A NMR and DFT Investigation - American Chemical Society

Oct 30, 2013 - compounds with tetragonal melilite-like layered structure. In such compounds the local structure and properties may be very different f...
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Local versus Average Structure in LaSrAl3O7: A NMR and DFT Investigation Chiara Ferrara,† Cristina Tealdi,† Alfonso Pedone,‡ Maria Cristina Menziani,‡ Aaron J. Rossini,§ Guido Pintacuda,§ and Piercarlo Mustarelli*,† †

Department of Chemistry, Physical Chemistry Division and INSTM, University of Pavia, Viale Taramelli 16, 27100 Pavia, Italy Department of Chemical and Geological Sciences, University of Modena and Reggio Emilia, Via G. Campi 183, 41125 Modena, Italy § Centre de RMN à très hauts champs − Institut des Sciences Analytiques, 5 Rue de La Doua, 69100 Villeurbanne, France ‡

S Supporting Information *

ABSTRACT: LaSrAl3O7 belongs to the family of M2T1(1)T2(2)O7 compounds with tetragonal melilite-like layered structure. In such compounds the local structure and properties may be very different from the average ones, mainly due to cationic disorder on the M site. In this work, solid-state 27Al NMR spectroscopy and periodic density functional theory calculations were used to highlight the differences between local and average order in the LaSrAl3O7 crystal. The 27Al isotropic chemical shifts and quadrupolar coupling constants were computed by employing the gauge including projector augmented wave (GIPAW) and PAW formalisms, respectively. An impressive linear relationship between the computed quadrupolar coupling constant CQ and the Al tetrahedra (T1, T2) distortion was observed. In particular, our calculations showed that the distortion of the T2 environment is determined by the La/Sr speciation around the apical nonbridging oxygen.

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INTRODUCTION Oxides with general formula M2T1(1)T2(2)O7 belong to the large family of tetragonal melilite-like structure (space group P421m). In this layered structure, T1 and T2 sites are tetrahedrally coordinated, whereas M is an 8-fold coordinated site. The T1 site is usually occupied by divalent or small trivalent cations, whereas the T2 site can host divalent, trivalent, tetravalent, or pentavalent cations. The M position is occupied by large divalent cations or trivalent lanthanides. This peculiar layered structure is flexible in accepting a high degree of substitution, doping, and exchange of cations on the different available sites. This leads to a huge variety of compounds showing peculiar and interesting properties. As an example, M2MgSi2O7 (M = Ba, Sr, Ca) is a class of materials characterized by high thermal and chemical stability accompanied by good biocompatibility. 1 The ease in accommodating rare earths (Eu, Dy) into the structure1,2 gives origin to luminescence effects, thus making these materials good candidates for a wide range of applications such as materials for tunable solid-state lasers, phosphors, photoluminescence, mechano-luminescence, and light-emitting diode (LED) technology.1−3 Al-containing compounds such as Ca2Al2SiO7 and CaYAl3O7 also present similar properties.4,5 The Ba2MgGe2O7 compound presents unusual nonlinear laser cascade effects and was therefore suggested for Raman laser frequency converters.6 The refractive indices and the optical dispersion of these Mg/Ge oxides can be controlled by © 2013 American Chemical Society

substitutions on all three cation sites, leading to the possibility of new classes of nonlinear optical materials. LaSrAl3O7, similarly to the Si-melilite series, does present luminescence properties if doped with lanthanides (Eu, Tb). It has also been proposed as a good candidate material for diode laser pumping and tunable laser generation.4,7 LaSrGa3O7 is also currently under study as a possible electrolyte for solid oxide fuel cells (SOFCs).8−13 This compound becomes an interstitial oxide ion conductor when excess oxygen is introduced within its structure by modifying the cation stoichiometry to La1+xSr1−xGa3O7+δ (δ = x/2) where x ≈ 0.5. The A2CoSi2O7 (A = Ca, Sr) series shows a large magnetocapacitance, which indicates a strong coupling between magnetic and dielectric properties. This effect is directly related to a peculiar 2D tetrahedral networking between SiO4 and CoO4 units. In fact, the Ba2CoSi2O7 composition that has the same general formula but a different crystallographic structure, is characterized by other properties.14 From this brief analysis, it is clear that many structural and functional properties of these materials are related to the presence of defects and to a given cation ordering on the available crystallographic sites. To address these aspects, therefore, it is mandatory to obtain a deep knowledge of the Received: March 27, 2013 Revised: October 16, 2013 Published: October 30, 2013 23451

