NMR Investigations in Li1.3Al0.3Ti1.7(PO4) - ACS Publications

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NMR Investigations in Li Al Ti (PO) Ceramics Part I: Structural Aspect Joël Emery, Tomas Salkus, Alla Abramova, Maud Barré, and Antanas Feliksas Orliukas J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06764 • Publication Date (Web): 09 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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The Journal of Physical Chemistry

NMR Investigations in Li1.3Al0.3Ti1.7(PO4)3 Ceramics Part I: Structural Aspect Joël Emery and Tomas Šalkus2, ,Alla Abramova1, Maud Barré1* and Antanas Feliksas Orliukas2 1

Institut des Molécules et Matériaux du Mans IMMM, UMR CNRS 6283, LUNAM, Université du

Maine, 72085 Le Mans Cedex 9, France 2

Faculty of Physics, Vilnius University, Saulėtekio al. 9/3, LT-10222 Vilnius, Lithuania

E–mail: [email protected]

Tel +33(0)2 43 83 33 53

[email protected] [email protected]

ACS Paragon Plus Environment

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Abstract : Because of its high Li+ conductivity, the family Li1+xAlxTi2-x(PO4)3 has already been widely studied and previous structural characterizations reported that aluminum occupied two types of sites in the NASICON framework : first one in octahedral coordination corresponding to Ti/Al substitution, second one in tetrahedral coordination corresponding to P/Al substitution. In this work we show that it is possible to synthesize samples presenting only the Ti/Al substitution in octahedral site which is more consistent with the formulation. Static local properties of our samples were characterized by multinuclear Nuclear Magnetic Resonance (NMR) and X-ray diffraction. The MAS NMR aluminum spectrum is characterized by a strong parameter of asymmetry (ηQ = 0.9) indicating that aluminum ions are situated in sites which lost their axial symmetry. This loss of symmetry is accompanied with an increase of the number of chemical sites of the phosphorus among which some are characterized by broad lines. The strong asymmetry quadrupolar parameter, together with the strong broadening of the

31

P lines assigned to phosphorus with 3 Ti4+ and one Al3+ are marks of

M2(IV)PO4 skeleton’s distortion. Multinuclear NMR experiment also allowed us to analyze the abnormal behavior of the lithium quadrupolar parameter νQ in relation with the flexibility of M2(IV)PO4 skeleton characteristic of NASICON family.

1. Introduction Solid electrolytes with fast Li+ ion transport are attractive materials for applications in CO2 gas sensors,1 and solid electrolyte batteries.2 In particular, lithium compounds with NASICON (Natrium Super ionic conductor) structure and formula LiM2(PO4)3 have been extensively studied.3-33 It has been shown that the bulk ionic conductivity of LiTi2(PO4)3, σb = 1.5×10–4 S⋅m–1 at room temperature,25 increases by several orders of magnitude if Ti4+ is partially substituted by Al3+,25 Sc3+, Fe3+, or Y3+ ions.3-25 The bulk ionic conductivity at 298 K for Li1.3Al0.3Ti1.7(PO4)3(LATP) was found to be 1. 10-1 S⋅m–1 with the activation energy Eb=0.25 eV for the bulk and Egb=0.29 eV for the brain boundary.12-25


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The Li+ ion transport number in these compounds was found to be ti = 1.22,32 In such samples the large enhancement of the conductivity cannot be only explained by the increase of the lithium content of the compound. From the Ohm law σLi=nLiqLiµLi (where nLi represents the number of charge carriers, qLi their charge and mLi their mobility), we can deduce that the electrochemical mobility of lithium in the NASICON structure is also greatly improved. LATP samples were first studied in references (11, 15, 17,18) and after in references (23, 24, 27). Li1.3Al0.3Ti1.7(PO4)3 belongs to the rhombohedral symmetry (space group R 3 c ) with six formula units per cell. The lattice parameters of LATP are a = 8.504 Å and c = 20.881 Å.11 R 3 c space group is typical for NASICON-type structure. The NASICON-type framework is built up of M2(PO4)3 (where M = Ti, Ge, Al, Y) units in which two MO6 octahedra and three PO4 tetrahedrons share oxygen atoms.33 It results in a system of three dimensional ‘channels’ in which Li ions move. It has been previously stated that the ionic conductivity can be altered by controlling the channel sizes via one or more of the above substitutions since, although the overall crystal symmetry remains the same,20 the lattice dimensions depend on the size of the structural cations. Moreover, substitution of a tetravalent Ti4+ by trivalent Al3+ involves an increase of the amount of charge carrier Li+ to keep electro-neutrality of the formula. Such a substitution modifies the flexibility proprieties of the skeleton. In NASICON structure, different positions suitable for Li ions, with different denominations depending on the authors, were pointed out: the M1 site (6b position in s.g. R3c ),34 at the center of an elongated octahedral oxygen environment at the intersection of the three conduction channels, corresponds to the cage commonly occupied by metallic cations in many NASICON compounds; the M2 site (18e position) is in a 8-10 oxygen environment at each bend of the conduction channels in the framework.35 the M1’ site (36f position), in the neighborhood of M1 site, was reported by Catti et al. in LiZr2(PO4).36 Finally, Arbi et al. recently mentioned an intermediary site, called M3 thereafter (36f position), along the conduction channels.24 In the case of Li1.2Al0.2Ti1.8(PO4)3, they showed that M1 sites are preferentially occupied while the occupancy of M3 sites increases with


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the substitution rate Al/Ti. This leads to the creation of vacancies in M1 site and gives rise for lithium mobility between M1 and M3 positions, the distance between the positions is ~3.5 Ǻ.24 The high ionic conductivity of Li1.3Al0.3Ti1.7(PO4)3 and peculiarities of the ionic migration stimulate further investigations. The samples of this series were the object of a large number of studies. 11, 15, 17, 18, 23, 24, 27

All these papers report results on samples with aluminium in two sites: the octahedral one and

the tetrahedral one. This last site was attributed to Al3+/P5+ exchanges in the network, although this hypothesis is not consistent with the nominal NASICON formula: tetrahedral sites should be fully occupied by P5+ ions leaving no available space for Al3+ cations. Thus, the existence of Al3+ in tetrahedral coordination could be explained by the presence, even in very low quantities, of impurities based on phosphates. In the first part of our study we focused on the structural aspects of this type of compound. Particularly, our attention was concerned on the sample itself and we have shown that it is possible to synthesize samples without aluminium in the tetrahedral site. The crystalline structure of this sample was determined by X-ray powder diffraction (XRPD) and the static local properties were studied by Nuclear Magnetic Resonance (NMR): at room temperature by the Magic Angle Spinning (MAS) experiments and in temperature range from 120 K to 420 K on static sample (non-spinning sample or static mode). The high resolution MAS NMR (MAS mode) experiments were performed in order to precise local structure while the experiment on static sample (static mode) allowed us to obtain temperature behavior of the 7Li and 31P NMR spectral parameters. 2. Experimental procedure The powders of Li1.3Al0.3Ti1.7(PO4)3 were synthesized using modified Pechini process.37 This polymerizable complexes method was already detailed in previous papers.38 Stoichiometric ratios of metallic Ti, lithium carbonate Li2CO3, ammonium dihydrogenophosphate NH4H2PO4 (Fluka, 99%), and aluminium nitrate Al(NO3)3. 9H2O were used as starting reagents. The procedure is summarized in ACS Paragon Plus Environment

