Reorientational Dynamics in Y(BH4)3·xNH3 (x = 0, 3, and 7): The Impact of NH3 on BH4- Dynamics.

The reorientational dynamics of Y(BH4)3·xNH3 (x = 0, 3, and 7) was studied using quasielastic neutron scattering (QENS) and neutron spin echo (NSE). The results showed that changing the number of NH3 ligands drastically alters the reorientational mobility of the BH4- anion. From the QENS experiments, it was determined that the BH4- anion performs 2-fold reorientations around the C2 axis in Y(BH4)3, 3-fold reorientations around the C3 axis in Y(BH4)3·3NH3, and either 2-fold reorientations around the C2 axis or 3-fold reorientations around the C3 axis in Y(BH4)3·7NH3. The relaxation time of the BH4- anion at 300 K decreases from 2 × 10-7 s for x = 0 to 1 × 10-12 s for x = 3 and to 7 × 10-13 s for x = 7. In addition to the reorientational dynamics of the BH4- anion, it was shown that the NH3 ligands exhibit 3-fold reorientations around the C3 axis in Y(BH4)3·3NH3 and Y(BH4)3·7NH3 as well as 3-fold quantum mechanical rotational tunneling around the same axis at 5 K. The new insights constitute a significant step toward understanding the relationship between the addition of ligands and the enhanced ionic conductivity observed in systems such as LiBH4·xNH3 and Mg(BH4)2·xCH3NH2.

2D crystal structure and anisotropic magnetism of GdAu6.75-xAl0.5+x (x ≈ 0.54).

Exploration of the gold-rich part of the ternary Gd-Au-Al system afforded the intermetallic compound GdAu6.75-xAl0.5+x (x ≈ 0.54) which was structurally characterized by single crystal X-ray diffraction (Pnma, a = 18.7847(4) Å, b = 23.8208(5) Å, c = 5.3010(1) Å). GdAu6.75-xAl0.5+x crystallizes in a previously unknown structure type featuring layers of Gd2(Au, Al)29 and Gd2(Au, Al)28 clusters which are arranged as in a close-packing parallel to the ac plane. The Gd substructure corresponds to slightly corrugated 36 nets (dGd-Gd = 5.30-5.41 Å) which are stacked on top of each other along the b direction with alternating short (5.4, 5.6 Å, within layers) and long distances (6.4 Å, between layers). The title compound has been discussed with respect to a quasicrystal approximant (1/1 AC) GdAu5.3Al in the same system. The magnetic properties of GdAu6.75-xAl0.5+x were found to be reminiscent to those of some ternary ACs, with sharp peaks in the temperature dependent magnetization, and metamagnetic-like transitions. The material becomes antiferromagnetic below 25 K; magnetometry results suggest that the antiferromagnetic state is composed of ferromagnetic ac planes, coupled antiferromagnetically along the b direction.

Interplay between the Reorientational Dynamics of the B3H8 - Anion and the Structure in KB3H8.

The structure and reorientational dynamics of KB3H8 were studied by using quasielastic and inelastic neutron scattering, Raman spectroscopy, first-principles calculations, differential scanning calorimetry, and in situ synchrotron radiation powder X-ray diffraction. The results reveal the existence of a previously unknown polymorph in between the α'- and ß-polymorphs. Furthermore, it was found that the [B3H8]- anion undergoes different reorientational motions in the three polymorphs α, α', and ß. In α-KB3H8, the [B3H8]- anion performs 3-fold rotations in the plane created by the three boron atoms, which changes to a 2-fold rotation around the C 2 symmetry axis of the [B3H8]- anion upon transitioning to α'-KB3H8. After transitioning to ß-KB3H8, the [B3H8]- anion performs 4-fold rotations in the plane created by the three boron atoms, which indicates that the local structure of ß-KB3H8 deviates from the global cubic NaCl-type structure. The results also indicate that the high reorientational mobility of the [B3H8]- anion facilitates the K+ cation conductivity, since the 2-orders-of-magnitude increase in the anion reorientational mobility observed between 297 and 311 K coincides with a large increase in K+ conductivity.

Fluctuating lattice constants of indium under high pressure.