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a 2.5 mm probe, operating at the Larmor frequency of 260.6 MHz. All the spectra were collected at 298 K, and the chemical shifts were referenced to a standard aqueous solution of Al(NO3)3 0.1 M. Both static and MAS spectra (25 kHz) were recorded using both rotor-synchronized spin−echo and wholeecho sequences with a 90° pulse of 1.76 μs (ν1 = 47 kHz), and 64 scans were acquired with a recycle delay 15.0 s. To account for the effects of nonideal excitation of NMR signals from large and small CQ sites, several MAS 27Al NMR spectra were acquired with a variety of r.f. fields and pulse tip angles (Figure S5, Supporting Information). The line shape of the spectra was insensitive to both the pulse tip angle and r.f. field, suggesting that the excitation efficiency does not strongly depend upon CQ under the experimental conditions employed. A 2D rotorsynchronized split-t1 (constant time)33 3Q-MAS spectrum was acquired at 16.4 T. “Hard pulses” (ν1 = 208 kHz) were used for the creation and conversion of triple quantum magnetization, and a soft CT selective refocusing π-pulse (3.52 μs, ν1 = 47 kHz) was employed. The 2D spectrum was acquired with 48 scans in the direct dimension and 112 increments in the indirect dimension, with the t1 increment equal to one rotor period (40 μs). All the spectra were processed using the TopSpin 3.0 software package (Bruker). The spectra were bestfitted with both TopSpin 3.0 SOLA (Bruker) and QuadFit packages.34 The latter was employed to implement the Czjzek distribution model,35 which allows for a continuous distribution of the quadrupolar parameters for a single site. This enables the accurate simulation of the asymmetric line shapes typical of disordered polycrystalline materials. In the case of 27Al NMR spectra, the Czjzek distribution accurately reproduced the highfield “tail” feature of the spectra.36 The spectra were simulated by keeping the Czjzek parameter d = 5. DFT Calculations. Simulations were performed in P1 symmetry on 2 × 2 × 4 supercells containing approximately 250 atoms. Due to the practical limitation of the supercell dimensions, and to ensure a good statistic treatment, six disordered distributions of the La and Sr cations on the available crystallographic sites were generated and analyzed, while maintaining the overall LaSrAl3O7 composition. All the distributions were obtained by means of a simple pseudorandom number generation routine. A Gaussian-like behavior of the cation distribution was found for both sites: the majority (36%) of Al1 sites is surrounded by 4 La and 4 Sr atoms, and 36% of Al2 sites are surrounded by 3 La and 3 Sr. The distributions of La/Sr cations around Al1 and Al2 sites of the six structural models are reported in Figure S1 of the Supporting Information (SI). The six structural models were optimized by means of density functional theory (DFT) calculations using the CASTEP code,37 and the NMR parameters were obtained by using PAW38 and GIPAW15,39 formalisms. The generalized gradient approximation (GGA) PBE functional40 was employed, and the core−valence interactions were described by ultrasoft pseudopotentials. For 17O, the 2s and 2p orbitals were considered as valence states with a core radius of 1.3 Å; for 27Al, a core radius of 2.0 Å was used with 3s and 3p valence orbitals; for Sr, a core radius of 2.0 Å was used with 4s, 4p, and 5s valence states; while for La, a core radius of 2.0 Å was used with 5s, 5p, 6s, and 5d valence states and a core radius of 1.6 Å was used for the 4f valence state. All calculations were performed at the Γ point since several tests have shown that it provides converged results for systems having side boxes larger than 13 Å (as in this case,

