local relaxation in lanthanum silicate oxyapatites by raman scattering and mas-nmr

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Research ArticleReceived: 23 September 2010 Accepted: 29 November 2010 Published online in Wiley Online Library: 23 February 2011

(wileyonlinelibrary.com) DOI 10.1002/jrs.2878

Local relaxation in lanthanum silicateoxyapatites by Raman scatteringand MAS-NMRStephanie Guillot,a Sophie Beaudet-Savignat,a Sebastien Lambert,a

Pascal Roussel,b Gregory Tricot,b Rose-Noelle Vannierb

and Annick Rubbensb∗

To characterize the local relaxation in the structure of lanthanum silicate oxyapatite materials, six compositions withdifferent cation and oxygen stoichiometries (La8Ba2Si6O26, La9BaSi6O26.5, La10Si5.5Mg0.5O26.5, La9.33SiO26, La9.67SiO26.5 andLa9.83Si5.5Al0.5O26.5) were investigated by combining Raman scattering and 29Si and 27Al magic-angle spinning nuclear magneticresonance (MAS-NMR) spectroscopies. Only [SiO4]4− species were evidenced and the hypotheses of [Si2O7]6− and [Si2O9]8−entities were ruled out. Both oxygen excess and cation vacancies induce local distortions in the structure, which leads tononequivalent [SiO4]4− species, characterized by different 29Si MAS-NMR signals and by splitting of Raman signals. Copyrightc© 2011 John Wiley & Sons, Ltd.

Keywords: apatite; lanthanum silicate; Raman scattering; 29Si MAS-NMR; 27Al MAS-NMR

Introduction

Because of their good chemical stability and good ionic con-duction properties,[1 – 3] lanthanum silicate oxyapatite materialsare generating considerable interest for their potential applica-tion as electrolytes in solid oxide fuel cells (SOFCs) operating atintermediate temperatures.

With La10−x x(SiO4)6O2+δ as the general formula, they exhibitthe apatite-type structure which is described in the P63/m (C6h)space group[4] and consists of isolated SiO4 tetrahedra spacedwith La cations located in two sites: a 7-coordinated one, 4f(C3); and a 9-coordinated one, 6h (Cs). In the initial model, twoadditional oxide ions were located in channels along the c-axis,2a (C3h), bordered by the La(6h) sites. The structure is very flexibleand can accept a wide range of partial substitutions on bothSi and La sites. Solid solutions with Ca, Sr, Ba and Mg[3,5 – 7] onthe lanthanum site and with Al,[8 – 15] Mg[15 – 18] and Ge[19,20] onthe silicon site were evidenced. Mg was a special case because,as an ambi-site, it can substitute both for Si and La.[16] Thepossibility of cation vacancies was also clearly demonstrated,these vacancies being located on the La(4f) site,[21 – 23] whichis also the preferred one for La doping. Most of these apatitematerials exhibit an oxygen excess and show a general increasein conductivity when the oxygen content increases. At 500 ◦C,conductivities in the range 6.6 × 10−3[3] to 1.2 × 10−2 S.cm−1[5]

were reported for La9Ba1Si6O26.5 as against 3.3 × 10−9 S.cm−1

for the stoichiometric composition La8Ba2Si6O26.[3] Interestingly,although it does not contain oxygen excess, La9.33Si6O26 exhibitsreasonable conductivity with 1.1 × 10−4 S.cm−1 at 500 ◦C[3]

compared with 1.3 × 10−3 S.cm−1 for La9.67Si6O26.5.[3] It wasexplained by the creation of Frenkel defects in the structure.[2]

Despite the fact that a diffusion mechanism through interstitialsites is usually invoked to explain oxide ion diffusion in thesematerials, the actual location of these interstitials is still a subject

of controversy. Oxygen interstitial positions at the periphery ofchannels were first predicted by computer modeling[24,25] andthen confirmed by neutron powder diffraction in various apatites,although refinements led to unrealistic distances (less than 1 Å)to the oxygen site belonging to the adjacent silicates.[14,16,21,26 – 28]

The possibility of a local relaxation of the tetrahedron was mootedto explain such an artefact.

