A(II)GeTeO₆ (A = Mn, Cd, Pb): Non-Centrosymmetric Layered Tellurates with PbSb₂O₆‑Related Structure

A­(II)­GeTeO₆ (A = Mn, Cd, Pb), new non-centrosymmetric (NCS) honeycomb-layered tellurates, were synthesized and characterized. A­(II)­GeTeO₆ (A = Mn, Cd, Pb) crystallize in trigonal space group <i>P</i>312 (No. 149) of edge-sharing Ge⁴⁺O₆ and Te⁶⁺O₆ octahedra, which form honeycomb-like-layers in the <i>ab</i>-plane with A­(II) (A = Mn, Cd, Pb) cations located between the layers. Their crystal structures are PbSb₂O₆-related, and the ordering of Ge⁴⁺ and Te⁶⁺ in octahedral environment breaks the inversion symmetry of the parent PbSb₂O₆ structure. The size of A­(II) cation in six coordination is an important factor to stabilize PbSb₂O₆-based structure. Temperature-dependent optical second harmonic generation measurements on A­(II)­GeTeO₆ confirmed non-centrosymmetric character in the entire scanned temperature range (0 to 600 °C). The materials exhibit a powder SHG efficiency of ∼0.37 and ∼0.21 times of KH₂PO₄ for PbGeTeO₆ and CdGeTeO₆, respectively. Magnetic measurements of MnGeTeO₆ indicate anti-ferromagnetic order at <i>T</i>_N ≈ 9.4 K with Weiss temperature of −22.47 K

Pressure-Modulated Conductivity, Carrier Density, and Mobility of Multilayered Tungsten Disulfide

Tungsten disulfide (WS₂) is a layered transition metal dichalcogenide (TMD) that differs from other two-dimensional (2D) compounds such as graphene due to its unique semiconducting, tunable-band-gap nature. Multilayered WS₂ exhibits an indirect band gap <i>E</i>_g of ∼1.3 eV, along with a higher load-bearing ability that is promising for strain-tuning device applications, but the electronic properties of multilayered WS₂ at higher strain conditions (<i>i</i>.<i>e</i>., static strain >12%) remain an open question. Here we have studied the structural, electronic, electrical, and vibrational properties of multilayered WS₂ at hydrostatic pressures up to ∼35 GPa experimentally in a diamond anvil cell and theoretically using first-principles <i>ab initio</i> calculations. Our results show that WS₂ undergoes an isostructural semiconductor-to-metallic (S–M) transition at approximately 22 GPa at 280 K, which arises from the overlap of the highest valence and lowest conduction bands. The S–M transition is caused by increased sulfur–sulfur interactions as the interlayer spacing decreases with applied hydrostatic pressure. The metalization in WS₂ can be alternatively interpreted as a 2D to 3D (three-dimensional) phase transition that is associated with a substantial modulation of the charge carrier characteristics including a 6-order decrease in resistivity, a 2-order decrease in mobility, and a 4-order increase in carrier concentration. These distinct pressure-tunable characteristics of the dimensionalized WS₂ differentiate it from other TMD compounds such as MoS₂ and promise future developments in strain-modulated advanced devices

Pressure-Induced Phase Transitions and Correlation between Structure and Superconductivity in Iron-Based Superconductor Ce(O_0.84F_0.16)FeAs

High-pressure angle-dispersive X-ray diffraction experiments on iron-based superconductor Ce­(O_0.84F_0.16)­FeAs were performed up to 54.9 GPa at room temperature. A tetragonal to tetragonal isostructural phase transition starts at about 13.9 GPa, and a new high-pressure phase has been found above 33.8 GPa. At pressures above 19.9 GPa, Ce­(O_0.84F_0.16)­FeAs completely transforms to a high-pressure tetragonal phase, which remains in the same tetragonal structure with a larger <i>a</i>-axis and smaller <i>c</i>-axis than those of the low-pressure tetragonal phase. The structure analysis shows a discontinuity in the pressure dependences of the Fe–As and Ce–(O, F) bond distances, as well as the As–Fe–As and Ce–(O, F)–Ce bond angles in the transition region, which correlates with the change in <i>T</i>_c of this compound upon compression. The isostructural phase transition in Ce­(O_0.84F_0.16)­FeAs leads to a drastic drop in the superconducting transition temperature <i>T</i>_c and restricts the superconductivity at low temperature. For the 1111-type iron-based superconductors, the structure evolution and following superconductivity changes under compression are related to the radius of lanthanide cations in the charge reservoir layer