subtle differences between the local and the average structure, as disclosed by spectroscopic and diffraction techniques, respectively. From the point of view of the local structure, solid-state NMR is a powerful tool for the structural analysis of highly disordered and/or compositionally very complex systems, where diffraction techniques, as well as other spectroscopies (e.g., IR and Raman), undergo failure or do not provide sufficient details. Moreover, recent developments of the Gauge Including Projector Augmented Wave method (GIPAW) especially devised for the computation of NMR parameters in extended solids (described within periodic boundary conditions, plane wave basis sets, and density functional theory (DFT-GIPAW))15,16 allowed the successful prediction of chemical shifts and quadrupolar coupling constants of various crystalline and amorphous solids.16−22 Thus, it is not surprising that the combination of onedimensional (1D) and two-dimensional (2D) NMR spectroscopy with periodic DFT-GIPAW calculations can provide relevant information on complex solid phases.23−27 In this work we present a striking example of this combined experimental/computational approach applied to the structural investigation of LaSrAl3O7, which is intended at the same time as a model system and as a material with promising applications in SOFCs. Such an approach has been recently applied to K2NiF4-type28 and melilite29 structures by choosing 27Al as the main NMR nuclear probe. As a matter of fact, 27Al (I = 5/2) is a quadrupolar nucleus characterized by a manageable electric quadrupolar moment, Q (14.7 fm2). This enables highresolution measurements of chemical shifts and provides deep insight into the details of local ordering, such as the cation distribution around the position of interest.

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METHODOLOGY Synthesis. The synthesis of a polycrystalline sample of nominal composition LaSrAl3O7 was carried out by a modified sol−gel Pechini route. Stoichiometric amounts of La(NO3)3· 6H2O (Sigma, 99.9%), Sr(NO3)2 (Sigma, >99.0%), and Al(NO3)2·9H2O (Sigma, >98.0%) were dissolved in water and mixed with ethylene diamine tetra-acetic acid (EDTA). A homogeneous aqueous solution was obtained after mixing and heating (70 °C). The pH factor was adjusted to 8−9 by addition of NH4OH to promote the complexation of the metal ions. The complexes were then polymerized by adding ethylene glycol as the gelation agent, and the gel was dried slowly at ∼80−100 °C under continuous stirring. After the complete evaporation of water, the gel was heated at 1000 °C to eliminate the organic component. Subsequently, the resulting powders were pressed and heated to 1100 °C for 48 h. Diffraction Analysis. The X-ray diffraction (XRD) pattern of the sample was collected at 298 K using a Bruker D8 powder diffractometer (Cu Kα radiation) operating in Bragg−Brentano θ−θ geometry. The data were recorded in the 2θ range 10− 100° with a step size of 0.02 (2θ) and a constant counting time of 10 s per step. The pattern was analyzed according to the Rietveld refinement method30,31 with the use of the FullProf software package.32 Refined parameters were zero shift, scale factor, background, lattice parameters, atomic positions, and isotropic thermal factors. Solid-State NMR Measurements. Solid-state 27Al magic angle spinning (MAS) NMR spectra were collected both on a 700 MHz (16.4 T) spectrometer (Bruker) equipped with a 2.5 mm probe, operating at the Larmor frequency of 182.4 MHz, and on a 1 GHz (23.5 T) spectrometer (Bruker) equipped with 23452

