Pure, Pr3+, and Yb3+ doped Ca-fluoroapatite (Ca-FAP) crystals were grown by the Czochralski method. The effective distribution coefficient keff for Yb3+ was 0.5. For Pr3+ a very high keff of 1.4 was obtained. Values for keff are discussed in terms of an elastic model accounting for the strain energy originating from the difference in the size of Ln3+ ions. The Ln3+ concentrations were measured by absorbance spectroscopy, by inductively coupled plasma optical emission spectroscopy, and by electron microprobe analyse

Caprez, A.

Hulliger, J.

Mikhail, P.

Schwecke, C.

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3210HelpCommentsWelcomeJournal ofMATERIALS RESEARCHhttps://doi.org/10.1557/JMDownloaded from https:/wCzochralski growth of Yb3+ and Pr3+ doped Ca-fluoroapatiteA. Caprez, P. Mikhail, C. Schwecke, and J. HulligerDepartment of Chemistry and Biochemistry, University of Berne, CH-3012 Berne, Switzerland(Received 19 March 1997; accepted 12 August 1997)Pure, Pr3+, and Yb3+ doped Ca-fluoroapatite (Ca-FAP) crystals were grown by theCzochralski method. The effective distribution coefficient keff for Yb3+ was 0.5. For Pr3+a very high keff of 1.4 was obtained. Values for keff are discussed in terms of an elasticmodel accounting for the strain energy originating from the difference in the size of Ln3+ions. The Ln3+ concentrations were measured by absorbance spectroscopy, by inductivelycoupled plasma optical emission spectroscopy, and by electron microprobe analyses.The apatite structural family [A10(MO4)6X2],represented by a large class of natural and syntheticcompounds, crystallizes in a few different, but re-lated, hexagonal and pseudohexagonal structures.1,2Ca5(PO4)3F (Ca-FAP), apatite, belongs to the hexagonalspace group P63ym with Z ­ 2.3 The ten Ca2+ ionsare distributed over two crystallographically differentsites: 4 Ca2+ are at site I with site symmetry 3 and9-fold coordination by O2–; 6 Ca2+ are at site II with 6-fold O2– and one F– coordination.4 Lanthanide dopantspredominantly enter the Ca(II) site which provides alow energy charge compensation mechanism5: The Ln3+ion substitutes for Ca2+(II), whose nearest neighbor F–ion is replaced by an O2–.6 Because only one F– ion isavailable for bonding to every three Ca(II) ions, at mostonly two Ln3+ ions for every ten Ca2+ in the unit cellcan enter the structure without creating an additionalcharge compensation mechanism.5 In this study wefound that co-doping with, e.g., Na+ had no influenceon the effective distribution coefficient keff of Yb3+.Yb3+ doped Ca-FAP and Sr-FAP are currently ofinterest because of efficient solid state laser proper-ties when pumped by InGaAs diode sources.7–9 Re-cently, a diode-pumped, Q-switched Sr–FAP : Yb3+ laser(1047 nm) has been demonstrated.9In this work we have explored Ca-FAP crystalgrowth for two reasons: (i) determination of the effectivedistribution coefficients keff of Yb3+ and Pr3+, and (ii) thepotential of the Ca-FAP host-lattice as a new material forPr3+ hyperfine pumped optical hole burning (H-OHB).An important parameter for the application of OHB inoptical memories is the homogeneous linewidth of theholes, which is influenced by flip-flop processes of thenuclear magnetic moments of the host material.10 In aprevious study we have discussed possible low nuclearspin density host materials for Pr3+ of Eu3+ H-OHB.10However, a major drawback of selected compounds isthat they showed only low doping levels and someeffects of disorder in the nearest neighborhood of Ln3+due to a heterovalent mechanism of substitution. Al-J. Mater. Res., Vol. 12, No. 12, Dec 1997R.1997.0419ww.cambridge.org/core. University of Basel Library, on 30 May 2017 at 16:3though Ca-FAP will provide nuclear spins at P and Fsites, it nevertheless bears the possibility of a particularmechanism for the charge compensation of Ln3+. Themechanism discussed above implies that a Pr3+ ion inthe Ca-FAP lattice would be surrounded by 7 O2– ionswhose only isotope with nonvanishing