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

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NMR Investigations in Li1.3Al0.3Ti1.7(PO4)3 Ceramics PART III: Local Dynamical Aspect Seen from Aluminium and Phosphorus Sites Joël Emery, Tomas Salkus, and Maud Barré J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11712 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 5, 2016

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The Journal of Physical Chemistry

NMR Investigations in Li1.3Al0.3Ti1.7(PO4)3 Ceramics PART III: Local Dynamical aspect seen from Aluminium and Phosphorus Sites.

Joël Emery1* and Tomas Šalkus2, Maud Barré1* 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 33-02 43 83 33 53

[email protected] Tel 33-02 43 83 33 53 [email protected]

Abstract. In compound Li1.3Al0.3Ti1.7(PO4)3, the investigation by solid state NMR of the T1 and T2 relaxation times of

27

Al and

31

P nuclei are used to study, versus temperature, the local dynamic

properties at different lattice points. Our results can be summarized in the following way: 1 ACS Paragon Plus Environment


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(i)

Inside the NASICON framework, these two nuclei do not undergo any diffusion motion.

(ii)

The motion of the skeleton M(IV)(PO4)3 induces fluctuations in the P-O and Al-O bonds giving rise to quadrupolar fluctuations for 27Al and chemical shift fluctuations for 31P.

(iii)

The static distortions induced by these motions allow explaining some structural aspects.

1- Introduction In part I of this work structural aspects of the modified system Li1+xAlxTi2-x(PO4)3 (with the acronym LATP) was reported.1 The main purpose of that first part was to synthesize and structurally characterize samples presenting a substitution Al3+/Ti4+ exclusively on octahedral site of the NASICON framework (Figure 2b of ref. 1). The acronym NASICON (NAtrium Super Ionic CONductor) represents a structural family of general formula AxM2(XO4)3 where A can be alkali, alkaline earth metal or rare earth, M is a transition metal and X is a small cation (P5+, As5+, Si4+). The first phase of this family, NaZr2(PO4)3 was pointed out by Sljukic et al. in 1967.2 Its structure, determined in 1968 by Hagman and Kierkegaard, consists in a tridimensional framework of PO4 tetrahedra and ZrO6 octahedra sharing oxygens.2 This pattern forms the NASICON skeleton Zr2(PO4)3, or more generally M2(PO4)3, in which Na (or A) cations are distributed. Li1.3Al0.3Ti1.7(PO4)3 belongs to the rhombohedral symmetry (space group R 3c , Z=6) which is typical for NASICON structure. The lattice parameters of Li1.3Al0.3Ti1.7(PO4)3 are a = 8.504 Å and c = 20.881 Å .4-6 Let us recall the main results obtained in part I (apart the synthesis). - In the case of 27Al, both Magic Angle Spinning (MAS) and static modes give evidence to a single site with disorder. This disorder, which prevents to observe any second order quadrupolar structure on the central transition line, is explained by the disorder in the Al-O bonds. Spinning side bands of MAS spectrum allow us to determine quadrupolar parameter CQ=250 kHz and ηQ=0.9.

2 ACS Paragon Plus Environment

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- In MAS mode, the single crystallographic site of 31P splits in 6 different chemical environments which account for the distribution of aluminum in its neighborhood and the distortion of the NASICON skeleton. - In both static and MAS modes, a well resolved quadrupolar structure is observed on 7Li spectra evidencing only one chemical site while the chemical formulation enforces at least two crystallographic sites. Actually, two cages of NASICON framework M1 and M3 were identified to host the lithium. The single chemical site evidenced by NMR, together with the increase against temperature of the quadrupolar parameter CQ (Figure 4d of ref. 1), were explained by the lithium motion inside equivalent triangular oxygen planes between the M1 and M3 cages. Part II was devoted to lithium motion studied in the temperature range from 120 K to 420 K on the Li1.3Al0.3Ti1.7(PO4) sintered ceramic sample studied in reference 1.7 These NMR studies consisted measurements of relaxation times of the Zeeman energy T1Z=T1 , T2, T1ρ, and of the quadrupolar energy relaxation time T1Q for 7Li. The T1 of 6Li results contributed to evidence that the relaxation of the lithium is due to transferred hyperfine fluctuations and particle diffusion. A model accounting for all the experimental results was proposed and validated. This part III is devoted to the study of the dynamics of aluminum and phosphorus ions in the same Li1.3Al0.3Ti1.7(PO4) sample and in the same temperature range as for part II. This dynamical aspect was observed through the measurements of T1 and T2 relaxation times of 27Al and 31P nuclei. So far no joint study of the dynamics on the site of the phosphor and the aluminum was realized in the substituted compounds. These dynamic results allowed establishing a plan of the static distortion of the lattice which was abundantly used in the part I to establish several important structural results. That is why the analysis of these NMR results of this part is particularly detailed.



