The minerals of carletonite group, fluorcarletonite, KNa4Ca4[Si8O18](CO3)4(F,OH)·H2O and carletonite, Na4Ca4[Si8O18](CO3)4(OH,F)·H2O, were investigated using a multi-method approach. A detailed comparative chemical study of the minerals was carried out using electron probe microanalysis and Fourier transform infrared spectroscopy. Using X-ray techniques and the results obtained, geometrical and distortion characteristics of the mineral structures are calculated and the successful crystal-structure refinement of these two natural compounds are given. Using spectroscopic and luminescence methods and ab initio calculations, it is shown that hole defects (CO3)•– are responsible for the colouration of the samples studied. Luminescence due to 5d–4f transition in Ce3+ ions is observed in both investigated compounds. Moreover, luminescence attributed to intrinsic luminescence, corresponding to the decay of electronic excitations of (CO3)2– complexes in the carletonite sample, is registered for the first time in phyllosilicates. An analysis of the optical absorption spectra and g-tensor values suggests that (CO3)•– defects in the crystal structure are localised in the C1 positions. Identification of these specific properties for these sheet silicates, with a two-dimensional infinite tetrahedral polymerisation, indicates that carletonites could be prospective materials for novel phosphors and luminophores.

First-principles simulations and high-pressure experiments were used to study the stability of BaCO3 carbonates at high pressures. Witherite, which is orthorhombic and isotypic with CaCO3 aragonite, is stable at ambient conditions. As pressure increases, BaCO3 transforms from witherite to an orthorhombic post-aragonite structure at 8 GPa. The calculated bulk modulus of the post-aragonite structure is 60.7 GPa, which is slightly less than that from experiments. This structure shows an axial anisotropicc ompressibility and the a axis intersects with the c axis at 70 GPa, which implies that the pressure-induced phase transition reported in previous experimental study is misidentified. Although a pyroxene-like structure is stable in Mg- and Ca-carbonates at pressures >100 GPa, our simulations showed that this structure does not appear in BaCO3.

The crystal structure of hohmannite, Fe3+2[O(SO4)2]·8H2O, was studied by means of single-crystal X-ray diffraction (XRD) and vibrational spectroscopy. The previous structural model was confirmed, though new diffraction data allowed the hydrogen-bond system to be described in greater and more accurate detail. Ab initio calculations were performed in order to determine accurate H positions and to support the experimental model obtained from XRD data. Infrared and Raman spectra are presented for the first time for this compound and comments are made on the basis of the crystal structure and the known literature for sulfate minerals.

The alanates (complex aluminohydrides) have relatively high gravimetric hydrogen density and are among the most promising solid-state hydrogen-storage materials. In this work, the crystal structure and electronic structure of pure and mixed-alkali alanates were calculated by ground-state density-functional band-structure calculations. The results are in excellent correspondence with available experimental data. The properties of the pure alanates were compared, and the relatively high stability of the Li3AlH6 phase was pointed out as an important difference that may explain the difficulty of hydrogenating lithium alanate. The alkali alanates are nonmetallic with calculated band gaps around 5 eV and 2.5–3 eV for the tetra- and hexahydrides. The bonding was identified as ionic between the alkali cations and the aluminohydride complexes, while it is polar covalent within the complex. A broad range of hypothetical mixed-alkali alanate compounds was simulated, and four were found to be stable compared to the pure alanates and each other: LiNa2AlH6, K2LiAlH6, K2NaAlH6, and K2.5Na0.5AlH6. No mixed-alkali tetrahydrides were found to be stable, and this was explained by the local coordination within the different compounds. The only alkali alanate that seemed to be close to fulfilling the international hydrogen density targets was NaAlH4.

Among many Sn-based intermetallics, Cu6Sn5 (η and η′) is ubiquitous in modern solder interconnects. Using the published structural models of η and η′ and also related structures, the total energies and equilibrium cohesive properties are calculated from first-principles employing electronic density-functional theory, ultrasoft pseudopotentials, and both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange-correlation energy. The accuracy of our calculations is assessed through comparisons between theoretical results and experimental measurements for lattice parameters, elastic properties, and formation and transformation energies. The ambient-temperature experimental lattice constants of η and η′ are found to lie between the LDA and GGA level calculated zero-temperature lattice constants. The Wyckoff positions in the structural models of η and η′ agree very well with the ab initio results. The calculated formation energy of η′ lies between −3.2 and −4.0 kJ/mol, which is more positive by about 3 to 4 kJ/mol compared to reported experimental data obtained by solution calorimetry. Our systematic differential scanning calorimetry (DSC) experiments show that the η′ → η transformation enthalpy is 438 ± 18 J/mol, which is about 66% higher than the literature value. In view of our DSC results on heating and cooling, the nature of η′ → η and η → η′ is discussed. Our experimental bulk modulus of η and η′, and the heat of η′ → η transformation agree very well with the ab initio total energy calculations at the GGA level. Based on these results, we conclude that other isotropic elastic moduli (Young’s modulus, shear, and Poissons ratio) of η and η′ phases measured by pulse-echo technique are representative of their actual properties. The scatter in experimental elastic constants in the literature may be attributed to various factors, such as the measurement technique (pulse-echo versus nanoindentation), type of specimen (bulk, Cu6Sn5-layer in diffusion couple, thin-film), and anisotropy effects (particularly in Cu6Sn5-layer in diffusion couples).

