Fivefold Coordination Research Articles

Understanding Defects in Amorphous Silicon with Million-Atom Simulations and Machine Learning.

The structure of amorphous silicon (a-Si) is widely thought of as a fourfold-connected random network, and yet it is defective atoms, with fewer or more than four bonds, that make it particularly interesting. Despite many attempts to explain such "dangling-bond" and "floating-bond" defects, respectively, a unified understanding is still missing. Here, we use advanced computational chemistry methods to reveal the complex structural and energetic landscape of defects in a-Si. We study an ultra-large-scale, quantum-accurate structural model containing a million atoms, and thousands of individual defects, allowing reliable defect-related statistics to be obtained. We combine structural descriptors and machine-learned atomic energies to develop a classification of the different types of defects in a-Si. The results suggest a revision of the established floating-bond model by showing that fivefold-bonded atoms in a-Si exhibit a wide range of local environments-analogous to fivefold centers in coordination chemistry. Furthermore, it is shown that fivefold (but not threefold) coordination defects tend to cluster together. Our study provides new insights into one of the most widely studied amorphous solids, and has general implications for understanding defects in disordered materials beyond silicon alone.

Understanding Defects in Amorphous Silicon with Million‐Atom Simulations and Machine Learning

AbstractThe structure of amorphous silicon (a‐Si) is widely thought of as a fourfold‐connected random network, and yet it is defective atoms, with fewer or more than four bonds, that make it particularly interesting. Despite many attempts to explain such “dangling‐bond” and “floating‐bond” defects, respectively, a unified understanding is still missing. Here, we use advanced computational chemistry methods to reveal the complex structural and energetic landscape of defects in a‐Si. We study an ultra‐large‐scale, quantum‐accurate structural model containing a million atoms, and thousands of individual defects, allowing reliable defect‐related statistics to be obtained. We combine structural descriptors and machine‐learned atomic energies to develop a classification of the different types of defects in a‐Si. The results suggest a revision of the established floating‐bond model by showing that fivefold‐bonded atoms in a‐Si exhibit a wide range of local environments–analogous to fivefold centers in coordination chemistry. Furthermore, it is shown that fivefold (but not threefold) coordination defects tend to cluster together. Our study provides new insights into one of the most widely studied amorphous solids, and has general implications for understanding defects in disordered materials beyond silicon alone.

Molecular insight into the initial hydration of tricalcium aluminate

Portland cement (PC) is ubiquitously used in construction for centuries, yet the elucidation of its early-age hydration remains a challenge. Understanding the initial hydration progress of tricalcium aluminate (C3A) at molecular scale is thus crucial for tackling this challenge as it exhibits a proclivity for early-stage hydration and plays a pivotal role in structural build-up of cement colloids. Herein, we implement a series of ab-initio calculations to probe the intricate molecular interactions of C3A during its initial hydration process. The C3A surface exhibits remarkable chemical activity in promoting water dissociation, which in turn facilitates the gradual desorption of Ca ions through a metal-proton exchange reaction. The dissolution pathways and free energies of these Ca ions follow the ligand-exchange mechanism with multiple sequential reactions to form the ultimate products where Ca ions adopt fivefold or sixfold coordination. Finally, these Ca complexes reprecipitate on the remaining Al-rich layer through the interface-coupled dissolution-reprecipitation mechanism, demonstrating dynamically stable inner-sphere adsorption states. The above results are helpful in unmasking the early-age hydration of PC and advancing the rational design of cement-based materials through the bottom-up approach.

Local Structure in Disordered Melilite Revealed by Ultrahigh Field 71Ga and 139La Solid-State Nuclear Magnetic Resonance Spectroscopy.

Multinuclear Nuclear Magnetic Resonance (NMR) spectroscopy of quadrupolar nuclei at ultrahigh magnetic field provides compelling insight into the short-range structure in a family of fast oxide ion electrolytes with La1+xSr1-xGa3O7+0.5x melilite structure. The striking resolution enhancement in the solid-state 71Ga NMR spectra measured with the world's unique series connected hybrid magnet operating at 35.2 T distinctly resolves Ga sites in four- and five-fold coordination environments. Detection of five-coordinate Ga centers in the site-disordered La1.54Sr0.46Ga3O7.27 melilite is critical given that the GaO5 unit accommodates interstitial oxide ions and provides excellent transport properties. This work highlights the importance of ultrahigh magnetic fields for the detection of otherwise broad spectral features in systems containing quadrupolar nuclei and the potential of ensemble-based computational approaches for the interpretation of NMR data acquired for site-disordered materials.

