Controlling electronic population through chemical doping is one way to tip the balance between competing phases in materials with strong electronic correlations. Vanadium dioxide exhibits a first-order phase transition at around 338 K between a high-temperature, tetragonal, metallic state (T) and a low-temperature, monoclinic, insulating state (M1), driven by electron-electron and electron-lattice interactions. Intercalation of VO2 with atomic hydrogen has been demonstrated, with evidence that this doping suppresses the transition. However, the detailed effects of intercalated H on the crystal and electronic structure of the resulting hydride have not been previously reported. Here we present synchrotron and neutron diffraction studies of this material system, mapping out the structural phase diagram as a function of temperature and hydrogen content. In addition to the original T and M1 phases, we find two orthorhombic phases, O1 and O2, which are stabilized at higher hydrogen content. We present density functional calculations that confirm the metallicity of these states and discuss the physical basis by which hydrogen stabilizes conducting phases, in the context of the metal-insulator transition.

In the past years, magnetism-driven ferroelectricity and gigantic magnetoelectric effects have been reported for a number of frustrated magnets featuring ordered spiral magnetic phases. Such materials are of high-current interest due to their potential for spintronics and low-power magnetoelectric devices. However, their low-magnetic ordering temperatures (typically <100 K) greatly restrict their fields of application. Here we demonstrate that the onset temperature of the spiral phase in the perovskite YBaCuFeO5 can be increased by more than 150 K through a controlled manipulation of the Fe/Cu chemical disorder. Moreover, we show that this novel mechanism can stabilize the magnetic spiral state of YBaCuFeO5 above the symbolic value of 25 °C at zero magnetic field. Our findings demonstrate that the properties of magnetic spirals, including its wavelength and stability range, can be engineered through the control of chemical disorder, offering a great potential for the design of materials with magnetoelectric properties beyond room temperature.

The renewed interest of mechanochemistry as an ecofriendly synthetic route has inspired original methodologies to probe reactions, with the aim to rationalize unknown mechanisms. Recently, Friščić et al. ( Nat. Chem. 2013 , 5 , 66 - 73 , DOI: 10.1038/nchem.1505 ) monitored the progress of milling reactions by synchrotron X-ray powder diffraction (XRPD). For the first time, it was possible to acquire directly information during a mechanochemical process. This new methodology is still in its early stages, and its development will definitively transform the fundamental understanding of mechanochemistry. A new type of in situ ball mill setup has been developed at the Materials Science beamline (Swiss Light Source, Paul Scherrer Institute, Switzerland). Its particular geometry, described here in detail, results in XRPD data displaying significantly lower background and much sharper Bragg peaks, which in turn allow more sophisticated analysis of mechanochemical processes, extending the limits of the technique.

Interaction of solid KBH 4 with liquid Al(BH 4 ) 3 at room temperature yields a solid bimetallic borohydride KAl(BH 4 ) 4 . According to the synchrotron X-ray powder diffraction, its crystal structure (space group Fddd, a = 9.7405(3), b = 12.4500(4), and c = 14.6975(4) Å) contains a substantially distorted tetrahedral [Al(BH 4 ) 4 ]− anion, where the borohydride groups are coordinated to aluminum atoms via edges. The η -coordination of BH 4− is confirmed by the infrared and Raman spectroscopies. The title compound is the first aluminum-based borohydride complex not stabilized by halide anions or by bulky organic cations. It is not isostructural to bimetallic chlorides, where more regular tetrahedral AlCl 4 − anions are present. Instead, it is isomorphic to the LT phase of TbAsO 4 and can be also viewed as consisting of two interpenetrated dia-type nets where BH 4 ligand is bridging Al and K cations. Variable temperature X-ray powder diffraction, TGA, DSC, and TGA-MS data reveal a single step of decomposition at 160°C, with an evolution of hydrogen and some amount of diborane. Aluminum borohydride is not released in significant amounts; however, some crystalline KBH 4 forms upon decomposition. The higher decomposition temperature than in chloride-substituted Li−Al (70°C) and Na−Al (