More recently, from neutron diffraction data, we found evidencefor two cavities in the structure of the oxygen-excess La9.67Si6O26.5

at (0, ∼0.50, ∼0.50) and (0.06, 0.10, 0).[4] With distances of 2.09 and2.13 Å from the silicate oxygen atoms, the first cavity was too smallto incorporate an extra oxide ion. The second cavity was more than2.3 Å from the silicate oxygen atoms but only 2 Å from the oxygenin the 2a (C3h) sites located in the La(6h) channels, which precludedsimultaneous occupancy of the two sites. However, Fourier mapconfirmed nucleon density in this cavity, and a realistic model wasproposed with oxygen atoms in the channels shared between 4esites close to the initial 2a sites with z = 0.28 and 12i sites at (−0.01,0.04, 0.06) corresponding to the cavity. This model was in goodagreement with the oxygen diffusion pathways recently proposedby Bechade et al. using computer modeling techniques.[29]

Nevertheless, cation vacancies and oxygen excess may alsoimply local relaxation in the structure which cannot be identified

∗ Correspondence to: Annick Rubbens, Univ. Lille Nord de France, CNRS UMR 8181,Unite de Catalyse et de Chimie du solide, Universite Lille 1, Polytech’Lille, EcoleNationale Superieure de Chimie de Lille, 59652 Villeneuve d’Ascq Cedex, France.E-mail: [email protected]

a CEA Le Ripault, BP 16, 37260 Monts, France

b Univ. Lille Nord de France, CNRS UMR 8181, Unite de Catalyse et de Chimie dusolide, Universite Lille 1, Polytech’Lille, Ecole Nationale Superieure de Chimie deLille, 59652 Villeneuve d’Ascq Cedex, France

J. Raman Spectrosc. 2011, 42, 1455–1461 Copyright c© 2011 John Wiley & Sons, Ltd.

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by diffraction techniques alone. 29Si and 27Al nuclear magneticresonance (NMR)[30 – 34] and Raman scattering[32 – 33,35 – 36] were alsocarried out on these systems. Local distortions were evidenced,and the formation of [Si2O7]6− groups by condensation of [SiO4]4−units was proposed to explain the creation of additional interstitialoxide ion defects in cation-deficient apatites.[30] The possibility ofSi2O9 unit (involving the connectivity between two pentahedralsilica) was also suggested to explain interchannel conduction.[31]

To verify these previous assumptions and to gain adeeper understanding of the local relaxation in the struc-ture, six apatites with different cation and oxygen stoichiome-tries were investigated in the present study: La8Ba2Si6O26,La9BaSi6O26.5, La10Si5.5 Mg0.5O26.5, La9.33SiO26, La9.67SiO26.5 andLa9.83Si5.5Al0.5O26.5, and were studied by combining Raman scat-tering and 29Si and 27Al magic-angle spinning (MAS) NMR.

Experimental

The apatites were prepared by high-temperature solid-statereaction from stoichiometric amounts of La2O3 (Rhodia 99.99%),amorphous SiO2 (Cerac 99.99%), BaCO3 (Neyco 99.9%), MgO (Cerac99.95%) and Al2O3 (Sumitomo ‘High purity’). The La2O3 and SiO2

powders were first annealed at 1100 ◦C in order to achievecomplete decarbonation and dehydroxylation. Stoichiometricamounts of the starting materials were then ball-milled for 24 hin ethanol, before being dried and ground. In order to obtainpure apatite powders required for solid-state NMR and Ramanspectroscopy, the mixtures were directly calcinated at 1625 ◦C inair for 30 min.

