Abstract The thermodynamic stability of quasicrystals (QCs) and their relationship to compositionally adjacent phases in intermetallic phase diagrams remain incompletely understood. We examined the icosahedral QC i-ScZn (Sc12Zn88), which is embedded in the Sc–Zn system between the cubic 1/1 approximant crystal (AC) phase ScZn6 (Sc14.3Zn85.7) and the ScZn12 phase (Sc7.7Zn92.3) adopting the tetragonal ThMn12 structure. Single crystalline (or single grain) samples of ScZn6, Sc12Zn88, and ScZn12 were obtained by solution growth from ScxZn100–x melts under appropriate composition and temperature conditions. Variable-temperature powder X-ray diffraction and thermal analysis experiments reveal that as-cast Sc12Zn88 undergoes annealing-induced decomposition into Sc12+xZn88–x (x ≈ 1) and ScZn12, indicating the existence of a temperature-dependent homogeneity range of the i-ScZn phase. Upon heating, i-ScZn decomposes peritectically at approximately 625 °C into ScZn6 and liquid. Interpretation of the phase equilibria is complicated by pronounced kinetic undercooling of approximately 30 °C associated with the crystallization of ScZn12, which decomposes on heating at 505 °C into i-ScZn and liquid, but crystallizes only upon cooling to 475 °C. The present results support prior suggestions of an enthalpy-driven stabilization of i-ScZn, while establishing Sc12+xZn88–x (x ≈ 1) as the thermodynamically stable QC composition at low temperatures.

The temperature-induced orthorhombic to cubic phase transition in Li2C2 is a prototypical example of a solid to solid phase transformation between an ordered phase, which is well described within the phonon theory, and a dynamically disordered phase with rotating molecules, for which the standard phonon theory is not applicable. The transformation in Li2C2 happens from a phase with directionally ordered C2 dimers to a structure, where they are dynamically disorderd. We provide a description of this transition by employing ab initio molecular dynamics (AIMD) based stress-strain thermodynamic integration on a deformation path that connects the ordered and dynamically disordered phases. The free energy difference between the two phases is obtained. The entropy that stabilizes the dynamically disordered cubic phase is captured by the behavior of the stress on the deformation path. We also show that a combination of the stress-strain thermodynamic integration and machine learning force field methodologies appears as a promising path in studies of dynamically disordered materials.

High-pressure forms of LiX (X = Si or Ge) that adopt the simple tetragonal P4/mmm CuAu structure were synthesized by reacting stoichiometric Li12Si7/5Si mixtures and by transforming I41/a-LiGe (MgGa structure) at similar to 12.5 GPa and 410 and 265 degrees C, respectively. P4/mmm-LiGe was recovered in quantitative yield as a metastable phase at ambient pressure, whereas P4/mmm-LiSi was partially converted into a hitherto unknown, kinetically more stable polymorph. The structures of the P4/mmm phases consist of alternately stacked square planar nets of X (d Si-Si = 2.595 & Aring;, and d Ge-Ge = 2.761 & Aring;) and Li atoms. Density functional theory (DFT)-based electronic structure calculations reveal pronounced polarity, Li0.83+Si0.83- and Li0.84+Ge0.84-, together with strong covalent bonding between X atoms. Electron-phonon coupling calculations within the Migdal-Eliashberg framework predict superconducting transition temperatures of similar to 7 K for P4/mmm-LiGe and similar to 6 K for P4/mmm-LiSi. For LiGe, magnetic susceptibility measurements show a sharp diamagnetic transition at 6.3 K, in support of the theoretical result.

Hydrogenation at gigapascal pressures can produce hydrides with potential superconducting, ionic, and hydrogen-storage properties. We studied the La-Al-H and La-Si-H systems up to 20 GPa using structure prediction and in situ synchrotron diffraction. In La-Al-H, only rhombohedral LaAlH6 is stable. The La-Si-H system forms an orthorhombic monohydride, LaSiH, at low pressure, while LaSiH2 and LaSiH7 are predicted to be stable at 20 GPa, and LaSiH6 is slightly unstable. LaSiH2 is structurally related to the monohydride, whereas LaSiH6 and LaSiH7 feature SiH62- units characteristic of hydridosilicates. Calculations predict superconductivity in LaSiH2 and LaSiH6 with Tc ≈ 10 and 6 K. Experimentally, LaSiH2 formation is indicated at 20 GPa, but higher hydrides were not observed due to decomposition into LaH3 and Si, suggesting that pressures above 20 GPa are required to stabilize these phases at synthesis temperatures.

