Materials Research Bulletin 60 (2014) e238–e241

Influence of 6s2 lone pair electrons of Bi3+ on its preferential siteoccupancy in fluorapatite, NaCa3Bi(PO4)3F – An insight from Eu3+

luminescent probe

N. Lakshminarasimhan *, U.V. VaradarajuMaterials Science Research Centre and Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India

A R T I C L E I N F O

Article history:Received 20 June 2013Received in revised form 5 June 2014Accepted 25 August 2014Available online 27 August 2014

Keywords:A. ApatitesB. LuminescenceC. Infrared spectroscopyC. X-ray diffractionD. Crystal structure

A B S T R A C T

Eu3+ luminescence was used as a structural probe in understanding the preferential site occupancy oflone pair cation, Bi3+, in fluorapatite by comparing the photoluminescence (PL) emission spectral featureswith that of in analogous La3+ based fluorapatite. The fluorapatites, NaCa3Bi0.95Eu0.05(PO4)3Fand NaCa3La0.95Eu0.05(PO4)3F, were synthesized by conventional high temperature solid state reactionmethod and characterized by powder X-ray diffraction (XRD) and FT-IR spectroscopy. The Eu3+ PLresults revealed a difference in the emission spectral features in NaCa3Bi0.95Eu0.05(PO4)3F andNaCa3La0.95Eu0.05(PO4)3F. This difference in Eu3+ PL emission can be attributed to the difference in itssite occupancy in the studied fluorapatites.

ã 2014 Elsevier Ltd. All rights reserved.

Contents lists available at ScienceDirect

Materials Research Bulletin

journa l homepage: www.elsevier .com/ locate /matresbu

1. Introduction

Apatites with the general formula M5(TO4)3X [M = alkalineearth and rare earth elements, T = P, As, Si and Ge; X = F, Cl, OH andO] find potential applications as bone materials, laser hosts, oxideion conductors, etc. [1–4]. The crystal chemistry of apatites hasbeen well established and apatite structure is flexible toaccommodate different combinations of monovalent and multiva-lent ions (e.g. Na+, Pb2+, Ln3+, Bi3+ etc.) in the M site with minoralterations in the space group symmetry [1,5–8]. The apatite unitcell contains two types of M sites namely, M(I) (Wyckoff4f position) and M(II) (Wyckoff 6h position) with 9- and 7-foldcoordination, respectively, as shown in Fig. 1 for Ca5(PO4)3F (spacegroup P63/m). The fluorine atom is coordinated exclusively to theCa(II) atom. The oxygen atoms that are coordinated to Ca(I) and Ca(II) atoms are from ��PO4 group. The M(I) coordination environ-ment has a C3 symmetry and the M(II) site has a distortedcoordination environment with Cs symmetry. Substantial amountof structural information is available for lone pair cation, Pb2+,containing fluor- and hydroxyapatites [9–11]. However, the

* Corresponding author. Present address: Functional Materials Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi 630 006, Tamil Nadu,India. Tel.: +91 4565 241292; fax: +91 4565 227779.

E-mail addresses: nlnsimha@gmail.com, laksnarasimhan@cecri.res.in(N. Lakshminarasimhan).

http://dx.doi.org/10.1016/j.materresbull.2014.08.0370025-5408/ã 2014 Elsevier Ltd. All rights reserved.

available structural information on Bi3+ based apatites is verylimited. Among the various apatites, only a few bismuth containingapatites are known [1]. The crystal chemistry of Bi3+ basedcompounds is quite interesting. Due to the presence of the 6s2 lonepair of electrons, Bi3+ induces a structural distortion and thisresults in the ferroelectric, piezoelectric and non-linear opticalproperties as observed in various Bi3+ based compounds [12].It will be of interest to synthesize Bi3+ containing apatites wherea distorted coordination environment is already present. The6s2 lone pair of electrons of Bi3+ may have a strong role indetermining the site occupancy of Bi3+ in apatites.