spin (O17, I ­5y2, m(N) ­ 21.9 mN) has a very low natural abundance(0.04%).11 In the first coordination sphere of the Pr3+ ionno nuclear magnetic moments could hence provide an ef-ficient mechanism of broadening the homogeneous line.In the following we describe the Czochralski growthof Ca-FAP doped either by Yb3+ or Pr3+ and the de-termination of the effective distribution coefficients keff .Obtained values are discussed in terms of an elasticmodel accounting for the strain energy originating fromthe difference in the size of Ln3+ ions.For Czochralski pulling we used the equipmentas described recently.12 Crystals were pulled froma 22 (Ø) 3 25 mm Ir crucible under a constant Arflow of 50 mlymin. CaCO3 (Aldrich, purity: 99.95%),CaHPO4 (Aldrich, purity: 99%), and CaF2 (Aldrich,purity: 99.9%) were used as starting materials. In afirst step stoichiometric amounts of the raw materialswere shaken in a polyethylene bottle and sintered ina platinum crucible at 1100 –C for 60 h in a N2 flowof about 100 mlymin. Crystals grown from a chargeprepared in this way were opaque and consisted of atleast two phases. A possible explanation is given by theReaction (1):3CaHPO4ssd 1 CaCO3ssd 1 CaF2ssd! Ca5sPO4d3Fssd 1 H2Osgd 1 CO2sgd 1 HFsgd .(1)Formation of H2O yielded a certain amount ofhydroxyapatite in the starting powder, which led toinclusions during crystal growth. Precursor material washence prepared in two steps: (i) water-free Ca3(PO4)2was obtained first according to Reaction (2) in air at a 1997 Materials Research Society5:34, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms.Communicationshttps://doi.Downloadetemperature of 1000 –C (60–80 h).2CaHPO4ssd 1 CaCO3ssd ! Ca3sPO4d2ssd1 H2Osgd 1 CO2sgd . (2)In a second step (ii), Ca3(PO4)2 obtained fromReaction (2) and CaF2 were reacted. The latter was driedunder a flow of 5% HFy95% N2 gas (20–50 mlymin) at1000 –C for 24 h. To prevent water contamination, theCa3(PO4)2 powder was taken from the furnace directlyafter finishing Reaction (2). Hot Ca3(PO4)2 was mixedwith the CaF2 in a preheated mortar. After transferringinto a hot Ir crucible, it was heated up in 2 h to 1700 –Cin order to complete Reaction (3):3Ca3sPO4d2ssd 1 CaF2ssd ! 2Ca5sPO4d3Fssd . (3)For Reactions (2) and (3) stoichiometric amounts ofthe starting materials were used. The melt (11–13 g) wassuperheated for 1 h 50 –C above the melting point. Spon-taneous nucleation was initiated on a tapered Ir wire andcrystals became single grain after , 2 mm of pulling.Spontaneous nucleation and seed selection producedcrystals growing along the c-axis. For a pulling rate of1.2 mmyh and a rotation of 45 rpm pale blue-greenishcrystals of 30 mm in length and 10 mm in diameterwere obtained. The boules showed opaque inclusions inthe core extending along the c-axis. These defects maybe caused by a lower curvature of the convex growthinterface at larger diameters. The trapping of gas bubblesin the core of Ca-FAP crystals is a known imperfection.4Slices with sufficient optical quality for spectroscopicmeasurements could be cut from the obtained crystals.During growth experiments (24 h) vapor losses,which may lead to the described imperfections in thecore of the crystal, were observed. Electron microprobe(EMP) and powder diffraction analysis of the losses,which were collected from the wall of the Czochralskiapparatus, showed that substantial amounts of CaF2are lost during growth. In order to minimize theselosses, an insulation lid (mullite) was placed at about50 mm above the top of the crucible. Additionally,this particular insulation reduced the axial temperaturegradient down to , 20–30 –Cycm. This was importantto avoid cracking during the cooling process. A furtherprecaution which proved to be a determinant in avoidingcracks was a slow cooling rate of 25 –Cyh.For doping experiments Pr6O11 (Fluka, purity:99.9%) or Yb2O3 (Fluka, purity: 99.9%) was addedto the melt. Other growth parameters were the same asfor pure Ca-FAP.Small pieces of a crystal grown at a Pr3+ concen-tration in the melt of c0 ­ 0.07 mol% (according to allatoms of Ca-FAP) were dissolved in HNO3 (0.7 molar)and analyzed with the inductively coupled plasma opticalJ. Mater. Res., Vol. 12org/10.1557/JMR.1997.0419d from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 16:3emission spectroscopy (ICP). The ICP technique led tocPr ­ 0.1 mol% in the crystal. Solutions of PrCl3 ? 