Crystallization of LiAlSiO₄ Glass in Hydrothermal Environments at Gigapascal Pressures–Dense Hydrous Aluminosilicates

High-pressure hydrothermal environments can drastically reduce the kinetic constraints of phase transitions and afford high-pressure modifications of oxides at comparatively low temperatures. Under certain circumstances such environments allow access to kinetically favored phases, including hydrous ones with water incorporated as hydroxyl. We studied the crystallization of glass in the presence of a large excess of water in the pressure range of 0.25–10 GPa and at temperatures from 200 to 600 °C. The <i>p</i> and <i>T</i> quenched samples were analyzed by powder X-ray diffraction, scanning electron microscopy, and IR spectroscopy. At pressures of 0.25–2 GPa metastable zeolite Li-ABW and stable α-eucryptite are obtained at low and high temperatures, respectively, with crystal structures based on tetrahedrally coordinated Al and Si atoms. At 5 GPa a new, hydrous phase of LiAlSiO₄, LiAlSiO₃(OH)₂ = LiAlSiO₄·H₂O, is produced. Its crystal structure was characterized from single-crystal X-ray diffraction data (space group <i>P</i>2₁/<i>c</i>, <i>a</i> = 9.547(3) Å, <i>b</i> = 14.461(5) Å, <i>c</i> = 5.062(2) Å, β = 104.36(1)°). The monoclinic structure resembles that of α-spodumene (LiAlSi₂O₆) and constitutes alternating layers of chains of corner-condensed SiO₄ tetrahedra and chains of edge-sharing AlO₆ octahedra. OH groups are part of the octahedral Al coordination and extend into channels provided within the SiO₄ tetrahedron chain layers. At 10 GPa another hydrous phase of LiAlSiO₄ with presently unknown structure is produced. The formation of hydrous forms of LiAlSiO₄ shows the potential of hydrothermal environments at gigapascal pressures for creating truly new materials. In this particular case it indicates the possibility of generally accessing pyroxene-type aluminosilicates with crystallographic amounts of hydroxyl incorporated. This could also have implications to geosciences by representing a mechanism of water storage and transport in the depths of the Earth

Single Crystal Growth and Thermodynamic Stability of Li₁₇Si₄

Single crystals of Li₁₇Si₄ were synthesized from melts Li_<i>x</i>Si_{100–<i>x</i>} (<i>x</i> > 85) at various temperatures and isolated by isothermal centrifugation. Li₁₇Si₄ crystallizes in the space group <i>F</i>4̅3<i>m</i> (<i>a</i> = 18.7259(1) Å, <i>Z</i> = 20). The highly air and moisture sensitive compound is isotypic with Li₁₇Sn₄. Li₁₇Si₄ represents a new compound and thus the lithium-richest phase in the binary system Li–Si superseding known Li₂₁Si₅ (Li_16.8Si₄). As previously shown Li₂₂Si₅ (Li_17.6Si₄) has been determined incorrectly. The findings are supported by theoretical calculations of the electronic structure, total energies, and structural optimizations using first-principles methods. Results from melt equilibration experiments and differential scanning calorimetry investigations suggest that Li₁₇Si₄ decomposes peritectically at 481 ± 2 °C to “Li₄Si” and melt. In addition a detailed investigation of the Li–Si phase system at the Li-rich side by thermal analysis using differential scanning calorimetry is given

Single Crystal Growth and Thermodynamic Stability of Li₁₇Si₄

Single crystals of Li₁₇Si₄ were synthesized from melts Li_<i>x</i>Si_{100–<i>x</i>} (<i>x</i> > 85) at various temperatures and isolated by isothermal centrifugation. Li₁₇Si₄ crystallizes in the space group <i>F</i>4̅3<i>m</i> (<i>a</i> = 18.7259(1) Å, <i>Z</i> = 20). The highly air and moisture sensitive compound is isotypic with Li₁₇Sn₄. Li₁₇Si₄ represents a new compound and thus the lithium-richest phase in the binary system Li–Si superseding known Li₂₁Si₅ (Li_16.8Si₄). As previously shown Li₂₂Si₅ (Li_17.6Si₄) has been determined incorrectly. The findings are supported by theoretical calculations of the electronic structure, total energies, and structural optimizations using first-principles methods. Results from melt equilibration experiments and differential scanning calorimetry investigations suggest that Li₁₇Si₄ decomposes peritectically at 481 ± 2 °C to “Li₄Si” and melt. In addition a detailed investigation of the Li–Si phase system at the Li-rich side by thermal analysis using differential scanning calorimetry is given