Phase purity was monitored by X-ray diffraction (XRD) atroom temperature (PANalytical X-pertPro diffractometer, CuKα1 = 15406 Å, with Ge monochromator and PiXel detector). XRDpatterns were analyzed using EVA and JANA2000 software.[37 – 38]

A single phase with the apatite structure was found for all theapatite samples. The obtained powder chemical composition wasconfirmed by inductively coupled plasma spectrometry.

In addition, La2SiO5 and La2Si2O7 were synthesized to be usedas references of isolated [SiO4]4− and [Si2O7]6−, respectively. Thesematerials were prepared by calcination of stoichiometric amountsof the starting materials for 10 h at 1500 ◦C. Their purity wasmonitored by XRD. It revealed a pure phase for La2Si2O7 andtraces of apatite as a secondary phase in La2SiO5.

Raman spectra were recorded at room temperature with the647.1 nm excitation line of a Spectra Physics krypton ion laser. Toavoid any degradation of the sample, all the compounds werestudied with a very low laser power (3–4 mW at the sample). Fouraccumulations of a few seconds were used for each spectral range.The beam was focused onto the samples using the macroscopicconfiguration of the apparatus. No damage of the material by thelaser was observed. The scattered light was analyzed with a RamanDilor XY800 spectrometer equipped with an optical multichannelcharge coupled device detector cooled by liquid nitrogen. Inthe required 15–1300 cm−1 range, the spectral resolution isapproximately 0.5 cm−1. Acquisition and data processing wereperformed with LABSPEC software.

Fitting of the spectra was carried out with the LABSPEC software.A pseudo-Voigt function was used because each Raman lineshape is very often composed of both Lorentzian and Gaussiancomponents. This makes fewer constraints than with a nonlineardeconvolution fitting (only the band wavenumbers are givenhere).

Ram

an In

tens

ity

Wavenumber/cm-1

La2SiO5

La2Si2O7

La9.33Si6O26

La9.67Si6O26.5

La9.83Si5.5Al0.5O26.5

La10Si5.5Mg0.5O26.5

La9BaSi6O26.5

La8Ba2Si6O26

200 400 800 1000600

Figure 1. Raman spectra of different lanthanum silicates.

The optimization of the fitting parameters (band wavenumber,band amplitude, full-width at half-maximum, band surface andGaussian/Lorentzian ratio) was carried out iteratively with themethod of maximum gradient.

29Si solid-state MAS-NMR experiments were conducted at79.94 MHz on a 9.4 T Bruker spectrometer equipped with a 7 mmprobe operating at a spinning frequency of 5 kHz. Single pulseacquisitions were obtained using a radio frequency (rf) field of50 kHz, a 90◦ pulse of 5 µs and 256–512 scans separated by adelay of 300 s. Higher recycle delay did not change the relativeproportions of the different silicate signals, indicating that ourspectra can be considered as quantitative. Relative proportionswere deduced from the spectrum simulations performed withthe Dmfit software.[39] 27Al MAS-NMR analysis was carried outat 208.57 MHz on a 18.8 T Bruker spectrometer with a 3.2 mmprobe allowing a spinning frequency of 20 kHz. The single pulseacquisition was performed with 25 kHz (determined on a liquid), apulse length of 1 µs (corresponding to a 30◦ pulse) and 1024 scansseparated by a delay of 0.5 s.

Results

Raman scattering

Figure 1 shows Raman spectra recorded for all the studiedapatites and compared with those of La2SiO5 and La2Si2O7

used as references. The La2SiO5 structure is composed of onlyisolated tetrahedra [SiO4]4−[40] whereas La2Si2O7 is constitutedby only pyrosilicates.[41] The spectra corresponding to the apatitecompounds are in good agreement with the spectra reportedby Lucazeau et al.,[35] Rodriguez-Reyna et al.[36] and more recentlyby Orera et al.[32] Two relatively intense bands are observed near850 and 385 cm−1. The first one is followed by high wavenumbercomponents with lower intensity beyond 1000 cm−1, and thesecond displays components between 325 and 600 cm−1. Thesebands are also observed for La2SiO5, which is made up of isolatedsilicate tetrahedra. They are close to the fundamental frequenciesfor free [SiO4]4− ion: 819 and 956 cm−1 for ν1(A1) and ν3(F2)

wileyonlinelibrary.com/journal/jrs Copyright c© 2011 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2011, 42, 1455–1461

