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Wet chemical synthesis of Eu2+ activated fluoro-elpasolite phosphors
Journal of Alloys and Compounds 599 (2014) 49–52
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Wet chemical synthesis of Eu2+ activated fluoro-elpasolite phosphors Vartika S. Singh b, C.P. Joshi a, P.L. Muthal c, S.M. Dhopte c, S.V. Moharil b,⇑ a
Physics Department, Shri Ramdeobaba K.N. Engineering College, Katol Road, Nagpur 440 013, India Department of Physics, R.T.M. Nagpur University, Nagpur 440010, India c National Environmental Engineering Research Institute, Nehru Marg, Nagpur, India b
a r t i c l e
i n f o
Article history: Received 21 December 2013 Received in revised form 15 February 2014 Accepted 17 February 2014 Available online 26 February 2014 Keywords: Fluoro elpasolite Fluoride Phosphors Luminescence
a b s t r a c t A simple precipitation synthesis of fluoro-elpasolites is reported for the first time. X-ray diffraction results show phase formation for K2NaAlF6 and K2LiAlF6 precipitated from aqueous solutions. Intense Eu2+ emission in both the hosts is also reported for the first time. K2NaAlF6 exhibits both line and band emissions which are attributed to Eu2+ at K+ and Na+ sites, respectively. Only line emission is observed for K2LiAlF6. The emission is very strong, intensity being comparable to that of a commercial UV lamp phosphor. The photoluminescence spectra are explained on the basis of the known energy level scheme for Eu2+. It is suggested that fluoro elpasolite phosphors obtained by the new route can be useful for making further studies on these phosphors. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Several halide based, Ce-doped elpasolites with structural formula A2BLnX6 (A = large alkali ion, B = smaller alkali ion, Ln = tivalent lathenide ion, X = I, Cl or Br ion) have shown interesting radioluminescence properties that are useful for application as scintillation detectors [1–5]. They exhibit excellent energy resolution, reasonably good photon yield and ability to discriminate between neutrons and gamma rays. This is by virtue of the fact that they exhibit cross luminescence (core–valence luminescence). Thus, scintillation output under irradiation by gamma rays consists of both the cross luminescence and Ce3+ emission, while that corresponding to neutrons contains only the latter. However, there are not many studies on fluoro-elpasolites despite the fact that the original elpasolite, K2NaAlF6, contains fluorine as anion. Fluoro-elpasolites could be more attractive as they are much less hygroscopic compared to other halo-elpasolites and amenable to crystal growth. They are also preferable due to greater resistance to color centre formation and better optical quality owing to hardness. Compared to rare earth based fluoro-elpasolites, those based on aluminum can be economic and easier to process. Apart from Ce3+, Eu2+ can also exhibit efficient luminescence in fluoride hosts. Several Eu2+ activated fluorides such as fluoroperovskites [6] LiBaF3 [7], KMgF3 [8,9], and NaMgF3 [10,11], fluorides of alkaline earth metals such as CaF2 [12], BaMgF4 [13,14], colquiriites LiCaAlF6 [15,16] and LiSrAlF6 [17] are already known to be efficient ⇑ Corresponding author. Tel.: +91 712 2042086; fax: +91 712 2249875. E-mail address: [email protected] (S.V. Moharil). http://dx.doi.org/10.1016/j.jallcom.2014.02.097 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
phosphors and find applications in many diverse fields such as solid state lasers, scintillators, X-ray imaging, dosimetry of ionizing radiations using thermally stimulated and optically stimulated luminescence (TL and OSL). However, there is no information available on luminescence of Eu2+ in fluoro-elpasolites. In fact in a review article [18] tabulating Eu2+ fluoride hosts, elpasolites are conspicuous by absence. In this paper we report new results on synthesis and luminescence of Eu2+ activated fluoro-elpasolites. 2. Experimental Fluorides are generally prepared by solid state reaction from stoichiometric mixtures of initial compounds preliminarily purified by different methods. Fluorides are sensitive to hydrolysis [19] and this factor must be considered during the process of crystal growth. Purification of fluorides is necessary to eliminate traces of water and to hinder the formation of oxyfluorides [20]. These treatments are carried out under fluorine gas or HF-gas stream at moderate temperatures which makes the solid state techniques cumbersome and difficult. In recent years, some wet chemical syntheses for preparation of OH free fluorides have been described [21–23]. The fact that the presence of chlorine ions prevents hydrolysis is exploited in these methods. We have tried the wet chemical route for the synthesis of fluoro-elpasolites. All the reagents used were of the Analytical Reagent grade. Aqueous solutions of alkali chlorides and AlCl36H2O were mixed in stoichiometric proportions. Solutions were heated to 85 °C and HF was added drop wise. Resulting precipitate was filtered, washed repeatedly with distilled water, dried and used in further experiments. e.g. K2NaAlF6 was prepared by dissolving 1.223 g KCl, 0.484 g NaCl and 2 g of AlCl36H2O, separately in minimum amount of triply distilled, deionized water. The solutions were thoroughly mixed in a PTFE beaker, stirred and heated to 85 °C and about 5 ml of HF (48%) was added drop wise till precipitation occurred. Amount of HF is rather critical. Less amount leads to incomplete precipitation and thus low yield, while excess amount again leads to lower yield due to very fine
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V.S. Singh et al. / Journal of Alloys and Compounds 599 (2014) 49–52
size/re-dissolution of the precipitate. Resulting precipitate was filtered, washed repeatedly with distilled water, dried and used in further experiments. Procedure is same for preparing Eu2+ doped samples except that desired quantity of freshly prepared europium chloride solution was added prior to precipitation. Amount of dopant refers to this value and actual concentration incorporated was not measured. As prepared Eu- doped samples did not show PL emission. A possible reason could be that the dopant is not well dispersed or it is not incorporated in divalent form. For reducing it to Eu2+ the powder was heated at 750–800 °C for 1 h in a reducing atmosphere provided by burning charcoal. X-ray diffraction patterns were recorded on a Rigaku MiniFlex II diffractometer. Photoluminescence spectra were recorded on a Hitachi F-4000 spectro-fluorimeter with spectral slit width of 1.5 nm in the spectral range 220–700 nm.