Information about the local site symmetry can be obtained byluminescent structural probes [13]. Due to its hypersensitivenature, Eu3+ luminescence has been extensively utilized as astructural probe, especially in apatites [14–18]. In our earlier work,Eu3+ luminescence was used as a structural probe in elucidatingthe site occupancy of Bi3+ in phosphate oxyapatite BiCa4(PO4)3Oand also in newly synthesized silicate oxyapatites, ALa3Bi(SiO4)3Oand ALa2Bi2(SiO4)3O [A = Ca, Sr and Ba] [19,20]. Using Eu3+

luminescence we found that the crystal structures of these Bi3+

containing silicate oxyapatites are different from that of idealapatites and they could be structurally related to the previouslyreported BiCa4(VO4)3O with apatite-related structure [7]. Thepresent study aims at elucidating the preferential site occupancy ofBi3+ in fluorapatite by comparing the Eu3+ luminescence in NaCa3Bi(PO4)3F and NaCa3La(PO4)3F where La3+ without any lone pair ofelectrons is nearly similar in size to that of Bi3+. The difference in

Fig. 2. Observed (red circles), calculated (black line) and difference profile (blue linein the bottom) of (a) NaCa3Bi0.95Eu0.05(PO4)3F and (b) NaCa3La0.95Eu0.05 (PO4)3F; theunidentified reflections in (b) are marked with the symbol*. (For interpretation ofthe references to colour in this figure legend, the reader is referred to the webversion of this article.)

Fig. 1. (a) The unit cell of fluorapatite Ca5(PO4)3F. (b) The Ca(I) and (c) the Ca(II) coordination environments in Ca5(PO4)3F are also shown to reveal the difference in theirsurroundings (colour online).

N. Lakshminarasimhan, U.V. Varadaraju / Materials Research Bulletin 60 (2014) 238–241 239

Eu3+ PL emission is expected to reveal the difference in itssite occupancy which can be directly related with the preferentialsite occupancy of Bi3+ in the studied fluorapatite.

2. Experimental

2.1. Synthesis

The compounds NaCa3Bi0.95Eu0.05(PO4)3F and NaCa3La0.95Eu0.05(PO4)3F were synthesized by conventional high temperature solidstate reaction method. The reactants were high purity Bi2O3 (Cerac,99.9%), La2O3 (Indian Rare Earths, 99.9%), CaCO3 (Cerac, 99.95%),Eu2O3 (Indian Rare Earths, 99.9%), NaF (Sigma–Aldrich, 99%) andNH4H2PO4(Merck, 99%). La2O3was preheated at 1100 �C overnight inorder to remove the adsorbed moisture and carbonate impurities.Stoichiometric quantities of the reactants were thoroughly groundand heated at 300 �C for 6 h, 700 �C for 12 h, 950 �C for 24 h and finallyat 1100 �C for 24 h with intermittent grindings.

2.2. Characterization

The compounds were characterized by powder X-ray diffraction(XRD) (D8 Advance, Bruker) using Cu-Ka radiation at roomtemperature. Rietveld profile matching by Le Bail method wascarried out for both NaCa3Bi0.95Eu0.05(PO4)3F and NaCa3La0.95Eu0.05(PO4)3F using FullProf program [21]. FT-IR spectra were recordedusing KBr disc technique (IFS 66V, Bruker). Photoluminescenceexcitation and emission spectra were recorded for the powdersamples at room temperature using a spectrofluorometer (FP-8500,Jasco).