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figure 1. As a final step to obtain the NASICON powders, the synthesized precursors were annealed at 900°C for 2h. These powders were directly used to perform XRD and MAS NMR. For NMR investigations on static sample at different temperatures, some ceramic pellets were prepared from powder pressed uniaxially (~4000 bars) and then isostatically (~5000 bars), and finally sintered at 1000 °C for 2 hours in air. X-ray powder diffraction (XRPD) diagrams were recorded in air at room temperature with Cu Kα radiation on a PANalytical X’pert Pro diffractometer equipped with the X’celerator detector in the 2θ range from 5 to 140 deg with step 0.017 deg (counting time 280 s/step). The powder was sieved out through 63 µm mesh in order to avoid any preferred orientation of grains. Structure refinement was carried out by the Rietveld method using Fullprof profile refinement program.39, 40 Nuclear Magnetic Resonance (NMR) experiments were performed on an Avance III DSX300 spectrometer (Bruker) working at Larmor frequencies ν0 = 121.495 MHz, ν0 = 116.642 MHz and

ν0 = 78.204 MHz for

31

P, 7Li and27Al nuclei, respectively.

momentum I = ½. 7Li (respectively

27

31

P is a nucleus with nuclear angular

Al) is a quadrupolar nucleus with I=3/2 (respectively I = 5/2).

MAS NMR experiments were performed at room temperature on powder while variable temperature experiments were performed on sintered sample. The details of the experimental setting can be found in tables S1-S4 of SI and in ref. (41,42). Typically, the lengths of the pulses were t90(7Li)=3.5µs (nonselective), t90(6Li)=12 µs, t90(31P)=3.5µs, texc(27Al)=1.µs (non selective) and the delays between transients were between 5T1 and 10T1 . It is well established that in addition to the chemical shift, the quadrupolar interaction itself can be a rich source of local information about solid. Among the three studied nuclei, two are quadrupolar ones characterized by the further quadrupolar contribution given by the Hamiltonian: (1)

HkQ

=

ωkQ

+2

∑

m =−2

k (−1)m V−km Tm for the kth site and


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C kQ e2qk Q = ωkQ = = 2 πν kQ 2I(2I − 1)h 2

(2)

where X km ( X = T or V) is the mth component of the second order irreducible tensor X k

k

k

of the kth

(

nucleus. T stands for the spin part of the quadrupolar interaction while eqV βLk , αLk

)

(the

anisotropic spatial part) accounts for the local electric field gradient felt by the kth7Li site (the angles r βLk and αLk identify the direction of the static magnetic field B0 in the principal axis system of the

electric field gradient tensor): (3)

eqk V0k ( θ, φ) =

 e2qk Q  1 (k) 3 2 2 sin2 βLk cos 2αLk  .  3 cos βLk − 1 + η 2I ( 2I − 1) h  2 8 

(

)

The electric field gradient is defined with its amplitude C kQ (kHz) or ωkQ (kHz) and its asymmetry parameter η kQ . For the half integer quadrupolar spins, this interaction acts at first perturbation order on the satellite transition and at second perturbation order on the central and satellites transitions. In a powder, the k1 satellite transition anisotropy generated by this first order term is equal to ∆ωQ( ) = 3ωkQ (satellites

transitions), while the central line will have an anisotropy due to the second order term equal to

(4)

) 3( ( ) 25  ∆ω =  I(I + 1) −  k 2 Q

k (1) ωQ

9 

4  16ω0

2

75 ( ω ) =

k 2 Q

16

ω0

.

When it is enough important, this term of the second order gives a particular shape to the central line of a half-integer spin spectrum. The disorder in the quadrupolar parameters has characteristic effects on the spectra of the quadrupolar nuclei with half integer spins. By acting in first order of perturbation on the satellites transitions and in second order on the central transition, the intensity of the latter is strongly enhanced with regards to that of satellites.


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Together with solid echo, it is possible to obtain more precise information on quadrupolar parameter and dynamical effects by performing Spin Alignment Echo (SAE) experiments allowing to create quadrupolar and/or dipolar order(s).43-50 The original Jeener-Broekaert sequence

43

(π/2)x--tp--(π/4)y--

tm--(π/2)θ--tp--Acq allows to create quadrupolar order and/or dipolar order. With short tp, preparation period (tp=10 µs in our experiments) we select the strongest interaction. Short pulse duration (t90 liquid = 3 µs) allows to avoid distortion of the signal. With fixed tm mixed time, we obtain the Spin Alignment Echo (SAE) spectrum. 2D experiment with variable tm is used for relaxation of quadrupolar order (T1Q). Our experiments were carried out by using the modified Jeener-Broekaert sequence with a fourth pulse between the first (π/2)x pulse and the first (π/4)y pulse: (π/2)x--tp/2-(π)x--tp/2--(π/4)y--tm--(π/2)θ-tp--Acq.45 The fourth (π)x pulse refocuses the effects of resonance offset and B0 inhomogeneity. Without quadrupolar interaction, it is impossible to create quadrupolar order but it is possible to obtain dipolar order. Not taking into account spin relaxation effects, the signal amplitude generated by the Jeener-Broekaert sequence evolving under quadrupolar interaction HQ (see relation (1)) is given by: (5)

(

)

SQ 2 tp , tm , t =

(

)

9 sin ωQ ( 0 ) t p sin ( ωQ ( t m ) t ) 20

where brackets ..... indicate the powder averaging. This signal gives two antiphase peaks associated to the two satellite transitions and without central line. Relation (5) assumes that the quadrupolar frequencies (see equ; (2)) are well defined during the dephasing (ωQ(0) during tp) and the detection periods (ωQ(tm) during t). It means that during these periods ωQτQ>>1. If we are not interested in quadrupolar energy relaxation, such an experiment gives precise information on quadrupolar parameter νQ and we are able to detect different sites with different non zero νQ while solid echo experiments detect all the sites (with νQ≠0 and also νQ=0). Furthermore experimental data can be fitted with the function


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(6)

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 βSAE ( t p )     S 02 t p , t m , t p =  S 0 + S1 exp  −  t m / τ SAE t p    exp  − t m /T1eff        

(

)

( )

(

β0

)

.  