3. Results and discussion Early work on Li3AlF6–K3AlF6 has been reviewed by Grjotheim et al. [24]. K2LiAlF6 exists in several polymorphs. The various forms are summarized by Tressaud et al. [25]. Fig. 1 shows the XRD pattern of K2LiAlF6. Stick pattern of ICDD 86-2056 is also shown for the comparison. An excellent match is seen. Indexing is same as that given in ICDD 86-2056. Stick pattern of cubic phase (ICDD 86-2057) is also shown for comparison. Though some lines are common in three patterns, it is clearly seen that the XRD pattern of K2LiAlF6 prepared in this work matches very well with the stick pattern of ICDD 86-2056 and not at all with the cubic phase. K2LiAlF6 which is described as HT form (R3 m) by Graulich et al. [26] is thus formed by simple precipitation method. It is remarkable that Graulich et al. obtained cubic form for K2LiAlF6 prepared by hydrothermal method and Tressaud et al. obtained HT phase by quenching the sample from high temperature whereas we have obtained the HT form by co-precipitation at 85 °C. In the system KF–NaF–AlF3 two compounds, K2NaAlF6 (ICDD 76-0517) and K2NaAl3F12 (ICDD 44-0575) which contain all three cations (K, Na and Al), are known. An excellent matching with cubic phase K2NaAlF6 was observed for the compound prepared in this work (Fig. 2). Indexing of various planes is same as that given in ICDD 76-0517, and hence not shown in the figure. Signature of K2NaAl3F12 phase is a line around 15.64. Absence of this line in diffraction pattern for our sample clearly rules out the formation of this phase. Formation of other possible phases in NaF–AlF3 and
Fig. 1. XRD pattern of K2LiAlF6. Stick patterns of ICDD 86-2056 and 2057 are also shown for the comparison. It is seen that the pattern for the synthesized compounds matches excellently with ICDD 86-2056, but not with the cubic phase (ICDD 86-2057).
Fig. 2. XRD pattern of K2NaAlF6. Stick pattern of ICDD 76-0547 is also shown for the comparison.
KF–AlF3 systems was also ruled out by similar considerations (absence of distinguishing lines in the recorded XRD pattern). Phase pure K2NaAlF6 is thus formed using the procedure described here. Relative intensities of prominent lines differ from the standard pattern, most probably due to growth of oriented crystallites during the precipitation. Intense photoluminescence was observed in both the Eu2+ activated phosphors (Fig. 3). Intense emission in form of a broad line (or narrow band) peaking at 358 nm was obtained upon 254 nm excitation for K2NaAlF6:Eu2+ (Fig. 3, curve a). There is also another broad and relatively weak band at longer wavelength having a maximum at about 400 nm. Excitation spectra for these two emissions are different (Fig. 3, curves b and c), showing that they arise from Eu2+ at crystallographically inequivalent sites. Excitation for the line emission is in form of 2 broad overlapping bands with some structure. Maximum is at 268 nm and shoulders can be seen around 255, 302 and 312 nm. Entire excitation for the band emission appears to be shifted to right when compared to the excitation for the line emission. The shift indicates that the corresponding Eu2+ centre is experiencing stronger crystal field resulting in higher red shift. For line emission the band around 262 nm is most prominent while for the band emission this is in form of a shoulder and the band at 314 nm showing considerable structure is prominent. Compared to PL emission spectrum of K2NaAlF6:Eu2+, that for K2LiAlF6:Eu2+ (Fig. 4, curve a) is simpler in that there is only one kind of emission. There is an intense line emission around 358 nm without indication of band emission. Excitation spectrum is somewhat similar to that for K2NaAlF6:Eu2+ (Fig. 3, curve b). The prominent excitation peak is at 261 nm with shoulders around 238 and 245 nm. There is a staircase like structure on the long
Fig. 3. Eu2+ emission in K2NaAlF6 host (a) emission for 254 nm excitation, (b) excitation for 358 nm emission and (c) excitation for 400 nm emission.
V.S. Singh et al. / Journal of Alloys and Compounds 599 (2014) 49–52
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Fig. 4. Eu2+ emission in K2LiAlF6 host (a) Emission for 254 nm excitation, (b) excitation for 358 nm emission, (c) emission spectrum for the commercial lamp phosphor SrB4O7:Eu2+ (Sylvania 2052) is also shown for the comparison (curve c, excitation by 254 nm) and (d) SrB4O7:Eu2+ excitation for 368 nm emission.
wavelength side with shoulders appearing at 279, 297, 313, 325 and 341 nm. Eu2+ emission in K2LiAlF6:Eu2+ is very intense. Emission spectrum for the commercial lamp phosphor SrB4O7:Eu2+ (Sylvania 2052) is also shown for the comparison (Fig. 4, curve c, excitation by 254 nm). It is seen that if peak height is taken as indication, emission in K2LiAlF6:Eu2+ is more intense. Area wise, emission intensity of K2LiAlF6:Eu2+ is about 82% of that of SrB4O7:Eu2+. However, the emission of the latter is distributed over wider spectral range. Compared to excitation spectrum of SrB4O7:Eu2+ (Fig. 4, curve d), that of K2LiAlF6:Eu2+ is shifted to slightly longer wavelengths, yet it overlaps very well with 253.7 nm emission of Hg vapour lamp. Observed spectra can be very well explained on the basis of known energy level structure for Eu2+. Three types of emissions associated with Eu2+, viz. f–f line emission, normal d–f band emission and anomalous emission, have been understood using these diagrams. Eu2+ emission arises from the lowest band of 4f65d1 configuration to 8S7/2 state of 4f7 configuration. The excitation arises from the transition from 8S7/2 state of 4f7 configuration to the states belonging to 4f65d1 configuration. The ground state electronic configuration of Eu2+ is 4f7. This results in a 8S7/2 level for the ground state. The next f7 manifold (6Pj) lies approximately 28,000 cm 1 higher. The lowest lying 4f65d levels begin near 34,000 cm 1 and are labeled 8HJ for the free ion [27]. The 4f65d levels experience much more crystal field splitting than the 4f7 levels due to the increased spatial extent of the 5d orbitals and often are the metastable state, or the lowest excited state, when the Eu2+ ion is incorporated in a crystalline host. The effect of the cubic crystal field on the 5d electron is to split the 5d orbitals into two components t2g and eg. The isotropic part of the exchange interaction between 5d and 4f electrons results in an exchange splitting into states with total spins of S = 7/2 and 5/2. Thus for the absorption spectra of Eu2+ in the solids, the lowest energy band arises from the state described by the notation |4f6(7FJ)eg, S = 7/2> [27]. The lowest energy configuration corresponds to the situation where 7 FJ(4f6) state couples to the 5d eg orbital such that all spins are parallel. Spectral positions of these bands vary a great deal from lattice to lattice [28]. The most commonly observed emission is the dipole and spin allowed d–f-emission starting from the relaxed 4f6(7F0) 5d1 level. Due to allowed nature of the transition, d–f emission is intense. When position of the band corresponding to f–d transition lies above f–f levels, line emission corresponding to 6Pj ? 8S7/2 transitions of 4f7 configuration is then observed [29,30]. A third type of emission involving the Eu2+ ions is often characterized by a very large Stokes (5000–10,000 cm 1) shift, very broad (>4000 cm 1) emission bands, and deviating temperature
Fig. 5. Co-ordination sphere for cations in K2NaAlF6 6-coordination of Na+, 6coordination of Al3+ and 12-coordination of K+ are depicted.