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2- Experimental procedure Nuclear Magnetic Resonance (NMR) experiments were performed on an Avance DSX300 spectrometer (Bruker) working at Larmor frequencies ν0 =121.495 MHz and ν0=78.204 MHz for and,

27

Al nuclei, respectively.

31

P is a nucleus with a nuclear angular momentum I = ½.

27

31

P

Al is

quadrupolar nuclei with I = 5/2 spin value. Dynamical properties of 31P and 27Al nuclei were studied in the temperature range 120 K-420 K. The experimental proceeding for the temperature control was already reported.

8,9

Setting parameters are

summarized in the SI file of reference (1). The T1 spin lattice relaxation time, which probes the Zeeman energy exchange between spin system and the lattice, were obtained by using the saturation-recovery sequence. In the case of 31P all the transition are saturated then observed. In the case of 27Al, because the satellite transitions are unobservable in the static mode, all transitions are saturated and measured with a non-selective excitation but the analyses were performed on the central transition. Pulse durations were t90(31P)=3.5 µs and texc(27Al)=1.5µs. T2 experiments were performed with the CarrPurcell-Meiboom-Gill sequence the delay between transients was taken tdel≈10T1 although a saturation comb before each transient was used.10 27

Al is a quadrupolar nucleus with spin I=5/2 evolving under the quadrupolar interaction HQ given

by:

HiQ = ωiQ

+2

∑ (−1)

m

V−( 2m) ( i ) Tm( 2) ( i )

for the kth site and

( 1)

m =−2

CiQ e2qi Q i = ωQ = = 2πνiQ 2I(2I − 1)h 2

( 2)

Several difficulties lie in the studies of relaxation of quadrupolar nuclei. They are mainly due to the

contribution of magnetic relaxation between adjacent levels (∆m=±1) and quadrupolar relaxation with (∆m=±1, ±2) on one hand and the multiplicity of sites on another hand, both leading to a multiplicity of 4 ACS Paragon Plus Environment

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relaxation times. Another complication is due to the value of the spin: for a system with half integer spin I, in absence of rapid spin exchange transition, 2I T1 values are expected in the slow regime. 11-21 This number decreases because of to the symmetry in the relaxation matrix, and it depends upon the type of fluctuations (quadrupolar, dipolar homo-nuclear, hetero-nuclear, etc...). For example, in the slow regime, for quadrupolar relaxation in the case of I=5/2 (27Al) the recovery of the central transition, after it was saturated is given by M(t)

 4W1   3W1   33W1  = 1 − 0.06 exp  − t  − 0.85exp  − t  − 0.09 exp  − t M (∞)  5   2   10 

( 3)

with W1 the one quantum transition probability and we suppose that the double quanta transition probability is equal to W1=W2.17 Wq is related to the spectral density J( 2)q ( qω0 ) . The two following properties will help us in the analysis:

Remark I It is important to note that each parameter Aq has its own symmetry given by the corresponding spherical harmonics. So it is possible to report asymmetrical phenomena by differentiating the amplitude and the correlation times of these parameters.

Remark II In the case of half integer spin, the homo-dipolar and quadrupolar fluctuations do not bring any adiabatic contribution to the transverse relaxation rates R2 of the central line. Thus R2 has a maximum at the same temperature position as R1.

Remark III Eq ( 3) shows that even in the case where there are only quadrupolar fluctuations it will be difficult to observe the different relaxation times owing to their weak contributions.



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The DMFIT software is used to fit the spectra and to obtain the linewidths, lines positions (in Hz or ppm) and intensities, and quadrupolar splitting.22

31

P spectra are referenced from H3PO4 (85 %),

27

Al

from Al(NO3)3. Results are expressed either in Hertz or in ppm (X(Hz)=X(ppm)ν0(MHz)). In the ( 2)

different tables, the amplitudes of the parameters A q =

(A ) ( 2)

2

1

2

q

are expressed in kHz. Logarithm

plots provide a more sensitive test of non-exponentiality. So, at first all the experimental relaxation data for both

27

Al and

31

P nuclei were processed in the logarithm scale at each temperature. Then, these

values were checked in the linear scale.