T1eff is an effective relaxation time which takes into account the different relaxation mechanism. The second term in parenthesis, accounts for the loss of intensity in the correlation due to ionic motion and is well describe by a Kohlrausch-Williams-Watts function with the Kohlrausch exponent βSAE(tp).51 The damping due to relaxation is also expressed by a stretched exponential function with stretching parameter β0. If the ratio (7) S∞ = S0 ( S0 + S1 ) is S∞≠0 usually the amplitude S20 decay proceeds in two steps at fixed tp and variable tm. In the first step (short tm) the decay is due to individual jumps of the ions and the second one is due to the spin lattice relaxation. This ratio (7) directly reflects the inverse of lithium sites equivalently participating in the diffusion process. Certainly this assertion holds only if these Li sites are equally populated. The DMFIT software is used to fit the spectra and to obtain the peak linewidth, peak position (in Hz or ppm), percentage and quadrupolar splitting.52 31P spectra are referenced from H3PO4(85 %), 7Li from LiCl,

and

27

Al

from

Al(NO3)3.

Results

are

expressed

either

in

Hertz

or

in

ppm

(X(Hz)=X(ppm)ν0(MHz)).

3.

Experimental Results 3.1 XRPD results XRD pattern of Li1.3Al0.3Ti1.7(PO4)3 recorded at room temperature is shown in figure 2(a). Rietveld

refinement of XRD diagram was carried out using NASICON framework model. All peaks were indexed in R 3 c space group evidencing no impurity. Occupancy rates were refined in both octahedral site, with Ti4+ and Al3+, and tetrahedral site with P5+ and Al3+. The best reliability figures were obtained with tetrahedral site fully occupied by phosphorus while octahedral site is shared by titanium (~85%)


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and aluminium (~15%). Despite the lack of precision of XRD for light atoms which prevented us to determine directly Li positions, we tried to refine different propositions previously reported in literature. Several distributions of these cations among the already known M1 (6b), M1’ (36f), M2 (18e) and M3 (36f) sites were tested through Rietveld refinement. Fully occupied M1 and partially occupied (~5%) M3 sites led to the best refinement (lowest reliability factors). It is worth noting that, using XRD for the refinement of lithium positions, the M1 and M1’ sites in the NASICON cage were not distinguishable and led to similar results. The fitting parameters of XRD pattern, atom positions, and distances between neighbor atoms in the unit cell are presented in table 1, table 2, and table 3 respectively. The NASICON structure is presented on figure 2(b). We can recognize the characteristic NASICON framework formed of M2(PO4)3 entities organized along c axis leaving free antiprismal cages for Li+ cations. The preservation of symmetric elements of R 3 c space group shows a statistical distribution of cations on their sites: octahedral sites occupied by Ti4+ and Al3+ are not distinguishable by XRD as well as occupied and empty M3 Li+ sites. Indeed, XRD shows averaging electronic densities on these sites. Anyway, these different chemical environments may be distinguished by NMR which is a sensitive local probe. Particularly 31P nuclei may be very sensitive to the presence of titanium or aluminium as second neighbors. Therefore, the different chemical environments of 31P, predicted from the structural resolution, are given in table 4 with their probabilities. Only the configurations with Al/Ti second neighbors are considered. Eight different chemical environments are thus reported. In the same way, 27

Al may be sensitive to the presence or absence of lithium as second neighbor. Unfortunately, the lack

of precision of XRD results on lithium positions prevents us from calculating the probabilities of different environments. It is worth noting that measures of transverse relaxation time T2 on 7Li and

27

Al highlighted the

instability of samples and the necessity of a waiting time before stabilization. The samples of the last synthesis, the results of which are presented here, have undergone several cycles from ambient ACS Paragon Plus Environment

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temperature to 120°C, which allowed stabilizing samples more quickly. These cycles do not modify the structure at all. These cycles were stopped when the T2 values were stabilized. Such a behavior is understandable by a relaxation of the strains in time.

3.2 NMR results Chemical shift interaction (31P) and quadrupolar interaction (7Li and

27

Al) contribute to the line

position in the spectra of the three nuclei under consideration while homo and hetero dipolar interactions broaden their lines. Anisotropy of all the interactions also contributes to the line broadening.

3.2.1

27

Al results

The room temperature

27

Al high resolution MAS spectrum recorded with the spinning frequency

νR=10 kHz with a solid echo sequence is presented in figure 3(a). The left inset gives

27

Al spectrum

recorded in the MAS synchronous mode (in which the spectral width is equal to the sample spinning frequency νR=25 kHz) while the right inset gives the spectrum recorded in the static mode (without sample spinning) used in variable temperature experiments. The MAS spectrum is a specific powder MAS spectrum for a half quadrupolar spin nucleus with an intensive line flanked by its spinning side bands. The most intensive line which corresponds to the isotropic contribution of the central transition (CT=+1/2↔-1/2 transition) is characteristic of a central line with a weak second order quadrupolar contribution smoothed by disorder. In order to obtain a better resolution, we recorded a MAS synchronized spectrum with νR = 25 kHz at room temperature. This spectrum gives evidence to a single line at -16.1 ppm which is typical for aluminum in octahedral site.53This line is a quadrupolar structure less line and is fitted with a Gaussian-Lorentzian line. The calculated spectrum is obtained with the parameters given in table 5. The quadrupolar parameters νQ=274 kHz and ηQ=0.9 were determined from the spinning side bands with DMFIT. With this software, each spinning side band is individually fitted. In our case, experimental and calculated ACS Paragon Plus Environment

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are confused. Then from their intensities Massiot software calculates the asymmetry and anisotropic parameters. It is worth noting that ab-initio calculation with WIEN’s program gives νQ=827 kHz in the range of the experimental value but with ηQ=0 at variance of the experimental value.54 The apparent discrepancy between the experimental and calculated values originates from the Al3+position which was taken at the 12c site exactly (with 3 symmetry). Figure 3(b) gives the calculated spectrum obtained with the SATRAS (SAtellite TRAnsition Spectrum) method and from the parameters given in table 4.55 The intensities of the spinning side bands given by the calculation do not agree with those observed. Also, the central line possesses a quadrupolar structure not observed experimentally. We can note that for the value of νQ=274 kHz, DMFIT also gives a central line with a second order quadrupolar structure which we smooth by introducing some broadening due to disorder in its environment. In static mode (without MAS), only the central transition is observed and no quadrupolar structure is observed on this transition because they are smoothed by the dipolar broadening and disorder. Nevertheless, dipolar broadening is averaged in MAS spectrum but even in this case, no second order quadrupolar structure is evidenced owing to disorder. The intensities of the spinning side bands originate from the first order quadrupolar interaction contribution given by the Hamiltonian (1) and acting on the satellites transitions. These spinning side bands are images of the central transitions modulated by the amplitudes of the satellite transitions (ST=±1/2↔±3/2; 3/2±↔±5/2 transitions). However the intensities of the latter do not correspond to those calculated (see fig. 3b). This is a mark of the disorder (in the dipolar and quadrupolar interactions) which cannot be averaged by the MAS. Satellites being more sensitive to the quadrupolar disorder than the central transition, they are more broadened and their amplitudes are much more reduced than that of the central transition. Thus, with our synthesis process we are able to produce Li1.3Al0.3Ti1.7(PO4)3 sample with aluminium only in an octahedral site. Nevertheless, the experimental quadrupolar value ηQ = 0.9 means that


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aluminum site has lost its axial symmetry (Al site substitutes Ti on 12c site of R 3 c space group on 3 axis) due to disorder in the P-O and Al-O bonds.

