Magic Angle Spinning NMR Study on Inversion Behavior and Vacancy

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Magic Angle Spinning NMR Study on Inversion Behavior and Vacancy Disorder in Alumina-Rich Spinel Bingtian Tu, He Zhang, Hao Wang,* Weimin Wang, and Zhengyi Fu State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, People’s Republic of China

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S Supporting Information *

ABSTRACT: Solid state magic angle spinning nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) were used to investigate the inverse behavior and vacancy disorder in alumina-rich spinel, Mg1−xAl2(1+x/3)O4 (0 ≤ x ≤ 0.86). Simulation and integration of NMR spectra have been developed for probing the population of Al located in different coordinated environments. Rietveld profile refinements were performed for powder XRD spectra by combining with NMR analysis. With changes in the composition, inversion disorder and cationic vacancies coexisting in tetrahedral and octahedral coordinations fluctuate in amount in the crystal lattice. The coordination polyhedra in the crystal structure can adjust the volume to variations of composition, anion parameter, and inverse parameter. This opens a window to the design and functionalization of spinel materials.

1. INTRODUCTION The spinel group of compounds crystallizing in a cubic crystal system form a great number of versatile materials with outstanding electrical, optical, magnetic, and mechanical properties. The spinel is the magnesium aluminum member of the spinel group oxides, which has the formula of MgAl2O4. MgAl2O4 spinel has received extensive attention due to its excellent chemical stability, wear resistance, thermomechanical properties, and optical transparency.1 Without birefringent scattering losses, fully dense spinel polycrystalline ceramics possess up to 80% transparency from ultraviolet (UV) to midwave infrared (MWIR) light, which can find application in lenses, transparent armors, and dome materials.2 Benefiting from the large solubility of Al2O3 in MgAl2O4 at high temperature, which is so-called alumina-rich spinel, the crystal structure, electronic, optical, and mechanical properties of spinel can be tuned and optimized to a certain extent.1,3−5 Cation inversion and vacancy (V) distribution in the crystal structure make it difficult to establish the relationship among composition, crystal structure, and intrinsic properties of alumina-rich spinel. In addition, from geological and radiological viewpoints,6,7 a comprehensive understanding of spinel crystal structure, which could accommodate multiple atomic disorders, is still vitally important. In the crystal structure of normal MgAl2O4, the oxygen atoms form a face-centered-cubic close-packed arrangement. Mg and Al cations reside on 1/8 tetrahedral sites and 1/2 octahedral sites with symmetries of 8a and 16d, respectively. For synthetic MgAl2O4, inversion involves the exchange of Mg © XXXX American Chemical Society

and Al, which is quantified as i, the fraction of tetrahedral sites occupied by exchanged Al cations. The exchange of cations in the spinel structure introduces intrinsic variation in the crystal lattice and physical properties. Hazen and Yang proposed that the bulk modulus of MgAl2O4 relates to the degree of inversion.8 From a series of first-principles simulations based on density functional theory (DFT), Li et al. found that the disorder in MgAl2O4 spinel increases both the bulk modulus and shear modulus but decreases the lattice parameters.9 Due to the lack of experimental results, several groups developed theoretical simulations to investigate the effect of inversion on the mechanical properties of MgAl2O4.10−12 They demonstrated that effects of cation disorder on structure and properties can not be ignored in spinel compounds. Alumina-rich spinel can be considered as a solid solution of MgAl2O4 and Al8/3O4 (spinel-type alumina, γ-Al2O3) with the formula of Mg1−xAl2(1+x/3)O4 (0 ≤ x ≤ 0.86). In the crystal structure of Mg1−xAl2(1+x/3)O4, 8x/3 vacancies per unit cell are formed to maintain the charge balance. Neutron diffraction, Xray diffraction (XRD), and nuclear magnetic resonance (NMR) were used to study the distribution of cation vacancies in Mg1−xAl2(1+x/3)O4.13,14 Infrared reflection and Raman spectroscopy also supplied valuable information for the study of structure of Mg1−xAl2(1+x/3)O4.5 There is still considerable disagreement on the location of vacancies, even though most of the recent studies supported that the cationic vacancy Received: April 17, 2018

A

DOI: 10.1021/acs.inorgchem.8b01034 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry prefers to locate in octahedral coordination. In addition, the inversion behavior plays an important role in the radiationresistant properties and has been widely discussed in MgAl2O415−17 but is rarely considered in alumina-rich spinel. In this work, 27Al magic angle spinning (MAS) NMR offers a powerful tool for investigating the inverse behavior in Mg1−xAl2(1+x/3)O4. In combination with Rietveld refinement of XRD, the distribution of cations and vacancy disorder in the crystal structure of alumina-rich spinel were clearly represented. Then, the adjustments of anion parameter, inversion parameter, polyhedral volume, and bond angle with the change in composition were systematically investigated.

