ARTICLE pubs.acs.org/JPCC
Mechanism of Ni1xZnxO Formation by Thermal Treatments on NiO Nanoparticles Dispersed over ZnO Fernando Rubio-Marcos,*,†,‡ Cristina V. Manzano,§ Julian J. Reinosa,|| Juan J. Romero,|| Pascal Marchet,† Marisol S. Martin-Gonzalez,§ and Jose F. Fernandez|| †
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Laboratoire de Science des Procedes Ceramiques et de Traitements de Surface, UMR 6638 CNRS, Universite de Limoges, Centre Europeen de la Ceramique, 12, rue Atlantis, 87068 Limoges Cedex, France ‡ Departamento de Fisica de Materiales, Universidad Complutense de Madrid, 28040 Madrid, Spain § IMM—Instituto de Microelectronica de Madrid (CNM-CSIC), Issac Newton 8, PTM, E-28760, Tres Cantos, Madrid Spain Instituto de Ceramica y Vidrio, CSIC, Kelsen 5, 28049 Madrid, Spain ABSTRACT: The formation of Ni1xZnxO rock salt solid solution is obtained by thermal treatments applied to NiO nanoparticles supported on ZnO micrometric particles. The high vapor pressure of ZnO produces a Zn-rich atmosphere during thermal treatment. The Zn ions tend to be adsorbed by the highly reactive NiO nanoparticles. When heated to temperatures high enough, over 500 C, the Zn ions react with the NiO nanoparticles forming the rock salt material. By variation of the treatment temperature, the composition of this rock salt structure can be varied through the whole solubility range.
1. INTRODUCTION Transition metal oxides (TMOs) doped by different elements are versatile functional ceramics which may exhibit electric, optical, catalytic, and magnetic properties. For example, ZnO doped by 5% Mn have been theoretically predicted by Dietl to present room temperature ferromagnetism (RTFM).1 The exceptional physicochemical characteristics of transition metals, like the several possible oxidation and spin states, the similarity in ionic radii, and the tendency to occupate the same site into the crystalline structure are behind this versatility. As a consequence, these compounds find applications in a wide spectrum of technological fields. One deceptively simple mixed-metal oxide system, Ni1xZnxO, is based on the very stable rock salt NiO lattice and contains Zn2+ substituting for Ni2+.25 Both metals are formally M2+, their stable oxidation state in solid-state oxides, but the octahedral coordination of the rock salt structure is unusual for zinc, which is typically found tetrahedrally coordinated.6 Then this unusual coordination of Zn2+ ion in the nickel oxide lattice can potentially result in new and interesting properties. NiO adopts a rock salt crystal structure with a unit cell lattice parameter of ao = 4.1777 Å at 300 K.7 The material is antiferromagnetic, with a Neel temperature of 523 K,8 and the alignment of the unpaired electron spins in alternating (111) planes causes a slight distortion to rhombohedral symmetry of approximately 1% along the Æ111æ direction.9,10 Although Ni2+ is 3d8, pure NiO is an insulator with a band gap of 4.2 eV11 resulting from the strong Coulomb interactions among the highly correlated d-electrons r 2011 American Chemical Society
and the localization of these electrons through hybridization with adjacent oxygen 2p levels. The Ni1xZnxO solid solution is known to be nominally facecentered cubic (fcc) rock salt for zinc concentrations up to the solubility limit (about 0.3), although, as occurring for the parent NiO structure, a slight rhombohedral distortion lowers the symmetry to the R3m space group.12 Magnetic studies12 have been performed on the Ni1xZnxO solid solution and showed that the antiferromagnetic structure persists throughout the range 0 e x e 0.3, with Neel temperature decreasing with increasing zinc content. The solid solution follows Vegard’s law,13 in which the unit cell parameter increases linearly with zinc concentration from ao = 4.1777 Å for x = 0 to 4.2107 Å for x = 0.3, indicating a homogeneous solution in which Zn2+ ions randomly substitute for Ni2+ in bulk cation lattice positions. Since Zn2+ configuration is 3d10 while Ni2+ is 3d8, substituting Zn2+ for Ni2+ can be anticipated to affect both the magnetic and the electronic structures of the parent NiO lattice. The present study explores the Ni1xZnxO solid solution in the system formed by ZnO supported NiO nanoparticles submitted to different thermal treatments. It is shown that Zn2+ ions diffuse into NiO lattice producing a rock salt solid solution at surprisingly low temperatures due to the high surface reactivity of the nickel oxide nanoparticles. The structural and Received: February 23, 2011 Revised: June 7, 2011 Published: June 10, 2011 13577
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optical changes of the system are thoroughly studied, demonstrating that ZnO particles are also modified during this process. These modifications also affect the surface of ZnO in the contact region with the NiO nanoparticles, while the main part of the particle remains unaltered. Then these modifications should be considered as proximity effects, and they are proof of the importance to develop highly selective and sensitive measurement techniques for the study of interactions at nanoscale level.