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where the supercell has side boxes between 15 and 16 Å).20−22,41 Wave functions were expanded in plane waves with a kinetic energy cutoff of 600 eV, a value which has been already demonstrated to be suitable for total energy and NMR chemical shift convergence.42 It is well-known that standard GGA/DFT is not recommended for elements with localized d or f empty states since it provides energy levels too stabilized; these generate excessive covalent interactions with anionic p states, and as a consequence, incorrect electronic properties such as the DOS, band gaps, and NMR parameters of oxygen connected to these elements are generated. For example, recent investigations have shown that it is necessary to add an onsite Hubbard correction on the 3d and 4f orbitals of Sr and La to adjust their position relative to Sr (4p)43 and La (5d) levels.44 In this work this problem has been solved by artificially shifting the local potentials of Sr2+ (4d0) and La3+ (4f0) to higher energy compared to the default definition proposed by the Material Studio Package, as previously proposed by Profeta et al.45 for the Ca2+ ion (3d0) and by Sadoc et al.46 for Sc3+ (3d0) and La3+ (4f0). Following Profeta’s procedure, the 3d and 4f energy levels of Sr2+ and La3+ have been empirically shifted by 2.49 and 20.4 eV, respectively. These are the values for which the chemical shifts of 17O in SrO and La2O3 oxides match the experimental values previously reported by Turner et al.47 The variation of the isotropic chemical shifts of 17O with respect to the 3d0 and 4f0 energetic shift is reported in Figure S2 and S3 of the SI. The calculation of the density of states (DOS) of La2O3 using the PBE functional with the original and shifted pseudopotentials (PPs) has been compared (Figure S4, SI) with the one obtained by using the hybrid PBE0 functional,48 which provides a better description of the covalency in these systems as well as band gaps in good agreement with the experimental ones.49 In the PBE0 calculations norm-conserving pseudopotentials (NCPPs) with a higher energy cutoff value (1000 eV) have been used because USPPs are not yet supported with hybrid functionals. As expected the partial DOS computed with the shifted potential is in very good agreement with those obtained by using the PBE0 functional. The effect of the shift applied on the 4f orbitals is clearly observed in the conduction band; the shape of the La2O3 conduction band having mostly a La 4f character for the not-shifted potentials becomes closer to the one obtained using the hybrid functional, where 4f orbitals do not contribute at all. The band gap of La2O3 is mainly imposed by the position of La (5d) states in the conduction band and is, respectively, calculated to 3.77 eV using the PBE functional with shifted potentials. As expected, the use of the hybrid functional gives a higher band gap value of 5.7 eV which is exactly equal to the experimental one.50 However, it is worth noting that although the energy shift heavily affects the 17O isotropic shift, further tests on the LaAlO3 crystal structure reported in Table S1 of the SI have shown that they do not affect the chemical shift and electric field gradients of Al ions. Outputs of DFT-GIPAW calculations were processed by the fpNMR package.51,52 This code allows the simulation of NMR spectra (MAS, MQMAS) under various experimental conditions as supplied by the end user and the statistical analysis of the NMR parameters. Isotropic chemical shifts (δcalc iso ) were evaluated from the calculated isotropic magnetic shielding, δcalc iso , 27 using the formula δcalc iso = −(σiso − σref). For the Al nucleus, σref

= 555.19 ppm was determined using Corundum as the reference crystalline solid phase.21,42 Quadrupolar coupling constants, CQ, of 27Al were calculated with a scaled quadrupole moment (Q) of 140.4 mB. This value was obtained by multiplying the accepted “experimental value” of 146.6 mB53 by a scaling factor calculated via linear fitting of the plot of experimental and theoretical 27Al CQ values in crystalline Anorthite (CaAl2Si2O8), as described in ref 54.

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RESULTS AND DISCUSSION Figure 1 presents the X-ray powder diffraction pattern of LaSrAl3O7 refined by means of the Rietveld method. The

Figure 1. Rietveld-refined XRD powder diffraction data for LaSrAl3O7 showing the observed, calculated, and difference profiles.

pattern can be indexed in the tetragonal P-421m (no. 113) space group. It is possible to observe a spurious reflection at 34°. Other less intense extra peaks are observed at 23.5° and 48°, which can be associated with the LaAlO3 phase. A twophase refinement was therefore applied with the aim to quantify the impurity phase, which was shown to be approximately 3%. The results of this refinement are reported in Table 1. Table 1. Refined Crystallographic Parameters for LaSrAl3O7a Al1 Al2 La/Sr O1 O2 O3

x

y

z

0.0 0.8582 (6) 0.6621 (2) 0.5 0.863 (1) 0.913 (1)

0.0 0.6418 (6) 0.8379 (2) 0.0 0.637 (1) 0.835 (1)

0.0 0.036 (2) 0.4911 (6) 0.830 (4) 0.704 (2) 0.189 (2)

a

Space group P-421m, a = 7.8966 (1) Å, c = 5.23145 (9) Å, Biso = 0.348, Rexp = 15.23, Rwp = 16.1, +Rp = 11.5.