Porous Ice Phases with VI and Distorted VII Structures Constrained in Nanoporous Silica

High-pressure compression of water contained in nanoporous silica allowed fabrication of novel porous ice phases as a function of pressure. The starting liquid nanoporous H₂O transformed to ice VI and VII at 1.7 and 2.5 GPa, respectively, which are 0.6 and 0.4 GPa higher than commonly accepted pressures for bulk H₂O. The continuous increase of pressure drives the formation of a tetragonally distorted VII structure with the space group <i>I</i>4<i>mm</i>, rather than a cubic <i>Pn</i>3<i>m</i> phase in bulk ice. The enhanced incompressibility of the tetragonal ice is related to the unique nanoporous configuration, and the distortion ratio <i>c</i>/<i>a</i> gradually increases with increasing pressure. The structural changes and enhanced thermodynamic stability may be interpreted by the two-dimensional distribution of silanol groups on the porous silica surfaces and the associated anisotropic interactions with H₂O at the interfaces

Sodium Ion Transport Mechanisms in Antiperovskite Electrolytes Na₃OBr and Na₄OI₂: An <i>in Situ</i> Neutron Diffraction Study

Na-rich antiperovskites are recently developed solid electrolytes with enhanced sodium ionic conductivity and show promising functionality as a novel solid electrolyte in an all solid-state battery. In this work, the sodium ionic transport pathways of the parent compound Na₃OBr, as well as the modified layered antiperovskite Na₄OI₂, were studied and compared through temperature-dependent neutron diffraction combined with the maximum entropy method. In the cubic Na₃OBr antiperovskite, the nuclear density distribution maps at 500 K indicate that sodium ions hop within and among oxygen octahedra, and Br^– ions are not involved. In the tetragonal Na₄OI₂ antiperovskite, Na ions, which connect octahedra in the <i>ab</i> plane, have the lowest activation energy barrier. The transport of sodium ions along the <i>c</i> axis is assisted by I^– ions

YCrWO₆: Polar and Magnetic Oxide with CaTa₂O₆‑Related Structure

A new polar and magnetic oxide, YCrWO₆, was successfully synthesized and characterized. YCrWO₆ crystallizes in polar orthorhombic space group <i>Pna</i>2₁ (no. 33) of edge-sharing dimers of CrO₆ and WO₆ octahedra, which are connected by corner-sharing to form a three-dimensional framework structure with Y³⁺ cations located in the channels. The structure of YCrWO₆ is related to that of CaTa₂O₆; however, the ordering of Cr³⁺ and W⁶⁺ in the octahedral sites breaks the inversion symmetry of the parent CaTa₂O₆ structure. X-ray absorption near edge spectroscopy of YCrWO₆ confirmed the oxidation state of Cr³⁺ and W⁶⁺. Temperature-dependent optical second harmonic generation measurements on YCrWO₆ confirmed the noncentrosymmetric character and evidenced a noncentrosymmetric-to-centrosymmetric phase transition above 800 °C. Piezoresponse force microscopy measurements on YCrWO₆ at room temperature show strong piezoelectric domains. Magnetic measurements of YCrWO₆ indicate antiferromagnetic order at <i>T</i>_N of ∼22 K with Weiss temperature of −34.66 K

Mn₂(Fe_0.8Mo_0.2)MoO₆: A Double Perovskite with Multiple Transition Metal Sublattice Magnetic Effects

Transition-metal-only perovskite oxides can introduce additional magnetic functionality with robust magnetoelectric properties but are rare. In this work we prepared a new transition-metal-only perovskite Mn₂(Fe_0.8Mo_0.2)­MoO₆ at high pressure and temperature. Uniquely, Mn₂(Fe_0.8Mo_0.2)­MoO₆ was discovered as a line phase upon composition modulation that was motivated from the above-room-temperature multiferroic Mn₂FeMoO₆ corundum phase. It exhibits ferrimagnetic Fe–Mo sublattice (<i>T</i>_C = 194 K) and Mn sublattice antiferromagnetic (<i>T</i>_m ∼ 45 K) transitions. Below <i>T</i>_m the two sublattice orderings are coupled and give rise to canted components in both. A first-order field-induced transition is also observed below 45 K. Mn₂(Fe_0.8Mo_0.2)­MoO₆ is a Mott variable range hopping semiconductor. These findings for the first time show that either an exotic perovskite or a corundum phase can be achieved by composition modulation besides the pressure effect