2. EXPERIMENTAL DETAILS Polycrystalline Mg1−xAl2(1+x/3)O4 samples were prepared by a conventional solid-state method, which has been employed to synthesize spinel-type MgO·1.5Al2O3, AlON, and MgAlON in our previous studies.18−20 Commercial MgO (∼99.9%) and α-Al2O3 (∼99.99%) powders were used to synthesize Mg1−xAl2(1+x/3)O4 with designed compositions of x = 0, 0.12, 0.25, 0.53, 0.6. The XRD patterns were recorded on a PANalytical X’Pert Pro X-ray diffractometer equipped with monochromated Cu Kα radiation (1.540598 Å, 40 kV, 40 mA). The mass ratios of Al content in asprepared samples were quantitatively determined by ICP-AES analysis (Optima Model 4300DV; PerkinElmer Instruments, Boston, MA). The Rietveld crystal structure refinements were performed using the Fullprof program. The solid-state 27Al MAS NMR experiments were performed on a Bruker Avance III 500 WB (11.7 T) spectrometer operating at a frequency of 130.43 MHz. A DVT triple-resonance 2.5 mm o.d. Bruker CP/MAS probe was used with a spinning frequency of 30.0 kHz. The spectra were recorded with a single pulse excitation using a short pulse length (π/12) and a recycle delay of 1 s (the rf field strength was 166.67 kHz, and the tip angle was π/12). 27Al chemical shifts were referenced using Al(NO3)3 1 M aqueous solution as an external reference (0 ppm). The NMR parameters chemical shift δiso and quadrupolar coupling constants CQ were extracted from fitting the curve using the CzSimple model in the Dmfit package.21,22

Figure 1. Observed and calculated patterns obtained by Rietveld structural refinement of the XRD data of Mg1−xAl2(1+x/3)O4. The crystallographic data are reported in the Supporting Information. The final reliability factors are given in Table S1.

AlVI, respectively, in synthesized Mg1−xAl2(1+x/3)O4 samples. All of the resonance peaks have asymmetric line shapes with a steep low-field edge and a trailing high-field edge. These features result from a distribution of the electric field gradient of AlIV and AlVI which leads to a distribution in the quadrupolar coupling constants. To estimate the inverse parameter with increasing Al content, the quadrupolar line shapes of experimental 27Al MAS NMR spectra have been simulated. The fitting of spectra was achieved using the CzSimple model,21,22 including firstorder contribution to the spinning side bands, as well as second-order interactions with the central component. Comparisons of experimental and simulated spectra for all samples are shown in Figure 2. It can be seen that the simulated results agree well with experimental spectra. In previous investigations on MgAl2O4, three resonance peaks for Al central transitions have been proposed to interpret 27Al MAS NMR spectra, while two peaks of AlVI appear in the spectra when they are recorded at a low magnetic field (e.g., 7.04 T).23,24 The spinning sidebands overlapped with the central transition, and it was hard to quantitatively analyze the Al populations. Therefore, higher magnetic field and spinning speed are necessary. As shown in Figure 2, the second resonance peak of AlVI is almost shaded out in the central transition and the spinning sidebands were completely separated from the central transition at a higher field of 11.7 T. The central transitions in observed spectra have further been well-fitted using two resonance peaks with the CzSimple