Metal amide and hydrogen (MNH2-H2) system is recognized as a promising reversible hydrogen storage system due to its high hydrogen capacity and lower operating temperature. However, slow reaction rate for the Li system with the highest hydrogen capacity is an important issue to be solved for practical use. In this thesis, modification of the reaction properties for the LiNH2-H2 system is carried out from thermodynamic and kinetic points of view. Particularly, the novel ammonia synthesis technique is proposed by applying the LiNH2-H2 system and Amide-imide system. Lithium hydride-Potassium hydride (LiH-KH) complex synthesized by ball-milling has been focused in order to modify the kinetic properties of the reaction between LiH and ammonia. The LiH-NH3 system is recognized as one of the most promising hydrogen storage system because it generates hydrogen at room temperature by ammonolysis reaction. Moreover, the starting system can be regenerated below 300 °C and possesses more than 8.0 wt.% hydrogen capacity. From the experimental results, it is confirmed that the hydrogen generation from the reaction between ammonia and the LiH-KH complex shows much higher reaction rate than that of the simple summation of each component as a synergetic effect. Then, a double-cation amide MNH2 (LiK(NH2)2) phase, which could not be assigned to any reported amides so far, is formed as the reaction product. Moreover, in the hydrogenation of LiK(NH2)2, two processes were confirmed at the different temperatures. After the low temperature hydrogenation, KH-lithium amide (LiNH2) composite is generated as the hydrogenated product. It is noteworthy that the hydrogenation temperature of the composite is dramatically lower than that of LiNH2 itself, which should be due to the interaction between LiNH2 and KH such as a eutectic melting phenomenon. II An ammonia synthesis technique from lithium nitride (Li3N) based on the reactions of "Amide-imide system" and "Amide-hydrogen system" is proposed. Namely, Li3N is hydrogenated below 300 °C under 0.5 MPa hydrogen atmosphere, and then LiNH2 and LiH are formed as products. Furthermore, the reaction between LiNH2 and hydrogen proceeds below 250 °C under 0.5 MPa of hydrogen flow condition, which results in the formation of ammonia and LiH. In this study, a new method of ammonia synthesis is proposed at laboratory scale using above two reactions. This method is capable of being operated under more moderate conditions than those of Haber-Bosch process. The proposed method is investigated for various reaction system such as open system, closed gas circuit system, and closed gas exchange system using couple of hydrogen storage alloys. As a result, it is experimentally clarified that the ammonia can be synthesized below 300 °C and 0.5 MPa with realistic reactions rate by non-equilibrium reaction field under certain hydrogen flow rate even in the closed system.

Proline has been widely used for various cocrystallization applications, including pharmaceutical cocrystals. Combining enantiopure and racemic flurbiprofen and proline, we discovered 18 new crystal structures. Liquid-assisted grinding proved highly efficient to explore all the variety of crystal forms. A unique combination of state-of-the-art characterization techniques, comprising variable temperature in situ X-ray diffraction and in situ ball-milling, along with other physicochemical methods and density functional theory calculations, was indispensable for identifying all the phases. Analyzing the results of in situ ball-milling, we established a stepwise mechanism for the formation of several 1:1 cocrystals via an intermediate 2:1 phase. The nature of the solvent in liquid-assisted grinding was found to significantly affect the reaction rate and, in some cases, the reaction pathway.

