A simple and straightforward mechanochemical synthesis of the far-from-equilibrium zinc aluminate, ZnAl2O4, and its response to thermal treatment

Zinc aluminate (ZnAl2O4) nanoparticles with an average size of about 10 nm are synthesized via one-step mechanochemical processing of the ZnO : γ-Al2O3 stoichiometric mixture at ambient temperature. The mechanochemically induced formation of the phase is followed by XRD and 27Al MAS NMR. High-resolution TEM studies reveal a non-uniform nanostructure of mechanosynthesized aluminate consisting of ordered grains surrounded or separated by disordered surface and interfacial regions. Due to the capability of 27Al MAS NMR to probe the local environment of the Al cations, valuable insights into the short-range structure of ZnAl2O4 on the Angstrom length scale are provided. It is demonstrated that the as-prepared aluminate possesses a partly inverse spinel structure with a far-from equilibrium arrangement of cations and distorted polyhedra, which are spatially confined to the surface and interfacial regions with a volume fraction of ca. 50% and a thickness of ca. 1 nm. The response of the nanostructured ZnAl2O4 to subsequent thermal treatment is further investigated. It turned out that the thermally induced grain growth is accompanied by a release of microstrain, by a shrinkage of the lattice parameter, as well as by a variation in the oxygen parameter and metal–oxygen bond lengths. Evidence is given of the thermally induced redistribution of cations approaching their equilibrium positions. Upon heating above 1100 K, mechanosynthesized ZnAl2O4 relaxes towards a structural state that is similar to the bulk one.


Introduction
The ability of spinels to redistribute their cations over crystallographically nonequivalent positions has attracted considerable interest from many scientists. The cubic spinel structure (space group Fd 3m) is characterized by close-packed arrays of oxygen atoms with one eighth of the tetrahedral and one half of the octahedral sites occupied by heterovalent cations (Fig. 1). To emphasize the site occupancy on the atomic level, the structural formula of 2-3 spinels of the type M1 2+ M2 2 3+ O 4 (where 2-3 refer to the valences of the M1 and M2 cations) may be written as (M1 1Àl M2 l )[M1 l M2 2Àl ]O 4 , where parentheses and square brackets enclose cations that are either tetrahedrally (A) or octahedrally [B] coordinated by oxygen anions, respectively. l represents the so-called degree of inversion that is dened as the fraction of the (A) sites occupied by trivalent (M2) cations. Spinel compounds with l ¼ 0 are denoted as normal spinels, whereas those with l ¼ 1 are called fully inverse spinels. The value of l rd ¼ 2/3 corresponds to a random distribution of cations over the (A) and [B] positions. 1 It is well recognized that physico-chemical properties of spinels are determined to a large extent by their degree of inversion. [2][3][4] Thus, a detailed understanding of the functional behavior of spinels relies on careful characterization of their cation distribution. In its equilibrium state, zinc aluminate (ZnAl 2 O 4 , gahnite) possesses the structure of a normal spinel (l c ¼ 0) with the following crystal chemical formula: (Zn) [ attention has been paid to several of its multifunctional applications such as catalyst and catalyst support, UV-transparent support conductor, sensor, dielectric and optical material. [6][7][8] The conventional solid state, i.e., ceramic, synthesis of ZnAl 2 O 4 requires long periods of calcination of the reaction precursors at considerably high temperatures. 9 In many cases, this causes the loss of zinc due to its high volatility and, consequently, it results in the formation of multiphase products and the degradation of microstructural and functional properties of the aluminate. Various wet chemistry-based routes, including, e.g., hydrothermal, 10 sol-gel, 11 combustion, 12 coprecipitation, 9 complexation, 13 solvothermal 6 and sonochemical 14 methods, have also been developed to synthesize nanosized ZnAl 2 O 4 powders. Most of the solution chemistry-based routes, however, still require calcination steps, at relatively low temperatures. Non-conventional mechanochemical synthesis (mechanosynthesis) has been recognized as an alternative low-temperature route; in general, it provides an efficient one-step and facile access to nanomaterials. 15 In this context, the present work focuses on the one-step synthesis of nanocrystalline ZnAl 2 O 4 via mechanochemical processing of a ZnO + g-Al 2 O 3 mixture at ambient temperature. Although the mechanosynthesis of nanocrystalline ZnAl 2 O 4 has already been reported in a few papers, 9,16 to the best of our knowledge there is no report in the literature focusing on the defect state or the disordered local structure of ZnAl 2 O 4 prepared by nonconventional mechanochemical routes.