3. RESULTS AND DISCUSSION 3.1. Phase Determination. All of the Mg1−xAl2(1+x/3)O4 samples were initially characterized by powder XRD to confirm the phase purity. Figure 1 shows the powder XRD patterns of Mg1−xAl2(1+x/3)O4 samples. The Bragg reflections of MgAl2O4 are included in Figure 1 for comparison. There are no additional peaks observed in the patterns, indicating the absence of any impurities in Mg1−xAl2(1+x/3)O4 samples. The Al contents in five samples were then quantitatively determined to be 37.9, 39.7, 41.5, 45.7, and 46.8 wt % by ICP-AES, which are very close to the designed nominal proportions. Thus, the chemical formulas of the obtained alumina-rich spinel can be expressed as MgAl 2 O 4 , Mg 0.88 Al 2.08 O 4 , Mg 0.75 Al 2.16 O 4 , Mg0.47Al2.35O4, and Mg0.40Al2.40O4. 3.2. 27Al MAS NMR Investigation. The objectives of the 27 Al MAS NMR experiments were to resolve and calculate relative populations of tetrahedral (AlIV) and octahedral (AlVI) aluminum. Furthermore, the site occupancies of AlIV and AlVI of all samples can be calculated with quantitative analysis. From the deconvolution of 27Al MAS NMR spectra, as illustrated in Figure 2, the spinning sidebands are well separated from the central transition peaks, enabling us to extract quantitative information from NMR experiments. The dipole interaction and chemical shift anisotropy with the order of kilohertz are averaged out by the fast MAS with 30 kHz frequency. There are two major resonances observed at around 60 and 10 ppm corresponding to the characteristics of AlIV and B

DOI: 10.1021/acs.inorgchem.8b01034 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

analysis were used in our XRD structure refinement of alumina-rich spinel. The occupancies of Mg and vacancies at both tetrahedral and octahedral sites were refined with charge balance constraint according to quantitative results. From a Rietveld refinement, as shown in Figure 1, the calculated results are all in good agreement with the experimental data. The crystallographic data and final reliability factors of structure refinements are given in Table S1. The observed and calculated XRD patterns and their difference for Mg1−xAl2(1+x/3)O4 are also presented in the Supporting Information. The lattice parameter (a), anion parameter (u), and occupancy of Mg in alumina-rich spinel are given in Table 2. As shown in Figure 3a, the lattice parameter of synthesized alumina-rich spinel shows a linear dependence on the concentration of Al8/3O4, x, and follows the empirical Vegard law. It means that the lattice parameter of Mg1−xAl2(1+x/3)O4 solid solution mostly relates to the variation of composition. The refined crystal lattice parameters are in good agreement with previous results obtained from XRD refinement without considering the inverse parameter and locating aluminum vacancies at 16d sites.3,25,26 A linear relationship between lattice parameter and x was obtained from fitting our data: a = 8.0873 − 0.1826x. This is also the case for the anion parameter; as shown in Figure 3b, the anion parameter of Mg1−xAl2(1+x/3)O4 decreases linearly with increasing x. However, the anion parameters resolved in our experiments are slightly lower than those results from single-crystal refinement.13,26 3.4. Cation Inversion and Vacancy Site Preference. In alumina-rich spinel, the inversion parameter (i) is defined as the fraction of Mg ions interchanging their positions with octahedral Al ions.13,27 In previous studies,28,29 the aluminum vacancies were supposed to reside only on octahedral sites, and the Mg entering into the octahedral site generates the inverse phenomena in alumina-rich spinel. By a combination of 27Al MAS NMR and XRD refinement, the inversion parameter and occupancy of vacancies were quantitatively determined. As given in Table 2, it is interesting that inverse disorder and cationic vacancies coexist at both tetrahedral and octahedral sites. As x increases, the amounts of cation inversion and vacancies at both 8a and 16d sites fluctuate in the crystal structure of Mg1−xAl2(1+x/3)O4 to achieve the stability of the crystal. When the inversion behavior is taken into account, the ideal distribution of cationic vacancies in alumina-rich spinel can be described by three schemes: located at 16d site, random distribution, or located at 8a site. This leads to the ratio of tetrahedral to octahedral aluminum ions r = n[AlIV]/n[AlVI], having the three descriptions r16d, rrand, and r8a, respectively.

Figure 2. 27Al MAS NMR spectra (11.7 T) of Mg1−xAl2(1+x/3)O4 and the individual contributions of AlIV, AlVI, and spinning sidebands (ssb). The chemical shifta (δiso) and quadrupolar coupling constants (CQ) of AlIV and AlVI were resolved and are indicated in Table 1.

model. The products of isotropic chemical shift, quadruplar coupling constant, and the population of Al are given in Table 1. Integrals of the spectra indicate the population of nuclei in different coordinations; thus, the proportion of AlIV and AlVI can be derived from the integrated intensities of the central transition peaks. Further, the occupancies of Al in tetrahedral and octahedral coordinations were calculated and are given in Table 1. Details of calculations are described in the Supporting Information. 3.3. Crystal Structure Refinement. Due to the very similar X-ray atomic form factors of Al and Mg, the XRD Rietveld refinement cannot distinguish the disordered distributions of Mg and Al in Mg1−xAl2(1+x/3)O4. The site occupancies of AlIV and AlVI obtained from 27Al MAS NMR