3. Results and discussion

3.1. Phase formation

The hexagonal fluorapatite phase formation ofNaCa3Bi0.95Eu0.05(PO4)3F was confirmed by comparing its powderXRD pattern with the standard pattern of NaCa3Bi(PO4)3F (ICDD #00-033-1220). The XRD pattern of NaCa3La0.95Eu0.05(PO4)3F issimilar to that of bismuth analogue. However, no standard pattern

Fig. 4. Photoluminescence excitation spectra of (a) NaCa3Bi0.95Eu0.05(PO4)3F (redline, lem. = 614 nm) and (b) NaCa3La0.95Eu0.05(PO4)3F (blue line, lem. = 617 nm). (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

240 N. Lakshminarasimhan, U.V. Varadaraju / Materials Research Bulletin 60 (2014) 238–241

is available for NaCa3La(PO4)3F. Few unidentified reflections withvery weak intensity are seen in the pattern of NaCa3La0.95Eu0.05

(PO4)3F (marked with * in Fig. 2). Fig. 2 shows the observed,calculated and difference profiles of NaCa3Bi0.95Eu0.05(PO4)3F andNaCa3La0.95Eu0.05(PO4)3F by Rietveld Le Bail fitting in the spacegroup P63/m and the corresponding Bragg positions are alsoshown. In both compounds, the reflections are matching withthe generated Bragg positions revealing their isostructural nature.The obtained hexagonal ‘a’ and ‘c’ lattice parameters ofNaCa3Bi0.95Eu0.05(PO4)3F are 9.4463 and 6.9473 Å, respectively.The lattice parameters of NaCa3La0.95Eu0.05(PO4)3F are 9.4572 and6.9509 Å. The larger lattice parameters of lanthanum compound isdue to the larger ionic radius of La3+ (1.06 Å) when compared tothat of Bi3+ (1.02 Å) [22].

3.2. FT-IR spectroscopy

Information about the nature of phosphate group and anydistortion in its geometry can be obtained from the IR spectro-scopic results. In apatite crystal structure, the oxygen atomsof ��PO4 groups are coordinated with the metal ions in M(I) andM(II) sites. Due to the presence of 6s2 lone pair of electrons on Bi3+,certain distortions in the ��PO4 group are expected. The FT-IRspectra of NaCa3Bi0.95Eu0.05(PO4)3F and NaCa3La0.95Eu0.05(PO4)3Fare shown in Fig. 3. The absorption bands at around 1050, 550 and600 cm�1 are due to the stretching and bending vibrations of ��PO4

group [23]. The bands are broader in the case of NaCa3Bi0.95Eu0.05

(PO4)3F than those of NaCa3La0.95Eu0.05(PO4)3F. A similar broaden-ing in the absorption bands of ��PO4 group has been observed as aresult of minor perturbations with increasing Pb2+ content inPb10 � xSrx(PO4)6F2 [11]. From the structure refinement, it wasobserved that lead atom incorporation in apatite structure resultsin the slight displacement of phosphorous atoms [9]. Note that thepresence of Bi3+ with a lone pair of electrons (6s2) is similar toPb2+ (5s2) making such distortions in ��PO4 group in the presentstudy. This broadening of absorption bands of ��PO4 group wasattributed to the presence of Bi3+ in oxyapatite Bi0.95Eu0.05Ca4(PO4)3O in comparison with La0.95Eu0.05Ca4(PO4)3O [19]. Distor-tions in ��SiO4 group due to the presence of lone pair cation Bi3+

were also observed with silicate oxyapatites [20]. The broaderabsorption bands of ��SiO4 group in ALa2Bi2(SiO4)3O whencompared to the bands in ALa3Bi(SiO4)3O [A = Ca, Sr and Ba] wereattributed to the higher amount of Bi3+ that is responsible for largerdistortions in the ��SiO4 group in the former series. Since Bi3+ iscoordinated to oxygen atoms from ��PO4 (or ��SiO4) group, the

Fig. 3. FT-IR spectra of (a) NaCa3Bi0.95Eu0.05(PO4)3F (red line) and (b) NaCa3-La0.95Eu0.05(PO4)3F (blue line). (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

stereochemically active 6s2 lone pair of electrons have directinfluence on the stretching and bending vibrations of thesurrounding tetrahedral groups. Thus, from the infrared spectro-scopic results, we could infer that the presence of 6s2 lone pair ofelectrons of Bi3+ in NaCa3Bi0.95Eu0.05(PO4)3F results in largerdistortion of ��PO4 group as compared to lanthanum analogue.