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Local relaxation in lanthanum silicate oxyapatites

Wavenumber/cm-1

750 800 850 900 950 1000

Ram

an In

tens

ity La9.33Si6O26

La9.67Si6O26.5

La9.83Si5.5Al0.5O26.5

La10Si5.5Mg0.5O26.5

La9BaSi6O26.5

La8Ba2Si6O26

Figure 2. Raman spectra in the 750–1050 cm−1 range corresponding to[SiO4]4− stretching modes for different oxyapatites (The dotted line showsthe shift of the band extremum with the material composition).

847848849850851852853854855856857

7.5 8 8.5 9 9.5 10 10.5

Lanthanum content in the material

aver

age

wav

enum

ber/

cm-1

of th

esy

mm

etric

al s

trec

tchi

ng m

ode

(a)

(b)

Figure 3. Average wavenumber evolution of the [SiO4]4− ν1(A1) bandin the function of the lanthanum content for apatites: (a) with cationicvacancies and (b) with dopant of the La site (The average wavenumberrefers to the first three components).

stretching modes, respectively, and 340 and 527 cm−1 for ν2(E)and ν4(F2) bending modes, respectively. The very weak bandobserved at 728 cm−1 for La9.83Si5.5Al0.5O26.5 is assigned to thesymmetrical stretching motion of [AlO4]5− tetrahedron species, inagreement with Lucazeau et al.[35] The similar features observedfor all the apatite compounds suggest no phase transition and atetrahedral silicate anion. An extra oxygen linked to Si atoms wassuggested by the group of Slater,[42] but an [SiO5]6− entity wouldgenerate other internal modes which are not detected here.

It is worth noting that the band located at 742 cm−1 in theLa2SiO7 spectrum, which corresponds to Si–O–Si bridge vibration,is not found in the apatite Raman spectra, which rules out thehypothesis of Samson[34] on the formation of [Si2O7]6− in cation-deficient apatites.

For all the apatite Raman spectra, a broadening of the main bandaround 850 cm−1, corresponding to the symmetrical stretchingmotion of [SiO4]4− species, was observed (Fig.2), especially inpresence of a cation vacancy. Figure 3 shows the variation of theaverage wavenumber as a function of the lanthanum content inthe sample. Two sets of data can be distinguished: (1) samplessubstituted on the lanthanum site without a cation vacancy;(2) samples that exhibit cation vacancies. For samples substituted

Table 1. Fitted parameters after deconvolution in the 800–880 cm−1

spectral range

Samples

Bandwavenumbers

in cm−1

Full-width athalf-maximum

in cm−1

Gaussian/Lorentzian

ratio

La8Ba2Si6O26 842 9.10 0.61

847 8.03 1

854 9.36 1

864 20.08 1

La9Ba1Si6O26.5 843 9.85 0.50

849 9.46 1

856 10.89 0.65

869 20.12 1

La9.33Si6O26 847 9.88 0.50

856 8.36 0.55

866 13.78 1

880 12.27 1

La9.67Si6O26.5 847 11.20 0.50

856 9.91 0.79

864 12.62 0.88

877 17.55 1

La9.83Si5.5Al0.5O26.5 847 10.88 0.50

855 9.28 1

864 11.55 0.60

879 22.79 1

La10Si5.5 Mg0.5O26.5 845 11.03 0.50

852 9.44 0.75

860 9.97 0.68

875 23.53 1

on the lanthanum site without a cation vacancy, the band positionshifts toward lower wavenumbers when the barium contentincreases. This indicates an increase of the Si–O bond lengths,which can be explained by a decrease of the interaction of the extraoxygen in the La(6h) channels, as the number of oxygen interstitialsdecreases when the dopant concentration increases. Indeed, thesilicate must be seen as an entity in interaction with its surrounding,namely the extra oxide in the structure. In contrast, a shift of theband positions toward higher wavenumbers is observed when thenumber of cation vacancies increases. This indicates a decrease ofthe Si–O bond lengths, which is in good correlation with a lack ofpositive charge in the silicate oxygen surroundings, the repulsionof the interstitial oxides being minimized in presence of cationvacancies.

