The aim of this critical review is to provide a broad but digestible overview of mechanochemical synthesis, i.e. reactions conducted by grinding solid reactants together with no or minimal solvent. Although mechanochemistry has historically been a sideline approach to synthesis it may soon move into the mainstream because it is increasingly apparent that it can be practical, and even advantageous, and because of the opportunities it provides for developing more sustainable methods. Concentrating on recent advances, this article covers industrial aspects, inorganic materials, organic synthesis, cocrystallisation, pharmaceutical aspects, metal complexes (including metal–organic frameworks), supramolecular aspects and characterization methods. The historical development, mechanistic aspects, limitations and opportunities are also discussed (314 references).

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Mechanochemistry: opportunities for new and cleaner synthesis

The electronic structure of the perovskite LaCoO$_3$ for different spin states of Co ions was calculated in the LDA+U approach. The ground state was found to be a nonmagnetic insulator with Co ions in a low-spin state. Somewhat higher in energy we found two intermediate-spin states followed by a high-spin state at significantly higher energy. The calculation results show that Co 3$d$ states of $t_{2g}$ symmetry form narrow bands which could easily localize whilst $e_g$ orbitals, due to their strong hybridization with the oxygen 2$p$ states, form a broad $\sigma^*$ band. With the increase of temperature which is simulated by the corresponding increase of the lattice parameter, the transition from the low- to intermediate-spin states occurs. This intermediate-spin (occupation $t_{2g}^5e_g^1$) can develop an orbital ordering which can account for the nonmetallic nature of LaCoO$_3$ at 90 K$<$T$<$500 K. Possible explanations of the magnetic behavior and gradual insulating-metal transition are suggested.

https://arxiv.org/pdf/cond-mat/9709060.pdf

Intermediate-spin state and properties of LaCoO_3

Perovskite oxides offer efficient and cheap electrocatalysts for both oxygen reduction reactions and oxygen evolution reactions (ORR/OER) in diverse oxygen-based electrochemical technologies. In this study, we report a facile strategy to enhance the electrocatalytic activity of CaMnO3 by introducing oxygen defects. The nonstoichiometric CaMnO(3-δ) (0 < δ ≤ 0.5) was prepared through thermal reduction of pristine perovskite microspheres and nanoparticles, which were synthesized from thermal-decomposition of carbonate precursors and the Pechini route, respectively. The as-prepared samples were analyzed by chemical titration, structural refinement, thermogravimetric analysis, and energy spectrometry. In 0.1 M KOH aqueous solution, the nonstoichiometric CaMnO(3-δ) with δ near 0.25 and an average Mn valence close to 3.5 exhibited the highest ORR activity (36.7 A g(-1) at 0.70 V vs RHE, with onset potential of 0.96 V), which is comparable to that of benchmark Pt/C. Density functional theory (DFT) studies and electrical conductivity measurement revealed that the enhanced ORR kinetics is due to facilitated oxygen activation and improved electrical properties. Besides high activity, the nonstoichiometric perovskite oxides showed respectable catalytic stability. Furthermore, the moderate oxygen-defective CaMnO(3-δ) (δ ≈ 0.25) favored the OER because of the improved electrical conductivity. This study makes nonstoichiometric CaMnO(3-δ) a promising active, inexpensive bifunctional catalytic material for reversible ORR and OER.

Nonstoichiometric perovskite CaMnO(3-δ) for oxygen electrocatalysis with high activity.