ScZn12 and YZn12 were synthesized by solution growth from Zn-rich melts and their body-centered tetragonal ThMn12 structure (I4/mmm) characterized from single crystal X-ray diffraction. Both compounds can be classified as polar intermetallics featuring a framework of slightly reduced Zn atoms as a consequence of partial charge transfer from Sc (Y). A structural analysis of this framework highlights a biface-capped tetrahedron Zn6 moiety as fundamental building unit. The connectivity of these units generates a rigid and regular 20-atom Zn cage that accommodates the rare-earth (RE) atoms. Electronic structure calculations based on density functional theory (DFT) revealed pronounced hybridization between Zn sp bands and high-lying, formally unoccupied RE d states. This interaction enhances a pseudogap at the Fermi level in the electronic density of states. Consequently, ScZn12 and YZn12, along with isostructural lanthanoid zincides, can be regarded as electron compounds with an optimal electron-per-atom (e/a) ratio of 2.08 (or 2.25 when expressed as electrons per Zinc atom). The rigidity of the Zn framework suggests that the size of the 20-atom Zn polyhedron is ideally matched by formally trivalent middle-to-late lanthanides as well as for Y, whose size is comparable to that of Dy, whereas Sc is comparatively undersized. Despite the seemingly loose coordination environment of Sc, phonon calculations confirm the dynamic stability of ScZn12. The calculated formation energies for REZn12 (RE = Sc, Y, Sm) are-13.1,-18.0, and-18.9 kJ/mol-atom, respectively, consistent with a lower thermodynamic stability of ScZn12.

Perovskite-type oxyhydrides stand out as hydride-ion conductors of relevance for diverse technological applications, but fundamental questions surrounding the relationship between the mechanism of hydride-ion diffusion and the local structure of these materials remain to be elucidated. Here, in a quasielastic neutron scattering (QENS) study of two perovskite-type oxyhydrides of barium titanate, BaTiO2.67H0.12 square 0.21 (square refers to anion vacancies) and BaTiO2.88H0.12, we establish that the mechanism of hydride-ion diffusivity relies on hydride-ion jumps to nearest-neighbor anion vacancies. Combined analyses of QENS and structural data for BaTiO2.67H0.12 square 0.21 show that the diffusion process is characterized by two different time scales, possibly related to diffusion in regions featured by different concentrations of anion vacancies. It follows that designing materials with specific concentrations of anion vacancies may be an effective route to optimize hydride-ion conductivity toward specific applications.

Hydridosilicates featuring SiH6 octahedral moieties represent a rather new class of compounds with potential properties relating to hydrogen storage and hydride ion conductivity. Here, we report on the new representative BaSiH6 which was obtained from reacting the Zintl phase hydride BaSiH∼1.8 with H2 fluid at pressures above 4 GPa and subsequent decompression to ambient pressure. Its monoclinic crystal structure (C2/c, a = 8.5976(3) Å, b = 4.8548(2) Å, c = 8.7330(4) Å, β = 107.92(1)°, Z = 4) was characterized by a combination of synchrotron radiation powder X-ray diffraction, neutron powder diffraction, and DFT calculations. It consists of complex SiH6 2- ions (d Si-H ≈ 1.61 Å), which are octahedrally coordinated by Ba2+ counterions. The arrangement of Ba and Si atoms deviates only slightly from an ideal fcc NaCl structure with a ≈ 7 Å. IR and Raman spectroscopy showed SiH6 2- bending and stretching modes in the ranges 800-1200 and 1400-1800 cm-1, respectively, in agreement with a hypervalent Si-H bonding situation. BaSiH6 is thermally stable up to 95 °C above which decomposition into BaH2 and Si takes place. DFT calculations indicated a direct band gap of 2.5 eV and confirmed that at ambient pressure BaSiH6 is a thermodynamically stable compound in the ternary Ba-Si-H system. The discovery of BaSiH6 consolidates the compound class of hydridosilicates, accessible from hydrogenations of silicides at gigapascal pressures (<10 GPa). The structural properties of BaSiH6 suggest that it presents an intermediate (or precursor) for further hydrogenation at considerably higher pressures to the predicted superconducting polyhydride BaSiH8 [Lucrezi, R.; et al. npj Comput. Mater. 2022, 8, 119] whose structure is also based on a NaCl arrangement of Ba and Si atoms but with Si in a cubic environment of H.