The results of decomposition for all the studied samples aregiven in Table 1. As suggested by Lucazeau et al.,[35] the profilesare more Gaussian than Lorentzian. Figure 4 shows the spectraldecomposition for La9.33Si6O26. The two main bands observed inthe range of the ν1(A1) [SiO4]4− vibration cannot be assigned to Ag

and E2g components because of their similar intensity, full-widthat half-maximum and a significant splitting for this vibrationalmotion (8.5 cm−1). The three obtained components at 847, 856and 866 cm−1 rather characterize three different environments ofthe SiO4 tetrahedra. For La8Ba2Si6O26 which is fully stoichiometric,the ν1(A1) band at lower wavenumber is narrower and moresymmetric, which is in good agreement with less disorder in thiscomposition (no extra oxygen, no cation vacancy). For the ν3(F2)asymmetrical stretching motion, the different components spread

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Figure 4. Stretching spectral range deconvolution of the [SiO4]4− anion inLa9.33Si6O26.

Ram

an In

tens

ity

350 400 450 500 550 600

Wavenumber/cm-1

La9.33Si6O26

La9.67Si6O26.5

La9.83Si5.5Al0.5O26.5

La10Si5.5Mg0.5O26.5

La9BaSi6O26.5

La8Ba2Si6O26.5

Figure 5. Raman spectra in the 325–700 cm−1 range corresponding to[SiO4]4− bending modes in different oxyapatites (The dotted lines are usedas reference marks of shoulders discussed in Refs 32 and 33).

from 865 to 1050 cm−1 except for La8Ba2Si6O26. The multiplenumber of components proves the sensitivity of the silicatetetrahedra to their environment. Our results are similar to those ofJo et al.[33] if we expect the non-observation of the line located at861 cm−1 described by these authors as due to a substructure ofthe silicate anion as suggested by Slater et al.[42] We have anotherhypothesis to interpret this difference: we believe that this lineis likely due to a La2SiO5 impurity in the La10Si6O27 composition,because two other bands at 315 and 337 cm−1, fingerprints ofLa2SiO5, are also detected in their Raman spectra.

In the spectral domain that characterizes the bending modes(Fig. 5), two shoulders are observed for all compositions except forLa8Ba2Si6O26. The first one observed around 356–360 cm−1 (Fig. 6)was associated to the presence of a nearby extra oxygen site inthe structure by Orera et al. for La8+xSr2−xSi6O26+x/2 (0 ≤ x ≤ 1.0)compositions.[32] Frenkel defects would justify this band evenfor composition La9.33Si6O26 which does not contain oxygenexcess. Orera et al.’s conclusions were confirmed by the increase in

Figure 6. Bending spectral range deconvolution of [SiO4]4− anion in(a) La9.33Si6O26 and (b) La8Ba2Si6O26.

intensity of this band with the oxygen excess content. However, inour case, this band is less pronounced for barium-substitutedcompositions and its intensity increases with the amount ofcationic vacancies. Thus, it could also be related to the amountof cation vacancies. This fact is supported by the second shoulderobserved for the ν4(F2) band around 537–545 cm−1 (Fig. 6), whichexhibits the same behavior. For this last spectral range, Bechade[43]

assigned the 570 cm−1 component to La–O vibration with theoxygen of the conduction channel. However, at this wavenumberthis band is more likely a component of the asymmetric bendingmotion of the silicate tetrahedron, the La–O stretching motionbeing at lower wavenumber.