The average structure of LaSrAl3O7 (Figure 2a) is described by a tetragonal symmetry (space group P4̅21m). In this structure, two distinct crystallographic sites for Al are present (named Al1 and Al2), and three different O positions are available (named O1, O2, and O3 in the following); however, La and Sr are distributed over the same crystallographic site. The structure is composed of AlO 4 tetrahedra. Each Al 1 O 4 tetrahedron is linked to four other Al2O4 units by four bridging 23453

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environment of the different sites occupied by the quadrupolar nucleus. From the point of view of the structural information, the use of 27Al as the nuclear probe offers some relevant advantages. Since the 27Al CQ values are generally on the order of a few megahertz, both 1D and 2D NMR spectra can be easily obtained at high magnetic fields due to the favorable scaling of the quadrupolar interaction. It is usually possible to establish good correlations between the isotropic chemical shift and the Al coordination number in the structure. In particular, in the case of the aluminum oxides, the atomic coordination IV, V, and VI are well-resolved and related to different chemical shift ranges, i.e., 50−80 ppm, 30−40 ppm, and −10 to 15 ppm, respectively.55 Figure 3 shows the 1D 27Al MAS spectra of the sample LaSrAl3O7 acquired at room temperature at 23.5 T (a) and 16.4

Figure 2. (a) LaSrAl3O7 crystal structure (Al1 green, Al2 blue, O red, La/Sr pink). (b) Representation of the first cationic coordination sphere for the two aluminum sites. Al1O4 neighbors eight cation sites and Al2O4 neighbors six cation sites. First cationic coordination sphere for the La sites.

oxygens (BO), whereas Al2O4 tetrahedra are linked to three other AlO4 polyhedra (two Al1O4 and one Al2O4 units), leading to an overall multiplicity of the Al sites of 1 Al1:2 Al2. A relevant feature of the Al2O4 unit is the presence of a nonbridging oxygen (NBO) along the c direction. The tetrahedral network extends in the ab plane; along the c axis these layers are spaced by La/Sr cationic slabs, where a single crystallographic position is available for both La and Sr. Figure 2b illustrates in greater detail that the Al2O4 tetrahedra are surrounded by six La/Sr positions in the first cationic coordination sphere, whereas the Al1O4 tetrahedra are surrounded by eight La/Sr positions. The average structure derived from the diffraction data treats the M site with mixed occupancy; i.e., the single site is occupied by the two different cations La3+ and Sr2+ with the same occupancy probability (0.5) and an associated formal charge +2.5. This implies that all the Al1O4 units are characterized by only one value for all the Al1−O3 distances. Al2O4 tetrahedra, in contrast, present three distinct bonds with oxygens in different crystallographic positions (Figure 2b). On the local scale, however, La and Sr may be nonrandomly distributed, and their reciprocal arrangements may induce local distortions and deviations from the ideal average structure; this will be reflected in the local environment of the individual Al sites. 27Al solid-state NMR is one of the techniques of choice to investigate this aspect. For quadrupolar nuclei such as 27Al, the second-order quadrupolar interaction often gives rise to broadened central transition NMR spectra and shifts the position of the NMR spectra away from the isotropic chemical shift.55 Both the line shape of the NMR spectrum and the apparent chemical shift are related to the electric field gradient (EFG) at the nuclear site, which in turn is related to the degree of spherical symmetry about the nucleus. Furthermore, the existence of structural disorder results in additional broadening due to distributions of the quadrupolar parameters, the quadrupole coupling constant, CQ, and the asymmetry parameter, ηQ. Although nonlocal and ionic effects must be also considered in determining the EFG tensor, careful work performed on gehlenite Ca2Al2SiO7 recently pointed out that CQ is substantially determined by the local electronic structure and hence depends on the distortion of the tetrahedral environment.29 Therefore, a proper analysis of the NMR line shape and shift can provide detailed information on the local

Figure 3. 1D MAS 27Al solid-state spin−echo NMR spectra: (a) experimental spectrum at 23.5 T; (b) experimental spectrum at 16.4 T; (c) 23.5 T spectrum best-fitted in terms of the Czjzek distribution; (d) 16.4 T spectrum best-fitted in terms of the Czjzek distribution.