In strongly correlated systems, interactions give rise to critical fluctuations surrounding the quantum critical point (QCP) of a quantum phase transition. Quasicrystals allow the study of quantum critical phenomena in aperiodic systems with frustrated magnetic interactions. Here, we study the magnetic field and temperature scaling of the low-temperature specific heat for the quantum critical Yb-Au-Al quasicrystal. We devise a scaling function that encapsulates the limiting behaviors as well as the area where the system goes from a temperature-limited to a field-limited quantum critical region, where the magnetic field acts as a cutoff for critical fluctuations. The zero-field electronic specific heat is described by a power-law divergence, C_{el}/T∝T^{−0.54}, aligning with previously observed ac-susceptibility and specific-heat measurements. The field dependence of the electronic specific heat at high magnetic fields shows a similar power law C_{el}/T∝B^{−0.50}. In the zero-field and low-field region, we observe two small but distinct anomalies in the specific heat, located at 0.7 and 2.1 K.

Clathrate hydrates are crystalline compounds in which guest molecules are encaged within an ice-like lattice. They occur naturally and possess properties of significant interest for energy and storage applications. Here, we report the thermal conductivity kappa of structure I CO2 clathrate hydrate across a broad temperature range (90-265 K) and at pressures up to 1.2 GPa. Similar to structure II clathrate hydrates, kappa decreases with decreasing temperature, displaying almost identical temperature dependence. However, the absolute values are 10-30% lower. Notably, kappa of CO2 clathrate hydrate is among the lowest observed for structure I clathrate hydrates, with kappa = (426 +/- 8) mW m-1 K-1 under stable conditions at 270 K and 1 MPa. Furthermore, the isothermal dependencies of kappa on density rho and pressure p-parameters crucial for thermal modeling at elevated pressures-are relatively weak, with (d ln kappa/d ln rho) = 1.2 +/- 0.2 and (d ln kappa/dp) = (12 +/- 1) % GPa-1. The measurements show significantly lower kappa values and a different temperature dependence compared with previously reported simulation results. Nevertheless, the experimental data confirm the simulation prediction that kappa for CO2 clathrate hydrate is significantly lower than for other structure I clathrates. Our findings further indicate that kappa in both structures I and II clathrate hydrates tends to decrease with increasing van der Waals radius of the guest molecules, as reviewed here. This trend may arise from enhanced distortion and anharmonicity within the ice framework. We tentatively propose that the pronounced anharmonicity of the clathrate hydrate lattice leads to frequent phonon-phonon scattering, effectively suppressing phonon-mediated heat transport and resulting in predominantly diffusive thermal conduction.

Transport and heat capacity measurements under pressure must reconcile the limited available space and complicated geometry of a high-pressure cell with the need for multiple electrical connections. One solution for diamond anvil cells is to use customized diamonds with deposited electrical leads. Here, we instead address the problem through a split-gasket approach, intended for diamond anvil cells at moderate pressures and low temperature. A key component is the use of a substrate with lithographically defined leads, which enables connections to components such as thermometer, heater, and/or sample within the confined sample volume of the cell. The design includes an elaborate BeCu gasket sandwich with a preparation method that ensures electrical contact integrity. Using this configuration, we bring 12 leads to within 100 μm of the center of the diamond anvil at a pressure of about 2 GPa, comparable to the pressure reached with a regular gasket, demonstrating the setup's capability for high-pressure experiments. The split-gasket approach may come at the cost of reduced maximum pressure, but brings versatility and reproducibility, and alleviates the experimental efforts of maintaining multiple electrical leads both intact and electrically isolated.