Table 1. Isotropic Chemical Shifts (δiso), Quadrupolar Coupling Constants (CQ), Populations of Al with Different Coordinations (%), and Site Occupancies of AlIV and AlVI of All Samples δiso (ppm) Mg1−xAl2(1+x/3)O4 MgAl2O4 Mg0.88Al2.08O4 Mg0.75Al2.16O4 Mg0.47Al2.35O4 Mg0.40Al2.40O4

IV

Al

71.65(4) 72.37(7) 72.91(5) 73.05(4) 72.89(6)

CQ (MHz) Al

VI

14.31(3) 13.94(2) 14.54(2) 14.50(1) 14.44(1)

IV

population (%) VI

Al

Al

3.39(2) 4.77(4) 6.35(3) 6.50(1) 6.38(2)

5.03(1) 4.57(1) 4.85(1) 4.86(1) 4.93(1) C

IV

Al

86.57 82.99 77.56 69.80 68.53

occupancy VI

Al

13.43 17.01 22.44 30.20 31.47

IV

Al

0.2685 0.3541 0.4858 0.7104 0.7553

AlVI 0.8657 0.8637 0.8395 0.8209 0.8224

DOI: 10.1021/acs.inorgchem.8b01034 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 2. Lattice Parameters (a), Anion Parameters (u), Inversion Parameters (i), and Occupancies of Mg in Mg1−xAl2(1+x/3)O4a Mg1−xAl2(1+x/3)O4

a (Å)

u

MgIV

MgVI

VIV

VVI

i

MgAl2O4 Mg0.88Al2.08O4 Mg0.75Al2.16O4 Mg0.47Al2.35O4 Mg0.40Al2.40O4

8.08967 8.06445 8.03860 7.99103 7.97897

0.26137 0.26081 0.25993 0.25801 0.25793

0.7315 0.6317 0.5004 0.2710 0.2117

0.1343 0.1230 0.1263 0.1002 0.0941

0.0000 0.0142 0.0138 0.0185 0.0330

0.0000 0.0133 0.0342 0.0789 0.0835

0.269 0.280 0.336 0.425 0.471

a

All data were resolved from powder XRD structure refinement by combining with 27Al MAS NMR analysis.

Figure 4. Ratios of tetrahedral to octahedral aluminum ions (r), with different inverse parameters versus composition of Al8/3O4, x, in Mg1−xAl2(1+x/3)O4. r16d, rrand, and r8a correspond to the vacancies disributed at 16d, distributed randomly, and distributed at 8a sites, respectively. Figure 3. Linear dependence of lattice parameter (a) and anion parameter (b) on the concentration of Al8/3O4, x, in the synthesized Mg1−xAl2(1+x/3)O4.

r16d =

3x + 3(1 − x)i 6 − x − 3(1 − x)i

(1)

rrand =

8x + 9(1 − x)i 18 − 2x − 9(1 − x)i

(2)

r8a =

2x + 3(1 − x)i 6 − 3(1 − x)i

(3)

The derivations of eqs 1−3 are detailed in the Supporting Information. The ratio n[AlIV]/n[AlVI] is shown in Figure 4, where the calculated r values in this work are also indicated. Obviously, the cationic vacancies prefer to occupy the octahedral sites, which agrees well with previous reports.13,29 However, small amounts of vacancies enter into the tetrahedral sites to attain the stability of alumina-rich spinel crystal. Previous investigations on inverse behavior in MgAl2O4 demonstrated that the anion parameter decreases with increasing inversion parameter. As shown in Figure 5, the anion parameter and inversion parameter of MgAl2O4 are consistent with the results of neutron diffraction refinements.13 It is interesting that the anion parameter decreases with the increase of inversion parameter and composition of Al in Mg1−xAl2(1+x/3)O 4. This may elucidate how the anion parameters obtained in this work are slightly lower than the published data. A linear relationship (as indicated in Figure 3b) between anion parameter and x could be obtained from

Figure 5. Variation of anion parameter versus inversion paremter of Mg1−xAl2(1+x/3)O4.

