Shalabh Gupta

Solvent-free mechanochemical synthesis of alane, AlH3: effect of pressure on the reaction pathway

Nearly quantitative mechanochemical synthesis of non-solvated AlH3 from lithium aluminium hydride (LiAlH4) and aluminium chloride (AlCl3) has been achieved at room temperature under reasonably low pressure of hydrogen (210 bar) or inert gas (125 bar for He or 90 bar for Ar). X-ray diffraction, solid-state 27Al NMR spectroscopy, and temperature programmed desorption analyses of as-milled materials reveal a nearly complete conversion of a 3 : 1 (molar) mixture of LiAlH4 and AlCl3 to a 4 : 3 (molar) mixture of AlH3 and LiCl in ca. 30 min. By applying pressure of 210 bar or less (depending on the gas: hydrogen, helium, or argon), competing reactions leading to formation of metallic aluminium can be completely suppressed. X-ray diffraction and NMR analyses of products extracted at various stages of the mechanochemical reaction between LiAlH4 and AlCl3 reveal, for the first time, that the solid-state transformation proceeds with LiAlCl4 as an intermediate. Evidently, the critical pressure required to suppress the formation of metallic aluminium depends on the rate at which mechanical energy is supplied during milling. For example, the critical pressure is reduced from 210 bar to 1 bar of hydrogen when the milling speed of a standard planetary mill is reduced from 300 rpm to 150 rpm, although at the expense of sluggish kinetics and much longer reaction time.

BaAuxZn13-x: Electron-Poor Cubic NaZn13- Type Intermetallic and Its Ordered Tetragonal Variant

Cubic NaZn(13)-type (Fm-3c, Z = 8) BaAu(x)Zn(13-x) compounds in the regions 1 ≤ x ≤ 5.4 (a = 12.418(1)-12.590(1) Å) and 6.4 ≤ x ≤ 8 (a = 12.630(1)-12.660(1) Å) plus an ordered tetragonal variant near x = 6 (P4/nbm; a = 8.8945(4) Å, c = 12.646(1) Å; Z = 4) have been synthesized and characterized by means of X-ray diffraction. Although the cubic structure contains Zn-centered, mixed (Zn, Au) icosahedra connected in alternate orientations via mixed tetrahedral stars (TS), the icosahedron vertices are ordered in the tetragonal structure. Both the inner and the outer tetrahedra in the TS in the cubic phase consist of mixed Au and Zn atoms, whereas the tetragonal phase features three different coloring schemes: inner Zn and outer Au tetrahedra, vice versa, or mixed Au and Zn sites on both inner and outer tetrahedra. Barium atoms center 24-atom snub cuboctahedra. Ordering of Au and Zn in the tetragonal phase achieves the largest number of heteroatomic Au-Zn contacts and yields relatively larger Hamilton populations (-ICOHPs) compared with homoatomic counterparts according to LMTO-based electronic structure calculations and analysis. Larger overlap populations are also observed for inter- versus intraicosahedral interactions. The densities-of-states data suggest the phase is metallic with highly dispersed Au d bands and nearly free-electron-like s and p bands for both Au and Zn.

BaHg2Tl2. An unusual polar intermetallic phase with strong differentiation between the neighboring elements mercury and thallium.

High yields of the novel BaHg(2)Tl(2) are achieved from reactions of the appropriate cast alloys at approximately 400 degrees C. (Isotypic SrHg(2)Tl(2) also exists.) The tetragonal barium structure (P4(2)/mnm, a = 10.606 A, c = 5.159 A) was refined from both single-crystal X-ray and neutron powder diffraction data in order to ensure the atom site assignments although distances and calculated atom site population also support the results. The Hg and Tl network atoms are distinctive in their functions and bonding. Parallel chains of Hg hexagons and of Tl tetrahedra along c are constructed from polyhedra that share opposed like edges, and these are in turn interconnected by Hg-Tl bonds. Overall, the number of Tl-Tl bonds per cell exceeds the Hg-Hg type by 20:12, but these are approximately 1:2 each in bonding according to their average -ICOHP values (related to overlap populations). Barium is bound within a close 15-atom polyhedron, 12 atoms of which are the more electronegative Hg. LMTO-ASA calculations show that scalar relativistic effects are particularly important for Hg 5d-6s mixing in Hg-Hg and Hg-Tl bonding, whereas relatively separate Tl 6s and 6p states are more important in Tl-Tl interactions. The 6p states of Hg and Tl and 5d of Ba define a dominant conduction band around E(F), and the phase is metallic and Pauli-like paramagnetic. The thallium characteristics here are close to those in numerous alkali-metal-Tl cluster systems. Other active metal-mercury phases that have been studied theoretically are all distinctly electron-richer and more reduced, and without appreciable net 5d, 6s contributions to Hg-Hg bonding.