T (b), with a standard spin−echo (SE) sequence. The first information that can be extracted from the spectra is the presence of two distinct Al sites in the structure. This is clearer in the 23.5 T spectrum. The observed chemical shift range (60−80 ppm) confirms the tetrahedral coordination of both sites. Although the two crystallographic sites are quite similar, which is reflected by the partial overlapping of the two NMR peaks, a striking difference between them is related to the presence of a NBO in Al2. The spectra do not show the features characteristic of second-order quadrupolar broadening; rather they exhibit an asymmetric shape with a steep edge at low field (more evident in the 23.5 T spectrum) and with a tail at high field. This last feature is generally associated with disorder and the presence of a distribution of the quadrupolar parameters, CQ and ηQ.56 The experimental spectra were then best-fitted using a Czjzek distribution of the quadrupolar parameters (panels c and d). The results, reported in Table 2, showed a good agreement between the values obtained at the two magnetic fields. The parameter σ, which is the only parameter optimized in the Czjzek distribution,57 controls the width of the distribution of the EFG elements. The extraction of the mean values and the distribution widths of the quadrupolar parameters is not simple. We performed some sensitivity tests on the CQ maximum value (the minimum one being kept at 0.1 MHz) and found that its effect becomes negligible when the higher values given by DFT calculations (see below) are reached. The ηQ distribution limits were 0 and 1. We also 23454

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Table 2. NMR Parameters Obtained by the Best-Fit of the Spectrum of Figure 3 site 23.5 T 16.4 T DFT a

1 (Al1) 2 (Al2) 1 (Al1) 2 (Al2) Al1 Al2

CQ average (MHz)

5.0a 7.1a

σ (MHz)

area (%)

δiso (ppm)

agreement factor (%)

0.6 0.9 0.6 0.7

26.0 74.0 33.5 66.5 33.3 66.7

75.6 84.2 76.0 83.8 75.6 81.8

97.7 96.7 -

Average of the values reported in Figure 4.

performed some sensitivity tests on the parameter d, where d = 5 gives a pure Czjzek model and d = 1 a model with geometrical constraints. Again, there were no strong differences among the examined cases, the best-fit errors being always below 4%. The best-fits for d = 1 are reported in the Supporting Information. To obtain more detailed information on the distribution of the quadrupolar parameters, to correlate them with the local structure features, as well as to assign the two NMR peaks to the two nonequivalent crystallographic sites, the combination of more sophisticated 2D NMR experiments (MQMAS) and DFT calculations is mandatory. Figure 4 reports the δiso vs CQ correlations obtained from DFT calculations for the two Al sites with all the possible

Figure 5. Experimental 3Q MQMAS spectrum. The 2D spectrum clearly indicates the presence of two distinct sites.