Revision of the Li–Si Phase Diagram: Discovery and Single-Crystal X‑ray Structure Determination of the High-Temperature Phase Li_4.11Si

Silicon has been regarded as a promising anode material for future lithium-ion batteries, and Li–Si phases play an important role. A detailed reinvestigation of the Li-rich part of the binary Li–Si phase diagram revealed the existence of a new phase, Li_4.106(2)Si (Li_16.42Si₄). Li_16.42Si₄ forms through the peritectic decomposition of the Li-richest phase Li₁₇Si₄ at 481–486 °C and was characterized by single-crystal X-ray diffraction (<i>a</i> = 4.5246(2) Å, <i>b</i> = 21.944(1) Å, <i>c</i> = 13.2001(6) Å, space group <i>Cmcm</i>, <i>Z</i> = 16), differential scanning calorimetry, and theoretical calculations. Li_16.42Si₄ represents a high-temperature phase that is thermodynamically stable above ∼480 °C and decomposes peritectically at 618 ± 2 °C to Li₁₃Si₄ and a melt. Li_16.42Si₄ can be retained at room temperature. The structure consists of 3 and 10 different kinds of Si and Li atoms, respectively. Two Li positions show occupational disorder. Si atoms are well-separated from each other and have only Li atoms as nearest neighbors. This is similar to Li₁₇Si₄ and Li₁₅Si₄ compositionally embracing Li_16.42Si₄. The SiLi_<i>n</i> coordination polyhedra in the series Li₁₅Si₄, Li_16.42Si₄, and Li₁₇Si₄ are compared. Li₁₅Si₄ exclusively features coordination numbers of 12, Li_16.42Si₄ of 12 and 13, and Li₁₇Si₄ reveals 13- and 14-coordinated Si atoms. The band structure and density of states of Li_16.42Si₄ were calculated on the basis of two ordered model structures with nominal compositions Li₁₆Si₄ (a hypothetical Zintl phase) and Li_16.5Si₄. Both reveal a metallic character that is analogous to Li₁₇Si₄. In contrast, the electronic structure of Li₁₅Si₄ is characteristic of a p-doped semiconductor

Revision of the Li–Si Phase Diagram: Discovery and Single-Crystal X‑ray Structure Determination of the High-Temperature Phase Li_4.11Si

Silicon has been regarded as a promising anode material for future lithium-ion batteries, and Li–Si phases play an important role. A detailed reinvestigation of the Li-rich part of the binary Li–Si phase diagram revealed the existence of a new phase, Li_4.106(2)Si (Li_16.42Si₄). Li_16.42Si₄ forms through the peritectic decomposition of the Li-richest phase Li₁₇Si₄ at 481–486 °C and was characterized by single-crystal X-ray diffraction (<i>a</i> = 4.5246(2) Å, <i>b</i> = 21.944(1) Å, <i>c</i> = 13.2001(6) Å, space group <i>Cmcm</i>, <i>Z</i> = 16), differential scanning calorimetry, and theoretical calculations. Li_16.42Si₄ represents a high-temperature phase that is thermodynamically stable above ∼480 °C and decomposes peritectically at 618 ± 2 °C to Li₁₃Si₄ and a melt. Li_16.42Si₄ can be retained at room temperature. The structure consists of 3 and 10 different kinds of Si and Li atoms, respectively. Two Li positions show occupational disorder. Si atoms are well-separated from each other and have only Li atoms as nearest neighbors. This is similar to Li₁₇Si₄ and Li₁₅Si₄ compositionally embracing Li_16.42Si₄. The SiLi_<i>n</i> coordination polyhedra in the series Li₁₅Si₄, Li_16.42Si₄, and Li₁₇Si₄ are compared. Li₁₅Si₄ exclusively features coordination numbers of 12, Li_16.42Si₄ of 12 and 13, and Li₁₇Si₄ reveals 13- and 14-coordinated Si atoms. The band structure and density of states of Li_16.42Si₄ were calculated on the basis of two ordered model structures with nominal compositions Li₁₆Si₄ (a hypothetical Zintl phase) and Li_16.5Si₄. Both reveal a metallic character that is analogous to Li₁₇Si₄. In contrast, the electronic structure of Li₁₅Si₄ is characteristic of a p-doped semiconductor