Fig. 6. Co-ordination sphere for cations in K2LiAlF6 6-coordination of Li+, 6coordination of Al3+ and 12-coordination of K+ are depicted.
behavior. This ‘‘anomalous’’ emission has been attributed to auto-ionization of the 5d electron to conduction band level. The electron is localized on the cations around the hole that stays behind on Eu2+; and an impurity trapped exciton state is created. The ‘‘anomalous’’ emission is the radiative transfer of the electron back to the ground state of Eu2+. Auto-ionization can also be a cause for absence of Eu2+ luminescence. In the excitation spectrum, usually two groups of bands are observed. These can be interpreted in terms of Eu2+ energy levels split by crystal field [31–33]. In 4f65d1 configuration there are six 4f electrons which, in the absence of any interaction with the 5d electron, would arrange themselves into seven levels, 7F0 through 7F6, analogous to the ground states of Eu+3. There are a total of 49 states contained within these seven spin–orbit energy levels which will be further split by the crystal field. There is also a single 5d electron. Under the influence of a cubic crystal field, but ignoring any interaction with the 4f6 electrons, the states of the 5d electron would be split into two bands, upper t2g and the lower eg. When combined with the 49 states of the 4f6 configuration this state yields a total of 196 nearly degenerate states. If the interaction between the 4f6 electrons and the 5d electron is sufficiently strong, the states are strongly mixed and much of this degeneracy can be lifted. The 4f65d system therefore can potentially exhibit a large number of distinct optical transitions in its absorption or excitation
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spectra. In many cases, however, the interaction is weak and the composite 4f65d system retains much of the character of the uncoupled 4f6 and 5d levels. Superimposed on the lower 2eg bands are seven narrow bands, referred to as a ‘‘staircase spectrum’’, which retain the character of the seven 4f6 levels. These are discussed in details by Ryan et al. [33]. No structure is seen in the 2t2g band probably because the spin orbit splitting that occurs in 2t2g blurs out the structure. 2eg band is not affected by spin orbit splitting and staircase structure can be seen. In the case of strong interaction between the 4f6 electrons and the 5d, one would expect that very little of the characteristic 4f6 structure would remain and that the absorption spectrum would reflect only the nature of the composite 4f65d system. In the limit of weak exchange the levels resemble the 4f6 levels and are reflected as a ‘‘staircase structure’’ in the excitation spectrum. Photoluminescence spectra reported in this work can be interpreted on the basis of the general description given above and the co-ordination spheres of Eu2+ in the elpasolite hosts. In elpasolite K2NaAlF6 there are three types of cationic sites; 12 coordinated K+ sites, 6 coordinated Al3+ and 6 coordinated Na+ sites (Fig. 5). Size wise, Eu2+ is more likely to occupy alkali sites. The charge balance may be achieved by creation of cation vacancies. If the compensating vacancies are away from Eu2+, then K+ site has cubic Td symmetry [34]. If vacancies are within coordination sphere of Eu2+, site symmetry will be lowered to tetragonal (C4v) or trigonal (e.g., C3v) and one expects more splitting in the excitation spectrum. The line emission may be attributed to Eu2+ at 12 coordinated K+ site. Such emission has been observed for KMgF3:Eu2+ where Eu2+ is at cubic, 12 coordinated position. Excitation spectrum for K2NaAlF6:Eu2+ also resembles that for KMgF3:Eu2+. Band emission at 400 nm may be attributed to Eu2+ at 6 coordinated Na+ site (Oh symmetry). In hexagonal elpasolite K2LiAlF6 also there are three types of cationic sites. The K2LiAlF6 framework presents a group of three octahedra interconnected through faces, with three cations (Al3+, K+, Li+) and 12 F ions. The central octahedrally coordinated position (3a or 3b) is occupied by Al3+ ion with the local symmetry D3d. Li+ ions occupy the two external sites, in the 6c position (local symmetry C3v) [35]. This group is connected with another similar trimeric group through AlF6 octahedra. The MF12 and AlF6 share a corner [25]. Al3+ ions are on 2 types of sites denoted as Al1 and Al2, both having 6-coordination. F ions are at 1.812 nm for Al1 and at 1.797 nm for Al2. Li+ ion also has 6-coordination, with 6 F ions at average distance of 2.06 nm. K+ ions are again on 2 types of sites denoted as K1 and K2, both having 12 coordination. Depending on separation from K+, 12 F ions are divided into 3 sub-groups containing 3, 3 and 6 ions (Fig. 6). The two inequivalent K+ positions also have local C3v symmetry. Line emission in K2LiAlF6 Eu2+ can be attributed to Eu2+ at 12 coordinated K+ site in C3v symmetry, if it is assumed that the charge compensating defects are away from Eu2+. Both the cations, Li+ and Al3+ are perhaps too small to be substituted by Eu2+ and hence we see no other emission in K2LiAlF6 Eu2+. Similarity of the excitation spectra for K2LiAlF6 Eu2+, K2NaAlF6 Eu2+ (line emission) and KMgF3:Eu2+ supports this argument. Staircase structure is seen to some extent in the excitation spectrum of K2LiAlF6 Eu2+. It is not seen for K2NaAlF6 Eu2+. It may be surmised that the interaction between 5d electron and 4f6 electrons is strong for K2NaAlF6 Eu2+ which blurs the staircase structure, but considerably weak for K2LiAlF6 Eu2+ and the staircase structure is observable even though the excitation spectrum is recorded at the room temperature. Observation of line emission corresponding to 6P7/2 ? 8S7/2 transition in both the elpasolites indicate weak crystal field and hence the position of the lowest band corresponding to 4f65d1