3.3. Photoluminescence studies

Photoluminescence excitation spectra of NaCa3Bi0.95Eu0.05

(PO4)3F and NaCa3La0.95Eu0.05(PO4)3F are shown in Fig. 4. Sharplines are observed in the spectrum of NaCa3Bi0.95Eu0.05(PO4)3F at394, 464 and �530 nm corresponding to 7F0! 5L6,7F0! 5D2 and7F0! 5D1 excitations of Eu3+, respectively. The broad bandsobserved in the UV region are due to Eu3+–O2�(F�) charge transfer(c.t.) transitions. In general, Eu3+–F� c.t. occurs in the vacuum UV(VUV) region since this c.t. process involves the transfer of electronfrom highly electronegative F� ion [24]. It is known that theabsorptions due to Eu3+ (O2� to Eu3+ c.t. transition) and Bi3+

(1S0–3P1 transition) occur in the UV region [25]. Thus, the broadband observed at 265 nm is due to Eu3+–O2� c.t. and theoverlapping band at �320 nm in NaCa3Bi0.95Eu0.05(PO4)3F couldbe due to Bi3+ absorption (1S0–3P1). A similar absorption band was

Fig. 5. Photoluminescence emission spectra of (a) NaCa3Bi0.95Eu0.05(PO4)3F (redline) and (b) NaCa3La0.95Eu0.05(PO4)3F (blue line); lexc. = 394 nm. (For interpretationof the references to colour in this figure legend, the reader is referred to the webversion of this article.)

N. Lakshminarasimhan, U.V. Varadaraju / Materials Research Bulletin 60 (2014) 238–241 241

observed in addition to Eu3+–O2� c.t. band in oxyapatiteBi0.95Eu0.05Ca4(PO4)3O [19]. In the case of NaCa3La0.95Eu0.05(PO4)3F,the Eu3+–O2� c.t. band centered at 259 nm is more intense than theother excitation lines of Eu3+ and this result is different from theobserved spectral features in bismuth analogue (Fig. 4a). This isdue to the fact that in La3+ based apatite, the O2� ion is lesspolarized resulting in the facile charge transfer to Eu3+ and ourresult agrees well with similar observations made in various La3+

based host lattices [14].The photoluminescence emission spectra of NaCa3Bi0.95Eu0.05

(PO4)3F and NaCa3La0.95Eu0.05(PO4)3F under 394 nm excitationare shown in Fig. 5. The observed emission lines in the regions575–580, 585–595 and 611–630 nm are due to 5D0! 7F0, 5D0! 7F1and 5D0! 7F2 transitions of Eu3+, respectively. The relativeemission intensities of these transitions in the two fluorapatitesare different. In NaCa3Bi0.95Eu0.05(PO4)3F, the 5D0! 7F2 emission ismore intense than 5D0! 7F0 whereas these two emissions arenearly equal in intensity in the case of NaCa3La0.95Eu0.05(PO4)3F.There is a marked difference in the position and features of5D0! 7F2 emission transition in NaCa3La0.95Eu0.05(PO4)3F ascompared to the bismuth analogue. In spite of the fact that thespectra were recorded at room temperature, we find a cleardifference in the number of lines observed for 5D0! 7F1 and5D0! 7F2 transitions. The relatively higher number of splitting inthese emission transitions in NaCa3La0.95Eu0.05(PO4)3F reveals thatEu3+ occupies the less symmetric M(II) site in this lattice and in themore symmetric M(I) site in NaCa3Bi0.95Eu0.05(PO4)3F. Further,the presence of forbidden 5D0! 7F0 transition with relativelyhigher intensity than 5D0! 7F1 transition in both host latticesindicates that the site symmetry of Eu3+ could be Cn, Cnv or Cs [26].According to the crystal structure of fluorapatite, Ca5(PO4)3F withspace group P63/m, the Ca(I) site has C3 symmetry and the Ca(II)site has Cs symmetry [5]. Thus, the otherwise forbidden 5D0! 7F0transition of Eu3+ becomes allowed when it occupies any of thesesites [13]. According to the local charge compensation modelprovided by Blasse based on Pauling’s electrostatic principle topredict the site occupancy of rare earth cations in apatites, smallercations occupy the M(II) site which is coordinated to free anionssuch as F�, O2� etc. [15]. This model was successful in predictingthe Eu3+ site occupancy from the photoluminescence results,especially from the position of Eu3+–O2� c.t. band and 5D0! 7F0emission line in M2Ln8(SiO4)6O2 [M = Mg and Ca; Ln = La, Gd and Y][15]. Based on this, Eu3+ might occupy M(II) site in bothNaCa3Bi0.95Eu0.05(PO4)3F and NaCa3La0.95Eu0.05(PO4)3F consideringits smaller size (0.95 Å) as compared to host lattice cations (Na –