2. EXPERIMENTAL DETAILS Nanodispersion Procedure of ZnO/NiO Mixture. The raw materials used in this study were nanoparticles of nickel oxide (NiO, 99.99%) and microparticles of ZnO (99.0%). These analytical grade powders were dried at 110 C for 2 h before dry mixing. The mixture of ZnO microparticles with 5 wt % of NiO nanoparticles (hereafter named as ZN5-Nps) was prepared by mixing the appropriate amounts of NiO nanoparticles and ZnO microparticles by a previously described dry nanodispersion procedure.1416 The dry dispersion process consisted of shaking a ZnO/NiO mixture and 1 mm diameter ZrO2 balls in a 60 cm3 nylon container during 5 min at 50 rpm using a tubular-type mixer. The heat treatments were performed in a tubular furnace under air atmosphere and at temperatures ranging from 500 to 900 C for 12 h. Thermal Characterization. Simultaneous thermogravimetric and differential thermal analyses were carried out on samples of the ZnO/NiO mixed powder before the calcinations process using a NETZSCH STA 409/C analyzer. Around 50 mg of powder was placed in a Pt/Rh crucible and heated up to 1100 C with a heating rate of 3 C/min. The measurements were performed in a flowing air atmosphere. Morphology and Structural Characterization. The particle size and morphology of the powders were evaluated using secondary electron images of field emission scanning electron microscopy (FE-SEM, Hitachi S-4700) and transmission electron microscopy (TEM, Hitachi H-7100 175, with an accelerating voltage of 120 kV). For TEM investigations, powders were suspended in isopropanol, and a drop of this suspension was deposited on a holey carbon-coated film supported on a 400 mesh copper grid. The crystalline structure was determined by X-ray diffraction analysis (XRD, Siemens D5000, Munich, Germany, Cu KR radiation). The lattice parameters were refined by a global simulation of the full diagram using the fullprof program employing crystallographic information obtained from the International Center for Diffraction Data (ICDD) PDF-2 database in the form of cards 36-1451 and 47-1049 for ZnO and NiO, respectively. Vibrational and Optical Properties Characterization. Raman spectra were measured in air atmosphere at room temperature, using 514 nm radiation from an Ar+ laser operating at 10 mW. The signal was collected by a microscope Raman spectrometer (Renishaw Micro-Raman System 1000) in the 1001100 cm1 range. For the photoluminescence (PL) characterization the sample was optically pumped at 355 nm with a tripled Nd:YAG pulsed laser (pulse duration 15 ns and repetition rate 20 kHz). Then the light emitted in the ultraviolet and/or visible range was filtered by suitable long pass filters and dispersed by a monochromator with 300 mm focal length (diffraction grating: 1200 lines/mm) and detected by means of a cooled photomultiplier connected to a lock-in amplifier.
Figure 1. TG-DTA curves of the ZnO/NiO mixture.