Mechanosynthesized complex oxides are oen inherently unstable because of their small constituent sizes, disordered structural state, and high chemical activity. 17 To gain insight into thermal stability and relaxation of structural disorder, the present experimental work also deals with the study of the response of mechanosynthesized ZnAl 2 O 4 when exposed to higher temperatures. For a comprehensive characterization of structural relaxation paths of the non-equilibrium product, we simultaneously apply X-ray diffraction (XRD), which is sensitive to medium-and long-range structural order, and 27 Al magic angle spinning (MAS) nuclear magnetic resonance (NMR), which reveals local magnetic and electronic structures. Moreover, the thermally induced evolution of the aluminate synthesized is systematically monitored with Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM).

Experimental
Solid precursors, zinc oxide (ZnO, 99.9% purity; Aldrich) and aluminium oxide (g-Al 2 O 3 , 99% purity; Aldrich), were used for the mechanosynthesis of ZnAl 2 O 4 . 5 g of the ZnO : g-Al 2 O 3 mixture was milled for various times (up to 2 h) in a high-energy planetary ball mill (Pulverisette 7 Premium line (Fritsch)). A grinding chamber (80 cm 3 in volume) and balls (10 mm in diameter) made of tungsten carbide were used. The ball-topowder weight ratio was 40 : 1. Milling experiments were performed in ambient atmosphere at 600 rpm. To investigate the thermally induced structural relaxation of mechanosynthesized ZnAl 2 O 4 , the material was subsequently annealed at various temperatures up to 1273 K in air for 4 hours.
In addition, polycrystalline ZnAl 2 O 4 (with the average crystallite size ca. 105 nm) was synthesized from the mixture of ZnO and g-Al 2 O 3 precursors following a conventional ceramic process. This sample served as reference material. Note that an excess of ZnO (5 wt%) with respect to the stoichiometric ratio was used to avoid the formation of a multiphase product. In this case, powdered reactants were hand-milled, pressed into pellets and sintered at 1273 K for 24 hours. This process was repeated four times, reaching the nal time of sintering of 120 hours.
The XRD patterns were collected using a D8 Advance diffractometer (Bruker) operating with Cu Ka radiation in Bragg-Brentano conguration. The generator was set up at 40 kV and 40 mA. The divergence and receiving slits were 0.3 and 0.1 mm, respectively. The patterns were recorded in the range of 20 to 105 2q with a step of 0.02 and a measuring time of 20 s. The JCPDS PDF database 18 was utilized for phase identication. Rietveld renements of XRD data of the asprepared and subsequently annealed samples were performed using the Fullprof computer program 19 utilizing regular Thompson-Cox-Hastings pseudo-Voigt prole parameters. In order to obtain proper geometry set-up and to eliminate instrumental broadening the instrumental resolution function was determined by the renement of the LaB 6 standard specimen. The cubic spinel structure of ZnAl 2 O 4 was visualized using the Diamond program. 20 The morphology of powders was studied using a combined eld-emission (scanning) transmission electron microscope (S)TEM (JEOL JEM-2100F). Prior to the TEM investigations, the powders were crushed in a mortar, dispersed in ethanol, and xed on a copper-supported carbon grid. 27  frequency for 27 Al). At both spectrometers the samples investigated were rotated in a 2.5 mm rotor at a spinning speed of 30 kHz. Typically, 64 scans were acquired with a repetition delay of 5 s. Spectra have been referenced to aqueous Al(NO 3 ). Since 27 Al is a half-integer quadrupole nucleus (spin-quantum number I ¼ 5/2) we used short excitation pulses close to a p/12 pulse to record spectra being useful for a quantitative analysis of site occupancies. This is especially important for Al sites with large quadrupole coupling constants. In general, a p/[4(I + 1/2)] pulse should be applied for such purpose. 21 Here, the degree of inversion was estimated from the intensity ratio of the NMR lines corresponding to (A)-and [B]-site Al ions, according to the FTIR experiments were carried out using a Tensor 27 (Bruker) spectrometer. The spectra were taken in transmission mode within the range of 1200-380 cm À1 .

Results and discussion
The mechanically induced formation of ZnAl 2 O 4 from the ZnO : g-Al 2 O 3 mixture was followed by XRD (Fig. 2). Aer 2 hours of intensive milling, all diffraction peaks above the background are attributed to mechanosynthesized ZnAl 2 O 4 (JCPDS PDF 82-1043). 18 The broad shape of XRD reections for mechanosynthesized aluminate, in contrast to the relatively narrow reections for bulk ZnAl 2 O 4 (see Fig. 2 at the bottom), provides clear evidence for a nanoscale nature of the oxide prepared via mechanosynthesis.