configuration lies above the 6P7/2 levels. Weak crystal field is quite common for F anion ligand in 12 coordination. 4. Conclusions New results on the Eu2+ photoluminescence in elpasolites K2LiAlF6:Eu2+ and K2LiAlF6:Eu2+ synthesized by wet chemical routes are reported. Intense emission is observed for these phosphors. Particularly, for K2LiAlF6:Eu2+ very strong emission around 358 nm comparable in intensity to that of a commercial phosphor (Sylvania 2052) is obtained. The excitation spectra show good overlap with 253.7 nm emission of Hg- vapour lamp. This can be useful for obtaining ‘‘black light’’ lamps. Simple synthesis route and strong PL make these phosphors suitable for further studies. Availability of elapsolite powders prepared by precipitation opens up several exciting possibilities such as obtaining hotpressed, transparent ceramics for scintillation studies. References [1] Kelly M. Nicholson, Sung Gu Kang, David S. Sholl, J. Alloys Comp. 577 (2013) 463–468. [2] Jong Kyu Cheon, Sunghwan Kim, Gul Rooh, J.H. So, H.J. Kim, H. Park, Nucl. Instr. Methods A 652 (2011) 205–208. [3] Urmila Shirwadkar, Jarek Glodo, Edgar V. van Loef, Rastgo Hawrami, Sharmishtha Mukhopadhyay, Alexei Churilov, William M. Higgins, Kanai S. Shah, Nucl. Instr. Methods A 652 (2011) 268–270. [4] Gul Rooh, H.J. Kim, H. Park, Sunghwan Kim, J. Lumin. 132 (2012) 713–716. [5] Peter A. Tanner, Chang-Kui. Duan, Guohua Jia, Bing-Ming. Cheng, J. Solid State Chem. 188 (2012) 105–108. [6] C.J. Dotzler, A. Edgar, G. VIctor, M. Williams, Patent US2010/0200741, 2010. [7] Vartika S. Singh, C.P. Joshi, S.V. Moharil, J. Alloys Comp. 579 (2013) 165–168. [8] Ruinian Hua, Jicheng Yu, Huiming Jiang, Chunshan Shi, J. Alloys Comp. 432 (2007) 253–257. [9] P.D. Belsare, C.P. Joshi, S.V. Moharil, V.K. Kondawar, P.L. Muthal, S.M. Dhopte, J. Alloys Comp. 450 (2008) 468–472. [10] C. Dotzlera G, V.M. Williams, A. Edgar, Appl. Phys. Lett. 91 (2007) 121910. [11] A.S. Pradhan, J.I. Lee, J.L. Kim, J. Med. Phys. 3 (2008) 85–99. [12] Byung-Chul Hong, Katsuyasu Kawano, J. Alloys Comp. 451 (2008) 276–279. [13] S. Janssens, G.V.M. Williams, D. Clarke, J. Lumin. 134 (2013) 277–283. [14] Shinobu Fujihara, Yoko Kishiki, Toshio Kimura, J. Alloys Comp. 333 (2002) 76– 80. [15] S. Neicheva, A. Gektin, N. Shiran, K. Shimamura, E. Villora, Radiat. Meas. 42 (2007) 811–814. [16] P.D. Belsare, C.P. Joshi, S.V. Moharil, S.K. Omanwar, P.L. Muthal, S.M. Dhopte, J. Alloys Comp. 464 (2008) 296–300. [17] A. Yamaji, Y. Yokota, T. Yanagida, N. Kawaguchi, Y. Futami, Y. Fujimoto, A. Yoshikawa, J. Cryst. Growth 352 (2012) 106–109. [18] P. Dorenbos, J. Lumin. 104 (2003) 239–260. [19] N. Kodama, T. Hoshino, M. Yamaga, N. Ishizawa, K. Shimamura, T. Fukuda, J. Cryst. Growth 229 (2001) 492–496. [20] I.N. Flerov, M.V. Gorev, K.S. Aleksandrov, A. Tressaud, J. Grannec, M. Couzi, Mater. Sci. Eng. 24 (1998) 81–151. [21] A. Mech, M. Karbowiak, L. Kepinski, A. Bednarkiewicz, W. Strek, J. Alloys Comp. 380 (2004) 315–320. [22] M. Karbowiak, A. Mech, A. Bednarkiewicz, W. Strek, J. Alloys Comp. 380 (2004) 321–326. [23] P. Boutinaud, R. Mahiou, Jean-Claude Cousseins, M. Bouderbala, J. Mater. Chem. 9 (1999) 125–128. [24] K. Grjotheim, J. Lutzow Holm, M. Malinovsky, S.A. Mikhaeil, Acta Chem. Scand. 25 (1971) 1695–1702. [25] Alain Tressaud, Jacques Darriet, Pascal Lagassie, Jean Grannec, Paul Hagenmuller, Mater. Res. Bull. 19 (1984) 983–988. [26] J. Graulich, St. Drueke, D. Babel, Z. Anorg. Allg. Chem. 624 (1998) 1460–1464. [27] J.K. Lawson, S.A. Payne, Phys. Rev. B 47 (1993) 14003–14010. [28] J.W.H. van Krevel, J.W.T. van Rutten, H. Mandal, H.T. Hintzen, R. Metselaar, J. Solid State Chem. 165 (2002) 19–24. [29] V. Mary, J. Hoffman, Electrochem. Soc. 118 (1971) 933–937. [30] V. Mary, J. Hoffman, Electrochem. Soc. 119 (1972) 905–909. [31] A.M. Srivastava, H.A. Comanzo, S. Camardello, S.B. Chaney, M. Aycibin, U. Happek, J. Lumin. 129 (2009) 919–925. [32] M.J. Freiser, S. Methfesskl, F. Holtzberg, J. Appl. Phys. 39 (1968) 900–902. [33] F.M. Ryan, W. Lehmann, D.W. Feldman, J. Murphy, J. Electrochem. Soc. 121 (1974) 1475–1481. [34] B.F. Aull, H.P. Jenssen, Phys. Rev. B 34 (1986) 6647–6655. [35] L.P. Sosman, F. Yokaichiya, H.N. Bordallo, J. Magn. Magn. Mater. 321 (2009) 2210–2215.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Wet chemical synthesis of Eu2+ activated fluoro-elpasolite phosphors Vartika S. Singh b, C.P. Joshi a, P.L. Muthal c, S.M. Dhopte c, S.V. Moharil b,⇑ a
Physics Department, Shri Ramdeobaba K.N. Engineering College, Katol Road, Nagpur 440 013, India Department of Physics, R.T.M. Nagpur University, Nagpur 440010, India c National Environmental Engineering Research Institute, Nehru Marg, Nagpur, India b
a r t i c l e
i n f o
Article history: Received 21 December 2013 Received in revised form 15 February 2014 Accepted 17 February 2014 Available online 26 February 2014 Keywords: Fluoro elpasolite Fluoride Phosphors Luminescence