Synthesis, crystal structure and Hirshfeld surface analysis of sodium bis(malonato)borate monohydrate

In the title salt, poly[aqua[μ4-bis(malonato)borato]sodium], {[Na(C6H4BO8)]·H2O} n or Na+·[B(C3H2O4)2]−·H2O, the sodium cation exhibits fivefold coordination by four carbonyl O atoms of the bis(malonato)borate anions and a water O atom. The tetrahedral B atom at the centre of the anion leads to the formation of a polymeric three-dimensional framework, which is consolidated by C—H...O and O—H...O hydrogen bonds. A Hirshfeld surface analysis indicates that the most significant contacts in the crystal packing are H...O/O...H (49.7%), Na...O/O...Na (16.1%), O...O (12.6%), H...H (10.7%) and C...O/O...C (7.3%).

Antimony Doped Hybrid Zinc Halide Crystals with Tunable Light Emission and Long‐Term Stability

AbstractAs a class of emerging photoluminescent materials, hybrid halide crystals have drawn research attention for their potential application in the fields of light‐emitting, security, and waveguide. Nevertheless, hybrid halide crystals containing antimony with long‐term stability and tunable light emission are still increasingly in demand. In this work, serial new hybrid halide crystals (BZA)2ZnCl4·2H2O:xSb3+ (x = 0–0.2, x represents the reaction ratio) and (BZA)2SbCl5 are synthesized (BZA = 2,4‐diamino‐6‐phenyl‐1,3,5‐triazine). In (BZA)2ZnCl4·2H2O:xSb3+ crystals, Sb3+ cations replace partial Zn2+ cations to form [SbCl4]− tetrahedron. Red light emission caused by the substitution of Sb3+ for Zn2+ enhances as the doping rate increases, resulting in the tunable emission from light blue to pink and finally to dark red. There are two kinds of Sb3+ in (BZA)2SbCl5 crystal. Sb(1) has a sixfold coordination with Cl to form a [Sb(1)Cl5]∞ 1D zigzag chain. Sb(2) atom adopts a fivefold coordination with Cl and is separated from each other by BZA+ cations. (BZA)2SbCl5 crystal shows bright orange‐yellow light emission with a photoluminescence quantum yield of 45%. Moreover, the organic–inorganic hybrid metal halide crystals containing antimony have excellent long‐term stability, with phase and luminescence keeping nearly unchanged after more than six months in ambient air.

Effect of graphene on the surface nanomechanical behavior and subsurface layer of GaN damage during nanogrinding using molecular dynamics simulation

Molecular dynamics simulations were used to investigate the surface nanomechanical behavior and subsurface layer damage of gallium nitride (GaN)/graphene composites when subjected to nanogrinding with diamond abrasive. The effects of graphene on GaN were analyzed based on parameters such as atomic displacement, forces, crystal structure, subsurface damage, atomic stress, and temperature. The simulation results indicate that graphene has a noteworthy impact on the surface morphology, such as the pile-up atoms on the workpiece surface and the elastic recovery on the machined surface. In addition, the addition of graphene can reduce the atomic temperature, stress, and normal and tangential forces. In terms of subsurface damage formation, graphene can suppress lattice damage and amorphization of GaN during nanogrinding. The addition of graphene can significantly reduce the dislocation length and the phase transition from wurtzite to a five-fold coordination hexagonal structure. Overall, the addition of graphene can improve the deformation mechanism on the GaN surfaces. This study provides technical references for the design of GaN with improved friction and wear resistance.

Mechanism of Al Coordination Change in Alkaline-earth Aluminosilicate Glasses: An Application of Bond Valence Model

Aluminum cations are generally present in four-fold ([4]Al3+) or five-fold coordination ([5]Al3+) in aluminosilicate slags, where the concentration of [5]Al3+ varies depending on the type of charge compensator, for example, Mg2+ and Ca2+. Although it has been reported that the amount of [5]Al3+ species increases with the replacement of CaO with MgO in the CaO–MgO–SiO2–Al2O3 system, the detailed mechanism underlying the change in the local structure near the aluminum cations remains unclear. Because the residual negative charge on the bridging oxygen between [4]Si4+ and [5]Al3+ ([4]Si4+–OBO–[5]Al3+) is larger than that of [4]Si4+–OBO–[4]Al3+, it is essential to understand the positive charge contributions of alkaline-earth cations to compensate for these negative charges on the bridging oxygens. In the present study, the valence of a single chemical bond near Mg2+ and Ca2+ cations in the chosen aluminosilicate glasses was determined using a simple empirical model, which enabled calculation of the bond valence from the observed interatomic distance of near alkaline-earth cations by synchrotron X-ray total scattering. Magnesium cations had a larger average bond valence (+0.39) than calcium cations (+0.31). The difference in the positive charge contribution from Mg2+ and Ca2+ should explain the variation in the coordination number of aluminum cations.