3. RESULTS AND DISCUSSION 3.1. Thermal and Morphological Characterization of the ZnO/NiO System. Thermal analysis of a mixture of ZnO with 5
wt % of NiO nanoparticles prepared by a dry-nanodispersion method is shown in Figure 1. Three weight losses can be observed on the TG curve at 80 C (I), 260 C (II), and 400 C (III), associated with endothermic peaks. The minor weight loss (I) observed between 80 and 150 C is attributed to desorption of water. The second weight loss step (II), associated with endothermic peaks at 260 C is ascribed to impurities or adsorbates of the ZnO, since the same mass reduction was observed on the TG of pure ZnO. The weight loss at 400 C (III, broad peak in DTA) is related to the generation of defects in the ZnO powder. These defects are related to the loss of oxygen, promoting the formation of oxygen vacancies (VO+)s and desorption of interstitial zinc (Zni+).17,18 Figure 2 shows the morphology of pure ZnO and NiO raw materials. The FE-SEM micrograph on Figure 2a shows the typical ZnO morphology, consisting mainly of elongated prismatic particles and nearly cubical particles, with sizes of 0.21.0 μm and an average particle size of 0.5 μm. The morphology of NiO particles, Figure 2b, is composed of small spherical particles with initial sizes of 2030 nm (see insert Figure 2b) which form globular agglomerates ranging from 1 to 10 μm. The FE-SEM images of the mixture of ZnO with 5 wt % of NiO at different temperatures are shown in Figure 3. The micrographs show that most of the NiO initial agglomerates disappear and the individual nanoparticles are anchored to the ZnO surfaces. The dispersion and great adherence of nanoparticles could indicate the appearance of ZnO/NiO interfaces between these materials, due to the high initial reactivity of the NiO and ZnO.19 The ZN5-Nps mixture thermally treated at 500 C, Figure 3a, shows ZnO particles covered by NiO nanoparticles with a mean diameter of ∼50 nm. It can be observed, from the inset of Figure 3a, that NiO nanoparticles are of ∼50 nm and are distributed on the surface of the ZnO particles. Raising the temperature to 700 C produced an increment in size of the NiO nanoparticles, with particle sizes increasing to ∼80 nm, Figure 3b. Nanoparticles up to ∼150 nm in size were present in the 900 C calcined sample, Figure 3c, although most of the particles show diameters of ∼100 nm. 3.2. Structural Characterization of the ZnO/NiO System. The X-ray diffraction patterns of the raw materials and the ZnO/ NiO powder at room temperature, displayed in logarithmic scale in Figure 4a, can be indexed on the basis of a mixture constituted by a majority of ZnO and a minority of NiO, without any peaks other than those of the pure materials. In order to check the effect 13578
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Figure 2. Field emission-scanning electron microscopy micrographs of the initial materials: (a) ZnO microparticles and (b) NiO nanoparticles agglomerates. The insert shows a detail of NiO spherical nanoparticles by FE-SEM.
Figure 3. Field emission-scanning electron microscopy micrographs of ZnO particles covered by NiO nanoparticles that adhered at the surface after low energy mixing, thermally treated at (a) 500 C (b) 700 C, and (c) 900 C during 12 h. The insert in panel (a) shows a transmission electron microscopy image of ZnO particles covered by NiO nanoparticles adhered at the surface after thermal treatment at 500 C.