to ∼5 MHz, whereas the one at higher chemical shifts is characterized by a broader CQ range (∼4−8 MHz). Therefore, we can conclude that the broader peak at lower field is from the Al2 site, whereas the sharper peak at higher field corresponds to the Al1 site. While the MQMAS experiment allowed a clear separation of the two sites, it must be stressed that an accurate determination of the distribution of the quadrupolar parameters, as well as of the relative peak intensities (which are related to the sites multiplicity), must be obtained from simulations of the 1D MAS spectrum (Figure 3) because the MQMAS spectrum is less accurate in describing sites affected by large CQ values and requires specific treatments.58 Having assigned the NMR features to the two distinct Al crystallographic sites, the next step is to correlate the local, and possibly the medium-range, structure of the sites (i.e., deviations from tetrahedral symmetry, nonbridging oxygens, next-neighbors nature, etc.) with the observed quadrupolar parameters and their distributions. In the case of 27Al NMR, several parameters were previously proposed to quantify the local order, including bond length- and bond angle-based tetrahedral distortions.55 However, because of the lack, in the case of 27Al NMR, of clear-cut relationships among direct geometric features and NMR observables, first-principles quantum mechanical calculations are mandatory to model the system at the local level. As stated before, both sites are in tetrahedral coordination and face cationic La/Sr planes. The cationic coordination sphere is constituted by La/Sr positions at an average distance of ∼4 Å for all the AlO4 units. All the tetrahedral units are linked to each other by forming a plane through bridging oxygen ions, and the two sites are expected to have similar NMR isotropic chemical shift and quadrupolar asymmetry because of similar first coordination spheres and coordination geometries. Despite the similarities of the two Al sites, however, significant differences in CQ are expected for the following reasons: (i) The crystallographic position of coordinating oxygens. In fact, Al1 is surrounded only by oxygens in O3 position, whereas Al2 is coordinated by two oxygens in position O3, one in position O1, and one in position O2. (ii) The presence of nonbridging oxygens. Being coordinated only to O3 positions, Al1 is always surrounded by oxygen coshared with other tetrahedral units. In contrast, Al2 is bound to a nonbridging oxygen in O2 position aligned with the c axis. (iii) The number and nature of ions in the first cationic coordination sphere. Al1O4 tetrahedra are surrounded by 8 La/ Sr positions, while Al2O4 units are surrounded by 6 La/Sr sites. Since CQ is related to the electric field gradient (EFG) at the

Figure 4. Theoretical (δiso, CQ) distributions for the Al1 and Al2 sites in the six computational structural models investigated.

combinations of the Sr and La cations. Al1 and Al2 yield wellseparated (δiso, CQ) distributions, and thus they should be easily distinguished in the MQMAS spectra. It is noteworthy that Al1 distribution is much more isotropic than Al2, irrespective of the stochastic distribution of cations around both the Al tetrahedra. This calls for a relevant role of the NBO in determining the (δiso, CQ) correlation (see below). In agreement with the simulations of the 1D MAS 27Al solidstate NMR spectra and the DFT calculations, the MQMAS 27Al NMR spectrum reported in Figure 5 clearly shows two sites. Isotropic chemical shifts of 76.5 and 84 ppm were obtained from simulations of the direct dimension slices with maximum intensity. However, the two sites are characterized by very different quadrupolar broadening. From the MQMAS spectrum, assignment of the two peaks to the two Al sites is straightforward: the peak at lower chemical shift is characterized by a relatively narrow CQ distribution, in the range ∼3 23455

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nucleus position, it is very sensitive to local structural distortions, and as outlined above, this explains why two distinct ranges of CQ are experimentally observed for the two distinct Al sites. The distortion, Δtetra, of the tetrahedral Al sites was calculated according to the following expression for each of the GIPAW DFT calculated structures with different La/Sr substitution patterns Δtetra =

1 4

⎛ li − lm ⎞ 2 ⎟ ∑⎜ lm ⎠ i=1 ⎝ 4

where li is the single Al−O distance within the tetrahedron, and lm is the average Al−O distance within the same tetrahedron.59 Figure 6 shows the calculated structural distortions plotted against the calculated CQ for each Al site. The upper panel

Figure 7. Theoretical CQ values vs Al2−O2 distance. The different colors are related to different cationic coordination for the nonbridging oxygen O2. Green: three Sr ions. Red: two Sr and one La ions. Blue: two La and one Sr ions. Pink: three La ions. The orange cross on the horizontal axis indicates the experimental (average) Al2−O2 distance obtained from Rietveld refinement of XRD.