In the spectral range corresponding to the lattice modes (Fig. 7),no significant shift of the most intense band located between 268and 280 cm−1 and assigned to stretching motion of the La–Obond was observed for the cation-deficient apatites containingonly lanthanum in the La(4f) sites. A broadening of the band hasto be noticed when cationic vacancies increased. In contrast, forbarium-substituted apatite, a decrease of this band wavenumberis noticed when the amount of barium increases. The widthof the bands can be easily explained by the dispersion theLa–O bond lengths in the materials which range from 2.284to 2.610 Å (the sum of the ionic radii of La3+ and O2− beingaround 2.5 Å). It is worth noting that the position of the mainband is mainly affected by the dopant on the La species whichare localized on the La(4f) sites (Fig. 8). No significant variation of


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150 200 250 300

Ram

an In

tens

ity

Wavenumber/cm-1

(a)

(b)

(c)

(d)

(e)(f)

Figure 7. Raman spectra in the 125–325 cm−1 range corresponding to thelattice modes of (a) La9.33Si6O26, (b) La9.67Si6O26.5, (c) La9.83Si5.5Al0.5O26.5,(d) La10Si5.5 Mg0.5O26.5, (e) La9BaSi6O26.5 and (f) La8Ba2Si6O26 (The dottedline is provided to appreciate the shift of this band with the cationicvacancies).

266

268

270

272

274

276

278

280

282

7.5 8 8.5 9 9.5 10 10.5

Lanthanum content in the material

wav

enum

bers

/cm

-1

(a)

(b)

Figure 8. Wavenumber evolution of the La–O band as a function of thelanthanum content for apatites: (a) with cationic vacancies and (b) withdopant of the La site.

its wavenumber is observed for cation-deficient apatites whichexhibit an average La–O distance of 2.494 Å for La(6h) sites and of2.515 Å for La(4f) sites.[43,44] The band position evolution towardlower wavenumbers obtained for the apatites with dopant onthe La site is in good agreement with an increase of the averageLa–O ionic bond length, i.e. 2.508 and 2.546 Å for La(6h) and La(4f)sites, respectively, observed for composition La9BaSi6O26.5.[44] Ashift of the band position toward lower wavenumbers was alsoobserved for the magnesium-substituted sample, which couldbe due to a small amount of magnesium in the La site sincemagnesium is an ambi-site substituent. In the recent works of Aliet al.,[45] the refinement of the La9.69(Si5.70 Mg0.30)O26.24 structureleads to an average La–O bond length of 2.512 and 2.496 Åfor La(4f) and La(6h) sites, respectively. These results corroboratethose obtained for our cation-deficient apatites and the averagewavenumber corresponding to this vibration would be 280 cm−1.Whatever the reason, we can conclude that the variation of theaverage wavenumber of the La–O bond in these apatites is notdue to the extra oxygen but rather to the doping in the La(4f) sites.

Table 2. 29Si NMR chemical shifts and relative intensity of all thestudied apatites

Composition Peak position (ppm)Relative intensity

(%)

La9.33Si6O26 −77.8/−79.4/−81.0/−85.0 54/8/31/7

La9.67Si6O26.5 −77.7/−79.3/−81.2/−85.0 59/15/22/4

La9.83Si5.5Al0.5O26.5 −77.8/−79.6/−81.6/−85.1 63/18/15/4

La10Si5.5 Mg0.5O26.5 −77.8/−79.6/−81.3/−85.1 73/15/11/1

La9Ba1Si6O26.5 −77.8/−80.2 82/18

La8Ba2Si6O26 −78.3 100

-2002040608010027Al Chemical shift / ppm

Figure 9. 27Al MAS-NMR spectrum acquired at 18.8 T on theLa9.83Si5.5Al0.5O26.5 sample.

