La(Fe,Si)13-based magnetic refrigerants obtained by novel processing routes

The structure and magnetic properties of LaFe_13−xSi_x and Co-substituted LaFe_11.8−xCo_xSi_1.2 alloys prepared by melt spinning, as well as of LaFe_11.57Si_1.43H_x hydrides prepared by reactive milling are investigated. The hysteresis in the temperature- and field-induced phase transitions is significantly reduced as compared with conventional bulk alloys, which makes these materials very attractive for magnetic refrigerant applications. The unusual combination of features characteristic of first- and second-order phase transitions in the La(Fe,Si)₁₃-based compounds is discussed on the basis of density-functional electronic structure calculations.     

Terminal Alkynes in the Coordination Sphere of Titanocene Complexes – Elementary Steps in Catalytic Dimerizations and Oligomerizations of Alkynes

The titanocene acetylene complex [Cp*₂Ti(η²-Me₃SiC≡CSiMe₃)] ( 14 ) reacts with 1-alkynes such as phenylacetylene ( 15 a ), 1-hexyne ( 15 b ), 1-dodecyne ( 15 c ) and trimethylsilylacetylene ( 15 d ) by ligand exchange and proton shift, to yield exclusively the 1-alkenyltitanocene acetylides [Cp*₂Ti(CH=CHR)(C≡CR)] ( 21 ) (R = Ph ( 21 a ), CH₃(CH₂)₃ ( 21 b ), CH₃(CH₂)₉ ( 21 c ), SiMe₃ ( 21 d )). The X-ray structure of 21 a is presented. In reaction of acetylene HC≡CH ( 15 e ) with 14 other products are formed. However, no intermediates, like [Cp*₂Ti(η²-RC≡CH)] ( 17 ), [Cp*₂Ti(H)C≡CR] ( 17 ) or [Cp*₂Ti=C=CHR] ( 22 ) in reactions of 14 with 15 are detectable. On the other hand, a stable titanocenehydride [Cp*₂Ti(H)OCH₃] ( 23 ) is obtained by oxidative addition of CH₃OH with Cp*₂Ti, generated from 14 .     

Synthesis and Crystal Structure of SnPd3D0.138(7) by Neutron Powder Diffraction

Palladium-rich intermetallic compounds have attracted attention for their use in heterogeneous catalysis and their interesting hydrogenation properties. SnPd₃ was synthesized from the elements using selenium as a mineralizing agent. The stoichiometric composition was confirmed by energy-dispersive X-ray spectra (Sn_0.98(5)Pd_3.02(5)). X-ray diffraction showed an ordered AuCu₃ type structure [Pm3 m, a = 397.799(1) pm]. SnPd₃ takes up hydrogen at 703 K under 5.0 MPa, resulting in a volume expansion of 0.4 %. The crystal structure was determined from neutron powder diffraction data of the deuteride. SnPd₃D_0.138(7) belongs to the cubic anti-perovskite type [Pm3 m, a = 398.338(11) pm]. Deuterium occupies [Pd₆] octahedral sites [d(Pd–D) = 199.169(6) pm] in a statistical manner.     

Surface science investigations of atomic layer deposition half-reactions using TaF5 and Si2H6

Fundamental adsorption and surface chemistry studies of TaF₅ and Si₂H₆ on a polycrystalline Ta surface were performed in ultra-high vacuum (UHV) in the range 303-523 K to understand if these precursors may be used in applications to grow Ta-based films by atomic layer deposition. TaF₅ uptake saturated in UHV conditions for less than 100 L exposure for adsorption on both clean Ta and on 144 L Si₂H₆-treated Ta. TaF₅ dissociatively adsorbed on clean Ta, and F ligands were determined to govern the self-limiting adsorption behavior. The extent of each “half-reaction”, the reaction of TaF₅ with a Si₂H₆-treated surface and the reaction of Si₂H₆ with a TaF₅-treated surface, increased with surface temperature and was dependent on SiH_xF_y byproduct desorption. Neither Si₂H₆ adsorption nor Si₂H₆ half-reaction reached saturation in UHV conditions, as measured by the surface Si concentration. F ligands were removed during the Si₂H₆ half-reaction, and residual F asymtotically approached a constant value.     

Reactive Gas Pressure Syntheses of Nitride‐Diazenides and Hydridometalates

Nitride‐diazenides and hydridometalates (exemplified by the chemical systems Sr (Ba)‐N₂ and Rb‐Pt‐H₂) are obtained by reactive gas pressure syntheses in autoclaves starting from the elements and binary precursors, respectively. The formation of the nitride‐diazenides can be described in terms of reversible redox‐intercalation processes and that of the hydridometalates as reversible solid‐gas redox reactions. Phase formation and stability regions of the intermediate compounds of the chemical systems under consideration are particularly controlled by the level of the applied gas pressure.     