Recent high-pressure investigations of elemental In have yielded controversial results. We show that the observed high-pressure face-centered orthorhombic (fco) structure can be explained as an intermediate state between two body-centered tetragonal (bct) structures with different c/a ratios (c/a < square root [2] and c/a > square root [2], respectively). In a pressure range from about 50 to 200 GPa these two bct structures correspond to local minima of the total energy with respect to orthorhombic distortion of the ground-state bct In structure. The fco saddle point represents a tiny barrier and even at low temperatures rapid structural fluctuations should occur. Such a situation has not been identified in any other elemental metal.

Metal-nonmetal transition in the boron group elements.

Structural competition in boron group elements has been studied by means of ab initio calculations. For boron we predict a structural change alpha-B-->alpha-Ga accompanied by a nonmetal-metal transition at a pressure of about 74 GPa. For Al and Ga we find an icosahedron based elemental modification (alpha-B) 0.22 and 0.05 eV/atom, respectively, higher in energy than the corresponding metallic ground state structures. In particular, the low energy difference for Ga raises expectations into the experimental feasibility of new modifications for these elements, especially in nanosized systems.

Al(22)Cl(20).12L (L = THF, THP): the first polyhedral aluminum chlorides.

Aluminum subhalides of the type Al(22)X(20).12L (X = Cl, Br; L = THF, THP) are the only known representatives of polyhedral aluminum subhalides and exhibit interesting multicenter bonding properties. Herein, we report on the synthesis and structural investigation of the first chlorides of this type. Additional investigations applying solid-state (27)Al NMR (MAS), XPS (of Al(4)Cp(4) and Al(22)X(20).12L), and quantum chemical calculations shed more light upon the structure of the molecules and possible Al modifications.

From V8Ga36.9Zn4.1 and Cr8Ga29.8Zn11.2 to Mn8Ga27.4Zn13.6: a remarkable onset of Zn-cluster formation in an intermetallic framework.

The series of isotypic compounds V8Ga41 --> V8Ga36.9Zn4.1 --> Cr8Ga29.5Zn11.2 --> Mn8Ga27.4Zn13.6 with the V8Ga41 structure type (space group R3, Z = 3) was prepared and structurally characterised by X-ray diffraction experiments (V8Ga41: a 13.9351(5), 14.8828(12); V8Ga36.9Zn4.1: a = 13.9244(7), c = 14.8660(9): Cr8Ga29.8Zn11.2: 13.7153(5), c = 14.6872(9); Mn8Ga27.4Zn13.6: a = 13.6033(6), c = 14.6058(16)). The site occupancies of the ternary compounds were refined from neutron powder-diffraction data and exposed a startling segregation of Zn and Ga, which finally resulted in the formation of separated Zn13 cluster entities-corresponding to almost ideal centred cuboctahedra or small pieces of fcc metal-in the Mn compound, which has the highest Zn content in the series. The homogeneity ranges of the underlying phases T8Ga41 xZnx were determined to be 0 < x < 4.1(3), 8.7(3) < x < 11.2(3) and 13.6(4) < x < 16.5(3) for T = V, Cr and Mn, respectively. The different ranges of composition of the phases reflect the requirement of an optimum electron concentration for a stable V8Ga41-type structure, which is in the narrow range between 159 and 165 electrons per formula unit. First-principles electronic-structure calculations could explain this fact by the occurrence of a pseudo gap in the density of states at which the Fermi level is put for this particular electron concentration. Furthermore the nature of the Zn/Ga segregation was revealed: T-Zn interactions were found to be considerably weaker than those for T-Ga. This places the Zn atoms as far as possible from the T atoms, thus leading to the formation of cuboctahedral Zn13 entities.

Gallium and indium under high pressure

Ga and In crystallize in unusual open ground-state crystal structures. Recent experiments have discovered that Ga under high pressure transforms into a close-packed structure, while this has so far not been observed for In. Results from first principles calculations explain in a simple way this difference in behavior. We predict a so far undiscovered transition of In to a close-packed structure at extreme pressures and show that the structure determining mechanism originates from the degree of s-p mixing of the valence orbitals. Group-III elements are shown to strongly disobey the standard corresponding-state rule.