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 Ce3+. The high-temperature Curie-Weiss fit yields an effective magnetic moment of approximately 2.54μB per Ce for both approximant systems, which is reduced to ∼2.0μB at temperatures below 10 K. The low-temperature specific heat is dominated by the Schottky anomaly originating from the splitting of the Ce3+ Kramers doublet, resulting in an entropy of Rln2 at around 10 K.

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, Cel/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 Cel/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.

Tsai-type 1/1 RE-Au-Al approximant crystal (AC) phases RE₁₄Au_xAl_86-x (RE = Gd, Tb) display a large homogeneity range (50 < x < 75) across which distinct changes of magnetic properties – from spin glass to ferromagnetic (FM) to antiferromagnetic (AFM) – and intricate changes in structural properties occur. Here we analyzed selected 1/1 Ho- and Sm-Au-Al AC phases in the range 53 < x < 71 with single crystal X-ray diffraction and magnetometry. We find that the Al/Au ordering behavior and associated structural changes with x are similar in both systems, and match that observed for RE = Gd, suggesting that the structural evolution with x is rather independent from the type of RE. On the other hand, the magnetic properties, and notably the type of long-range order obtained at large x, are strongly depending on the RE. Ho₁₄Au_xAl_86-x samples with x = 61–71 consistently displayed a ferrimagnetic behavior whereas Sm₁₄Au_xAl_86-x samples with x = 67 and 71 revealed AFM behavior.

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.

The thermal conductivity κ 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 κ characteristic of many crystalline clathrate hydrates, e.g., tetrahydrofuran (THF) CH. The magnitude of κ 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 κ = −0.621−40.1/T at atmospheric pressure (in SI-units). This is slightly less than κ 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 (M·17 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 κ 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.

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 ∼30^∘ 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·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.

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 LaGaH_1.66-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 (LaGaH_1.66 type) and the onefold structure (NdGaH_1.1 type) is estimated on the basis of phase–composition isotherms and neutron powder diffraction to be x = 1.15.

TiO2-II is a high pressure form of titania with a density about 2% larger than that of rutile. In contrast to the common polymorphs anatase, brookite and rutile its electronic structure and optical properties are poorly characterized. Here we report on a comparative electron-energy-loss-spectroscopy (EELS) study for which high resolution valence-loss and core-loss EELS data were acquired from nanocrystalline (<75 nm sized) titania particles with an energy resolution of about 0.2 eV. Electronic structure features revealed from titanium L3,2 and oxygen K electron energy loss near-edge structures show a strong similarity of TiO2-II with both rutile and brookite, which is attributed to similarities in the connectivity of octahedral TiO6 units with neighboring ones. From combined valence-loss EELS and UV-VIS diffuse reflectance spectroscopy data the band gap of TiO2-II was determined to be indirect and with a magnitude of-3.18 eV, which is very similar to anatase (indirect,-3.2 eV), and distinctly different from rutile (direct,-3.05 eV) and brookite (direct,-3.45 eV).

K₂SiH₆, crystallizing in the cubic K₂PtCl₆ structure type (Fm3̅m), features unusual hypervalent SiH₆^2– complexes. Here, the formation of K₂SiH₆ at high pressures is revisited by in situ synchrotron diffraction experiments, considering KSiH₃ as a precursor. At the investigated pressures, 8 and 13 GPa, K₂SiH₆ adopts the trigonal (NH₄)₂SiF₆ structure type (P3̅m1) upon formation. The trigonal polymorph is stable up to 725 °C at 13 GPa. At room temperature, the transition into an ambient pressure recoverable cubic form occurs below 6.7 GPa. Theory suggests the existence of an additional, hexagonal, variant in the pressure interval 3–5 GPa. According to density functional theory band structure calculations, K₂SiH₆ is a semiconductor with a band gap around 2 eV. Nonbonding H-dominated states are situated below and Si–H anti-bonding states are located above the Fermi level. Enthalpically feasible and dynamically stable metallic variants of K₂SiH₆ may be obtained when substituting Si partially by Al or P, thus inducing p- and n-type metallicity, respectively. Yet, electron–phonon coupling appears weak, and calculated superconducting transition temperatures are <1 K.