Effects of hydrogenation on magnetism of UNiGe

U-based intermetallic compound UNiGe absorbs hydrogen up to the stoichiometry UNiGeH_1.2. In analogy to other compounds with the TiNiSi-type of structure, the structure is modified into hexagonal. The zig-zag U-chains are stretched, the U-U spacing is largely enhanced and the ordering temperature increases up to 100 K. The ordered state has a spontaneous moment, but it is unlikely to be a simple ferromagnet.     

Beiträge zur Chemie des Phosphors. 230. Hexaisopropyl-tetradecaphosphan(6), P14i-Pr6, und Hexaisopropyl-hexadecaphosphan(6), P16i-Pr6 — Bildung und 31P-NMR-spektroskopische Strukturbestimmung

Contributions to the Chemistry of Phosphorus. 230. Hexaisopropyltetradecaphosphane(6), P₁₄i-Pr₆, and Hexaisopropylhexadecaphosphane(6), P₁₆i-Pr₆ — Formation and Structural Determination by ³¹P-NMR Spectroscopy Hexaisopropyltetradecaphosphane(6) ( 1 ) and hexaiso-propylhexadecaphosphane(6) ( 2 ) are formed together with other isopropylpolycyclophosphanes by the reaction of i-PrPCl₂ with P₄ and magnesium and have been enriched to 30 mol% and 10 mol%, respectively. According to ³¹P-NMR spectroscopic investigations, the novel conjuncto-phosphane skeletons of 1 and 2 are the annelation products of a P₅ ring with a P₁₁(5) or a P₁₃(5) partial skeleton, respectively, joined by a common P₂ bridge. Thus, 1 is 4,5,6,10,12,14-hexaisopropylpentacyclo-[9.2.1.0^2,9 .0^3,7 .0^8,13]tetradecaphosphane and 2 is 5,6,7,11,14,15-hexaisopropylhexacyclo[7.7.0.0^2,13 .0^3,10 .0^4,8 .0^12,16]hexadecaphosphane. The phosphorus hydrides P₁₄H₁₆ and P₁₆H₆ have the same skeletal structures which are also intermediate stages in the formation of Hittorf's phosphorus.     

Syntheses of the Antimonides R2Sb− (R = Ph,Mes, tBu,tBu2Sb) and Sb73− by Reactions of Organoantimony Hydrides or cyclo‐(tBuSb)4 with Li,Na, K,or BuLi

Reactions of R₂SbH with BuLi at ?70 °C in tetrahydrofuran (thf) lead to [R₂SbLi(thf)₃] [R = Ph ( 1 ) or R = Mes ( 2 )]. The antimonides [tBu₂SbK(pmdeta)] ( 3 ) (pmdeta = pentamethyldiethylenetriamine), [Li(tmeda)₂][tBu₄Sb₃]·benzene ( 4 ) (tmeda = tetramethylethylenediamine), and [tBu₄Sb₃Na(tmeda, thf)] ( 5 ) result from the reduction of cyclo‐(tBuSb)₄ by Li, Na, or K with pmdeta or tmeda in thf. The primary stibanes RSbH₂ [R = Mes ( 6 ), 2‐(Me₂NCH₂)C₆H₂ ( 7 )] are synthesized by reactions of RSbCl₂ with LiAlH₄. PhSbH₂ reacts with BuLi, and tmeda in toluene to give [Sb₇Li₃(tmeda)₃]·toluene ( 8 ). [Sb₇Na₃(pmdeta)₃]·toluene ( 9 ) is obtained from PhSbH₂, Na in liqu. NH₃, pmdeta and toluene. Crystal structures are reported for 1 – 5 and 9 .     

A First Principles Study of Hydrogen Storage in NaAlH4‐Related Complex Hydrides

We have applied first principles computations to predict the properties of complex hydrides related to the alanate NaAlH₄, a very promising class of systems for reversible hydrogen storage. The effect of partial substitution on the Na site (by Li or K), and on the Al site (by B or Ga) on the thermodynamic stability of NaAlH₄ and its decomposition product Na₃AlH₆ is investigated and evaluated by means of qualitative van't Hoff plots. From the calculated results we infer that the most promising improved hydrides are Na_1?xLi_xAl_1?yB_yH₄, obtained by a double substitution on the Na and on the Al sites of NaAlH₄.