3.2.2 7 Li results 7

Li single pulse spectra recorded in static mode at different temperature are sketched in fig. 4(a).

These are typical powder spectra of a I=3/2 quadrupolar spin with ηQ=0 in which the central transition (CT=+1/2↔-1/2 transition) and the satellites transitions (ST==±1/2↔±3/2 transitions) are clearly observed. It is worth noting that ηQ parameter remains equal to zero for each temperature The specific powder line shapes originate from the first order quadrupolar interaction contribution given by the Hamiltonian (1). This figure shows that the external transitions are better and better resolved at increasing temperature: due to motional narrowing, the broadening by the dipolar interactions and the disorder are averaged. It gives also evidence of the abnormal behavior of the CQ (or νQ) quadrupolar parameter which increases at increasing temperature when we expect a decrease of this parameter due to motional averaging. Solid echo and single pulse spectra recorded at RT in static mode are compared in figure 4b. The single pulse experiment gives a more intensive central transition contribution. This figure also shows that the solid echo pulse sequence (which favors the refocusing of the quadrupolar interaction) improves the ratio between the contributions of the satellite and central transitions. All these results show that the difference between the relative intensities of the central and satellite transitions observed at RT in the single pulse experiment originates from the loss of the first acquisition points of the Free Induction Decay in the dead time of the probe. The inset of this figure gives a fitting of the experimental data with a single quadrupolar contribution the parameter of which being CQ=48 kHz and ηQ=0. We attribute the weak discrepancy between experimental and calculated spectra to dipolar interaction and disorder, the last one acting at the first order of the perturbation on the satellite transitions and in the second order on the central transition. ACS Paragon Plus Environment

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Solid echo (SE) recorded at RT with MAS mode at 295 K and spinning frequency νR=5 kHz is reported on figure 4c. This spectrum is composed of the isotropic line (the most intensive one which corresponds mainly to the central transition) flanked by several spinning side bands (less intensive lines marked by *) due to the quadrupolar anisotropy. This spectrum is fitted by using the SATRAS method 55

. The isotropic line (right inset of fig. 4c) does not evidence any second order quadrupolar effect. This

is also observed at νR=25 kHz (at RT both using single pulse and SE). We need a single first order quadrupolar contribution whose parameters are given in table 5. The fit are rather well as it can be viewed on the insets of this figure which give an overview on the reconstruction of the spinning side bands and the isotropic line: these amplitudes are well accounted. Therefore only one site is evidenced from these high resolution MAS experiments. It is worth noticing that these values are closer to those obtained by SE in the static mode: CQ=48 kHz and ηQ=0. These results are very similar to the ones obtained in previous works 14-21, 31. When the motional averaging process operates the central line is averaged before the satellite ones. It's exactly what we observe on figure 4d), where the satellite transitions become more and more resolved. Furthermore, at RT, the solid echo leads to a nearly theoretical ratio 2/3 between the CT and the ST and we can deduce that no disorder is present at this temperature (disorder in the quadrupolar parameters would smooth the satellite transitions at first order and the central transition would appear enhanced). The discrepancy between calculated and experimental spectra originates from the orientation of the dipolar tensor with regard to the quadrupolar one and also because the motion is not an isotropic one. These results correspond to lithium in axial symmetry site (as in M1 site) or to dynamical occupation of M12 site around the same symmetric axis (M1 and M12 corresponding to 6b and 36f sites of R 3 c space group). However, the observed single value of CQ can also result from an average on the values taken on various sites visited by the lithium.


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Figure 4(d) gives the experimental CQ parameter together with the theoretical curve obtained with model of equation (8). In this model we suppose the increase of CQ at increasing temperature is due to jumps which allow the lithium to reach higher energy level with different CQ values.

3.2.3

7

Li Spin Alignment Echo (SAE)

Figure 5 gives an example of the SAE spectrum recorded at 400 K with the Jeener-Broekaert modified sequence 45. The sharpness of the satellite transition which is preserved down to 210 K allows us to: (i) confirm that there is a single contribution to the spectrum and (ii) precisely measure the νQ parameter (inset figure 5). The present results indicate that (i) there is no disorder at T>210 K in the quadrupolar parameters (ii) this parameter increases with the temperature. Finally, with the SAE experiments we observe again that ηQ remains null and that no change of the spectrum shape was observed at high temperature apart from a better resolution. So, the SAE experiments confirm the increase of the parameter CQ at increasing temperature (inset fig. 5), while a decrease is expected. This was also observed on fig. 4b where the spectrum appears to be more and more structured upon heating. This increase of CQ parameter at increasing temperature highlights an anisotropic motion of the lithium and/or a distortion of its site. The null value of the ηQ parameter which is conserved at each temperature (a least above 200 K) indicates that the residual electric field gradient remains (at least in average) symmetric. "In average" means that the lithium ions move around some “pseudo” local symmetry axis. This completely agrees with M. Catti et al. results 36. It is worth noting that ab-initio calculation with WIEN's program 54 shows that the axis of the Vzzk ( ∝ V0k ) component points towards the titanium ions

55

with a value CQ=20 kHz

at T=0 K while we find CQ=48 kHz at room temperature (CQ increases as temperature is increasing). Thus, experimental CQ value is in the range of the calculated one.


14

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3.2.4

31

P spectrum

The interest in the phosphorus embedded in oxygen tetrahedron lies in its sensitivity to the second neighbor as it was shown in several papers.17, 23, 25-27, 42 In figure 6, we report the high resolution (MAS experiment) NMR

31

P spectrum recorded at RT with the sample spinning at 25 kHz. The inset of this

figure gives the spectrum recorded in the static mode. This last spectrum is fitted with a single Lorentzian/Gaussian broad line. This line without structure is due to, on one hand, its broadening by the anisotropy of the chemical shift and dipolar interactions and on the other hand, by disorder. In LiTi2(PO4)3,23 the structural studies showed that the phosphorus ions are located in only 1 crystallographic site and only one line is observed at -27.5 ppm in the MAS

31

P spectrum. In the

modified systems Li1+xAlxTi2-x(PO4)3, even if the XRPD refinement leads to only one crystallographic site, the 31P spectrum is drastically modified. With the high resolution experiment (MAS mode), six 31P different chemical sites can be evidenced from the breakdown of the line slope, as indicated by the arrows in this figure 6. The reconstructed spectrum parameters are given in table 6. In figure 7, the

31

P line width together with the transverse relaxation rate R2 versus reciprocal

temperature are shown. Two interesting facts are evidenced: the weakness of these slopes and their opposite signs (negative for R2 and positive for δ). This means that: -

the line width does not follow any activated motion law and at infinite temperature, the line width reaches a finite value around 3000 Hz

-

transverse relaxation rate R2 and line width do not undergo exactly the same fluctuating process.