of the thermal treatment on the structure of the two pure compounds, the mixture was thermally treated at temperatures ranging from 500 to 900 C for 12 h. Some changes are observed in the XRD pattern after such thermal treatment, Figure 4b. The expected reaction between ZnO and NiO is the formation of a Ni1xZnxO rock salt solid solution, which possesses the same crystal structure as NiO. This would explain the fact that only the diffraction peaks of NiO are affected by thermal treatment. There is little difference between the ionic radii of Ni2+ (0.069 nm) and Zn2+ (0.074 nm) and, therefore, small changes in the a axis value due to Zn2+ substitution in NiO can be expected. The formation of the Ni1xZnxO rock salt solid solution requires a large diffusion of Zn atoms (not only those at the particle surface) and therefore is more favored by the longer annealing time. A careful examination of the XRD patterns reveals a pronounced evolution of the lattice parameters of the NiO phase, as shown in Figure 4c. The evolution observed in the NiO rock salt unit cell parameters, ao, is due to the formation of the Ni1-xZnxO rocksalt solid solution, even for samples treated at temperatures as low as 500 C. The reduction of diffusion temperatures has been previously observed in other nanostructured systems, such as ZnO/MnOx multilayers.20 The lattice parameters, ao, over the range at which the solid solution
forms, is found to increase linearly with thermal treatment temperature as the larger Zn2+ substitutes the smaller Ni2+ in the NiO lattice, Figure 4c. These results about the increase of the unit cell length with increasing thermal treatment temperature correspond to a variation from 4.1775 Å for the NiO at room temperature to 4.2112 Å for the ZnO/NiO mixture treated at 900 C. Additionally, the crystallite size (D) of the NiO nanoparticles at room temperature and Ni1xZnxO rock salt solid solution depending on the treatment temperature are calculated from the full width at half-maximum of the diffraction peaks by using Scherrer’s equation: D¼
Kλ ðB cos θÞ
where λ is the X-ray wavelength, B is the full width at halfmaximum of the diffraction peak, θ is the angle of diffraction, and K is a constant (having the value 0.9 in our case). Figure 5a shows the increase of the crystallite size as a function of the thermal treatment temperature. The average Ni1xZnxO nanophase crystallite size varied from 29 nm, for NiO phase at room temperature, to 70 nm, for the Ni1xZnxO solid solution at 900 C. These results are in good agreement with the tendency observed by SEM and TEM (Figures 2 and 3). 13579
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Figure 4. (a) XRD patterns corresponding to the raw materials and the ZnO/NiO mixture thermally treated at temperatures between 500 and 900 C during 12 h. (b) Magnified XRD diffraction patterns in the 2θ range from 42 to 44of the ZnO/NiO mixtures. (c) Variation of lattice parameters as a function of the temperature treatment.
Figure 5. Evolution of the crystallite size in the Ni1xZnxO nanophase and ZnO microparticles as a function of the thermal treatment temperature.
Figure 6. Lattice constant derived from experimental data (open circle) compared to those predicted from ref 21 (filled triangles).
Moreover, Figure 5b shows the crystallite size of the ZnO microparticles as a function of the thermal treatment temperature. It can be observed that the ZnO crystallite size remains constant until 800 C and increases a bit for higher temperatures.
A similar evolution with the temperature was observed in the lattice parameters of the ZnO; see Figure 4c. This behavior could be related to diffusion of Ni2+ ions on the ZnO lattice. Gaskell et al.21 recently reinvestigated the Ni1xZnxO solid solution. Their experimental data show that the lattice parameter, ao, follows the Vegard law up to x = 0.3, indicating that above 30% zinc concentration, the samples are phase-separated into wurtzite ZnO and a rock salt solid solution of constant composition, nominally Ni0.7Zn0.3O. These results are represented together with our experimental data in Figure 6. From this expression, we can calculate Ni1xZnxO rock salt compositions for the NiO nanoparticles dispersed on the ZnO surface under study, see Figure 6, reaching the limit of solid solution, Ni0.7Zn0.3O, in the sample with a larger thermal treatment, 900 C for 12 h. From this temperature, the solid solution limit is exceeded, and therefore ZnO lattice parameters are affected by the processes of Ni2+ diffusion in the ZnO lattice as can be observed in Figure 4c. 3.3. Vibrational and Optical Characterization of ZnO/NiO System. Figure 7a shows the Raman spectra for the ZnO and NiO as raw materials, and the ZN5-Nps samples for different thermal treatments. ZnO has a wurtzite structure, with two formulas per primitive cell with C3v symmetry. For this structure, the group theory predicts four Raman active modes A1 + E1 + 2E2.22 NiO exhibits a broad band at 500 cm1 due to the NiO stretching mode, with a shoulder at 410 cm1 indicative of the nonstoichiometry of the material23,24 and the high nickel vacancy concentration. It is obvious from Figure 7a that thermal treatment modifies the Raman spectra of the mixtures, producing a Raman intensity decrease and a wavenumber shift and broadening of the particular modes of ZnO. Raman peaks corresponding to the NiO nanoparticles are not observed due to the low Raman efficiency of this material, together with the low quantity of nanoparticles in the mixtures and the further intensity reduction expected on 13580
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Figure 7. (a) The Raman spectra of raw materials, ZnO/NiO mixture (RT), and thermally treated mixture at temperatures between 500 and 900 C during 12 h. (b) Magnified Raman spectra in the wavenumber ranges from 500 to 650 cm1 and from 800 to 1200 cm1 of the ZnO/NiO mixture.