a b s t r a c t A simple precipitation synthesis of fluoro-elpasolites is reported for the first time. X-ray diffraction results show phase formation for K2NaAlF6 and K2LiAlF6 precipitated from aqueous solutions. Intense Eu2+ emission in both the hosts is also reported for the first time. K2NaAlF6 exhibits both line and band emissions which are attributed to Eu2+ at K+ and Na+ sites, respectively. Only line emission is observed for K2LiAlF6. The emission is very strong, intensity being comparable to that of a commercial UV lamp phosphor. The photoluminescence spectra are explained on the basis of the known energy level scheme for Eu2+. It is suggested that fluoro elpasolite phosphors obtained by the new route can be useful for making further studies on these phosphors. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Several halide based, Ce-doped elpasolites with structural formula A2BLnX6 (A = large alkali ion, B = smaller alkali ion, Ln = tivalent lathenide ion, X = I, Cl or Br ion) have shown interesting radioluminescence properties that are useful for application as scintillation detectors [1–5]. They exhibit excellent energy resolution, reasonably good photon yield and ability to discriminate between neutrons and gamma rays. This is by virtue of the fact that they exhibit cross luminescence (core–valence luminescence). Thus, scintillation output under irradiation by gamma rays consists of both the cross luminescence and Ce3+ emission, while that corresponding to neutrons contains only the latter. However, there are not many studies on fluoro-elpasolites despite the fact that the original elpasolite, K2NaAlF6, contains fluorine as anion. Fluoro-elpasolites could be more attractive as they are much less hygroscopic compared to other halo-elpasolites and amenable to crystal growth. They are also preferable due to greater resistance to color centre formation and better optical quality owing to hardness. Compared to rare earth based fluoro-elpasolites, those based on aluminum can be economic and easier to process. Apart from Ce3+, Eu2+ can also exhibit efficient luminescence in fluoride hosts. Several Eu2+ activated fluorides such as fluoroperovskites [6] LiBaF3 [7], KMgF3 [8,9], and NaMgF3 [10,11], fluorides of alkaline earth metals such as CaF2 [12], BaMgF4 [13,14], colquiriites LiCaAlF6 [15,16] and LiSrAlF6 [17] are already known to be efficient ⇑ Corresponding author. Tel.: +91 712 2042086; fax: +91 712 2249875. E-mail address: [email protected] (S.V. Moharil). http://dx.doi.org/10.1016/j.jallcom.2014.02.097 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
phosphors and find applications in many diverse fields such as solid state lasers, scintillators, X-ray imaging, dosimetry of ionizing radiations using thermally stimulated and optically stimulated luminescence (TL and OSL). However, there is no information available on luminescence of Eu2+ in fluoro-elpasolites. In fact in a review article [18] tabulating Eu2+ fluoride hosts, elpasolites are conspicuous by absence. In this paper we report new results on synthesis and luminescence of Eu2+ activated fluoro-elpasolites. 2. Experimental Fluorides are generally prepared by solid state reaction from stoichiometric mixtures of initial compounds preliminarily purified by different methods. Fluorides are sensitive to hydrolysis [19] and this factor must be considered during the process of crystal growth. Purification of fluorides is necessary to eliminate traces of water and to hinder the formation of oxyfluorides [20]. These treatments are carried out under fluorine gas or HF-gas stream at moderate temperatures which makes the solid state techniques cumbersome and difficult. In recent years, some wet chemical syntheses for preparation of OH free fluorides have been described [21–23]. The fact that the presence of chlorine ions prevents hydrolysis is exploited in these methods. We have tried the wet chemical route for the synthesis of fluoro-elpasolites. All the reagents used were of the Analytical Reagent grade. Aqueous solutions of alkali chlorides and AlCl36H2O were mixed in stoichiometric proportions. Solutions were heated to 85 °C and HF was added drop wise. Resulting precipitate was filtered, washed repeatedly with distilled water, dried and used in further experiments. e.g. K2NaAlF6 was prepared by dissolving 1.223 g KCl, 0.484 g NaCl and 2 g of AlCl36H2O, separately in minimum amount of triply distilled, deionized water. The solutions were thoroughly mixed in a PTFE beaker, stirred and heated to 85 °C and about 5 ml of HF (48%) was added drop wise till precipitation occurred. Amount of HF is rather critical. Less amount leads to incomplete precipitation and thus low yield, while excess amount again leads to lower yield due to very fine
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size/re-dissolution of the precipitate. Resulting precipitate was filtered, washed repeatedly with distilled water, dried and used in further experiments. Procedure is same for preparing Eu2+ doped samples except that desired quantity of freshly prepared europium chloride solution was added prior to precipitation. Amount of dopant refers to this value and actual concentration incorporated was not measured. As prepared Eu- doped samples did not show PL emission. A possible reason could be that the dopant is not well dispersed or it is not incorporated in divalent form. For reducing it to Eu2+ the powder was heated at 750–800 °C for 1 h in a reducing atmosphere provided by burning charcoal. X-ray diffraction patterns were recorded on a Rigaku MiniFlex II diffractometer. Photoluminescence spectra were recorded on a Hitachi F-4000 spectro-fluorimeter with spectral slit width of 1.5 nm in the spectral range 220–700 nm.