A metal borohydride–ammonia borane complex, Mg(BH4)2(NH3BH3)2 was synthesized via a solid-state reaction between Mg(BH4)2 and NH3BH3. Different mechanochemical reaction mechanisms are observed, since Mg(BH4)2(NH3BH3)2 is obtained from α-Mg(BH4)2, whereas a mixture of Mg(BH4)2(NH3BH3)2, NH3BH3, and amorphous Mg(BH4)2 is obtained from γ-Mg(BH4)2. The crystal structure of Mg(BH4)2(NH3BH3)2 has been determined by powder X-ray diffraction and optimized by first-principles calculations. The borohydride groups act as terminal ligands, and molecular complexes are linked via strong dihydrogen bonds (<2.0 Å), which may contribute to the high melting point of Mg(BH4)2(NH3BH3)2 found to be ∼48 °C in contrast to those for other molecular metal borohydrides. Precise values for the 11B quadrupole coupling parameters and isotropic chemical shifts are reported for the two NH3BH3 sites and two BH4 – sites in Mg(BH4)2(NH3BH3)2 from 11B MAS NMR spectra of the central and satellite transitions and MQMAS NMR. The 11B quadrupole coupling parameters agree excellently with the electric field gradients for the 11B sites from the DFT calculations and suggest that a more detailed structural model is obtained by DFT optimization, which allows evaluation of the dihydrogen bonding scheme.

Hydrogen-storage properties of complex hydrides depend of their form, such as a polymorphic form or an eutectic mixture. This Paper reports on an easy and reproducible way to synthesize a new stable form of magnesium borohydride by pressureinduced collapse of the porous γ-Mg(BH 4 ) 2 . This amorphous complex hydride was investigated by temperature-programmed synchrotron X-ray diffraction (SXRD), transmission electron microscopy (TEM), thermogravimetric analysis, differential scanning calorimetry analysis, and Raman spectroscopy, and the dynamics of the BH 4 − reorientation was studied by spin−lattice relaxation NMR spectroscopy. No long-range order is observed in the lattice region by Raman spectroscopy, while the internal vibration modes of the BH 4 − groups are the same as in the crystalline state. A hump at 4.9 Å in the SXRD pattern suggests the presence of nearly linear Mg−BH 4 −Mg fragments constituting all the known crystalline polymorphs of Mg(BH 4 ) 2 , which are essentially frameworks built of tetrahedral Mg nodes and linear BH 4 linkers. TEM shows that the pressure-collapsed phase is amorphous down to the nanoscale, but surprisingly, SXRD reveals a transition at ∼90°C from the dense amorphous state (density of 0.98 g/cm 3 ) back to the porous γ phase having only 0.55 g/cm 3 crystal density. The crystallization is slightly exothermic, with the enthalpy of −4.3 kJ/mol. The volumetric hydrogen density of the amorphous form is 145 g/L, one of the highest among hydrides. Remarkably, this form of Mg(BH 4 ) 2 has different reactivity compared to the crystalline forms. The parameters of the reorientational motion of BH 4 groups in the amorphous Mg(BH 4 ) 2 found from NMR measurements differ significantly from those in the known crystalline forms. The behavior of the nuclear spin−lattice relaxation rates can be described in terms of a Gaussian distribution of the activation energies centered on 234 ± 9 meV with the dispersion of 100 ± 10 meV. ■ INTRODUCTIONHigh gravimetric and volumetric hydrogen densities are the main requirements for potential hydrogen-storage materials. Thus, complex hydrides and especially light metal borohydrides are extensively studied for this purpose. 1,2 Magnesium borohydride and its multiple polymorphs are very promising candidates, given its hydrogen desorption reversibility 3 and formation of reversible reactive hydride composites. 4 Magnesium borohydride exists in a form of various polymorphs, 5−10 having framework structures and very different densities. In particular, the recently discovered porous form of Mg(BH 4 ) 2 having 14.9 wt % covalently bound hydrogen can store an additional 3 wt % of physisorbed molecular H 2 at low temperatures. 5 The form of the solid hydride can be determinative for the decomposition reaction pathway and thus for the hydrogen desorption reversibility. For instance, many borohydrides yield upon heating stable [B 12 H 12 ] 2− species that decrease their hydrogen release potential and prevent their reversibility; see a review 11 . This mechanism is one o...