Revision of the Li–Si Phase Diagram: Discovery and Single-Crystal X‑ray Structure Determination of the High-Temperature Phase Li_4.11Si

Silicon has been regarded as a promising anode material for future lithium-ion batteries, and Li–Si phases play an important role. A detailed reinvestigation of the Li-rich part of the binary Li–Si phase diagram revealed the existence of a new phase, Li_4.106(2)Si (Li_16.42Si₄). Li_16.42Si₄ forms through the peritectic decomposition of the Li-richest phase Li₁₇Si₄ at 481–486 °C and was characterized by single-crystal X-ray diffraction (<i>a</i> = 4.5246(2) Å, <i>b</i> = 21.944(1) Å, <i>c</i> = 13.2001(6) Å, space group <i>Cmcm</i>, <i>Z</i> = 16), differential scanning calorimetry, and theoretical calculations. Li_16.42Si₄ represents a high-temperature phase that is thermodynamically stable above ∼480 °C and decomposes peritectically at 618 ± 2 °C to Li₁₃Si₄ and a melt. Li_16.42Si₄ can be retained at room temperature. The structure consists of 3 and 10 different kinds of Si and Li atoms, respectively. Two Li positions show occupational disorder. Si atoms are well-separated from each other and have only Li atoms as nearest neighbors. This is similar to Li₁₇Si₄ and Li₁₅Si₄ compositionally embracing Li_16.42Si₄. The SiLi_<i>n</i> coordination polyhedra in the series Li₁₅Si₄, Li_16.42Si₄, and Li₁₇Si₄ are compared. Li₁₅Si₄ exclusively features coordination numbers of 12, Li_16.42Si₄ of 12 and 13, and Li₁₇Si₄ reveals 13- and 14-coordinated Si atoms. The band structure and density of states of Li_16.42Si₄ were calculated on the basis of two ordered model structures with nominal compositions Li₁₆Si₄ (a hypothetical Zintl phase) and Li_16.5Si₄. Both reveal a metallic character that is analogous to Li₁₇Si₄. In contrast, the electronic structure of Li₁₅Si₄ is characteristic of a p-doped semiconductor

Lithium and Calcium Carbides with Polymeric Carbon Structures

We studied the binary carbide systems Li₂C₂ and CaC₂ at high pressure using an evolutionary and ab initio random structure search methodology for crystal structure prediction. At ambient pressure Li₂C₂ and CaC₂ represent salt-like acetylides consisting of C₂^2– dumbbell anions. The systems develop into semimetals (<i>P</i>3̅<i>m</i>1-Li₂C₂) and metals (<i>Cmcm</i>-Li₂C₂, <i>Cmcm</i>-CaC₂, and <i>Immm</i>-CaC₂) with polymeric anions (chains, layers, strands) at moderate pressures (below 20 GPa). <i>Cmcm</i>-CaC₂ is energetically closely competing with the ground state structure. Polyanionic forms of carbon stabilized by electrostatic interactions with surrounding cations add a new feature to carbon chemistry. Semimetallic <i>P</i>3̅<i>m</i>1-Li₂C₂ displays an electronic structure close to that of graphene. The π* band, however, is hybridized with Li-sp states and changed into a bonding valence band. Metallic forms are predicted to be superconductors. Calculated critical temperatures may exceed 10 K for equilibrium volume structures

Dynamics of Pyramidal SiH₃^– Ions in ASiH₃ (A = K and Rb) Investigated with Quasielastic Neutron Scattering

