This review focuses on key aspects of the thermal decomposition of multinary or mixed hydride materials, with a particular emphasis on the rational control and chemical tuning of the strategically important thermal decomposition temperature of such hydrides, Tdec. An attempt is also made to predict the thermal stability of as-yet unknown, elusive or even unknown hydrides. The future of a particularly promising class of materials for hydrogen storage, namely the catalytically enhanced complex metal hydrides, is discussed. The predictions are supported by thermodynamics considerations, calculations derived from molecular orbital (MO) theory and backed up by simple chemical insights and intuition.

Thermal decomposition of the non-interstitial hydrides for the storage and production of hydrogen.

Shape maps and polyhedral interconversion paths in transition metal chemistry

Skyrmion crystals are regular arrangements of magnetic whirls that exist in a wide range of chiral magnets. Because of their topology, they cannot be created or destroyed by smooth rearrangements of the direction of the local magnetization. Using magnetic force microscopy, we tracked the destruction of the skyrmion lattice on the surface of a bulk crystal of Fe(1-x)Co(x)Si (x = 0.5). Our study revealed that skyrmions vanish by a coalescence, forming elongated structures. Numerical simulations showed that changes of topology are controlled by singular magnetic point defects. They can be viewed as quantized magnetic monopoles and antimonopoles, which provide sources and sinks of one flux quantum of emergent magnetic flux, respectively.

Unwinding of a skyrmion lattice by magnetic monopoles.

Oxide superconductors in the system Tl2Ba2Can–1CunO4+2n (ref. 1) have transition temperatures (Tc) above 100 K, increasing with n. So far, stacking sequences up to n = 3 have been found in small crystals, and sequences with n > 3 have been seen in electron microscopy studies1. For large n, the stoichiometry of Tl2Ba2Can–1CunO4+2n approaches CaCuO2, a structure expected to consist only of CuO2 planes separated by Ca atoms. By analogy with Tl2Ba2Can–1CunO4+2n, the unit cell of this hypothetical phase is expected to be tetragonal with a = 3.86 A. Such a compound is not known in the Ca–Cu–O system, but Roth2 recently reported that small amounts of Sr on the Ca site can stabilize this simple structure. Here we report the growth of small single crystals of this phase, with composition (Ca0.86Sr0.14)CuO2, and their characterization by single-crystal X-ray diffraction. The crystals are tetragonal with space group P4/mmm, and the structure contains planar [CuO2]∞ layers separated by Ca and Sr atoms. The structure is a simple defect perovskite with ordered oxygen vacancies and can be regarded as the n = ∞ parent of the A2B2Can–1CunO4+2n (A = Bi, Tl; B = Sr, Ba) superconductors.

The parent structure of the layered high-temperature superconductors

Constructing magnetic molecular solids by employing three-atom ligands as bridges

Synthesis, single-crystal structure determination, and magnetic properties are reported for manganese carbodiimide, MnNCN. The presumably unstable but inert phase adopts the trigonal system (R3m) with a = 3.3583(4) A, c = 14.347(2) A, V = 140.13(3) A3, and Z = 3. Divalent manganese is octahedrally coordinated by nitrogen atoms at 2.26 A, and the NCN2- unit adopts the linear [NCN]2- carbodiimide shape with two CN double bonds of 1.23 A. MnNCN contains high-spin Mn(II) with five unpaired electrons and behaves like an antiferromagnet with an ordering temperature below 30 K.

Synthesis, crystal structure, and properties of MnNCN, the first carbodiimide of a magnetic transition metal.

The Crystal Structures of the Superconducting Oxides Bi2(Sr1−xCax)2CuO8−δ and Bi2(Sr1−yCay)3Cu2O10−δ with 0≤ x≤0.3 and 0.16≤y≤0.33

The antiferromagnetic structures of KFeS2, RbFeS2, KFeSe2, and RbFeSe2 and the correlation between magnetic moments and crystal field calculations

Synthesis, crystal structure, and magnetic properties of the semihard itinerant ferromagnet RhFe3N.

The ternary hydride Na2PdH2 was synthesized by the reaction of sodium hydride with palladium in a hydrogen atmosphere at 370 °C. The structure was derived from X-ray investigations on powdered samples and on a single crystal as well as from neutron diffraction experiments on the deuterated compound. Na2PdH2 crystallizes in the tetragonal space group I4/mmm and is isotypic with Na2HgO2. The atomic arrangement is characterized by a novel linear [PdH2] complex.

Paul Müller

Papers

Synthesis, crystal structure, and properties of MnNCN, the first carbodiimide of a magnetic transition metal.

The Crystal Structures of the Superconducting Oxides Bi2(Sr1−xCax)2CuO8−δ and Bi2(Sr1−yCay)3Cu2O10−δ with 0≤ x≤0.3 and 0.16≤y≤0.33

The antiferromagnetic structures of KFeS2, RbFeS2, KFeSe2, and RbFeSe2 and the correlation between magnetic moments and crystal field calculations

Synthesis, crystal structure, and magnetic properties of the semihard itinerant ferromagnet RhFe3N.

Na2PdH2, a hydride with a novel linear [PdH2] complex