Dag Noréus

Investigation of the perovskite related structures of NaMgH3, NaMgF3 and Na3AlH6

Abstract Two perovskite related metal hydrides, NaMgH 3 and Na 3 AlH 6 were structurally investigated using powder diffraction techniques. Single crystal X-ray data was also used to for the first time confirm that the structure of NaMgF 3 is analogous to the orthorombically distorted perovskite structure of GdFeO 3 space group Pnma (no. 62) . Looking for new ternary hydrides in the NaH–MgH 2 system, the only new compound found was NaMgH 3 which is isotypic with NaMgF 3 . The positions of the D atoms in Na 3 AlD 6 , isotypic with the low temperature phase of Na 3 AlF 6 , space group P2 1 /n (no. 14), could for the first time be determined from neutron diffraction data. The degree of distortion was discussed from the point of bonding distances and angles in the octahedra of the hydrides and fluorides. In the case of NaMgH 3 and NaMgF 3 , the angles of which the octahedra are rotated ( φ ) and atomic coordinates were calculated. Both NaMgH 3 and Na 3 AlH 6 appear to be less tilted but more deformed than their corresponding fluorides.

Structural determination of AMgNi4 (where A=Ca, La, Ce, Pr, Nd and Y) in the AuBe5 type structure

Abstract A number of new ternary magnesium based alloys AMgNi 4 (where A=Ca, La, Ce, Pr, Nd, and Y) have been synthesized by direct combination of the elements in the atomic ratio A:Mg:Ni=1:1:4. The crystal structures were determined by Guinier–Hagg X-ray powder diffraction. The compounds have a cubic SnMgCu 4 (AuBe 5 type) structure, which is related to the (C 15 ) MgCu 2 structure. All interatomic distances indicate metallic-type bonding.

Hydrogenation, purification, and unzipping of carbon nanotubes by reaction with molecular hydrogen: road to graphane nanoribbons.

Reaction of single-walled carbon nanotubes (SWNTs) with hydrogen gas was studied in a temperature interval of 400-550 °C and at hydrogen pressure of 50 bar. Hydrogenation of nanotubes was observed for samples treated at 400-450 °C with about 1/3 of carbon atoms forming covalent C-H bonds, whereas hydrogen treatment at higher temperatures (550 °C) occurs as an etching. Unzipping of some SWNTs into graphene nanoribbons is observed as a result of hydrogenation at 400-550 °C. Annealing in hydrogen gas at elevated conditions for prolonged periods of time (72 h) is demonstrated to result also in nanotube opening, purification of nanotubes from amorphous carbon, and removal of carbon coatings from Fe catalyst particles, which allows their complete elimination by acid treatment.

Structure and stability of high pressure synthesized Mg–TM hydrides (TM = Ti, Zr, Hf, V, Nb and Ta) as possible new hydrogen rich hydrides for hydrogen storage

A series of hydrogen rich Mg6–7TMH14–16 (TM = Ti, Zr, Hf, V, Nb and Ta) hydrides have been synthesized at 600 °C in a high pressure anvil cell above 4 GPa. All have structures based on a fluorite type metal atom subcell lattice with a ≈ 4.8 A. The TM atom arrangements are, however, more ordered and can best be described by a superstructure where the 4.8 A FCC unit cell axis is doubled. The full metal atom structure corresponds to the Ca7Ge type structure. This superstructure was also observed from electron diffraction patterns. The hydrogen atoms were found from powder X-ray diffraction using synchrotron radiation to be located in the two possible tetrahedral sites. One coordinates three Mg atoms and one TM atom and another coordinates four Mg atoms. These types of new hydrogen rich hydrides based on immiscible metals were initially considered as metastable but have been observed to be reversible if not fully dehydrogenated. In this work, DFT calculations suggest a mechanism whereby this can be explained: with H more strongly bonded to the TM, it is in principle possible to stepwise dehydrogenate the hydride. The remaining hydrogen in the tetrahedral site coordinating the TM would then act to prevent the metals from separating, thus making the system partially reversible.

Mechanically reversible conductor–insulator transition in Mg2NiH4

An irreversible conductor-insulator transition has been observed when heating Mg2NiH4 in the temperature interval 110 to 570 K. The disappearance of the electric conductivity is concomitant with the appearance of stacking faults (or microtwinning) in the Mg2NiH4 structure, as observed by powder x-ray diffraction. However, the stacking faults are sensitive to applied mechanical pressure or grinding, and by compressing the hydride sample in a tablet press, Mg2NiH4 regains its electric conductivity as the observable amount of stacking faults is reduced. These phenomena are attributed to peculiarities connected with the stabilization of the electron-rich tetrahedral d10 [Ni(0)H4] complex by the lattice. Formally low-valent oxidation states usually demand good electron-accepting ligands with suitable π* or d orbitals to relieve the high electron density at the central atom. This is not possible when hydrogen is the only ligand, but the easily polarizable H− ion helps to distribute electron density by outward b...