Near-infrared emission of Cr3+ in five-coordinated field

Cr3+-activated NIR phosphors have received increasing attention due to their versatile applications. Cr3+ generally shows tunable NIR emission by occupying a six-coordinated field (Oh). Herein, we discover broadband NIR emission of Cr3+ at the five-coordinated field in the phosphors BaXP2O7:Cr3+ (X = Mg/Zn) for the first time. The five-coordinated [MgO5] (D3h) or [ZnO5] (C4v) site provides a quasi-octahedral field. The symmetry of D3h and C4v fields is lower than that of Oh, thus leading to further splitting of the energy levels and generating the broadband NIR emission from 700 to 1100 nm. This work breaks through the traditional limitation of NIR luminescence of Cr3+ in the six-coordinated field and opens a new window to explore novel NIR phosphors of Cr3+ in five-fold coordination.

Unveiling the atomistic and electronic structure of NiII-NO adduct in a MOF-based catalyst by EPR spectroscopy and quantum chemical modelling.

The nature of the chemical bonding between NO and open-shell NiII ions docked in a metal-organic framework is fully characterized by EPR spectroscopy and computational methods. High-frequency EPR experiments reveal the presence of unsaturated NiII ions displaying five-fold coordination. Upon NO adsorption, in conjunction with advanced EPR methodologies and DFT/CASSCF modelling, the covalency of the metal-NO and metal-framework bonds is directly quantified. This enables unravelling the complex electronic structure of NiII-NO species and retrieving their microscopic structure.

Oxygen Reduction Reaction on Fe-N-C Catalysts: A Computational Spectroscopy Study

The oxygen reduction reaction (ORR) plays an important role in proton exchange fuel cells (PEFCs). In PEFCs, ORR is the cathodic half-cell reaction complementary to the oxidation of the fuel, but since ORR has slow kinetics, it requires high amounts of catalyst. State-of-the-art ORR catalysts are based on the expensive metal platinum. Even though the amount of platinum needed for ORR in PEFCs has been reduced significantly over the last decade, it is still the major contributor to the cost of PEFCs, thus hindering the commercialization and accessibility of this technology.[1]Iron and nitrogen doped carbon (Fe-N-C) catalysts have gained a lot of research attention due to their high ORR activity, which makes them potential substitutes for platinum-based catalysts. In Fe-N-C catalysts, iron is thought to be atomically dispersed as pseudo-molecular active centres with four- or fivefold nitrogen coordination spheres which are embedded in graphene layers. Since Fe-N-C catalysts are typically prepared via pyrolysis, they have a highly amorphous structure and can contain multiple iron phases, which makes them difficult to characterize structurally and spectroscopically. Consequently, there is still a scientific debate on the exact nature of the active site, in terms of iron spin and oxidation states and its precise coordination environment.[2-4]Fe-57 Mössbauer spectroscopy can provide direct insights on iron spin and oxidation states and is used successfully to characterise the amorphous Fe-N-C catalysts. Until recently, the interpretation of Mössbauer spectra was limited to comparisons with small reference complexes which lack the extended π-systems of Fe-N-C catalysts.[2] Since synthesis of such extended π-systems as references is difficult, we have developed a library of computational models that encompasses different structural motifs and electronic structures. With increasing use of in situ and operando experiments on Fe-N-C catalysts, the interest in computational models for the interpretation of experimental Mössbauer spectra has grown.[4-6] In this contribution, we present our density functional theory results for different molecular Fe-N-C models with extended π-systems and discuss their electronic structures and spectroscopic properties.[1] L. Osmieri, et al. Current Opinion in Electrochemistry 2021, 25, 100627.[2] U. I. Kramm, et al. Advanced Materials 2019, 31, 1805623.[3] S. Wagner, et al. Angewandte Chemie International Edition 2019, 58, 10486.[4] L. Ni, C. Gallenkamp, et al. Advanced Energy & Sustainability Research 2021, 2, 2000064.[5] J. Li, et al. Nature Catalysis, 2021, 4, 10.[6] X. Liu, et al. Chem 2020, 6, 3440.