3.2.5 Second moment M 2 In table S5 of SI, we report the contributions to the second moments (dipolar interaction contributions to line widths) for the various nuclei

58

.

These parameters strongly depend on

geometrical parameters and are related to the line width (without MAS) in the rigid lattice by the relation δ=(2Ln(2))1/2∆ for a Gaussian line (M2=∆2). Contributions are obtained positioning the lithium


15


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Page 16 of 50

ions at the centers of the cages while the ones of aluminum were evaluated by positioning the corresponding ions in the Ti4+ sites. All the calculated values are weaker than the experimental ones. There are three reasons for these discrepancies: -

we do not take into account the anisotropy of the quadrupolar interaction (7Li and

27

Al) and the

anisotropy of the chemical shift interaction (7Li, 27Al and 31P). -

the ions are not exactly at the exact crystallographic position.

-

the line shapes are not exactly Gaussian. Whatever the temperature and the nucleus, the free induction decay does not follow a gaussian law (M0exp(-∆2t2) with ∆ the second moment). The results presented in this work were obtained for the specific NASICON compound with

chemical composition Li1.3Al0.3Ti1.7(PO4)3 but should be valuable for other compounds presenting the same NASICON skeleton. However our experiments give evidence to the instability of the fresh samples and before going further in the sample studies it is necessary to wait for its stabilization (several weeks). We highlighted these instabilities because our experiments were performed several times before publishing our results. The measure of relaxation times (T1 and T2) on 7Li and/or

27

Al

nuclei are good tests to check the stabilization. At RT T2(7Li) lengthens when the sample ages. T2(7Li) and T1 T2(7Li) present hysteresis phenomena in temperature which disappear when the compound is stabilized. It can take several weeks. The experimental rigid lattice line width limit obtained is 5380 Hz for 7Li and around 2300 Hz for 27

Al. These values, which are higher than the calculated ones (4670 Hz for7Li and 1200 Hz for 27Al),

indicate that these nuclei experience some noticeable anisotropic contributions to the experimental values (chemical shift and quadrupolar) and/or disorder. At low temperature (120 K), 31P experimental line width is about 5500 Hz, more than the rigid lattice calculated value (3940Hz). Although this value is on an ascending weak slope (see figure 7), it suggests


16

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that the value of rigid lattice will be reached at very low temperature. This means that the phosphorus does not feel any activated motion.

4.

Discussion The indexation and then the Rietveld refinement of XRD pattern showed a pure Li1.3Al0.3Ti1.7(PO4)3

NASICON phase. MAS NMR investigations carried out on 27Al give evidence to a single contribution observed at -16.1 ppm which, without doubt, has to be assigned to the 27Al in octahedral site 53 within the LATP structure. Thus, XRD and MAS NMR show without doubt that in this pure NASICON phase, aluminium ions Al3+ occupy exclusively the octahedral sites of the NASICON framework (12c in R 3 c space group) sharing them with titanium ions Ti4+. Such results are quite different from those reported in previous works

11, 18, 19, 23

: the authors indicated a second line on the

27

Al spectrum at 39

ppm for the same composition. This line was attributed to Al3+ in a tetrahedral site. This observation was explained by either the presence of a secondary AlPO4 phase

19

or by aluminium ions occupying

partially the tetrahedral sites of the NASICON framework owing to some substitution Al/P

23

.

Nevertheless, from the formulation Li1.3Al0.3Ti1.7(PO4)3, we can note that the tetrahedral sites (18e in R 3 c space group) should be fully occupied by the phosphorus ions (Z = 6, 18 P

5+

per cell) since these

ions present a strong preference for tetrahedral coordination while aluminium may accept both tetrahedral and octahedral coordinations. Thus, our results demonstrate the purity of the sample synthesized by Pechini process, this soft chemistry route allowing a better control of the stoichiometry than classical solid state reaction. The 31P isotropic NMR MAS lines range around the position of the orthophosphate line at -27.5 ppm as found in non-substituted LiTi2(PO4). The line positions in the phosphorus spectrum are very sensitive to the second neighbor and the change of P-O-M bond angle which may affect the magnitude and/or the orientation of the chemical shift tensor. With the configuration envisaged in the table 4, we


17


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Page 18 of 50

expect to observe at least 8 lines. Without taking into account the lithium ion, the main contributions obtained from table 4 arise from: -

phosphorus with 4 titanium ions (Ti4a=18.2%, Ti4b=18.2%, Ti4c=4.5%) (See S. I.)

-

phosphorus with 3 titanium ions and one aluminium ion (Ti3a=16.2%, Ti3b=36.4%, Ti3c= 3%)

-

phosphorus with 2 close aluminium ions (Ti2a=2%, Ti2c=1.5%).

If we took the lithium into account, we would multiply the number of different phosphorus chemical sites. Anyway, these lines are not quite detectable because of their weak contributions or because several of them are staked, this may be due to the important local motion of the lithium ions. In order to attribute the lines observed on the 31P spectrum, we have to take into account the missing positive charge on the aluminum, with regard to the titanium, and the distortions engendered by the substitution. The charge compensation would lead to a shielding of the phosphorus that is not observed. Nevertheless, we can reasonably think that the charge compensation is distributed on the AlO6 octahedron and in the neighbor of this octahedron. The net effects are contributions on P-O bonds in PO4 tetrahedron. On another hand, the P-O bond distortions, due to the motion of the M2(IV)(PO4)3 skeleton, contribute also to the broadening. The flexibility of the M2(IV)(PO4)3 skeleton, already mentioned by several authors 58,59,60 and observed by Infrared Spectroscopy 30,60, gives rise to distortion in P-O and Al-O bonds. This flexibility allows a rotation of the oxygen ions around the c axis, alternatively clockwise and counterclockwise, involving a distortion of the PO4 group. All these displacements result in an elongation along the c axis of the LiO6 antiprism around M1 site. A more precise analysis, given in part II and reported in figure 7, of the 31P line width behavior together with the transverse relaxation rate behavior versus reciprocal temperature allow us to conclude that the 31P spectrum is very sensitive to the P-O bonds. For example, looking at the fig. 6, we observe that the MAS is more effective in the narrowing than the temperature. Then looking at figure 7, we can observe that the relaxation rate R2 does not follow the same behavior as the line width. Therefore, we can reasonably conclude that the weak discrepancies between the different contributions are due to ACS Paragon Plus Environment

18

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different P-O bonds and thus are related to the distortion of the [PO4]3-tetrahedrons. This conclusion agrees with Infrared observations