A representative TEM micrograph of nanocrystalline mechanosynthesized ZnAl 2 O 4 is shown in Fig. 3. It reveals that the aluminate consists of nanoparticles with a size distribution ranging from about 5 to 40 nm; the average crystallite size (D) is estimated to be approximately 10 nm. As shown in Fig. 3a and b, the nm-sized crystallites tend to agglomerate. They are found to be roughly spherical with a so-called core-shell structure consisting of ordered inner cores (grains) surrounded or separated by disordered surface regions (see Fig. 3c). The thickness t of the disordered surface shell estimated via high-resolution TEM was found to be about 1 nm.
To determine the phase evolution of the ZnO : g-Al 2 O 3 mixture during high-energy milling in greater detail and to provide insight into the local structural disorder of the aluminate nanoparticles, the mechanochemical reaction was also followed by 27 Al MAS NMR. High-resolution NMR has been proven to be highly useful to shed light on local magnetic and electric structures around the aluminium ions. In particular, this includes also local coordination and any distortions of the oxygen polyhedra. 15,23  The change in cationic order in spinels is usually induced by high temperature, 25 high pressure, 26 irradiation of the material with electrons, ions or neutrons, 27,28 and its particle size reduction to the nanometer range. 5 All of these processing parameters were found to affect the cation distribution towards random arrangement (l / l rd ). 15   temperature in the range of 973-1673 K, they found, with temperature increasing, a small increase in the degree of inversion. l increased from 0.01 to 0.06. The extraordinary high value of l ¼ 0.31(2), derived for our mechanosynthesized material, demonstrates the far-from-equilibrium nature that is accessible via the mechanochemical preparation route used.
By analogy with the non-uniform conguration of the mechanochemically prepared nanooxides, 15 the l value determined for mechanosynthesized ZnAl 2 O 4 can be considered as a mean value reecting the cation distribution within its ordered grains and disordered interfaces and surfaces. Note that the atomic congurations in these regions of spinel oxides prepared by mechanochemical routes are chiey characterized by a random arrangement of cations (l rd ). 30 In contrast, the ordered grains of nano-oxides were found to exhibit an equilibrium cation distribution (l c ). 30 Thus, the experimentally determined l value for mechanosynthesized ZnAl 2 O 4 can be expressed as l ¼ (1 À w)l c + wl rd , where w is the volume fraction of disordered regions. The estimated value of w ¼ 0.465 indicates that about 50% of the atoms in the aluminate mechanosynthesized are in a structurally disordered state. Note that the simultaneous presence of two spinel phases characterized by different inversion parameters (l c and l rd ) with w ¼ 0.80 has also been observed in ZnAl 2 O 4 irradiated with Au ions. 28 Assuming a spherical shape of the as-prepared nanoparticles and taking their average diameter (D ¼ 10 nm) as determined experimentally by TEM into account (see Fig. 3), one can deduce information on the thickness of the disordered interfacial regions in the nanomaterial [w ¼ 1 À (1 À 2t/D) 3 ]. The resulting t, which is 0.94 nm, is comparable to the unit cell dimension (a) of the material. We note that, in general, 1 nm is a typical thickness of grain boundaries or surface shell regions in nanostructured mechanochemically prepared oxides, such as spinels, olivines, perovskites, as well as orthorhombic and ilmenite-type complex oxides. 15 In the following, we will present and discuss the results obtained when mechanosynthesized ZnAl 2 O 4 is exposed to higher temperatures. As it is shown in Fig. 5, subsequent annealing of ZnAl 2 O 4 results in signicant narrowing of the diffraction peaks, indicating recrystallization of the sample accompanied by crystallite growth and a release of accumulated microstrain. At temperatures above 1148 K, tiny reections belonging to ZnO appear in the XRD pattern of the material heat treated. This is due to the thermally induced partial decomposition of the highly non-equilibrium ZnAl 2 O 4 . A similar behaviour has already been observed during thermal relaxation of mechanochemically treated ZnFe 2 O 4 . 31 Rietveld analyses of the XRD data of the samples thermally treated enabled us to quantitatively characterize their thermally induced evolution. As is seen in Fig. 6a, the lattice parameter of the mechanosynthesized material (a ¼ 8.121Å) was found to be larger than that of polycrystalline ZnAl 2 O 4 (a ¼ 8.088Å), which served as a reference here. The disappearance observed for the  lattice expansion with increasing annealing temperature can be ascribed to a structural relaxation of the sample towards its equilibrium structure. It was found that both the crystallite size and microstrain do not change signicantly with annealing temperature up to about 900 K (see Fig. 6b and c). The two properties, however, alter considerably at temperatures ranging from 900 K to 1273 K. Annealing also leads to a recovery of the local structure of the aluminate; Fig. 6d shows that the degree of inversion decreases with annealing temperature; l changes from 0.34 (2) to about 0.02(2) aer treatment