Monomeric Fe Species in Square Planar Geometry Active for Low Temperature NH3-SCR of NO

Monomeric Fe species in zeolites are considered to be the active sites for the low temperature activity toward ammonia-assisted selective catalytic reduction of nitrogen oxides (NH3-SCR) in exhaust gases. Herein, we report on a preparation method to synthesize single-site Fe/ZSM-5 by combination of dealumination with the use of a bulky iron complex to introduce Fe. Transient in situ XAS experiments at the Fe K-edge under dynamic NH3-SCR reaction conditions demonstrated the involvement of all iron atoms in the redox cycle and, with that, suggest that Fe species in the studied Fe/ZSM-5 catalyst are monomers. Simulation of experimental X-ray absorption data revealed that the Fe species in the sample obtained by this approach adopt a square planar geometry, which changes into fivefold coordination at low temperature by strongly binding either NO or NH3 as the ligand. The presence of monomeric Fe species in this Fe/ZSM-5 conveys molecular level insights into the temperature-dependent NH3-SCR activity and might prove useful in the study of other reactions over monomeric Fe species, such as methane partial oxidation or dehydroaromatization reactions.

Density Functional‐Based Tight‐Binding Simulations of Pristine and Aluminum‐Modified Silica

Using self-consistent density functional tight-binding simulations it is shown that Aluminum (Al) content in amorphous silica (a-SiO2) changes its ideal microscopic structure in a manner compatible to densification. Similar to the structure of pressure-densified a-SiO2, the Al-modified a-SiO2 comprises a network of Silicon (Si)-centered tetrahedra as well as unquenchable pentahedra and, to a smaller extent, hexahedra coordination defects. Al itself acts not only as a network former, with fourfold coordination, but also as a center for fivefold and sixfold coordination defects. Al content promotes densification since it shifts the potential energy minima at densities larger than in their pristine counterpart. Calculations uncover that Young's modulus (Y) and static dielectric constants (ε0) can be effectively doubled through densification. Oxygen starvation promotes network polymerization, which further increases Y and ε0. However, the small rings formation through Si─Si bonding and presence of undercoordinated Si introduce electronic states in the electronic band gap. The results provide guidance for the bottom-up design of amorphous silica with tunable microscopic structure and properties desirable for advancing electronic applications.

Fully Chelating N3O2-Pentadentate Planar Ligands Designed for the Strongest and Selective Capture of Uranium from Seawater.

Based on the unique fivefold equatorial coordination of UO22+, water-compatible pentadentate planar ligands, H2saldian and its derivatives, were designed for the strong and selective capture of UO22+ in seawater. In the simulated seawater condition (0.5 M NaCl + 2.3 mM HCO3-/CO32-, pH 8), saldian2- shows the strongest complexation with UO22+ to form UO2(saldian) (log β11 = 28.05 ± 0.07), which is more than 10 order of magnitude greater than amidoxime-based or -inspired ligand systems most commonly employed for U capture from seawater. Good selectivity for UO22+ from other metal ions coexisting in seawater was also demonstrated.

Variation in viscosity of aluminosilicate melts with MgO/CaO molar ratio: Influence of five-fold coordinated aluminum

Viscosity of aluminosilicate melts is important for estimating transport phenomena in industrial processes. Viscosity variations with increasing MgO/CaO molar ratio have been reported for CaO–MgO–SiO2 (CMS) and CaO–MgO–Al2O3–SiO2 (CMAS) systems; however, the viscosity variation was not clearly correlated with the change in the melt structure. The present study investigated the influence of the MgO/CaO molar ratio on the viscosity of CMAS melts at temperatures higher than the liquidus using the rotating crucible method. The viscosity decreased with increasing substitution of CaO by MgO in the chosen CMAS system, whereas the opposite behavior has been reported for the CMS system. Aluminum-27 magic-angle spinning nuclear magnetic resonance spectra of the CMAS glasses show that the fraction of aluminum atoms in five-fold coordination ([5]Al) increased with increasing substitution of CaO by MgO. Our findings indicate that the formation of [5]Al reduces the viscosity of aluminosilicate melts.