The present review focuses on links between structure, energetics and ion transport in oxygen-deficient perovskite oxides, ABO3−δ. The perfect long-range order, convenient for interpretations of the structure and properties of ordered materials, is evidently not present in disordered materials and highly defective perovskite oxides are spatially inhomogeneous on an intermediate length scale. Although this makes a fundamental description of these and other disordered materials very difficult, it is becoming increasingly clear that this complexity is often essential for the functional properties. In the present review we advocate a potential energy barrier description of the disordered state in which the possible local (or inherent) structures are seen to correspond to separate local minima on the potential energy surface. We interpret the average structure observed experimentally at any temperature as a time and spatial average of the different local structures which are energetically accessible. The local structure is largely affected by preferences for certain polyhedron coordinations and the oxidation state stability of the transition metals, and the strong long-range electrostatic interactions present in non-stoichiometric oxides imply that only a small fraction of the local energy minima on the potential energy surface are accessible at most temperatures. We will show that models neglecting the spatial inhomogeneity and thus the local structure serve as useful empirical tools for particular purposes, e.g. for understanding the main features of the complex redox properties that are so crucial for many applications of these oxides. The short-range order is on the other hand central for understanding ionic transport. Oxide ion transport involves the transformation of one energetically accessible local structure into another. Thus, strongly correlated transport mechanisms are expected; in addition to the movement of the oxygen ions giving rise to the transport, other ions are involved and even the A and B atoms move appreciably in a cooperative fashion along the transition path. Such strongly correlated or collective ionic migration mechanisms should be considered for fast oxide ion conductors in general and in particular for systems forming superstructures at low temperatures. Structural criteria for fast ion conduction are discussed. A high density of low-lying local energy minima is certainly a prerequisite and for perovskite-related A2B2O5 oxides, those containing B atoms that have energetic preference for tetrahedral coordination geometry are especially promising.

Oxygen-deficient perovskites: linking structure, energetics and ion transport

Recent advances in oxide thermoelectric materials and modules

Electronic structure and polarons in CaMnO 3-δ single crystals: Optical data

Temporal changes in magnetic properties of high-density CuO nanoceramics

A coordinated study of the magnetic, electrical, optical, and EPR properties of LaMnO3 single crystals doped by donors (7 at. % Ce) and acceptors (7 at. % Sr) revealed that in all cases, except undoped LaMnO3, charge segregation associated with large-scale crystal-field fluctuations occurs and the magnetic properties originate from the existence of ferromagnetic phase inclusions and localized ferrons in the matrix with a canted magnetic structure.

Charge segregation and a nonuniform magnetic state in donor-and acceptor-doped LaMnO 3

The mechanochemical method is shown to be a relatively simple method for producing nanostructural manganites LaMnO3 + δ with crystallite size D ≥ 10 nm. An increase in the treatment duration in a planetary mill from 1 to 13 h decreases the size D and increases microstrains. The Curie temperature of the nanostructural manganites decreases insignificantly and the phase transition is smeared as D decreases. A decrease in the unit-cell volume and the temperature dependences of the inverse magnetic susceptibility 1/χ(T) indicate an increase in the Mn4+ ion concentration with the milling duration. The variation of the magnetic properties of LaMnO3 + δ nanostructural powders is explained by the competition of a number factors, such as variations of the composition, the cation-sublattice defect structure, the size effect, and the microstrain level.

Magnetic susceptibility of nanostructural manganite LaMnO 3 + δ produced by mechanochemistry method

The magnetic properties of La2−2x
Sr1+2x
Mn2O7 polycrystals (x=0.3–0.4) are studied over a broad temperature range 80–600 K. Quasi-two-dimensional manganites have a complex magnetic structure that undergoes several transitions from one type of magnetic ordering to another. A specific feature of these manganites is a hyperbolic dependence of inverse susceptibility in the transition region from the magnetically ordered to paramagnetic state for T>360 K. This suggests the onset of ferrimagnetism. Electron irradiation to a fluence Φ=1×1018 electrons/cm2 is shown to have no effect on the long-range magnetic order while favoring the formation of paramagnetic polarons and of an inhomogeneous paramagnetic state.

T. I. Arbuzova

Papers

Electronic structure and polarons in CaMnO 3-δ single crystals: Optical data

Temporal changes in magnetic properties of high-density CuO nanoceramics

Charge segregation and a nonuniform magnetic state in donor-and acceptor-doped LaMnO 3

Magnetic susceptibility of nanostructural manganite LaMnO 3 + δ produced by mechanochemistry method

Magnetic properties of electron-irradiated quasi-layered manganites La 2-2x Sr 1+2x Mn 2 O 7 (x=0.3, 0.35, 0.4)