3. Results and discussion Early work on Li3AlF6–K3AlF6 has been reviewed by Grjotheim et al. [24]. K2LiAlF6 exists in several polymorphs. The various forms are summarized by Tressaud et al. [25]. Fig. 1 shows the XRD pattern of K2LiAlF6. Stick pattern of ICDD 86-2056 is also shown for the comparison. An excellent match is seen. Indexing is same as that given in ICDD 86-2056. Stick pattern of cubic phase (ICDD 86-2057) is also shown for comparison. Though some lines are common in three patterns, it is clearly seen that the XRD pattern of K2LiAlF6 prepared in this work matches very well with the stick pattern of ICDD 86-2056 and not at all with the cubic phase. K2LiAlF6 which is described as HT form (R3 m) by Graulich et al. [26] is thus formed by simple precipitation method. It is remarkable that Graulich et al. obtained cubic form for K2LiAlF6 prepared by hydrothermal method and Tressaud et al. obtained HT phase by quenching the sample from high temperature whereas we have obtained the HT form by co-precipitation at 85 °C. In the system KF–NaF–AlF3 two compounds, K2NaAlF6 (ICDD 76-0517) and K2NaAl3F12 (ICDD 44-0575) which contain all three cations (K, Na and Al), are known. An excellent matching with cubic phase K2NaAlF6 was observed for the compound prepared in this work (Fig. 2). Indexing of various planes is same as that given in ICDD 76-0517, and hence not shown in the figure. Signature of K2NaAl3F12 phase is a line around 15.64. Absence of this line in diffraction pattern for our sample clearly rules out the formation of this phase. Formation of other possible phases in NaF–AlF3 and
Fig. 1. XRD pattern of K2LiAlF6. Stick patterns of ICDD 86-2056 and 2057 are also shown for the comparison. It is seen that the pattern for the synthesized compounds matches excellently with ICDD 86-2056, but not with the cubic phase (ICDD 86-2057).
Fig. 2. XRD pattern of K2NaAlF6. Stick pattern of ICDD 76-0547 is also shown for the comparison.
KF–AlF3 systems was also ruled out by similar considerations (absence of distinguishing lines in the recorded XRD pattern). Phase pure K2NaAlF6 is thus formed using the procedure described here. Relative intensities of prominent lines differ from the standard pattern, most probably due to growth of oriented crystallites during the precipitation. Intense photoluminescence was observed in both the Eu2+ activated phosphors (Fig. 3). Intense emission in form of a broad line (or narrow band) peaking at 358 nm was obtained upon 254 nm excitation for K2NaAlF6:Eu2+ (Fig. 3, curve a). There is also another broad and relatively weak band at longer wavelength having a maximum at about 400 nm. Excitation spectra for these two emissions are different (Fig. 3, curves b and c), showing that they arise from Eu2+ at crystallographically inequivalent sites. Excitation for the line emission is in form of 2 broad overlapping bands with some structure. Maximum is at 268 nm and shoulders can be seen around 255, 302 and 312 nm. Entire excitation for the band emission appears to be shifted to right when compared to the excitation for the line emission. The shift indicates that the corresponding Eu2+ centre is experiencing stronger crystal field resulting in higher red shift. For line emission the band around 262 nm is most prominent while for the band emission this is in form of a shoulder and the band at 314 nm showing considerable structure is prominent. Compared to PL emission spectrum of K2NaAlF6:Eu2+, that for K2LiAlF6:Eu2+ (Fig. 4, curve a) is simpler in that there is only one kind of emission. There is an intense line emission around 358 nm without indication of band emission. Excitation spectrum is somewhat similar to that for K2NaAlF6:Eu2+ (Fig. 3, curve b). The prominent excitation peak is at 261 nm with shoulders around 238 and 245 nm. There is a staircase like structure on the long
Fig. 3. Eu2+ emission in K2NaAlF6 host (a) emission for 254 nm excitation, (b) excitation for 358 nm emission and (c) excitation for 400 nm emission.
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Fig. 4. Eu2+ emission in K2LiAlF6 host (a) Emission for 254 nm excitation, (b) excitation for 358 nm emission, (c) emission spectrum for the commercial lamp phosphor SrB4O7:Eu2+ (Sylvania 2052) is also shown for the comparison (curve c, excitation by 254 nm) and (d) SrB4O7:Eu2+ excitation for 368 nm emission.
wavelength side with shoulders appearing at 279, 297, 313, 325 and 341 nm. Eu2+ emission in K2LiAlF6:Eu2+ is very intense. Emission spectrum for the commercial lamp phosphor SrB4O7:Eu2+ (Sylvania 2052) is also shown for the comparison (Fig. 4, curve c, excitation by 254 nm). It is seen that if peak height is taken as indication, emission in K2LiAlF6:Eu2+ is more intense. Area wise, emission intensity of K2LiAlF6:Eu2+ is about 82% of that of SrB4O7:Eu2+. However, the emission of the latter is distributed over wider spectral range. Compared to excitation spectrum of SrB4O7:Eu2+ (Fig. 4, curve d), that of K2LiAlF6:Eu2+ is shifted to slightly longer wavelengths, yet it overlaps very well with 253.7 nm emission of Hg vapour lamp. Observed spectra can be very well explained on the basis of known energy level structure for Eu2+. Three types of emissions associated with Eu2+, viz. f–f line emission, normal d–f band emission and anomalous emission, have been understood using these diagrams. Eu2+ emission arises from the lowest band of 4f65d1 configuration to 8S7/2 state of 4f7 configuration. The excitation arises from the transition from 8S7/2 state of 4f7 configuration to the states belonging to 4f65d1 configuration. The ground state electronic configuration of Eu2+ is 4f7. This results in a 8S7/2 level for the ground state. The next f7 manifold (6Pj) lies approximately 28,000 cm 1 higher. The lowest lying 4f65d levels begin near 34,000 cm 1 and are labeled 8HJ for the free ion [27]. The 4f65d levels experience much more crystal field splitting than the 4f7 levels due to the increased spatial extent of the 5d orbitals and often are the metastable state, or the lowest excited state, when the Eu2+ ion is incorporated in a crystalline host. The effect of the cubic crystal field on the 5d electron is to split the 5d orbitals into two components t2g and eg. The isotropic part of the exchange interaction between 5d and 4f electrons results in an exchange splitting into states with total spins of S = 7/2 and 5/2. Thus for the absorption spectra of Eu2+ in the solids, the lowest energy band arises from the state described by the notation |4f6(7FJ)eg, S = 7/2> [27]. The lowest energy configuration corresponds to the situation where 7 FJ(4f6) state couples to the 5d eg orbital such that all spins are parallel. Spectral positions of these bands vary a great deal from lattice to lattice [28]. The most commonly observed emission is the dipole and spin allowed d–f-emission starting from the relaxed 4f6(7F0) 5d1 level. Due to allowed nature of the transition, d–f emission is intense. When position of the band corresponding to f–d transition lies above f–f levels, line emission corresponding to 6Pj ? 8S7/2 transitions of 4f7 configuration is then observed [29,30]. A third type of emission involving the Eu2+ ions is often characterized by a very large Stokes (5000–10,000 cm 1) shift, very broad (>4000 cm 1) emission bands, and deviating temperature
Fig. 5. Co-ordination sphere for cations in K2NaAlF6 6-coordination of Na+, 6coordination of Al3+ and 12-coordination of K+ are depicted.