Competing stabilisation mechanisms in Mg2NiH4

Abstract By using three different Mg 2 Ni alloys to synthesise Mg 2 NiH 4 an additional stabilisation mechanism of this system has been identified. It has been known for some time that the stacking faults at unit cell level in the lattice, microtwinning, stabilise this hydride thermodynamically, but the results in this study also reveal that the addition of free magnesium from the casting of the starting alloys have a similar stabilising effect on the hydride. It is also clear that these two stabilisation mechanisms are connected to each other, i.e. the microtwinning in Mg 2 NiH 4 is dependent not only on the thermal history of the sample but also of the amount of free magnesium added at the casting of the Mg 2 Ni alloy. The extra magnesium acts as a stabilising dopant to the Mg 2 NiH 4 system and in addition to this gives interesting colour effects of the low-temperature phase of Mg 2 NiH 4 .

Air‐Metal Hydride Battery Construction and Evaluation

An air-metal hydride battery (32 Ah) using a noble metal-free air electrode (300 cm{sup 2}) was constructed and tested. The battery was charged with a third nickel mesh electrode and discharged using oxygen, with discharge voltages of 0.87 V at 3 A and 0.58 V at 30 A. The peak power was 25 W (83 mW/cm{sup 2}) with oxygen and 15 W (50 mW/cm{sup 2}) with air. Energy density and power density per total electrode weight were 120 Wh/kg at 3 A and 75 W/kg at 30 A, respectively. A low initial Wh efficiency (41%) due to a high charging voltage (1.95 V at 6 A) was raised to 58% by replacing the Ni mesh with a sintered nickel electrode (capacity 7.5 Ah). The new battery design (air-MH-Ni hybrid) allows two discharge possibilities, the air-MH couple for low voltage and low rate and the NiMH couple for high voltage and high rate discharges.

Metallic properties in the series K2Pd(II)H4, Na2Pd(0)H2 and Li2Pd(0)H2 correlated with the stabilization of a formally zero-valent palladium-hydrogen complex

Abstract Two new hydrides, Li 2 PdH 2 and K 2 PdH 4 , are compared with Na 2 PdH 2 . Both Li 2 PdH 2 and Na 2 PdH 2 are found to be metals and their structures are characterized by linear, formally zero-valent PdH 2 complexes in an alkali atom framework. The structures are isomorphous with Na 2 HgO 2 . The palladium-hydrogen bond length is 1.68 A. The usual stabilization mechanism for a low formal oxidation state, by back donation to orbitals on the ligand, is inoperative when hydrogen is the only ligand and the metallic properties of Li 2 PdH 2 and Na 2 PdH 2 are attributed to a stabilization mechanism in which electrons are delocalized into a conduction band. In K 2 PdH 4 , with the Na 2 PtH 4 type structure, the larger and more electropositive potassium atom allows a more common four-coordinated d 8 square planar palladium complex to be formed. The palladium-hydrogen bond length is 1.63 A. The electrons are localized and K 2 PdH 4 is a yellow-green non-conducting powder.

Structural studies of TiNiH

Abstract A structure determination of TiNiH from X-ray powder diffraction data has been made. TiNiH was found to have a tetragonal unit-cell with the dimensions a = 6.221(1) and c = 12.363(3) A . The structure was determined and refined in space group I4/mmm. The X-ray system used for structural studies of powder samples, based on a focussing Guinier-Hāgg camera and an automatic film-scanner system is briefly described.

In situ neutron diffraction study of the kinetics of metallic hydride electrodes

Abstract Using a new design of electrochemical cell for in situ neutron powder diffraction of electrode material in alkaline media, and taking advantage of the D20 diffractometer at the Laue Langevin Institute (high flux and large position sensitive detector), we were able to follow the structural evolution of LaNi 5 -type metal hydride electrodes at high charge/discharge rates on surface-treated material characterised by transmission electron microscopy. From the evolution of the relative amounts and the unit cell volumes of the α and β phases at transients, we show that for surface-treated and activated materials, the main rate limitation is the kinetics of the α⇔β phase transformation, i.e. the mobility of the phase interface and not the diffusion coefficient of the hydrogen in the phases (here deuterium, for neutron diffraction reasons).