Rare-earth elements containing aperiodic quasicrystals and their related periodic approximant crystals can exhibit nontrivial physical properties at low temperatures. Here, we investigate the 1/1 and 2/1 approximant crystal phases of the Ce-Au-Al system by studying the ac susceptibility and specific heat at low temperatures and in magnetic fields up to 12 T. We find that these systems display signs of quantum criticality similar to the observations in other claimed quantum critical systems, including the related Yb-Au-Al quasicrystal. In particular, the ac-susceptibility at low temperatures shows a diverging behavior χ∝1/T as the temperature decreases as well as cutoff behavior in magnetic field. Notably, the field dependence of χ closely resembles that of quantum critical systems. However, the ac susceptibility both in zero and nonzero magnetic fields can be understood from the splitting of a ground state Kramers doublet of Ce^{3+}. The high-temperature Curie-Weiss fit yields an effective magnetic moment of approximately 2.54μ_{B} per Ce for both approximant systems, which is reduced to ∼2.0μ_{B} at temperatures below 10 K. The low-temperature specific heat is dominated by the Schottky anomaly originating from the splitting of the Ce^{3+} Kramers doublet, resulting in an entropy of Rln2 at around 10 K.

Perovskite-type oxhydrides such as BaTiO3−xHy exhibit mixed hydride ion and electron conduction and are an attractive class of materials for developing energy storage devices. However, the underlying mechanism of electric conductivity and its relation to the composition of the material remains unclear. Here we report detailed insights into the hydride local environment, the electronic structure and hydride conduction dynamics of barium titanium oxyhydride. We demonstrate that DFT-assisted solid-state NMR is an excellent tool for differentiating between the different feasible electronic structures in these solids. Our results indicate that upon reduction of BaTiO3 the introduced electrons are delocalized among all Ti atoms forming a bandstate. Furthermore, each vacated anion site is reoccupied by at most a single hydride, or else remains vacant. This single occupied bandstate structure persists at different hydrogen concentrations (y = 0.13−0.31) and a wide range of temperatures (∼ 100−300 K).

Rare earth monogallide (REGa) Zintl phases are attractive for their properties in hydrogen storage and magnetic cooling. However, the magnetic effects upon hydrogen additions in REGa are not well understood. This study aims to explore the magnetic effects in REGaHx using SQUID magnetometry and neutron powder diffraction. To avoid challenges due to absorption and high incoherent scattering in the neutron diffraction experiments, the compound NdGaDx (x = 0, 0.9, or 1.6) was chosen for examination. It was found that NdGa exhibits two ferromagnetic structures below the Curie temperature of 42 K. Just below 42 K the magnetic moments are oriented along the crystallographic c axis, and at 20 K a spin reorientation occurs where the moments turn similar to 30 degrees toward the a axis. Upon partial deuteration (x = 0.9), the magnetization decreases and two magnetic phases are observed, one intermediate incommensurate phase, and one canted ferromagnetic phase with the net magnetization aligning along the b axis. For the full deuteride (x = 1.6) only one incommensurate magnetic phase is observed at low temperatures. Magnetometry also reveals that there are no isotope effects when absorbing H or D. The absorption of H or D changes the Nd-Nd distances as well as the electronic structure, which results in a drastic change in the magnetic properties as compared to NdGa.

The reorientational dynamics of Y(BH4)(3)center dot xNH(3) (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 C-2 axis in Y(BH4)(3), 3-fold reorientations around the C-3 axis in Y(BH4)(3)center dot 3NH(3), and either 2-fold reorientations around the C-2 axis or 3-fold reorientations around the C-3 axis in Y(BH4)(3)center dot 7NH(3). The relaxation time of the BH4- anion at 300 K decreases from 2 x 10(-7) s for x = 0 to 1 x 10(-12) s for x = 3 and to 7 x 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 C-3 axis in Y(BH4)(3)center dot 3NH(3) and Y(BH4)(3)center dot 7NH(3) 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 center dot xNH(3) and Mg(BH4)(2)center dot xCH(3)NH(2).