Fig. 6. Co-ordination sphere for cations in K2LiAlF6 6-coordination of Li+, 6coordination of Al3+ and 12-coordination of K+ are depicted.
behavior. This ‘‘anomalous’’ emission has been attributed to auto-ionization of the 5d electron to conduction band level. The electron is localized on the cations around the hole that stays behind on Eu2+; and an impurity trapped exciton state is created. The ‘‘anomalous’’ emission is the radiative transfer of the electron back to the ground state of Eu2+. Auto-ionization can also be a cause for absence of Eu2+ luminescence. In the excitation spectrum, usually two groups of bands are observed. These can be interpreted in terms of Eu2+ energy levels split by crystal field [31–33]. In 4f65d1 configuration there are six 4f electrons which, in the absence of any interaction with the 5d electron, would arrange themselves into seven levels, 7F0 through 7F6, analogous to the ground states of Eu+3. There are a total of 49 states contained within these seven spin–orbit energy levels which will be further split by the crystal field. There is also a single 5d electron. Under the influence of a cubic crystal field, but ignoring any interaction with the 4f6 electrons, the states of the 5d electron would be split into two bands, upper t2g and the lower eg. When combined with the 49 states of the 4f6 configuration this state yields a total of 196 nearly degenerate states. If the interaction between the 4f6 electrons and the 5d electron is sufficiently strong, the states are strongly mixed and much of this degeneracy can be lifted. The 4f65d system therefore can potentially exhibit a large number of distinct optical transitions in its absorption or excitation
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spectra. In many cases, however, the interaction is weak and the composite 4f65d system retains much of the character of the uncoupled 4f6 and 5d levels. Superimposed on the lower 2eg bands are seven narrow bands, referred to as a ‘‘staircase spectrum’’, which retain the character of the seven 4f6 levels. These are discussed in details by Ryan et al. [33]. No structure is seen in the 2t2g band probably because the spin orbit splitting that occurs in 2t2g blurs out the structure. 2eg band is not affected by spin orbit splitting and staircase structure can be seen. In the case of strong interaction between the 4f6 electrons and the 5d, one would expect that very little of the characteristic 4f6 structure would remain and that the absorption spectrum would reflect only the nature of the composite 4f65d system. In the limit of weak exchange the levels resemble the 4f6 levels and are reflected as a ‘‘staircase structure’’ in the excitation spectrum. Photoluminescence spectra reported in this work can be interpreted on the basis of the general description given above and the co-ordination spheres of Eu2+ in the elpasolite hosts. In elpasolite K2NaAlF6 there are three types of cationic sites; 12 coordinated K+ sites, 6 coordinated Al3+ and 6 coordinated Na+ sites (Fig. 5). Size wise, Eu2+ is more likely to occupy alkali sites. The charge balance may be achieved by creation of cation vacancies. If the compensating vacancies are away from Eu2+, then K+ site has cubic Td symmetry [34]. If vacancies are within coordination sphere of Eu2+, site symmetry will be lowered to tetragonal (C4v) or trigonal (e.g., C3v) and one expects more splitting in the excitation spectrum. The line emission may be attributed to Eu2+ at 12 coordinated K+ site. Such emission has been observed for KMgF3:Eu2+ where Eu2+ is at cubic, 12 coordinated position. Excitation spectrum for K2NaAlF6:Eu2+ also resembles that for KMgF3:Eu2+. Band emission at 400 nm may be attributed to Eu2+ at 6 coordinated Na+ site (Oh symmetry). In hexagonal elpasolite K2LiAlF6 also there are three types of cationic sites. The K2LiAlF6 framework presents a group of three octahedra interconnected through faces, with three cations (Al3+, K+, Li+) and 12 F ions. The central octahedrally coordinated position (3a or 3b) is occupied by Al3+ ion with the local symmetry D3d. Li+ ions occupy the two external sites, in the 6c position (local symmetry C3v) [35]. This group is connected with another similar trimeric group through AlF6 octahedra. The MF12 and AlF6 share a corner [25]. Al3+ ions are on 2 types of sites denoted as Al1 and Al2, both having 6-coordination. F ions are at 1.812 nm for Al1 and at 1.797 nm for Al2. Li+ ion also has 6-coordination, with 6 F ions at average distance of 2.06 nm. K+ ions are again on 2 types of sites denoted as K1 and K2, both having 12 coordination. Depending on separation from K+, 12 F ions are divided into 3 sub-groups containing 3, 3 and 6 ions (Fig. 6). The two inequivalent K+ positions also have local C3v symmetry. Line emission in K2LiAlF6 Eu2+ can be attributed to Eu2+ at 12 coordinated K+ site in C3v symmetry, if it is assumed that the charge compensating defects are away from Eu2+. Both the cations, Li+ and Al3+ are perhaps too small to be substituted by Eu2+ and hence we see no other emission in K2LiAlF6 Eu2+. Similarity of the excitation spectra for K2LiAlF6 Eu2+, K2NaAlF6 Eu2+ (line emission) and KMgF3:Eu2+ supports this argument. Staircase structure is seen to some extent in the excitation spectrum of K2LiAlF6 Eu2+. It is not seen for K2NaAlF6 Eu2+. It may be surmised that the interaction between 5d electron and 4f6 electrons is strong for K2NaAlF6 Eu2+ which blurs the staircase structure, but considerably weak for K2LiAlF6 Eu2+ and the staircase structure is observable even though the excitation spectrum is recorded at the room temperature. Observation of line emission corresponding to 6P7/2 ? 