most distorted tetrahedra (i.e., the ones with the largest CQ) are those characterized by the smallest Al2−O2 bond lengths. As already stated, each NBO is near to three La/Sr cations in apical positions above it (see Figure 2b). When the various combinations of cations are marked with different colors, we observe well-defined islands in the correlation chart of Figure 7. In particular, the shortest Al2−O2 bond lengths, which correspond to the largest tetrahedron distortions (and CQ values), are found when three Sr cations occupy the positions above the NBO. In contrast, the longest bond lengths are observed when only La cations are present. Intermediate bond lengths are found in the case of mixed cation distributions. This result confirms that La3+ attracts the O in the apical nonbridging position more than Sr2+ due to the charge effects, as previously suggested for the LaSrAlO4 system.28 From Figure 7 it is also clear that the large CQ spread observed for Al2 sites is mainly due to the cation speciation in the NBO apical position. The presence of NBOs is, therefore, an important feature to introduce possible elements of disorder in layered structures, where different cations can be differently arranged around them. In the case of stochastic cation distributions, like in the present case, this could lead to interesting applications, for example, in thermoelectric materials where one of the optimization strategies is the decoupling of the phonon spectrum from the electronic spectrum.60 In other cases, it could be possible to force a nonrandom cation distribution, so giving rise to a “two-phase” material, at least from the NMR point of view.

Figure 6. Theoretical CQ values vs tetrahedral distortion parameter, Δtetra, for Al1 (top panel) and Al2 sites (lower panel).

reports the data related to the Al1 sites, whereas the lower panel refers to the Al2 ones. The first clear information that can be extracted from the visual inspection of these plots is that the Al1O4 tetrahedra are characterized by a smaller distribution of distortions (0.00−0.02 range) and by a relatively small distribution of calculated CQ values (3−8 MHz range). Conversely, for Al2O4 tetrahedra, the calculated tetrahedral distortions range approximately from 0.0 to 0.055 and the calculated CQ from 1 to 13 MHz. A linear trend can be observed between the tetrahedron distortion and the corresponding CQ value, with larger distortions leading to larger values of CQ. We also tried to correlate the Δtetra parameter with the nature of the surrounding cations in the first cationic coordination shell. To clarify this correlation, different colors were associated to different La/Sr ratios in Figure 6 (see caption). Unfortunately, we did not find a clearcut correlation between the degree of distortion and the La/Sr ratio in the first cationic coordination shell. For example, in the case of the Al2 site, the distortions of the La/Sr 3:3 combination are well distributed in the range 0−0.055, with the corresponding CQ values spanning from ∼1 to more than 12 MHz. In contrast, a clear relationship emerges if one considers the position of the La/Sr atoms with respect to NBOs. Figure 7 shows the correlation plot between the CQ values calculated for the Al2O4 tetrahedra and the Al2−O2 distance, i.e., the bond length involving the NBO. The plot shows a clear linear correlation between these two parameters. In particular, the

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CONCLUSIONS In this work we have addressed the problem of local vs average structure in the layered LaSrAl3O7 melilite-like compound. The use of both average (diffraction) and local experimental techniques (NMR), combined with DFT calculations, allowed us to obtain a detailed description of the structure. The main results of this study can be summarized as follows: •Two distinct Al tetrahedral sites are expected from the average crystal structure. The 1D and 2D NMR experiments confirmed the presence of these two Al moieties but highlighted a significant difference in the distribution of CQ for the two sites. 23456

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•DFT calculations allowed us to highlight a linear trend between the distortion parameter, Δ tetra , and the C Q distributions. Experimental and calculated CQ values were in very good agreement. •The presence of a NBO in the Al2O4 tetrahedra was the main cause of the differences in CQ distributions for the two Al sites. In particular, the wide range of Al2−O2 bond lengths was due to the speciation of La/Sr cations around the O2 position. We found a clear correlation between the Al2−O2 bond lengths and the O2 cationic coordination, with the smallest Al2−O2 distances being related to the unique presence of Sr, whereas the largest ones are due to all La cations. •The NMR data indicate that there is not preferential partitioning of the cations into different sites. In summary, this study highlights that a combination of average and local experimental approaches with DFT calculations is a very powerful tool to investigate the subtle differences between local and average order in complex oxides. The importance of our results is made clearer considering that such structural details often strongly influence the functional properties of these materials.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S6 and Tables S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Cariplo Foundation through project 2009-2623 is gratefully acknowledged. C.F. acknowledges a fellowship from the French Ministry for the Foreign Affairs (dossier n. 781569G).

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