A small band is observed near 250 cm−1, which is better definedin the La8Ba2Si6O26 and La9.33Si6O26 spectra. It is likely present inall compositions, hidden by the broadening of the side bands, dueto the disorder induced by cationic vacancies and extra oxygen,which leads to unresolved spectra.

To complete this study, 27Al and 29Si MAS NMR were carried out.

27Al NMR and 29Si NMR

It is well established that the 27Al chemical shift values can bedirectly related to the aluminium coordination state, tetrahedralaluminium [AlO4]5− species giving a signal between 80 and40 ppm, pentahedral aluminate [AlO5]7− between 40 and 0 ppmand octahedral aluminate [AlO6]9− between 0 and −60 ppm.[46]

The spectrum collected for La9.83Si5.5Al0.5O26.5 (Fig. 9) clearlyshows two peaks at 80 and 39.8 ppm. If the main peak at 80 ppm(93% of the total intensity) can be unambiguously associatedwith a [AlO4]5− unit, the second peak (7% of the total intensity)likely corresponds to [AlO5]7− although its associated chemicalshift is at the boundary between the tetrahedral and pentahedralregions. These data are in good agreement with results reported byKharlamova et al.[34] During the preparation of Al-doped apatite bymechanical activation, [AlO5]7− units were also evidenced by theseauthors. They were associated with the formation of amorphousAl/Si phase during the grinding process. These peaks disappearedafter calcination at 1200 ◦C. Traces of such an amorphous phase inour sample may explain this second peak and one cannot concludethat the apatite structure contains [AlO5]7− units. Figure 9 showsthe 27Al MAS-NMR region corresponding to the octahedral species.The lack of signal indicates the absence of octahedral aluminium inour study in contrast to Kharlamova et al.[34] who found evidence ofthe formation of LaAlO3 (containing octahedral aluminate species)as secondary phase which appeared to be more stable in theirconditions of preparation.


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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

29Si Chemical shift / ppm

-80 -90 -100-70-60 -110

Figure 10. 29Si MAS-NMR analyses performed at 9.4 T: (a) La2SiO5,(b) La2Si2O7, (c) La9.33Si6O26, (d) La9.67Si6O26.5, (e) La9.83Si5.5Al0.5O26.5,(f) La10Si5.5 Mg0.5O26.5, (g) La9BaSi6O26.5 and (h) La8Ba2Si6O26.

The 29Si MAS-NMR analyses are depicted in Fig. 10. Onlychemical shift in the range of −110 and −60 ppm are given herebut it is worth noting that no peak was found around −150 ppm,which rules out the possibility of SiO5 entity and therefore theformation of [Si2O9]10− entity as proposed by Kendrick et al.[31]

The La2SiO5 spectrum (Fig. 10(a)) exhibits a peak at −85.2 ppm,which can be assigned to [SiO4]4− (Q0 units) silica site surroundedby lanthanum, accompanied by a low intensity signal centeredat −79 ppm, whose presence can be related to the residualapatite detected by XRD. The 29Si MAS-NMR spectrum of La2Si2O7

(Fig. 10(b)) is composed of two peaks at −88.2 and −89.1 ppm,corresponding to the two [Si2O7]6− (Q1 units) sites surrounded bylanthanum, as expected from the structure.

While the spectrum of La8Ba2Si6O26, which is fully stoichiomet-ric, exhibits a single peak at −77.8 ppm (Fig. 10(h)) assigned tothe Q0 units, in good agreement with Raman spectroscopy, othercompositions display more than one peak. For La9Ba1Si6O26.5

(Fig. 10(g)), the spectrum contains a second contribution centeredat −80.2 ppm, in good agreement with Samson et al.,[30] who alsofound two peaks at −77.5 and −80.5 ppm for La9M(SiO4)6O2.5