30,60

in which IR line broadening was explained by distortions of the

orthophosphate units due to the insertion of Al3+ ions in the structure, these distortions being due to the ionic radius variations with the substitutions. The attribution of the lines will be then made considering the different phosphorus second neighbors as described above and taking into account that lithium is or is not in the cage (table 6): Line (1)=Ti4a, line (2) =Ti4b+Ti4c Line (3)=Ti3a, line (4)=Ti3b Line (5)=Ti2a, line (6)=Ti2c Doing this classification we also took into account line positions and line widths of the contributions and by considering that: The titanium

47

Ti and

49

Ti have very weak natural abundances with small gyro magnetic constant

leading to small line broadenings. The substitution Al3+/Ti4+ introduces some broadening due to some disorder as observed on

27

Al

spectra. We can go further in the analysis of the previous results by proposing a new interpretation. This one is not contradictory with the previous results and takes into account physical characteristics of the NASICONS under consideration. Let consider the 31P results as discussed above. For each of the configurations (0 Al, 1 Al, 2Al), we obtain a pair of lines which are understandable in the following way: the distortion of the skeleton M2(IV)(PO4)3 gives rise to compressed regions where the bonds P-O are shortened and regions where the bonds P-O are lengthened. In every pair of lines, one corresponds to the compressed zone while the other one is attributed to the dilated zone. The wide lines correspond to regions where the substitution gives rise to some disorder in the bonds (due to the presence of aluminum). Looking at the NASICON


19


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Page 20 of 50

framework, the case a (table 4), corresponding to two consecutive AlO6 octahedron along c axis, seems to be the one leading to the most important distortion. This confirms the attribution proposed above. The 27Al quadrupolar spectrum is characterized by a single line with quadrupolar parameter νQ=220 kHz and a strong asymmetry parameter ηQ=0.9 which corresponds to a strongly distorted aluminium environment. This is not consistent with the 3 axis implied by the 12c position of Al3+ in the R 3c space group and given by XRD. This discrepancy is not very surprising because XRD is sensitive to geometrical environment while NMR is sensitive to the chemical one. The possibility that the aluminum belongs to a secondary phase must be ruled out because such a phase was not observed in the XRD experiments. However, the distortion of aluminum site can be understandable by the distortion of the skeleton M2(IV)(PO4)3. As AlO6 octahedrons share oxygen atoms with PO4 tetrahedrons, we can conclude that aluminum ions, as phosphorus, are sensitive to the distortions which result from the motions of the skeleton M2(IV)(PO4)3. This entails a distortion of the environment of the aluminum, and thus, a loss of the axial symmetry of the site of this ion. Thus, Al3+ remains on 12c site but its environment is sensitive to distortion of the M2(IV)(PO4)3 skeleton. This is consistent with the second moment calculation which gives values very different than the experimental ones. Let us recall that these calculations were made with the aluminum exactly at the 12c crystallographic site. Certain authors had recommended that the improvement of the conduction had to be due to an improvement of the densification by the formation of an aluminum secondary phase which would form at the grain boundary 61, 62.Such a secondary phase reduces the grain boundary resistance and increases the densification of the ceramic as it was suggested by porosity value which is 34% in LiTi2(PO4)3 and only 4% for Li1.3Al0.3Ti1.7(PO4)3. Generally this densification is considered as the origin of the improvement of the conductivity in mixed systems. However, our results are not consistent with the existence of a neither secondary phase as XRD show neither crystallized impurity nor glassy phase


20

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(smoothed background) while NMR indicates only one aluminum contribution. We can guess that, actually, it is the disorder in the P-O and Al-O bonds and/or the vibrations of the skeleton, implied by the insertion of aluminum, which induce the improvement of conduction properties. The combination of MAS single pulse, static solid echo and spin alignment echo 7Li spectra allows us to conclude that there is a single axial (ηQ=0) magnetic site for this ion with parameters given in table 5. These features are observed above Tc0 (the satellite transitions and the central transition are well differentiated above this temperature). Spin alignment echo gives a precise determination of the νQ parameter. Nevertheless the observation of a single magnetic site for the lithium is contradictory to the formulation of the sample Li1.3Al0.3Ti1.7(PO4)3 which requires to place 7.8 Li in the elementary cell while we have only 6 accessible M1 (or M1’) sites. Thus, after we have placed 6 lithium ions on these sites, we have to place 1.8 lithium ions in the cell out of these sites leading to at least 2 contributions to the lithium spectrum. At RT, we recorded several spectra with various spectral widths and various recycle times with non-selective excitation, but we were unable to evidence any second contribution. An attentive study of these spectra turns out to be necessary below Tc0=170 K. At increasing temperature from Tc0, the quadrupolar features begin to be well marked on single pulse spectra, when they appear only above 190 K on echo spectra. Furthermore, above Tc0, it is difficult to account for the ST and the CT owing to disorder which acts differently on CT and ST, and also because of the motion. Thus, below Tc0, we expect that the different broadenings (disorder, anisotropy which are averaged by the slow motion) smooth the quadrupolar structure and both the central and satellite transitions are accounted by Gaussian/Lorentzian line shape in the ratio CT/ST=2/3. Therefore we followed the solid echo spectra from Tc0 down to 120 K in order to give evidence to a second contribution when the motion has no effect on the spectra. This way we observe that below Tc0, the spin echo spectra are well reconstructed with two Gaussian/Lorentzian (G/L) lines. In these temperature range these two contributions are nearly at the same position (the shift is less than 0.2 ppm) and with the ratio


21


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Page 22 of 50

corresponding to the theoretical value. So we can conclude that we have a single contribution to the spectrum as it was observed in MAS spectrum obtained at RT. This contribution is sensitive to some disorder at low temperature (see SE spectrum fig. 4d and fig. 5b). The disorder observed at low temperature indicates that the lithium ions are not centered on M1 sites but occupy general positions (M1’ and M3 or other). We analyzed more in detail the SAE between 180 K and 220 K where we observe a change of shape of the amplitudes of the SAE as reported in figure 5b. The normalized amplitude of SAE, given in figure 5c, indicates that in this temperature range the change is due to a modification of the plateau value and a modification of the slope. The first decrease is due to a further interaction and the shape change is due to a decrease of the Kohlrausch exponents. The further interaction is a dipolar one which modifies the contributions of the central and satellite transitions, increasing the ratio CT/ST. In order to account for the experimental data by means of (6), we have to introduce below T≤210 K, a distribution of residence times τSAE and of relaxation times T1Q which broaden below 210 K. These distributions are characterized by their Kohlrausch exponents (β0 for T1Q and βSAE for τSAE) which decrease at decreasing temperature (see table 7). It is clear that these small exponents imply that a significant part of the correlation decay will take place outside of the experimental time window i.e. at times shorter than 10 µs. These results indicate that the change of shape of the amplitudes of the SAE appears when the distribution of τSAE contains values of same order or lower than T1Q. These effects can be interpreted in terms of mixed mobile ions effects (MME) 63, 64: the broaden distribution of τSAE, is due to mobile and less mobile lithium. While the MME is accompanied by local structural changes as previously evidenced for phosphorus and aluminum, the global network is not affected. Let us now consider the CQ parameter behavior, as it can be seen figure 4d). The increase of this parameter at increasing temperature is surprising because we expect its decrease due to motional narrowing. This surprising effect was already observed in Li1,3Al0.3Ti1.7(PO4)3