nanoparticles. The changes observed on the ZnO Raman peaks must be related to diffusion and/or some interaction between NiO and ZnO particles. It is found that the ZnN5-Nps samples thermally treated at temperatures higher that 700 C present an increasing intensity of the Raman band at about 550 cm1. In particular, especially between 800 and 900 C, additional modes at about 525 and 570 cm1 appear, Figure 7b. The first one (∼525 cm1) has been observed in other ZnO based compositions, such as Zn1xMnxO,2527 Codoped ZnO nanorods,28 and Zn1xNixO,29 and is assigned to an impurity-related vibration associated with defects in the host lattice induced by the doping. The 570 cm1 Raman mode has been assigned by some authors to A1(LO)17,2527 or a disorder-increased A1(LO) mode of ZnO26 (this mode appears at ∼574 cm1 in pure material).17 These modes are then associated to the modifications caused on the ZnO by doping, and it can be an indication that the Ni ions in our case are diffusing into the ZnO at temperatures of about 700 C, a much lower temperature than the one suggested by the XRD data. This difference is due to the higher sensitivity of the Raman spectroscopy with respect to the XRD. Moreover, the thermal treatment induces significant changes in the position, full width at half-maximum (fwhm) and intensity of the E1(LO) and A1(LO) Raman modes. It should be noticed that the A1(LO) mode at ∼569 cm1 and the E1(LO) mode at ∼586 cm1 have relatively close wavenumbers and rise from background, which is originated from secondorder Raman scattering. The presence of impurities and/or defects can influence both modes. The E1(LO) mode slightly red shifts from ∼587 cm1 in ZnO powder to ∼580 cm1 in the mixture of ZnO with 5 wt % of NiO at 800 C for 12 h. As the E1(LO) mode is related to oxygen vacancies, its intensity increase and red shift with the thermal treatment points out serious oxygen deficiency in activated ZnO powders,17,30 although for the ZN5-Nps at 900 C this mode tends to disappear. A shift to lower wavenumber indicates a structural
Figure 8. (a) Room temperature PL spectra from the ZnO/NiO mixture and mixtures thermally treated between 500 and 900 C upon excitation with 355 nm light. (b) Magnified PL spectra in the wavelength range from 400 to 650 nm of the ZnO microparticles and ZnO/NiO mixtures as function of the thermal treatment. In the same figure, Gaussian fits of the individual bands centered at ∼465, ∼515, and ∼550 nm, and ascribed to oxygen vacancies (VO), zinc vacancies (VZn), and interstitial oxygen (Oi).
relaxation of the crystalline lattice due to reduction of structural stress.31 In addition, the A1(LO) Raman mode that corresponds to local vibration modes associated with intrinsic lattice defects may also be favored by the NiO nanoparticles proximity32 and thermal 13581
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Figure 9. Schematic representation of the attachment of nanoparticles to the outer surface of an individual ZnO microparticle obtained by the drynanodispersion method: (a) after the dry-nanodispersion process; (b) magnified ZnO surface covered by NiO nanoparticles hierarchically dispersed, where the NiOZnO interfaces can be observed; (c, d) schematic representation of the thermal evolution of the solid solution, Ni1xZnxO, formed between NiO and ZnO.