Synthesis, Structure, and Properties of the Layered Oxyselenide Ba₂CuO₂Cu₂Se₂

A new layered oxyselenide, Ba₂CuO₂Cu₂Se₂, was synthesized under high-pressure and high-temperature conditions and was characterized via structural, magnetic, and transport measurements. It crystallizes into space group <i>I</i>4/<i>mmm</i> and consists of a square lattice of [CuO₂] planes and antifluorite-type [Cu₂Se₂] layers, which are alternately stacked along the <i>c</i> axis. The lattice parameters are obtained as <i>a</i> = <i>b</i> = 4.0885 Å and <i>c</i> = 19.6887 Å. The Cu–O bond length is given by half of the lattice constant <i>a</i>, i.e., 2.0443 Å. Ba₂CuO₂Cu₂Se₂ is a semiconductor with a resistivity of ∼18 mΩ·cm at room temperature. No magnetic transition was found in the measured temperature range, and the Curie–Weiss temperature was obtained as −0.2 K, suggesting a very weak exchange interaction. The DFT+<i>U</i>_eff calculation demonstrates that the band gap is about 0.2 eV for the supposed antiferromagnetic order, and the density of state near the top of the valence band is mainly contributed from the Se 4p electrons

Hole Doping and Structural Transformation in CsTl_1–<i>x</i>Hg_<i>x</i>Cl₃

CsTlCl₃ and CsTlF₃ perovskites have been theoretically predicted to be superconductors when properly hole-doped. Both compounds have been previously prepared as pure compounds: CsTlCl₃ in a tetragonal (<i>I</i>4/<i>m</i>) and a cubic (<i>Fm</i>3̅<i>m</i>) perovskite polymorph and CsTlF₃ as a cubic perovskite (<i>Fm</i>3̅<i>m</i>). In this work, substitution of Tl in CsTlCl₃ with Hg is reported, in an attempt to hole-dope the system and induce superconductivity. The whole series CsTl_1–<i>x</i>Hg_<i>x</i>Cl₃ (<i>x</i> = 0.0, 0.1, 0.2, 0.4, 0.6, and 0.8) was prepared. CsTl_0.9Hg_0.1Cl₃ is tetragonal as the more stable phase of CsTlCl₃. However, CsTl_0.8Hg_0.2Cl₃ is already cubic with the space group <i>Fm</i>3̅<i>m</i> and with two different positions for Tl⁺ and Tl³⁺. For <i>x</i> = 0.4 and 0.5, solid solutions could not be formed. For <i>x</i> ≥ 0.6, the samples are primitive cubic perovskites with one crystallographic position for Tl⁺, Tl³⁺, and Hg²⁺. All of the samples formed are insulating, and there is no signature of superconductivity. X-ray absorption spectroscopy indicates that all of the samples have a mixed-valence state of Tl⁺ and Tl³⁺. Raman spectroscopy shows the presence of the active Tl–Cl–Tl stretching mode over the whole series and the intensity of the Tl–Cl–Hg mode increases with increasing Hg content. First-principle calculations confirmed that the phases are insulators in their ground state and that Hg is not a good dopant in the search for superconductivity in this system

The Metallic State in Neutral Radical Conductors: Dimensionality, Pressure and Multiple Orbital Effects

Pressure-induced changes in the solid-state structures and transport properties of three oxobenzene-bridged bisdithiazolyl radicals <b>2</b> (R = H, F, Ph) over the range 0–15 GPa are described. All three materials experience compression of their π-stacked architecture, be it (i) 1D ABABAB π-stack (R = Ph), (ii) quasi-1D slipped π-stack (R = H), or (iii) 2D brick-wall π-stack (R = F). While R = H undergoes two structural phase transitions, neither of R = F, Ph display any phase change. All three radicals order as spin-canted antiferromagnets, but spin-canted ordering is lost at pressures <1.5 GPa. At room temperature, their electrical conductivity increases rapidly with pressure, and the thermal activation energy for conduction <i>E</i>_act is eliminated at pressures ranging from ∼3 GPa for R = F to ∼12 GPa for R = Ph, heralding formation of a highly correlated (or bad) metallic state. For R = F, H the pressure-induced Mott insulator to metal conversion has been tracked by measurements of optical conductivity at ambient temperature and electrical resistivity at low temperature. For R = F compression to 6.2 GPa leads to a quasiquadratic temperature dependence of the resistivity over the range 5–300 K, consistent with formation of a 2D Fermi liquid state. DFT band structure calculations suggest that the ease of metallization of these radicals can be ascribed to their multiorbital character. Mixing and overlap of SOMO- and LUMO-based bands affords an increased kinetic energy stabilization of the metallic state relative to a single SOMO-based band system