The two alkali silanides ASiH₃ (A = K and Rb) were investigated by means of quasielastic neutron scattering, both below and above the order–disorder phase transition occurring at around 275–300 K. Measurements upon heating show that there is a large change in the dynamics on going through the phase transition, whereas measurements upon cooling reveal a strong hysteresis due to undercooling of the disordered phase. The results show that the dynamics is associated with rotational diffusion of SiH₃^– anions, adequately modeled by H-jumps among 24 different jump locations radially distributed around the Si atom. The average relaxation time between successive jumps is of the order of subpicoseconds and exhibits a weak temperature dependence with a small difference in activation energy between the two materials, 39(1) meV for KSiH₃ and 33(1) meV for RbSiH₃. The pronounced SiH₃^– dynamics explains the high entropy observed in the disordered phase resulting in the low entropy variation for hydrogen absorption/desorption and hence the origin of these materials’ favorable hydrogen storage properties

Structural and Vibrational Properties of Silyl (SiH₃^–) Anions in KSiH₃ and RbSiH₃: New Insight into Si–H Interactions

The alkali metal silyl hydrides <i>A</i>SiH₃ (<i>A</i> = K, Rb) and their deuteride analogues were prepared from the Zintl phases <i>A</i>Si. The crystal structures of <i>A</i>SiH₃ consist of metal cations and pyramidal SiH₃^– ions. At room temperature SiH₃^– moieties are randomly oriented (α modifications). At temperatures below 200 K <i>A</i>SiH₃ exist as ordered low-temperature (β) modifications. Structural and vibrational properties of SiH₃^– in <i>A</i>SiH₃ were characterized by a combination of neutron total scattering experiments, infrared and Raman spectroscopy, as well as density functional theory calculations. In disordered α-<i>A</i>SiH₃ SiH₃^– ions relate closely to freely rotating moieties with <i>C</i>_3<i>v</i> symmetry (Si–H bond length = 1.52 Å; H–Si–H angle 92.2 °). Observed stretches and bends are at 1909/1903 cm^–1 (ν₁, A₁), 1883/1872 cm^–1 (ν₃, E), 988/986 cm^–1 (ν₄, E), and 897/894 cm^–1 (ν₂, A₁) for <i>A</i> = K/Rb. In ordered β-<i>A</i>SiH₃ silyl anions are slightly distorted with respect to their ideal <i>C</i>_3<i>v</i> symmetry. Compared to α-<i>A</i>SiH₃ the molar volume is by about 15% smaller and the Si–H stretching force constant is reduced by 4%. These peculiarities are attributed to reorientational dynamics of SiH₃^– anions in α-<i>A</i>SiH₃. Si–H stretching force constants for SiH₃^– moieties in various environments fall in a range from 1.9 to 2.05 N cm^–1. These values are considerably smaller compared to silane, SiH₄ (2.77 N cm^–1). The reason for the drastic reduction of bond strength in SiH₃^– remains to be explored

Synthesis, Structure, and Properties of the Electron-Poor II–V Semiconductor ZnAs

ZnAs was synthesized at 6 GPa and 1273 K utilizing multianvil high-pressure techniques and structurally characterized by single-crystal and powder X-ray diffraction (space group <i>Pbca</i> (No. 61), <i>a</i> = 5.6768(2) Å, <i>b</i> = 7.2796(2) Å, <i>c</i> = 7.5593(2) Å, <i>Z</i> = 8). The compound is isostructural to ZnSb (CdSb type) and displays multicenter bonded rhomboid rings Zn₂As₂, which are connected to each other by classical two-center, two-electron bonds. At ambient pressure ZnAs is metastable with respect to Zn₃As₂ and ZnAs₂. When heating at a rate of 10 K/min decomposition takes place at ∼700 K. Diffuse reflectance measurements reveal a band gap of 0.9 eV. Electrical resistivity, thermopower, and thermal conductivity were measured in the temperature range of 2–400 K and compared to thermoelectric ZnSb. The room temperature values of the resistivity and thermopower are ∼1 Ω cm and +27 μV/K, respectively. These values are considerably higher and lower, respectively, compared to ZnSb. Above 150 K the thermal conductivity attains low values, around 2 W/m·K, which is similar to that of ZnSb. The heat capacity of ZnAs was measured between 2 and 300 K and partitioned into a Debye and two Einstein contributions with temperatures of θ_D = 234 K, θ_E1 = 95 K, and θ_E2 = 353 K. Heat capacity and thermal conductivity of ZnSb and ZnAs show very similar features, which possibly relates to their common electron-poor bonding properties