linear fitting of the data. This could benefit the estimation of anion parameter on the basis of the generally quantitative analysis of alumina-rich spinel. 3.5. Local Structure. The anion parameter u has many important crystallographic implications, including changes in bond angle, bond length, interstice volume, and the symmetry of the coordination polyhedron. In our theoretical investigations on composition-dependent crystal structures of alumina-rich spinel, it was found that the polyhedral volume could be tuned by changing the composition.30 The volumes of D

DOI: 10.1021/acs.inorgchem.8b01034 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

structural refinements. By a combination of NMR analysis and X-ray diffraction, the inverse parameters, anion parameters, and lattice parameters of Mg1−xAl2(1+x/3)O4 were systematically investigated. The lattice parameters of synthesized Mg1−xAl2(1+x/3)O4 show a linear dependence on the concentration of Al8/3O4 (x) and follow the empirical Vegard law. The lattice parameters of Mg1−xAl2(1+x/3)O4 solid solution mostly relate to the variation of composition, no matter whether the inversion and vacancy disorders were considered or not. The anion parameters also decrease linearly with increasing x, but the values are slightly lower than those in experiments without regard for inversion and vacancy disorder in the crystal lattice. A decreasing trend of anion parameter with the inversion parameter was found in Mg1−xAl2(1+x/3)O4. As the composition is varied, the cation inversion and vacancies at both 8a and 16d sites fluctuate in amounts to achieve the stability of the Mg1−xAl2(1+x/3)O4 crystal. The volume of coordination polyhedra could be tuned by inversion parameter, anion parameter, and composition. This study provides guidance for the design and functionalization of novel spinel materials with versatile applications.

occupied octahedral and tetrahedral interstices were calculated according to eqs 4 and 5 and are plotted in Figure 6 versus x. V8a =

V16d =

8 3ij 1y a jju − zzz 3 k 4{

2 16 3ij 1 1y yi a jj − uzzz jjju − zzz 3 k2 8{ {k

(4)

(5)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01034. Powder X-ray diffraction data for Mg1−xAl2(1+x/3)O4 alumina-rich spinel, crystallographic information, observed, calculated, and difference patterns obtained by Rietveld refinement of Mg1−xAl2(1+x/3)O4, calculation of site occupancies of AlIV and AlVI from NMR analysis, and derivation of eqs 1−3 (PDF) (PDF)

Figure 6. (a) Volume of occupied polyhedral coordination (V16d and V8a correspond to tetrahedral and octahedral sites, respectively) and (b) O−M−O bond angle (M denotes octahedral ions) as a function of composition x. The theoretical results from first-principles simulations are referenced and plotted for comparison.30

Accession Codes

CCDC 1838861−1838865 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

As shown in Figure 6a, theoretical results are also included for comparison.30 In the crystal structure of the spinel, a decrease in u leads to the contraction of occupied tetrahedral coordination. Meanwhile, the O−M−O bond angle (as indicated in an inset in Figure 6b) increases toward 90° and the crystal lattice tends to be ideal face-centered cubic. In the crystal structure of alumina-rich spinel, the more vacancies the cationic sites possess, the larger the polyhedral volume.30,31 For the reason that larger Mg ions substitute AlVI ions, the interstice of 16d sites slightly increases with an increase in x. Both experimental and theoretical results demonstrated that the coordination polyhedra can adjust the volume to variations of composition, anion parameter, and inversion parameter. This enabled us to develop novel spinel-based materials with versatile performance by doping functional ions in aluminarich spinel.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for H.W.: [email protected]. ORCID

Bingtian Tu: 0000-0002-0204-706X Hao Wang: 0000-0002-3511-3895 Zhengyi Fu: 0000-0002-4310-9247 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Nos. 51472195 and 51502219) and National Key R&D Program of China (No. 2017YFB0310500).

4. CONCLUSIONS 27 Al solid-state MAS NMR spectra of Mg1−xAl2(1+x/3)O4 powders have been recorded at a magnetic field of 11.7 T in this study. The chemical shift and populations of Al at octahedral sites can be distinguished from those of tetrahedrally coordinated Al. On the basis of deconvolution of 27Al MAS NMR spectra, the distributions of Al at different sites were quantitatively determined and then used in Rietveld

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DOI: 10.1021/acs.inorgchem.8b01034 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01034 Inorg. Chem. XXXX, XXX, XXX−XXX