(M = Ca, Sr, Ba), and with Orera et al.[32] in case of the Sr solidsolution La8+xSr2−x(SiO4)6O2+x/2. These authors also reported anincrease of the second peak intensity when the oxygen excesscontent increases. This second peak was therefore associated withthe oxygen excess and, according to these authors, it is correlatedwith a silicate group adjacent to an interstitial oxygen site. Whenlooking at cation-deficient apatites (Fig. 10(c–e)), it first appearsthat all spectra contain, in addition to the two peaks previously de-scribed, a third component centered at −85 ppm, this latter signalbeing previously reported by Samson[30] and Kharlamova et al.[34]

Nevertheless, our results differ from those previously reportedsince a simulation derived from the three-component system pro-posed by these authors does not fit properly our experimentaldata (Fig. 11(a)), suggesting the existence of an additional contri-bution. Figure 11(b) depicts a proper simulation obtained by theaddition of a single component centered at −79.5 ppm, indicat-

(1) (2) (3)

(1)(4)

(3)(2)

-90-85-80-75-7029Si Chemical shift / ppm

(a)

(b)

Figure 11. Simulation of the 29Si MAS-NMR spectrum of the La9.67Si6O26.5sample with (a) three and (b) four components.

ing a more complex silicate structure than expected. A good fit(not shown here) was also obtained with four components forLa10Si5.5 Mg0.5O26.5 which, as shown in the Raman section, is likelyto contain a small amount of magnesium on the lanthanum siteand for which a few cation vacancies may also exist. As shown byRaman scattering, the possibility of [Si2O7]6− is ruled out and theexistence of [SiO4]4− with different surroundings is more likely.It is worth noting that in our case the intensity of the peaks at−81 and −85 ppm increases with the amount of cation vacan-cies in the structure, whereas the intensity of the peak locatedaround −79.5 ppm increases with the oxygen excess content (18%La9.83Si5.5Al0.5O26.5, 15% for La9.67Si6O26.5 and La10Si5.5 Mg0.5O26.5

against 8% for La9.33Si6O26 which is supposed to contain a fewinterstitial oxygens and no peak for La8Ba2Si6O26) (Table 2). Itis also worth noting that the intensity of this latter peak is al-most the same as that observed for the peak at −80.2 ppm forLa9Ba1Si6O26.5 which contains the same oxygen excess. Whereasthe peaks at −81 and −85 ppm are likely the fingerprint of cationvacancies, we can assume that this additional peak is likely relatedto the oxygen excess in the structure.

Conclusions

The combination of Raman spectroscopy and 29Si NMR confirmsthe presence of [SiO4]4− with different surroundings in oxygen-excess apatites but especially for cation-deficient apatites. Theevolution of the wavenumber corresponding to the symmetricalstretching motion indicates a decrease of the Si–O bond lengthwhen the number of cation vacancies increases, whereas theshift of the band (which characterizes the La–O toward lowerwavenumbers when the barium content in the structure increases)is an indication of an increase of the bond lengths when thesubstitution ratio increases. Comparison of Raman spectra withthat of La2Si2O7 allows us to exclude the possibility of [Si2O7]6−

entity in the structure. The [SiO5]6− entity and therefore the


14

61


[Si2O9]10− entity are also ruled out since no characteristic 29Si MAS-NMR signal was found and it would generate other internal modeswhich were not detected by Raman scattering. At this stage, Ramanspectroscopy reveals no clear evidence of the oxygen excess in thestructure since it is not strongly linked to any cation, but we canconclude that both oxygen excess and cation vacancies lead to abroadening of the main bonds likely because of the local disorderinduced by these defects. As proposed by other authors,[30,32] thepeak observed around −80 ppm in the 29Si MAS-NMR spectra islikely a fingerprint of this oxygen excess, whereas the peaks at−81 and −85 ppm likely characterize the cation vacancies on thelanthanum site.

Acknowledgements

The authors are grateful to the Region Nord/Pas de Calais, EuropeFEDER, CNRS and French Minister of Science for funding the NMRfacilities. SG is grateful to the CEA for supporting her Ph.D. program.

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local relaxation in lanthanum silicate oxyapatites by raman scattering and mas-nmr

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