R5Pn3-type Phases of the Heavier Trivalent Rare-Earth-Metal Pnictides (Pn = Sb, Bi): New Phase Transitions for Er5Sb3 and Tm5Sb3

The syntheses and distributions of binary R(5)Pn(3) phases among the hexagonal Mn(5)Si(3) (M), and the very similar orthorhombic beta-Yb(5)Sb(3) (Y) and Y(5)Bi(3) (YB) structure types have been studied for R = Y, Gd-Lu and Pn = Sb, Bi. Literature reports of M and YB-type structure distributions among R(5)Pn(3) phases, R = Y, Gd-Ho, are generally confirmed. The reported M-type Er(5)Sb(3) could not be reproduced. Alternate stabilization of Y-type structures by interstitials H or F has been disproved for these nominally trivalent metal pnictides. Single crystal structures are reported for (a) the low temperature YB form of Er(5)Sb(3) (Pnma, a = 7.9646(9) A, b = 9.176(1) A, c = 11.662(1) A), (b) the YB- and high temperature Y-types of Tm(5)Sb(3) (both Pnma, a = 7.9262(5), 11.6034(5) A, b = 9.1375(6), 9.1077(4) A, c = 11.6013(7), 7.9841(4) A, respectively), and (c) the YB structure of Lu(5)Sb(3), a = 7.8847(4) A, b = 9.0770(5) A, c = 11.5055(6) A. Reversible, temperature-driven phase transitions (beta-Yb(5)Sb(3) left arrow over right arrow Y(5)Bi(3) types) for the former Er(5)Sb(3) and Tm(5)Sb(3) around 1100 degrees C and the means of quenching the high temperature Y form, have been esstablished. According to their magnetic susceptibilities, YB-types of Er(5)Sb(3) and Tm(5)Sb(3) contain trivalent cations. Tight-binding linear muffin-tin-orbital method within the atomic sphere approximation (TB-LMTO-ASA) calculations for the two structures of Tm(5)Sb(3) reveal generally similar electronic structures but with subtle Tm-Tm differences supporting their relative stabilities. The ambient temperature YB-Tm(5)Sb(3) shows a deep pseudogap at E(F), approaching that of a closed shell electronic state. Short R-R bonds (3.25-3.29 A) contribute markedly to the structural stabilities of both types. The Y-type structure of Tm(5)Sb(3) shows both close structural parallels to, and bonding contrasts with, the nominally isotypic, stuffed Ca(5)Bi(3)D and its analogues. Some contradictions in the literature are discussed.

Synthesis, structure, and bonding of BaTl4. Size effects on encapsulation of cations in electron-poor metal networks.

The synthesis, structure, and bonding of BaTl(4) are described [C2/m, Z = 4, a = 12.408(3), b = 5.351(1), c = 10.383(2) Å, β = 116.00(3)°]. Pairs of edge-sharing Tl pentagons are condensed to generate a network of pentagonal biprisms along b that encapsulate Ba atoms. Alternating levels of prisms along c afford six more bifunctional Tl atoms about the waists of the biprisms, giving Ba a coordination number of 16. Each Tl atom is bonded to five to seven other Tl atoms and to three to five Ba atoms. There is also strong evidence that Hg substitutes preferentially in the shared edges of the Tl biprisms in BaHg(0.80)Tl(3.20) to generate more strongly bound Hg(2) dimers. Cations that are too small relative to the dimensions of the surrounding polyanionic network make this BaTl(4) structure (and for SrIn(4) and perhaps EuIn(4) as well) one stable alternative to tetragonal BaAl(4)-type structures in which cations are bound in larger hexagon-faced nets, as for BaIn(4) and SrGa(4). Characteristic condensation and augmentation of cation-centered prismatic units is common among many relatively cation- and electron-poor, polar derivatives of Zintl phases gain stability. At the other extreme, the large family of Frank-Kasper phases in which the elements exhibit larger numbers of bonded neighbors are sometimes referred to as orbitally rich.

A benign synthesis of alane by the composition-controlled mechanochemical reaction of sodium hydride and aluminum chloride

AbstractSolid-state mechanochemical synthesis of alane (AlH3) starting from sodium hydride (NaH) and aluminum chloride (AlCl3) has been achieved at room temperature. The transformation pathway of this solid-state reaction was controlled by a stepwise addition of AlCl3 to the initial reaction mixture that contained sodium hydride in excess of stoichiometric amount. As in the case of previously investigated LiH–AlCl3 system, complete selectivity was achieved whereby formation of unwanted elemental aluminum was fully suppressed, and AlH3 was obtained in quantitative yield. Reaction progress during each step was investigated by means of solid-state NMR and powder X-ray diffraction, which revealed that the overall reaction proceeds through a series of intermediate alanates that may be partially chlorinated. The NaH–AlCl3 system presents some subtle differences compared to LiH–AlCl3 system particularly with respect to optimal concentrations needed during one of the reaction stages. Based on the results, we postulate that high local concentrations of NaH may stabilize chlorine-containing derivatives and prevent decomposition into elemental aluminum with hydrogen evolution. Complete conversion with quantitative yield of alane was confirmed by both SSNMR and hydrogen desorption analysis.

Solvent- and catalyst-free mechanochemical synthesis of alkali metal monohydrides

Alkali metal monohydrides, AH (A = Li–Cs) have been synthesized in quantitative yields at room temperature by reactive milling of alkali metals in the presence of hydrogen gas at 200 bar or less. The mechanochemical approach reported here eliminates problems associated with the malleability of alkali metals — especially Li, Na, and K — and promotes effective solid–gas reactions, ensuring their completion. This is achieved by incorporating a certain volume fraction of the corresponding hydride powder as a process control agent, which allows continuous and efficient milling primarily by coating the surface of metal particles, effectively blocking cold welding. Formation of high-purity crystalline monohydrides has been confirmed by powder X-ray diffraction, solid-state NMR spectroscopy, and volumetric analyses of reactively desorbed H2 from as-milled samples. The proposed synthesis method is scalable and particularly effective for extremely air-sensitive materials, such as alkali and alkaline earth metal hydrides. The technique may also be favorable for production in continuous reactors operating at room temperature, thereby reducing the total processing time, energy consumption and, hence, the cost of production of these hydrides or their derivatives and composites.