at 1273 K. The relaxation of the cation distribution towards its equilibrium state is accompanied by changes in the geometry of the structural units of the material. Fig. 6e shows the thermally induced variations in the cation-oxygen bond lengths in tetrahedrally and octahedrally coordinated polyhedra of ZnAl 2 O 4 . One can observe the opposite effect on the geometry of the polyhedra; while the cation-oxygen bond length in the tetrahedra expands with annealing temperature, the cation-oxygen bond length in the octahedra decreases. This alteration is obvious if we take into account the different radii of Zn 2+ and Al 3+ ions in (A) and [B] sites; the ions migrate from their nonequilibrium sites into the equilibrium ones; r(Zn 32 Finally, the oxygen parameter u was found to increase with increasing annealing temperature as it is shown in Fig. 6f. For the structurally relaxed material, obtained aer thermal treatment at 1273 K, this parameter takes a value of 0.264, which is close to that reported for well crystalline ZnAl 2 O 4 . 33 The response of the mechanosynthesized aluminate to changes in temperature was also followed by 27 Al MAS NMR. Fig. 7 shows the 27 Al MAS NMR spectra of the mechanosynthesized oxide that were recorded aer heat treatment at the various temperatures indicated. Annealing the sample at  temperatures of up to 523 K has no signicant effect on the shape of the two NMR lines observed demonstrating a rather high stability of the product against heat treatment. At temperatures above 773 K, however, gradual crystallization of the ZnAl 2 O 4 powders takes place. As expected, the spectral component corresponding to the Al 3+ (A) ions progressively vanishes because the mechanically induced inversion of the spinel structure gets lost. This is accompanied by a gradual narrowing of the NMR line shapes implying that the octahedra are increasingly less distorted aer the sample has been annealed at elevated T. The shi observed for the NMR lines also suggests the formation of an ordered state that is reached aer heat treatment.
It is interesting to note that, most likely, the relaxation path involves an intermediate state with Al 3+ ions located on the tetrahedral interstices 8b (see the asterisk in Fig. 7); these sites are normally not occupied by Al cations. 34 Simultaneously, with increasing temperature of heat treatment, a right-hand side broadening of the prole for the Al 3+ [B] line (ca. 0 ppm) disappears. This broadening can be attributed to Al cations located in additional, most likely 16c octahedrally coordinated sites. 34,35 The degree of inversion l, calculated from the spectral intensities of the sample annealed at 1273 K, is approximately 0.03(3), which is well comparable with that of the reference   (3)). In detail, the results on the relaxation process of far-from-equilibrium ZnAl 2 O 4 are listed in Table 1. Furthermore, FTIR spectroscopy was employed to provide information on the relaxation process. As shown in Fig. 8, the spectrum of the as-prepared aluminate is dominated by two broadened bands centred at about 695 and 537 cm À1 . They can be assigned to stretching vibrations in the oxide. A shoulder at about 790 cm À1 can be related to the vibrations of Al 3+ (A) ions. 36,37 With increasing annealing temperature the absorption bands become sharper and the peak centred at 537 cm À1 splits into two absorption maxima at n 2 ¼ 564 cm À1 and n 3 ¼ 504 cm À1 . This can be ascribed to the relaxation of the geometry of distorted polyhedra in ZnAl 2 O 4 . Since the position of vibrational modes is rather sensitive to the chemical nature of trivalent cations, i.e., to the bonding force between a trivalent cation and an oxygen anion, 38 the observed red shi indicates the redistribution of cations from their nonequilibrium sites towards the equilibrium ones. The latter is accompanied by the gradual disappearance of the shoulder at 790 cm À1 .

Conclusions
The present study demonstrates that nanostructured ZnAl 2 O 4 with an average crystallite size of about 10 nm in diameter can be prepared via simple and straightforward mechanochemical synthesis starting with a stoichiometric mixture of ZnO : g-Al 2 O 3 . The synthesis was carried out at ambient temperature and the reaction time was relatively short (2 h). It has been found that the as-prepared, nanostructured aluminate consists of ordered crystalline grains surrounded or separated by disordered interfacial regions characterized by a volume fraction of about 50%. 27 Al MAS NMR spectroscopy demonstrates that the nano-aluminate is characterized by distorted polyhedra; moreover, the oxide shows a far-from equilibrium arrangement of cations characterized by degree of inversion of l ¼ 0.31 (2). Fortunately, the range of thermal stability of the mechanosynthesized product extends up to ca. 523 K. Upon annealing at T > 773 K, the nonequilibrium cation distribution relaxes towards the equilibrium conguration. Simultaneously, the crystallites grow and the accumulated microstrain releases during annealing. This relaxation process is accompanied by a disappearance of the lattice expansion and variations in the cation-oxygen bond lengths. Thus, during heating, mechanosynthesized ZnAl 2 O 4 relaxes towards a structural state that is similar to that of the bulk oxide.