1.02 Å; Ca – 1.00 Å; La – 1.06 Å and Bi – 1.02 Å) in the apatite lattice[22]. Thus, the intensity of forbidden 5D0! 7F0 transition isexpected to be high in both host lattices and the results showed thesame. However, the relative emission intensity of 5D0! 7F0transition when compared to 5D0! 7F1 and 5D0! 7F2 transitionsreveals a clear difference between the two host lattices.

Since the M(II) site cation coordinates to the free F� anionforming a shortest M��F bond, the local site symmetry changesfrom Cs into pseudo C1v as viewed only along the M��F bond [16].For C1v symmetry, the transition 5D0! 7F0 is allowed and5D0! 7F2 is forbidden. This could result in the observed enhancedemission intensity for 5D0! 7F0 transition in NaCa3La0.95Eu0.05

(PO4)3F (Fig. 5b). An intense 5D0! 7F0 emission was observed inCa5(PO4)3F and it was attributed to Eu3+ in Ca(II) site with Cs

symmetry [17]. In the present study, the observed differences inthe emission spectral features of Eu3+ in NaCa3Bi0.95Eu0.05(PO4)3Fand NaCa3La0.95Eu0.05(PO4)3F clearly indicate that Eu3+ occupiesthe M(I) site in the former and M(II) site in the latter fluorapatite.This difference in Eu3+ site occupancy in fluorapatites can beattributed to the preferential occupancy of lone pair cation Bi3+ inM(II) site where large space is available to accommodate its 6s2

lone pair of electrons. Such preferential site occupancy of lone paircation (Pb2+) has been reported in calcium and strontium hydroxy-and fluorapatites based on the structure refinement results [9–11].The preferential site occupancy of Bi3+ in apatite-related BiCa4(PO4)3O was clearly evidenced in our earlier work using Eu3+

luminescence [19]. Thus, the 6s2 lone pair of electrons of Bi3+ leadsto the difference in the local site occupancy of Eu3+ in apatitecrystal structures. Our earlier results in oxyapatites and presentresults in fluorapatites reveal that the crystal chemistry of Bi3+

based apatites are of fundamental interest and open up newavenue in the structural chemistry of these apatites.

4. Conclusions

The application of Eu3+ luminescence as a structural probe inunderstanding the preferential site occupancy of Bi3+ in fluorapa-tite NaCa3Bi0.95Eu0.05(PO4)3F was made. From the difference in theemission spectral features of Eu3+, it has been found that Eu3+

occupies the less symmetric M(II) site in NaCa3La0.95Eu0.05(PO4)3Fand in M(I) site in NaCa3Bi0.95Eu0.05(PO4)3F due to the preferentialoccupancy of Bi3+ in M(II) site. The 6s2 lone pair of electrons plays avital role in determining the preferential site occupancy of Bi3+.Though our attempt gives a preliminary insight into the differencein the site occupancy of lone pair cations in apatites, a morecareful structural work is sought to clearly understand the crystalchemistry of Bi3+ based apatites. Such studies will also be useful inthe design and synthesis of new Bi3+ based apatites with potentialapplications in various fields.