Using an all-electron, full potential first-principles method, we have investigated the topology of charge density and elastic anisotropy of Ti3SiC2 polymorphs comparatively. By analyzing the charge density topology, it was found that the Ti–Si bonds are weaker in β than in α, resulting in a destabilizing effect and lower Young’s modulus in directions between a and c axes for β. On the other hand, the Si–C bonds (absent in α) are formed in β in the c direction. The formation of the Si–C bonds not only mitigates the destabilizing effect of the weaker Ti–Si bonds, but also results in larger Young’s modulus in the c direction. In contrast to the high elastic anisotrophy, the elastic anisotropy of Ti3SiC2 is very low.

We investigated adsorption and dissociation of water and HfCl4 on a Ge/Si(100) −(2 × 1) surface with a density-functional theory. The Si–Ge and Ge–Ge homodimers are used to represent the Si1−xGex surface. (i) Water first adsorbs on the bare Ge/Si(100) − (2 × 1) surface and then dissociates into OH and H. The activation energy for adsorption of water on the Ge–Ge homodimer is much higher than that on the Si–Ge heterodimer. (ii) HfCl4 dissociates upon adsorption on the Ge/Si(100) − (2 × 1) surface into HfCl3 and Cl. No net activation barrier exists during the adsorption of HfCl4 on both SiGe surface dimers. The molecular adsorption state is found to be metastable according to the calculation, which implies that the reaction tends to move toward to the product rather than trapping in HfCl4 adsorbed state. The difference in the potential energy surface between reactions on Si–Ge and Ge–Ge dimers is due to different bond strengths.

Bulk metallic glass forms when liquid metal alloys solidify without crystallization. In the search for iron-based bulk glass-forming alloys of the metal–metalloid type (Fe–B- and Fe–C-based), crystals based on the structural prototype C6Cr23 often preempt the amorphous phase. Destabilizing this competing crystal structure could enhance glass formability. We carried out first-principles total energy calculations of enthalpy of formation to identify third elements that can effectively destabilize C6Cr23. Yttrium appears optimal among transition metals, and rare earths also are suitable. Atomic size is the dominant factor.

The hypothetical wurtzite structure chromium chalcogenides were investigated through first-principle calculation within density-functional theory. All compounds are predicted to be true half-metallic ferromagnets with an integer Bohr magneton of 4 μB per unit. Their half-metallic gaps are 1.147, 0.885, and 0.247 eV at their equilibrium volumes for wurtzite-type CrM (M = S, Se, and Te), respectively. The half-metallicity can be maintained even when volumes are expanded by more than 20% for all compounds and compressed by more than 20%, 20%, and 5%, for CrS, CrSe, and CrTe, respectively.

Comprehensive results are presented on the influence of interfaces on electronic structure and giant magnetoresistance (GMR). Two structures were calculated for Co5/Cu3/Co5 and Co3/Cu/Co/Cu3/Co/Cu/Co3, where numbers stand for monolayer numbers of atoms, by employing the discrete variational method in the framework of the local spin density approximation. It has been found that the electron spin-dependent scattering is very strong at the interfaces compared to the interiors of the ferromagnetic layers, independent of the moment alignment configuration. The calculation results of total energy for various magnetization configuration revealed that antiferromagnetic exchange coupling was present between any of the ferromagnetic layers separated by Cu layers at zero field in the Co3/Cu/Co/Cu3/Co/Cu/Co3. The evaluated GMR ratio for the Co3/Cu/Co/Cu3/Co/Cu/Co3 (about 35.1%) was much larger than that of the Co5/Cu3/Co5 (about 21.0%), indicating large GMR effect could be expected with more interfaces when the thicknesses were the same. The result also indicated that the negative polarization of 4s electrons could reduce the GMR effect.