6H2O(Janssen Chemica, purity: 99.9%) in HNO3 (0.7 molar)were used for calibration. An EMP analysis of a boulegrown at c0 ­ 0.07 mol% Pr3+ yielded a doping level ofcPr ­ 0.14 mol%. In the case of the EMP measurementPrAlO3 served as a reference for Pr3+. The difference inthe Pr3+ concentrations may originate from the referencematerial used in the EMP analysis. PrAlO3 containsdifferent elements when compared to Ca-FAP : Pr3+ andtherefore, the software of the EMP calculated a differentcorrection factor for the reabsorption of the characteristicx-ray of the Pr3+. We assume that keff ­ cPryc0 ­ 1.4obtained by the ICP method is more accurate.The concentration for Yb3+ doping was determinedspectroscopically. Based on the cross sections s for thelines at 905 and 982 nm originating from the 2F7/2 !2F5/2 transition of Yb3+ and the measured absorbanceat the maximums of these lines, the doping level wascalculated. A sample grown at c0 ­ 0.28 mol% Yb3+and 0.5 mol% Na+ in the melt was cut with flat andparallel faces perpendicular to the c-axis for absorp-tion measurements with unpolarized light at room tem-perature (Fig. 1). With s905nm ­ 2.6 ? 10–20 cm2 ands982nm ­ 5.3 ? 10–20 cm2 for an electric field polarizationperpendicular to the c-axis (E ' c),13 we yielded cYb ­0.13 mol%, which corresponds to keff ­ 0.5.Qualitatively, the influence of cations co-doping hasbeen observed earlier.14 The value of keff for Yb3+ seemsnot to be significantly influenced by adding Na+ to themelt. This is also consistent with the fact that we couldnot detect Na+ by the proton induced gamma emission(PIGME)15 method in a crystal grown at melt con-centrations c0 ­ 0.6 mol% Na+ and 0.3 mol% Yb3+.The detection limit was 0.03 mol% Na+ and a basalt(BHVO-1)16 served as Na reference.FIG. 1. Optical absorption spectrum of Yb3+ doped Ca-FAP (cYb ­0.13 mol%) at room temperature. The thickness of the sample was3.24 mm and the spectrum was measured with unpolarized lightpropagating along the c-axis., No. 12, Dec 1997 32115:34, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms.Communicationshttps://doi.DownloadeA possible explanation for a large difference in thekeff for Yb3+ and Pr3+ could be due to the different ionicradii of the lanthanides. The electronic structures arevery similar and the oxidation states are the same. Theonly variable affecting keff is most likely the ionic radii(0.98 ˚A for Yb3+ and 1.14 ˚A for Pr3+, for a coordinationnumber of 8).17 Brice18 regards the lattice as an elasticcontinuum and calculates the strain energy originatingfrom the radius misfit between the substituted and thedoping ion. The effective distribution coefficient keffdepends on the misfit in the radius and on elasticproperties:lnskeffd ­ lnsk0d 24pENART3h 12rAsrj 2 rAd2 113srj 2 rAd3i, (4)where k0 is the effective distribution coefficient for adopant with a radius misfit of zero, E is the averagedYoung’s modulus, NA is the Avogadro number, T is themelting temperature of the material and rA and rj arethe effective ionic radii of the substituted ion (index A)and the dopant (index j), respectively. Relation (4) wasapplied to estimate the ratio between the distributioncoefficients of Yb3+ and Pr3+ (kYbeffykPreff ­ 0.58) usingan effective ionic radius of 1.12 ˚A (for a coordina-tion number of 8)17 for Ca2+. Young’s moduli for thedirection parallel¡Ek¢and perpendicular sE'd to the6-fold symmetry axis were calculated from the elasticconstants of hydroxy-apatite.19 The average value E ­13 ? sEk 1 2 ? E'd ­ 120 GPa was used for Eq. (4).Payne et al.13 give an average value of E ­ 119 GPa forCa5(PO4)3F. The measured ratio of the two distributioncoefficients is 0.36, being smaller than the calculatedvalue (0.58). This may be caused by an uncertainty in theionic radii, which after Brice18 can introduce an error of20%. However, the model predicts a significantly largerkeff for Pr3+.Optical absorption spectra for a Pr3+ doped crys-tal (cPr ­ 0.1 mol%) at room temperature to 11 Kmeasured with unpolarized light propagating along thec-axis indicated that the linewidth of the 3H4 ! 