treatment. This mode increases with annealing treatment. The anomalous enhancement of the LO mode has also been reported in doped ZnO3335 and ascribed to oxygen vacancies, zinc vacancies, and interstitial Zn. On the other hand, the high wavenumber region in the Raman spectra of ZnO is dominated by a broad asymmetric mode at ∼1154 cm1, containing contributions of 2A1(LO) and 2E1(LO) modes at the Γ point of the Brillouin zone.36 The intensity decrease of the most intensive second-order Raman mode is followed by a red shift (from ∼1154 cm1 in ZnO down to ∼1140 cm1 in ZN5Nps at 900 C) and broadening due to the annealing treatment. The intensity ratio of this mode to the first-order A1E1(LO) mode, I2LO/ILO, mode in ZnO and ZN5-Nps samples can be used to estimate the electronLO phonon coupling strength.37,38 The I2LO/ILO ratio is decreasing with thermal treatment from ∼1.92 in ZnO down to ∼1.35 in ZN5-Nps at 900 C, Figure 7b. In order to determine the possible origin of these defects, photoluminescence studies were performed; see Figure 8. The emission of ZnO/NiO mixture without further thermal treatment is dominated by near-band-edge (NBE) UV with a maximum at 382 nm (3.25 eV), which corresponds with the band gap of ZnO. In the visible range, weaker emission bands between 420 and 650 nm can be observed. These bands are associated to defects and/or impurities in the ZnO structure, since NiO emission at room temperature is negligible39 and, moreover, NiO quantity in the samples is clearly smaller than that of ZnO. The emission at 420650 nm is too wide to be associated to only one defect or impurity, so it should be deconvoluted into three Gaussian functions: a blue component centered at 465 nm (2.67 eV); a green one at 515 nm (2.41 eV), and a yellow one at 550 nm (2.25 eV). The blue emission can be associated with oxygen vacancies (VO), the green emission can be associated with zinc vacancies (VZn),40 and the yellow one to interstitial oxygen (Oi).41 During the thermal treatment, Figure 8, the relative intensity of the 420650 nm band increases versus the NBE band, being the visible band the most intense for temperatures higher than 600 C. This means that upon heating the mixture the number of defects (oxygen vacancies and zinc vacancies) increases, in agreement with the results obtained by Raman spectroscopy. At temperatures g700 C the blue emission decreases, since oxygen vacancies migrate at ∼636 C.40 For T > 900 C no
emission is detected in the visible range or at the NBE band of ZnO, indicating that ZnO has reacted with NiO, to a extent high enough to produce a high number of defects that prevent these emissions. This modification of the ZnO is consistent with the lattice constant change observed on Figure 4. Nevertheless, a narrow emission band is clearly observed at 372 nm (3.33 V) on the 900 C calcined sample. This band also appears for calcinations at lower temperatures, and its intensity is increasing with the treatment temperature. Then this emission could be associated with the Ni1xZnxO rock salt structure, whose amount increases with the treatment temperature. The phenomenology described here can be interpreted by a simple model based on the formation of Ni1xZnxO rock salt solid solution by different thermal treatments applied to the dispersion of NiO nanoparticles over ZnO micrometric particles. Figure 9a shows a schematic representation of an individual ZnO particle hierarchically coated with NiO nanoparticles at room temperature. Figure 9b shows a magnification of the ZnO surface, which illustrates the clean interfaces between ZnO microparticles and NiO nanoparticles. When this mixed material is heated at temperatures below 500 C in open air, part of the ZnO evaporates, due to the high Zn partial pressure. Part of the evaporated Zn cations are adsorbed by the NiO nanoparticles, which show a very high reactivity with these atoms, probably forming a capping surface over them, as depicted