8S7/2 transition in both the elpasolites indicate weak crystal field and hence the position of the lowest band corresponding to 4f65d1
configuration lies above the 6P7/2 levels. Weak crystal field is quite common for F anion ligand in 12 coordination. 4. Conclusions New results on the Eu2+ photoluminescence in elpasolites K2LiAlF6:Eu2+ and K2LiAlF6:Eu2+ synthesized by wet chemical routes are reported. Intense emission is observed for these phosphors. Particularly, for K2LiAlF6:Eu2+ very strong emission around 358 nm comparable in intensity to that of a commercial phosphor (Sylvania 2052) is obtained. The excitation spectra show good overlap with 253.7 nm emission of Hg- vapour lamp. This can be useful for obtaining ‘‘black light’’ lamps. Simple synthesis route and strong PL make these phosphors suitable for further studies. Availability of elapsolite powders prepared by precipitation opens up several exciting possibilities such as obtaining hotpressed, transparent ceramics for scintillation studies. References [1] Kelly M. Nicholson, Sung Gu Kang, David S. Sholl, J. Alloys Comp. 577 (2013) 463–468. [2] Jong Kyu Cheon, Sunghwan Kim, Gul Rooh, J.H. So, H.J. Kim, H. Park, Nucl. Instr. Methods A 652 (2011) 205–208. [3] Urmila Shirwadkar, Jarek Glodo, Edgar V. van Loef, Rastgo Hawrami, Sharmishtha Mukhopadhyay, Alexei Churilov, William M. Higgins, Kanai S. Shah, Nucl. Instr. Methods A 652 (2011) 268–270. [4] Gul Rooh, H.J. Kim, H. Park, Sunghwan Kim, J. Lumin. 132 (2012) 713–716. [5] Peter A. Tanner, Chang-Kui. Duan, Guohua Jia, Bing-Ming. Cheng, J. Solid State Chem. 188 (2012) 105–108. [6] C.J. Dotzler, A. Edgar, G. VIctor, M. Williams, Patent US2010/0200741, 2010. [7] Vartika S. Singh, C.P. Joshi, S.V. Moharil, J. Alloys Comp. 579 (2013) 165–168. [8] Ruinian Hua, Jicheng Yu, Huiming Jiang, Chunshan Shi, J. Alloys Comp. 432 (2007) 253–257. [9] P.D. Belsare, C.P. Joshi, S.V. Moharil, V.K. Kondawar, P.L. Muthal, S.M. Dhopte, J. Alloys Comp. 450 (2008) 468–472. [10] C. Dotzlera G, V.M. Williams, A. Edgar, Appl. Phys. Lett. 91 (2007) 121910. [11] A.S. Pradhan, J.I. Lee, J.L. Kim, J. Med. Phys. 3 (2008) 85–99. [12] Byung-Chul Hong, Katsuyasu Kawano, J. Alloys Comp. 451 (2008) 276–279. [13] S. Janssens, G.V.M. Williams, D. Clarke, J. Lumin. 134 (2013) 277–283. [14] Shinobu Fujihara, Yoko Kishiki, Toshio Kimura, J. Alloys Comp. 333 (2002) 76– 80. [15] S. Neicheva, A. Gektin, N. Shiran, K. Shimamura, E. Villora, Radiat. Meas. 42 (2007) 811–814. [16] P.D. Belsare, C.P. Joshi, S.V. Moharil, S.K. Omanwar, P.L. Muthal, S.M. Dhopte, J. Alloys Comp. 464 (2008) 296–300. [17] A. Yamaji, Y. Yokota, T. Yanagida, N. Kawaguchi, Y. Futami, Y. Fujimoto, A. Yoshikawa, J. Cryst. Growth 352 (2012) 106–109. [18] P. Dorenbos, J. Lumin. 104 (2003) 239–260. [19] N. Kodama, T. Hoshino, M. Yamaga, N. Ishizawa, K. Shimamura, T. Fukuda, J. Cryst. Growth 229 (2001) 492–496. [20] I.N. Flerov, M.V. Gorev, K.S. Aleksandrov, A. Tressaud, J. Grannec, M. Couzi, Mater. Sci. Eng. 24 (1998) 81–151. [21] A. Mech, M. Karbowiak, L. Kepinski, A. Bednarkiewicz, W. Strek, J. Alloys Comp. 380 (2004) 315–320. [22] M. Karbowiak, A. Mech, A. Bednarkiewicz, W. Strek, J. Alloys Comp. 380 (2004) 321–326. [23] P. Boutinaud, R. Mahiou, Jean-Claude Cousseins, M. Bouderbala, J. Mater. Chem. 9 (1999) 125–128. [24] K. Grjotheim, J. Lutzow Holm, M. Malinovsky, S.A. Mikhaeil, Acta Chem. Scand. 25 (1971) 1695–1702. [25] Alain Tressaud, Jacques Darriet, Pascal Lagassie, Jean Grannec, Paul Hagenmuller, Mater. Res. Bull. 19 (1984) 983–988. [26] J. Graulich, St. Drueke, D. Babel, Z. Anorg. Allg. Chem. 624 (1998) 1460–1464. [27] J.K. Lawson, S.A. Payne, Phys. Rev. B 47 (1993) 14003–14010. [28] J.W.H. van Krevel, J.W.T. van Rutten, H. Mandal, H.T. Hintzen, R. Metselaar, J. Solid State Chem. 165 (2002) 19–24. [29] V. Mary, J. Hoffman, Electrochem. Soc. 118 (1971) 933–937. [30] V. Mary, J. Hoffman, Electrochem. Soc. 119 (1972) 905–909. [31] A.M. Srivastava, H.A. Comanzo, S. Camardello, S.B. Chaney, M. Aycibin, U. Happek, J. Lumin. 129 (2009) 919–925. [32] M.J. Freiser, S. Methfesskl, F. Holtzberg, J. Appl. Phys. 39 (1968) 900–902. [33] F.M. Ryan, W. Lehmann, D.W. Feldman, J. Murphy, J. Electrochem. Soc. 121 (1974) 1475–1481. [34] B.F. Aull, H.P. Jenssen, Phys. Rev. B 34 (1986) 6647–6655. [35] L.P. Sosman, F. Yokaichiya, H.N. Bordallo, J. Magn. Magn. Mater. 321 (2009) 2210–2215.