17,19,25

and also in

22

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Li1.3Al0.15Y0.15Ti1.7(PO4)3 31, LiTi2(PO4)3 23, LiGe2(PO4)3 27 and LiSc2(PO4)3 23. It cannot be due to Al3+ substitution but is more certainly a property of the M2(IV)(PO4)3 skeleton containing Li+. The single contribution can be explained in the following way. In the NASICON network, the cages occupied by Li+ ions (M1, M1’ and M3 sites) are too large, with too long average distances Li-O (> 2.2

Å). Therefore, we can expect that the lithium shifts to a more appropriate off-centered position. In the case of crystallographic sites M1’ and M3, we can observe, inside the cages, that the lithium accommodates in a triangular oxygen plan with shorter Li-O distances (1.8 < d < 2.15 Å). This would mean that, despite the existence of two different crystallographic sites, the immediate environments of 7

Li represented by the closest oxygen atoms (which monitor the quadrupolar interaction) are similar

and the motion makes axial these sites. In this ways both in the two environments the quadrupolar parameters are the same. This explains the difference between experimental and calculated second moments and the disorder observed on the different spectra. By pursuing our argumentation, we cannot attribute the variations of CQ with the temperature (fig. 4(d)) to a process of jumps between two sites 1and 3 with the quadrupolar parameters CQ1 and CQ3 respectively. Such a process between asymmetric sites65 gives a static average CQ parameter: (8)

C Q = C Q3

W1 W3 . W1 (respectively W3 ) is the probability the ion + C Q1 W3 + W1 W3 + W1

jumps from site 1 to site 3 (respectively from site 3 to site 1). W1 = W01e

−

Ea1 kT

W3 = W03e

−

Ea3 kT

, W01

(respectively W03 ) is the jump attempt per second for the ion jumps from site 1 to site 3 (respectively from site 3 to site 1). The results in fig. 4(d) are obtained with W01=820 ± 50 s-1, W03=1800 ± 100 s-1, CQ1=300 ± 20 kHz, CQ3=24 ± 5 kHz , Ea1=0.17 ±0.05 eV Ea3=0.13±0.05 eV. However, at low temperature, where the effects of the motions are unimportant, we should have observed two very different components on the spectrum (CQ1=300 kHz and CQ2=24 kHz). These two components were not observed. Therefore, these variations are due to the distortions of the cages which


23


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Page 24 of 50

we also highlighted above in the cases of the phosphorus and the aluminum. These cage distortions are induced by non-symmetrical vibrations of the atoms59 which shift from their average position. As underlined by V.I. Pet’Kov et al.59, this motion leads to a modification of the size of the oxygen cage in which the lithium ion is located. The Li+ ion is submitted to a non-symmetrical (anharmonic) thermal vibration and the lithium electronic states become also dependent of the vibration state. In this case, already evocated in31, CQ fluctuates around an average valueCQ to which the NMR spectrum is sensitive; this average value depends on the temperature, because of the coupling of the electronic states with the states of vibration. The shift of the atomic positions is a characteristic observed in displacive phase transition66. Furthermore, the local motion of the lithium ions in the neighborhood of the plans of oxygen as it was described above, makes symmetric the quadrupolar tensor (ηQ=0). Finally, these motions which induce thermal expansion57 allow the Li+ ion to migrate from one site to another one

59

. It is worth noting that such motions could lead to displacive phase transition61. Such

phase transitions were observed in the LiTi2-xZrx(PO4)3 and LiTi2-xHfx(PO4)3 series27 in which the triclinic distortion is attributed to the bigger ionic radius of the M4+cation. This last distortion disappears when samples are heated. The thermal expansion hypothesis is supported by the behavior of the line width and R2 parameters of phosphorus which are reported in figure 7. This behavior is analyzed in terms of chemical shift fluctuations due to variations of the lengths of P-O bonds. In summary, at low temperature, considering the disorder highlighted by SAE, we cannot distinguish sites M1 and M3 and the spectrum is characterized by a widened quadrupolar contribution. When the temperature increases, the quadrupolar parameters of M1 and M3 change because of the lattice distortions, but remain identical because they are characterized by their immediate environment of three oxygen ions (as explain above) which give the main contribution to the CQ. The differences of length of connection Li-O are averaged at high temperature but participate in the low-temperature to disorder. The motions at long distance of the lithium are monitored by the motions of oxygens. The variations of CQ with the temperature are due to the distortions of the M2(IV)(PO4)3 skeleton of the ACS Paragon Plus Environment

24

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NASICON which causes the dilatation of the oxygen cage around the lithium site. This mechanism is also supported by the stabilization of samples freshly synthesized that we mentioned previously.

5. Conclusion The powder of Li1.3Al0.3Ti1.7(PO4)3 compound was synthesized by Pechini modified method and studied by XRPD and MAS high resolution and static NMR. The first important result lies in the fact that the synthesis process allows us to obtain samples with a single aluminum site corresponding to octahedral environment in substitution of Ti4+ ions, unlike what was observed by other authors. Magic Angle Spinning investigations at RT revealed the presence of 6 chemically different sites for 31P, while single distorted sites are found for aluminium and lithium in the lattice. For 31P, this multiplicity of sites is analyzed as the result of non-equivalent PO4 entities due to distortions of P-O bonds and to the substitution of Ti4+ by Al3+confirming the hypothesis used by different authors to explain the profile of Raman and IR spectra. Secondly, the high ηQ=0.9 experimental quadrupolar parameter value obtained for indicates a low symmetry of aluminum environment. From both XRPD,27Al and

31

27

Al nuclei

P NMR results it

seems reasonable to explain this phenomenon by the flexibility of the M(IV)(PO4)3 skeleton combined with slight distortion of the octahedral environment due to the substitution of Ti4+ ions by smaller trivalent cation Al3+. The spitting of the 31P NMR unique line observed in LiTi2(PO4)3 in six components is explained in terms of three pairs of lines attributed to 31P with 0 Al or 1 Al, or 2 Al, in its neighborhood. The pairs of lines are due to the presence of short and long P-O bonds due to the distortion of the M(IV)(PO4)3 skeleton. The behavior of the CQ parameter is a consequence of the off-site positions of the lithium ions, which prefer to lie inside a triangle of oxygen ions. Such triangular environments can be found between the column of M(IV)(PO4)3 units along c axis. The distortion of this triangle due to the distortion of the ACS Paragon Plus Environment