at Figure 9c. By thermal treatment at temperatures over 500 C the adsorbed Zn ions react with the NiO nanoparticles producing the formation of the Ni1xZnxO rock salt. The composition of this rock salt structure is clearly dependent on the temperature, since the higher the temperature, the higher the ZnO partial pressure, and hence the higher the quantity of Zn cations that can react with the NiO nanoparticles. Increasing the temperature over 900 C, the limit composition of the Ni1xZnxO solid solution is reached and Ni ions start to diffuse in the ZnO particles, as schematically depicted on Figure 9d. These results indicate that the nanoparticle depositions are stable up to high temperatures, implying that this method would be useful for preparation of new catalytic materials working even at high temperatures. Only at temperatures over 900 C have we seen evident modification of the substrate material due to reaction with the nanoparticles, although nanoparticles themselves, which would act as 13582
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4. CONCLUSIONS The formation of Ni1xZnxO rock salt compositions has been attained upon heating a nanodispersion of NiO nanoparticles over ZnO micrometric rods. These nanodispersions present clean interfaces, with no evidence of chemical reactions or diffusion among the contacting particles. Heating this material provokes partial desorption of Zn ions, due to the high partial pressure of ZnO, that react with the highly reactive NiO nanoparticles to form the rock salt structure. The composition of the rock salt material depends on the treatment temperature, and the whole solubility range can be obtained by heating at temperatures between 500 and 900 C. When temperatures were over 900 C, the solubility limit (x = 0.3) is reached and NiO reacts with the ZnO particles, producing a doping of the supporting particles. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors express their thanks to the MICINN (Spain) projects MAT 2010-21088-C03-01, MICINN ACI PLAN E (JAPON) ref: PLE2009-0073, and to the European Comission ERC-2008-Stg: 240497 for their financial support. M.S.M.G. acknowledges B. Alen from IMM for technical support. C. V. Manzano thanks the JAE-Predoc program for the research grant. Dr. F. Rubio-Marcos is indebted to the “Conseil Regional du Limousin” for a postdoctoral fellowship. ’ REFERENCES (1) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019–1022. (2) Navrotsky, A.; Muan, A. J. Inorg. Nucl. Chem. 1971, 33, 35–47. (3) Hahn, W. C.; Muan, A. J. Phys. Chem. Solids. 1961, 19, 338–348. (4) Kedesdy, H.; Drukalsky, A. J. Am. Chem. Soc. 1954, 76, 5941–5946. (5) Rigamonti, R. Gazz. Chim. Ital. 1946, 76, 974. (6) Vainsthtein, B. K.; Fridkin, V. M.; Indenbom, V. L. Modern Crystallography, 3rd ed.; Springer: New York, 2000; Vol. 2, p 156. (7) Smart, J. S.; Greenwald, S. Phys. Rev. 1951, 82, 113–114. (8) Marynowski, M.; Franzen, W.; El-Batanouny, M.; Staemmler, V. Phys. Rev. B 1999, 60, 6053–6067. (9) Jauch, W.; Reehuis, M. Phys. Rev. B 2004, 70, 195121. (10) Springthorpe, A. Phys. Status Solidi 1967, 24, K3–K4. (11) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Phys. Rev. B 1998, 57, 1505–1509. (12) Rodic, D.; Spasojevic, V.; Krusigerski, V.; Tellgren, R.; Rundlof, H. Phys. Status Solidi B 2000, 218, 527–536. (13) Denton, A.; Ashcroft, N. Phys. Rev. A 1991, 43, 3161–3164. (14) Fernandez, J. F.; Lorite, I.; Rubio-Marcos, F.; Romero, J. J.; Garcia, M. A.; Quesada, A.; Martin-Gonzalez, M. S.; Costa-Kr€amer, J. L. Patent Numbers WO2010010220-A1; ES2332079-A1, 2010, to Consejo Superior de Investigaciones Cientificas, CSIC. (15) Rubio-Marcos, F.; Calvino, V.; Ba~nares, M. A.; Fernandez, J. F. J. Catal. 2010, 275, 288–293. (16) Martin-Gonzalez, M. S.; Garcia, M. A.; Lorite, I.; Costa-Kr€amer, J. L.; Rubio-Marcos, F.; Carmona, N.; Fernandez, J. F. J. Electrochem. Soc. 2010, 157, E31–E35.
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