1D2transition exhibits only a low temperature dependence(Fig. 2). The half-width is decreasing only by a factor1.4 (for the lowest crystal field level), which wouldmean that the broadening is mainly inhomogeneous.The room temperature absorption spectrum of an Yb3+doped sample (cYb ­ 0.13 mol %) exhibits a half-widthof 2.49 nm for the line at 905 nm sE'cd (Fig. 1). Thisis in agreement with the values of 2.4 nm (for Ekc)obtained earlier.20,21Preliminary experiments22 on Ca-FAP : Pr3+ revealedno H-OHB processes. Maybe this is due to (i) the sitesymmetry m of the Pr3+ ion, which does not allow thetransition in the lowest crystal field level,10 or (ii) the3212 J. Mater. Res., Vol. 12org/10.1557/JMR.1997.0419d from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 16:3FIG. 2. Optical absorption spectra of Pr3+ doped Ca-FAP (cPr ­0.1 mol%) at 11 to 300 K. The thickness of the sample was 3.1 mm.All spectra were measured with unpolarized light propagating alongthe c-axis.presence of a broad homogeneous line leading to anoverlapping of the holes with the other hyperfine levelsof the electronic ground state.ACKNOWLEDGMENTSWe thank F. Graf and B. Plagemann of the groupof Professor U. Wild at the ETH Zurich for helpfuldiscussions and preliminary H-OHB experiments. Weare indebted to Dr. L. Diamond for an introduction toelectron microprobe analysis. We also thank Ch. Widmerand K. Noll of the group of Professor H. Ga¨ggelerand Professor U. Kra¨henbu¨hl (University of Berne) forproviding an ICP analyzer and performing the PIGMEexperiments and the group of Professor H. U. Gu¨del(University of Berne) for using spectroscopic equipment.Both crystal growth and the electron microprobe at theInstitute of Mineralogy and Petrography of the Univer-sity of Berne were supported by the Swiss NationalScience Foundation (projects 2000-043116.95/1 and 21-26579.89, respectively). Further support was providedby the Wander Foundation (University of Berne).REFERENCES1. E. R. Kreidler and F.A. Hummel, Am. Mineral, 55, 170 (1970).2. A. G. Cockbain, Mineral. Mag. 36, 654 (1968).3. W. L. Bragg, Atomic Structure of Minerals (Cornell Univ. Press,Ithaca, NY, 1937).4. G. B. Loutts and B.H. T. Chai, SPIE-The International Society forOptical Engineering 1863, 31 (1993).5. L. D. DeLoach, S. A. Payne, W. L. Kway, J. B. Tassano, S.N.Dixit, and W. F. Krupke, J. Lumin. 62, 85 (1994).6. J. L. Ouweltjes, Modern Materials (Academic Press, New York,1965), Vol. 5, p. 161., No. 12, Dec 19975:34, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms.Communicationshttps://doi.Downloade7. M.R. Dickinson, L.A.W. Gloster, N.W. Hopps, and T. A. King,Opt. Commun. 132, 275 (1996).8. C. D. Marshall, L.K. Smith, R. J. Beach, M. A. Emanual, K. J.Schaffers, J. Skidmore, S.A. Payne, and B.H. T. Chai, IEEEJ. Quantum Electron. 32, 650 (1996).9. L. A.W. Gloster, P. Cormont, A.M. Cox, T.A. King, and B.H. T.Chai, unpublished.10. A. Caprez, P. Meyer, P. Mikhail, and J. Hulliger, Mater. Res.Bull. 32, 1045 (1997).11. Handbook of Chemistry and Physics, 74th ed., edited by D.R.Lide and H. P. R. Frederikse (CRC Press, Cleveland, OH,1993–94), Sec. 11, p. 35.12. A. Caprez, P. Mikhail, and J. Hulliger, J. Cryst. Growth (1997,in press).13. S. A. Payne, L.K. Smith, L.D. DeLoach, W. L. Kway, J. B.Tassano, and W. F. Krupke, IEEE J. Quantum Electron. 30, 170(1994).J. 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Czochralski growth of Yb3+ and Pr3+ doped Ca-fluoroapatite

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