25


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Page 26 of 50

skeleton led to an increase of CQ when the temperature increases. These off-site positions allow to explain why we can enter 7.8 lithium ions in the cell while observing a single site on the NMR spectrum. This proposition is based on the phosphorus NMR results together with the orthophosphate Infrared spectroscopy. It takes also into account the existence of potential phase transition in such structures. Although our results and their analysis were obtained before the publication67 in March 2016 , it seemed important to us to quote this article. This work is devoted to the lithium localization in the material under consideration and we can compare their results to the ones presented here. Figures 1b of bond valence mismatch given in ref. (67) shows that lithium ions are delocalized in the neighborhoods M1’ of M1 site and M3’ of M3 site.67 Our results agree with this one. Effectively, only one contribution is observed on the NMR Li spectrum which broadens when temperature is decreasing. The analysis of this spectrum in connection with the chemical formulation of the sample allows specifying the off-center positions of these ions which prefer to lie inside a triangle of oxygen ions. Such triangular environments are found at the interface of M1’ and M3’ which are between the columns of M(IV)(PO4)3. The results on the dynamics of the lithium studied in the part II will confirm the formation of the Li-O bonds thanks to the highlighting of the transferred hyperfine interactions. In the reference (68) the importance of the P-O and Al-O bonds was underlined. By studying the behavior of R2(31P) and δ(31P) we showed that six components, observed on the NMR spectrum of 31P, line up by pairs associated with 0 Al or 1 Al or 2 Al in the neighborhood of 31P. These pairs are due to more or less long P-O bonds explained by M(IV)(PO3)4 skeleton distortion, and the width of these contributions is attributed to disorder in the lengths. The CQ parameter behavior versus temperature, together with the behavior of the disordered P-O and Al-O bonds, specify the thermal evolution of the lattice by showing that this one is due to the distortions of the M(IV)( PO4)3 skeleton. Our results are in agreement with the following schema. It is


26

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the second site which allows high conductivity. The lithium ion jumps from a M ' 1 site to a M ' 3 site, but in each of these sites the ion lithium remains in the neighborhood of an oxygen plan which confers it a quadrupolar average interaction. There is no impediment to jumps of lithium ions between sites with same symmetry and leading to the same spectra (in that case, the mobility is not observable by exchange experiments). We suggest that lithium ions lie dynamically in sites in the neighborhood of a triangular plane of oxygen atoms and that the jump may occur in any direction. The 27Al NMR spectrum is a characteristic one of a single octahedral site which is strongly distorted. Finally, we can add the following remarks: unfortunately the syntheses using exactly the same procedure never lead to pure phase when using 6Li isotope carbonate as reagent. These samples would have allowed obtaining more information about relaxation. Nevertheless, the failure of this synthesis makes us wonder the importance of the mass of the cation hosted in the NASICON cage and its effects on the cage stabilization.

Supporting Information Contains: Experiments and Apparatus Characteristics of the sequences Quadrupolar interaction, selective and non-selective excitations Quadrupolar alignment experiments Second moment calculation

Acknowledgment One of us, J. E., would like to thank Dr. Body from “Institut des Molécules et Matériaux du Mans”, for, her calculations on WIEN.


27


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References (1) Salam F.; Weppner W. Solid-state Potentiometric CO2 Sensor based on Li22Co3-MgO Electrolyte,

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Page 36 of 50

Table 1. Structure refinement results for Li1.3Al0.3Ti1.7(PO4)3.

Space group

R 3 c, Z = 6

Peak shape, η

pseudo-Voigt 0.309 (5)

Cell parameters/Å

a = 8.5098(1) c = 20.8305(4)

Unit cell volume/Å3

1306.40(4)

Half-width parameters

u = 0.009(1) v = 0.056(2) w = 0.0124(4)

Asymmetry parameters

P1 = –0.035(3) P2 = 0.0208(9)

RBragg/%

3.60

Rp/%

13.6

Rwp/%

12.1

Rexp/%

9.16

χ2

1.76


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Table 2. Atomic coordinates in Li1.3Al0.3Ti1.7(PO4)3.

site

o.f.

x/a

y/b

z/c

B (10-2 Å2)

Ti

12c

0.85

0

0

0.14154(5)

0.38(2)

Al

12c

0.15

0

0

0.14154(5)

0.38(2)

P

18e

1

0.2887(2)

0

1/4

0.50(3)

O1

36f

1

0.1851(3)

0.9916(3)

0.1894(1)

0.37(4)

O2

36f

1

0.1699(2)

0.4793(3)

0.2476(1)

0.37(4)

Li1

6b

1

0

0

0

0.992(5)

Li2

36f

0.05

0.08(1)

0.35(2)

0.056(6)

0.992(5)


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Page 38 of 50

Table 3. Distances (in Å) between atoms in Li1.3Al0.3Ti1.7(PO4)3 unit cell.

Ti|Al–O1

1.894(3) ×3

Ti|Al–O2

1.966(3) ×3

P–O1

1.526(3) ×2

P–O2

1.538(3) ×2

Li1–O2

2.259(3) ×6

Li2–O1

2.165(2) ×1

Li2–O2

2.191(3) ×1

Li2–O2

2.347(3) ×1

Li2–O2

2.147(2) ×1


38

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Table 4. Different environments of 31P with their ratio

31

P next neighbors

a : 2 Al3+ in 2 consecutive octahedrons

b : 2 distant Al

Probability

names

2 Al3+ + 2 Ti4+

~ 1.5 %

Ti2a

1 Al3+ + 3 Ti4+

~ 3.0 %

Ti3a

4 Ti4+

~ 4.5 %

Ti4a

1 Al3+ + 3 Ti4+

~ 36.4 %

Ti3b

4 Ti4+

~ 18.2 %

Ti4b

2 Al3+ + 2 Ti4+

~ 2.0 %

Ti2c

1 Al3+ + 3 Ti4+

~ 16.2%

Ti3c

4 Ti4+

~ 18.2 %

Ti4c

3+

3+

c : 2 Al in 2 not consecutive but adjoining octahedrons

Table 5. 7Li and 27Al spectrum parameters


39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

δiso(±0.1)

CQ(kHz)

ηQ(±0.1)

Al

-15.6

270 (±20)

0.9

Li

0.6

44 (±2)

0.0

Line 27

7

Page 40 of 50

Table 6. MAS spectrum parameters of 31P at room temperature.

Line

δiso(±0.1)

Width (ppm) (±0.05)

% (±1)

complex

1

–27.9

0.95

18

Ti4a

2

–27.4

1.14

23

Ti4b+Ti4c

3

-26.6

1.71

16

Ti3a

4

-26.

3.36

39

Ti3b+Ti3c

5

-25.6

0.78

2

Ti2a

6

-24.2

0.26

NMR Investigations in Li1.3Al0.3Ti1.7(PO4) - ACS Publications

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