Synthesis, structural, optical, thermal, magnetic and EPR investigations of the new phosphate KCuCr(PO4)2

New phosphate KCuCr(PO4)2 has been elaborated by solid-state reaction route and characterized by various techniques. The structure was determined from X-ray powder diffraction using Fullprof software based on Rietveld refinement. KCuCr(PO4)2 crystallizes in the monoclinic system, P21/n (no.14) space group, and with the unit cell parameter s: a = 7.861(2) Å, b = 9.828(2) Å, c = 9.002(2) Å, and β = 115.2° (1). Its structure consists of 3D - network built up from Cu2O8 units and CrO6 octahedra which construct rigid layers parallel to the bc plane. These layers connected by PO4 tetrahedra gave rise to large tunnels where K+ cations are located with nine-fold coordination. SEM analysis reveals an agglomeration state of the prepared material. DTA/TGA analysis shows good thermal stability of the crystalline powder indeed required for ceramic pigments. EPR spectrum presents a strong signal characterized a high dipole-dipole interaction (Cu2+- Cu2+, Cr3+- Cr3+ and Cu2+- Cr3+). The green color of the investigated material was mainly due to the presence of Cr3+ and Cu2+ ions in six and five-fold coordination, respectively. UV Visible spectrum consists of several bands between 400 and 850 nm, assigned to d–d transitions of Cu2+ and Cr3+ ions; besides, a strong band around 294 nm was attributed to the O–M (M = Cu, Cr) charge transfers. CIE L*a*b*and RGB parameters values were determined to examine the material's color properties as a green ceramic pigment. Magnetic measurements reveal an antiferromagnetic behavior with TN = 17 K and θCW = −128K.

Tridentate acylhydrazone copper(II) complexes with heterocyclic bases as coligands. Synthesis, spectroscopic studies, crystal structure and cytotoxicity assays

Two new penta-coordinated copper(II) hydrazone coordination compounds with 2,2́ bipyridine (I) or 1,10 phenanthroline (II) as coligands are synthesized and characterized in the solid state and in solution by spectroscopic methods (FTIR, Raman, UV–vis, EPR). Single crystal X-ray studies show that they have closely related molecular structures with the copper center in a distorted square pyramidal O2N3 environment. The N-acylhydrazone, 4-hydroxy-NŃ-(E)2-hydroxy-3-methoxybenzylidene]benzohydrazide, H2L, coordinates tridentate through its ONO donor atoms as monoanion (HL-) in the cationic complex (I), [Cu(HL)(bipy)](NO3). During the period that takes the synthesis reaction of compound (II), [Cu(L)(o-phen)] to complete, keto-amine to enol-imine tautomerization of H2L occurs and the hydrazone coordinates tridentate as dianionic ligand (L2-). The fivefold coordination sphere of the complexes is completed with the nitrogen atoms of the respective coligands. Cytotoxicity studies against bone (MG-63, IC50(I) = 5.6 ± 1.0 , IC50(II) = 3.5 ± 0.3), breast (MCF7, IC50(I) = 10.8 ± 1.9, IC50(II) = 4.0 ± 1.3), (MDA-MB-231, IC50(I) = 11.4 ± 0.6, IC50(II) = 5.3 ± 0.2) and lung cancer cells (A549, IC50(I) = 7.7 ± 0.7, IC50(II) = 7.0 ± 0.4) reveals increased effectiveness of both complexes compared with the free ligand, the copper nitrate salt and the reference metallodrug cisplatin (CDDP).

Pressure-induced C23–C37 transition and compression behavior of orthorhombic Fe2S to Earth’s core pressures and high temperatures

