated emission band.

zero phonon line positions for all three compounds are indicated in the figure. Using the values of Stokes shift and FWHM of emission curve EVI parameters such as Huang–Rhys factor and effective phonon energy were obtained by solving Eqs. (1) and (2). These values are reported in Table 1. It is observed that the analyzed systems shows rather low values of Stokes shift, intermediate values of Huang–Rhys factor, 1oSo5 which indicates intermediate coupling regime and low effective phonon energy. For Li3BF6 EVI parameters are calculated for both the emission centers, site(1) and site(2), which correspond to emission wavelengths 23,529 cm  1 and 25,575 cm  1 respectively. The intensity at optical center site(1) has reduced probably due to the presence of OH  center which acts as luminescence killer [13]. The OH  centers are introduced into the host possibly due to the hygroscopic nature of Li3BF6. Furthermore the value of phonon energy at site(1) is about 7 times higher than that site(2). Such nonrealistic value of effective phonon energy can be due to the presence of optical center OH  . Another possible explanation could be ‘‘anomalous emission of Eu2 þ ’’. The fluoride compounds have small values of phonon frequencies and Stokes shift [22]. But here at site(1) large value of Stokes shift and FWHM are observed. Such emission of Eu2 þ ion having large Stokes shift and very broad emission band has been ascribed to auto-ionization of 5d electron to the conduction band. The electron is localized on cation around the hole that stays behind Eu2 þ and an impurity trapped exciton state is created. Anomalous emission is radiative transfer of electron back to

ground state of Eu2 þ state.[22,23]. Several cases of anomalous emission have been discussed in the past [23–25]. In order to check the validity of obtained parameters the shape of emission spectra has been modeled using Eqs. (3) and (4). One of the quantities m which represents the number of phonons is calculated using Eq. (4) and ZPL position. Modeling of emission curve can also be done by allowing ZPL to vary until the best agreement between experimental and calculated energy curves is obtained. Ideally, emission and absorption/excitation spectra show mirror symmetry and zpl position is the point of intersection of emission and absorption/excitation spectra. But for real systems such symmetry is usually not observed, due to several reasons such as the presence of defect centers, luminescence centers, anharmonicity and interaction between different vibrational modes. The zpl position obtained by this method is reasonably close to those obtained by modeling. The modeled emission curve (shown with dotted line in Fig. 3) is found to match closely with experimental curve. This establishes the validity of estimated EVI parameters.

4. Conclusions In this work the electron vibrational interaction for LiMgBF6:Eu2 þ , Li2NaBF6:Eu2 þ and Li3BF6:Eu2 þ has been analyzed for the first time. The EVI parameters mainly Huang–Rhys factor, effective phonon energy, Stokes shift and zero phonon line position obtained for these

26

P. Bhoyar, S.J. Dhoble / Journal of Luminescence 139 (2013) 22–27

1.0

0.8

Normalised intensity

Normalised intensity

1.0

0.6

23450 cm-1 0.4

excitation 0.2

0.8

23060 cm-1 0.6

excitation

0.4

0.2

emission

emission 0.0 15000

20000

25000

30000

35000

0.0 15000

40000

20000

wave number (cm-1) site(1)

1.0

25000

30000

35000

40000

wave number (cm-1) site(2) 26000 cm-1

24500 cm-1

doublet spectra of Eu2+

normalised intensity

0.8

0.6

0.4

0.2

Excitation Emission 0.0

15000

20000

25000

30000

35000

40000

45000

wave number (cm-1) Fig. 4. Zero phonon line position for (a) LiMgBF6:Eu2 þ , (b) Li2NaBF6:Eu2 þ and (c) Li3BF6:Eu2 þ .

Table 1 EVI parameters for (a) LiMgBF6:Eu2 þ , (b) Li2NaBF6:Eu2 þ and (c) Li3BF6:Eu2 þ . Effective ZPL phonon energy 1 energy (cm ) (cm  1)

Compound

Stokes FWHM of emission curve shift 1 (cm  1) (cm )

Huang– Rhys factor S

LiMgBF6:Eu2 þ Li2NaBF6:Eu2 þ Li3BF6:Eu2 þ Site (1) Site (2)

1378 1104

961 907

1.82 4.33

363 118

23450 23060

5010 1359

3050 1171

1.31 2.80

1856 254

24500 26000

compounds are reported in Table 1. Making use of estimated values of Huang–Rhys factor and effective phonon energy the shape of emission band was predicted and it was found to be in good agreement with experimentally obtained emission spectra.

Acknowledgment One of the authors SJD is grateful to Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Government of India, for providing financial assistance to carry out this work under the research project (sanctioned letter No. 2011/37P/ 10/BRNS/144).

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