3- Results Dynamical NMR studies can be separated in two parts: on one hand low frequency regime evidenced by R2=1/T2 and linewidth, and on another hand high frequency regime given by R1=1/T1. R2 and the linewidth are intimately connected but this last one can be disturb by contributions which have nothing to do with the dynamics. Therefore, apart for

31

P, only R2 will be analyzed. In some figures several

temperatures Tc1, Tc1M, TcρΜ, Tc2M are identified. Tc1M≈195 K identifies the temperature of the relative minimum of R2(7Li); TcρΜ=225 K identifies the temperature of the R1ρ(7Li) maximum while Tc2M=250 K indicates the temperature of the relative maximum of both R2(7Li). Tc1M=320 K indicates the temperature of the maximum in R1(7Li) and R1Q(7Li).7

3.1 Low frequency range 3.1.1 31P Transverse relaxation rate (R2) and linewidth 31

P spectra recorded at RT are sketched in the Figure 1a. The broad line corresponds to the spectrum

recorded in static mode, while the narrow line corresponds to the MAS spectrum (νR=25 kHz). When the temperature varies, the linewidth of this broad line changes very weakly (Figure 1b). We can deduce that MAS is more effective in the line narrowing than temperature (motional averaging). In the 6 ACS Paragon Plus Environment

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static mode, a Gaussian/Lorentzian shape accounts for the

31

P spectra. Indeed the dipolar interaction,

the chemical shift anisotropy and the disorder smooth the line shape, making impossible to distinguish the different contributions observed in high resolution MAS mode (see ref. (1)). In Figure 1b, we compare the experimental transverse relaxation rate results R2(31P) with the line width δ(31P). At decreasing temperature, we observe that: -

the slope of R2 is very weakly negative (even zero) while the slope of the linewidth is clearly positive.

-

owing to the difference of slopes between R2 and the linewidth, we can conclude that these two physical quantities are not sensitive exactly to the same fluctuations.

-

at very high temperature the linewidth does not reach zero and a residual linewidth remains. It is consistent with the R2 behavior. Such results will be also observed in the case of 27Al.

The difference ∆= R2(31P) -δ(31P) between these two parameters versus reciprocal temperature is given as an inset of this figure 1b. ∆ clearly deviates from a linear law (best fitted with an exponential) clearly highlighting a saturation effect. Therefore, these straight lines are only unrefined approximations.

3.1.2 27Al transverse relaxation rate R2. Figure 2a gives part of the static spectrum restricted to central transition ( ± 1 2 ↔ m 1 2 ) which is the only observable transition in the static mode. This unique line is fitted by a Gaussian / Lorentzian line shape. This structure less line indicates that the second order quadrupolar is weak. Moreover the dipolar broadening and local disorder (see part I) lead to the disappearance of the quadrupolar structure of the central line (even in Magic Angle Spinning experiments (MAS), see part I).1



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The R2(27Al) parameter versus reciprocal temperature results are reported in Figure 2b. A unique R2 is detected as it is expected in this type of experiment when there is only one aluminum site and only the central line is analyzed. The first points (high temperature) of the experimental data seem to indicate that R2(27Al) does not decrease to zero and a residual contribution remains (at least in the temperature range under consideration). At decreasing temperature, R2(27Al) begins to increase from nearly 1000 s-1 up to a relative maximum (of about 2000 s-1) around 1000/T=3.7 (T=286 K). Then it decreases down to a relative minimum around 1000/T=5.2. Finally it increases slowly towards its rigid lattice limit (which is not reached).

3.2 High frequency range 3.2.1 31P Spin lattice relaxation R1. In Figure 3 we display the relaxation rates R1 versus temperature for the phosphorus. Although several components are evidenced in high resolution MASNMR (up to six components, see ref. 1), only two T1 are observed. In this temperature range the R1 values are weak. At decreasing temperature, the less intense component of R1 reaches its maximum at 1000/T~3.2 and seems to stabilize in this value. The studied domain of temperature did not allow evidencing the stabilization of the value of the the most intense component. A third very weak and constant component around 0.05 s-1 is also detected. We have attributed this component to dipolar hyperfine interaction which is too weak to be observed on the other nuclei.

3.2.2 27Al spin lattice relaxation rate R1. The experimental results of the spin lattice relaxation rate R1(27Al) are gathered in Figure 4a. In Figure 4b we compare the relaxation rates R1(27Al) and R2(27Al). Two branches of R1 are observed in the whole domain of temperature (T

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

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