The thermal conductivity kappa of cyclopentane clathrate hydrate (CP CH) of type II was measured at temperatures down to 100 K and at pressures up to 1.3 GPa. The results show that CP CH displays amorphous-like kappa characteristic of many crystalline clathrate hydrates, e.g., tetrahydrofuran (THF) CH. The magnitude of kappa is 0.47 W m-1 K-1 near the melting point of 280 K at atmospheric pressure, and it is almost independent of pressure and temperature T: ln kappa = -0.621-40.1/T at atmospheric pressure (in SI-units). This is slightly less than kappa of type II CHs of water-miscible solvents such as THF. Intriguingly, unlike other water-rich type II clathrate hydrates of water-miscible molecules M (M17 H2O), CP CH does not amorphize at pressures up to 1.3 GPa at 130 K and also remains stable up to 0.5 GPa at 240 K. This shows that CP CH is mechanically more stable than the previously studied water-rich type II CHs, and suggests that repulsive forces between CP and the H2O cages increase the mechanical stability of crystalline CP CH. Moreover, we show that kappa of an ice-CH mixture, which often arises for CHs that form naturally, is described by the average of the parallel and series heat conduction models to within 5% for ice contents up to 22 wt%. The findings provide a better understanding of the thermal and stability properties of clathrate hydrates for their applications such as gas storage compounds. Through the exchange of guest molecules from water-miscible molecules to cyclopentane, the mechanical stability of type II clathrate hydrates increases significantly.

NdGa hydride and deuteride phases were prepared from high-quality NdGa samples and their structures characterized by powder and single-crystal X-ray diffraction and neutron powder diffraction. NdGa with the orthorhombic CrB-type structure absorbs hydrogen at hydrogen pressures ≤ 1 bar until reaching the composition NdGaH(D)1.1, which maintains a CrB-type structure. At elevated hydrogen pressure additional hydrogen is absorbed and the maximum composition recovered under standard temperature and pressure conditions is NdGaH(D)1.6 with the Cmcm LaGaH1.66-type structure. This structure is a threefold superstructure with respect to the CrB-type structure. The hydrogen atoms are ordered and distributed on three fully occupied Wyckoff positions corresponding to tetrahedral (4c, 8g) and trigonal–bipyramidal (8g) voids in the parent structure. The threefold superstructure is maintained in the H-deficient phases NaGaH(D) x until 1.6 ≥ x ≥ 1.2. At lower H concentrations, coinciding with the composition of the hydride obtained from hydrogenation at atmospheric pressure, the unit cell of the CrB-type structure is resumed. This phase can also display H deficiency, NdGaH(D) y (1.1 ≥ y ≥ 0.9), with H(D) exclusively situated in partially empty tetrahedral voids. The phase boundary between the threefold superstructure (LaGaH1.66 type) and the onefold structure (NdGaH1.1 type) is estimated on the basis of phase–composition isotherms and neutron powder diffraction to be x = 1.15.

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

A multianvil cell assembly with octahedral edge length 25 mm has been adapted for high pressure investigations involving water-rich environments up to 6.5 GPa and 400 & DEG;C. Water-rich samples are confined in Teflon containers with a volume up to 300 mm(3). Applicability tests were performed between 250 and 400 & DEG;C by investigating the transformation of amorphous titania particles close to the rutile-TiO2-II (& SIM;5 GPa) phase boundary, and the transformation of amorphous silica particles close to the quartz-coesite (& SIM;2.5 GPa) and coesite-stishovite (& SIM;7 GPa) phase boundaries. The performed experiments employed 25.4 mm tungsten carbide anvils with a truncation edge length of 15 mm. The sample pressure at loads approaching 820 t was estimated to be around 6.5 GPa. The large volume multianvil cell is expected to have broad and varied application areas, ranging from the simulation of geofluids to hydrothermal synthesis and conversion/crystal growth in aqueous environments at gigapascal pressures.