Abstract The phase stability of orthorhombic Fe2S was explored to 194 GPa and 2500 K using powder and multigrain synchrotron X-ray diffraction techniques. Between 30 and 120 GPa, a C23-like (Co2P, Pnma, Z = 4) Fe2S structure is observed and determined to exhibit a highly compressible a axis. A softening of the a axis occurs between 120 and 150 GPa and a relative stiffening of the b and c axes accompanies this compressibility change. Above 150 GPa, the a axis stiffens as the b and c axes soften, and a C37-like (Co2Si, Pnma, Z = 4) Fe2S unit cell is measured. On the basis of these changes in unit cell geometry, a pressure-induced C23–C37 Fe2S phase transition is inferred between 120–150 GPa. The C23 and C37 (Pnma, Z = 4) structures are closely related and share the same site symmetries. Forming the C37 structure from the C23 structure requires a shortening of the a axis and lengthening of the b and c axes accompanied by a four- to fivefold coordination change. The softening of the a axis above 120 GPa may therefore indicate the onset of a coordination change, and the final compressibility change above 150 GPa may mark the completion of this phase transition. The presented pressure-temperature (P-T) stabilities of C23 and C37 structures of Fe2S are in agreement with and resolve the differing observations of two previous studies (Tateno et al. 2019; Zurkowski et al. 2022). As C37 Fe2S is observed to core-mantle boundary pressures and high temperatures, the C37 Fe2S density profile through Earth’s outer core was determined by fitting the C23 Fe2S equation of state (<120 GPa) and applying a 1.6% volume reduction based on the C37 Fe2S volume residuals to this fit. Comparing the density of liquid C37 Fe2S with that of liquid hcp-Fe (Dewaele et al. 2006) and the seismologically determined density deficit of Earth’s core (Irving et al. 2018), 13.9 ± 1.5 wt% and 8.6 ± 0.8 wt% sulfur are required to match the densities at the CMB and ICB, respectively, for a purely Fe-S core.

Fe-N-C Catalysts for Oxygen Reduction Reaction: A Computational Analysis of Electronic Structure and Spectroscopic Properties

In proton exchange fuel cells (PEFCs), the electrochemical reduction of oxygen (ORR) plays a key role as the complementary half-cell reaction to the oxidation of hydrogen. It requires the transport of four electrons per oxygen molecule associated with high activation barriers and slow kinetics and thus requires high amounts of catalyst. [1] State-of-the art catalysts for both reactions are based on platinum, an expensive metal.Iron and nitrogen doped carbon (Fe-N-C) catalysts show high ORR activity and are thus a very promising alternative to Pt catalysts. It is believed that in Fe-N-C catalysts, iron is atomically dispersed as pseudo-molecular active centres with four- or fivefold nitrogen coordination spheres which are embedded in a graphene matrix. However, the exact nature of the active site in terms of Fe spin and oxidation states, precise coordination environment and spatial extent of the graphene sheets are still debated, as is the influence of these factors on the ORR mechanism in Fe-N-C catalysts. [2-4]Unlike other spectroscopies, Mössbauer spectroscopy, which provides direct insights on iron spin and oxidation states, is used successfully to characterise the amorphous FeNC catalysts. Until recently, the interpretation of Mössbauer spectra was limited to comparisons with small reference complexes [2] which lack the extended π-systems of Fe-N-C catalysts. Recently, with increasing use of in situ and operando experiments on these catalysts, the interest in computational models for the interpretation of experimental Mössbauer spectra has grown. [4-6]. Using density functional theory, we show for different FeN4 environments how the electronic structure is influenced by the size and symmetry of the π-system. The influence is quantified in terms of structural parameters, relative spin state energies, spin distributions and spectroscopic properties.

Unusual cation coordination in nanostructured mullites

Abstract Nanocrystalline mullite-type bismuth-bearing complex oxides Bi2(M0.5Al0.5)4O9 (M=Fe3+, Ga3+) are prepared by high-energy ball milling of the corresponding microcrystalline counterparts. An unusual five-fold coordination of metal cations is revealed in nanostructured Bi2(M0.5Al0.5)4O9 by means of 27Al magic angle spinning nuclear magnetic resonance and 57Fe Mössbauer spectroscopies. The concentration of five-fold coordinated cations increases with decreasing crystallite size of a material at the expense of octahedrally coordinated ones. In addition to the nuclear spectroscopic methods, Rietveld analyses of the X-ray diffraction data of the as-prepared nanooxides show that the constituent tetrahedra, octahedra, and the newly formed structural units with five-fold cation coordination are strongly distorted. With decreasing crystallite size of mullites, the average volume of their octahedra increases whereas this parameter decreases for tetrahedra. The macroscopic behaviour of the non-equilibrium nanomullites is characterised by SQUID magnetometry. The Fe-containing mullites exhibit a superposition of a dominant antiferromagnetism and a weak ferromagnetism. The increase in both the remanent magnetization and the coercive field with decreasing crystallite size is attributed to the effect of spin canting. The latter is confined to the interfacial and surface regions of the nanomaterials, and arises due to both the mechanically induced deformation of constituent structural units and the formation of cation sites with the unusual five-fold coordination.