Acknowledgements

The author N. L. acknowledges the Council of Scientific andIndustrial Research, New Delhi for a research fellowship. Thestructural models were drawn using VESTA program, version 3[27]. CSIR-CECRI is acknowledged for powder XRD and PL facilities.

References

[1] T.J. White, D. Zhi Li, Acta Cryst. B 59 (2003) 1–16.[2] L.M. Rodríguez-Lorenzo, J.N. Hart, K.A. Gross, J. Phys. Chem. B 107 (2003)

8316–8320.[3] T.S. Davis, E.R. Kreidler, J.A. Parodi, T.F. Soules, J. Lumin. 4 (1971) 48–62.[4] J.E.H. Sansom, D. Richings, P.R. Slater, Solid State Ion 139 (2001) 205–210.[5] K. Sudarsanan, P.E. Mackie, R.A. Young, Mater. Res. Bull. 7 (1972) 1331–1338.[6] J.R. Tolchard, J.E.H. Sansom, P.R. Slater, M.S. Islam, J. Solid State Electrochem. 8

(2004) 668–673.[7] J. Huang, A.W. Sleight, J. Solid State Chem. 104 (1993) 52–58.[8] G. Buvaneswari, U.V. Varadaraju, J. Solid State Chem. 149 (2000) 133–136.[9] A. Bigi, A. Ripamonti, S. Brückner, M. Gazzano, N. Roveri, S.A. Thomas, Acta

Cryst. B 45 (1989) 247–251.[10] B. Badraoui, A. Bigi, M. Debbabi, M. Gazzano, N. Roveri, R. Thouvenot, Eur. J.

Inorg. Chem. 2002 (7) (2002) 1864–1870.[11] B. Badraui, A. Aissa, A. Bigi, M. Debbabi, M. Gassano, J. Solid State Chem. 179

(2006) 3065–3072.[12] M.W. Stoltzfus, P.M. Woodward, R. Seshadri, J.-H. Klepeis, B. Bursten, Inorg.

Chem. 46 (2007) 3839–3850.[13] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994.[14] G. Blasse, Chem. Phys. 45 (1966) 2356–2360.[15] G. Blasse, J. Solid Chem. 14 (1975) 181–184.[16] B.Piriou,D.Fahmi,J.Dexpert-Ghys,A.Taitai,J.L.Lacout, J.Lumin.39(1987)97–103.[17] R. Jagannathan, M. Kottaisamy, J. Phys. Cond. Matter 7 (1995) 8453–8466.[18] L. Boyer, B. Piriou, J. Carpena, J.L. Lacout, J. Alloys Compd. 311 (2000) 143–152.[19] N. Lakshminarasimhan, U.V. Varadaraju, J. Solid State Chem. 177 (2004)

3536–3544.[20] N. Lakshminarasimhan, U.V. Varadaraju, J. Solid State Chem. 178 (2005)

3284–3292.[21] T. Roisnel, J. Rodríguez-Carvajal, J. Mater. Sci. Forum 378 (2001) 118–123.[22] R.D. Shannon, C.T. Prewitt, Acta Cryst. B 25 (1969) 925–945.[23] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination

Compounds, Part A, 5th ed., John Wiley & Sons, Inc., 1997.[24] J.C. Krupa, A. Mayolet, P. Martin, in: A.P. Roy (Ed.), Luminescence and Electronic

Structure Using Synchrotron Radiation, Spectroscopy-Perspectives andFrontiers Narosa Publishing House, New Delhi, 1997.

[25] G. Blasse, Solid Chem. 4 (1972) 52–54.[26] G. Blasse, A. Bril, Philips Res. Rep. 21 (1966) 368–378.[27] K. Momma, F. Izumi, J. Appl. Crystallogr. 44 (2011) 1272–1276.