More Than 50 Years after Its Discovery in SiO2 Octahedral Coordination Has Also Been Established in SiS2 at High Pressure.

SiO2 exhibits a high-pressure-high-temperature polymorphism, leading to an increase in silicon coordination number and density. However, for the related compound SiS2 such pressure-induced behavior has not been observed with tetrahedral coordination yet. All four crystal structures of SiS2 known so far contain silicon with tetrahedral coordination. In the orthorhombic, ambient-pressure phase these tetrahedra share edges and achieve only low space filling and density. Up to 4 GPa and 1473 K, three phases can be quenched as metastable phases from high-pressure high-temperature to ambient conditions. Space occupancy and density are increased first by edge and corner sharing and then by corner sharing alone. The structural situation of SiS2 up to the current study resembles that of SiO2 in 1960: Then, in its polymorphs only Si-O4 tetrahedra were known. But in 1961, a polymorph with rutile structure was discovered: octahedral Si-O6 coordination was established. Now, 50 years later, we report here on the transition from 4-fold to 6-fold coordination in SiS2, the sulfur analogue of silica.

Superconductivity in Weyl semimetal candidate MoTe2.

Transition metal dichalcogenides have attracted research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry and wide spectrum of potential applications. Despite the fact that the majority of related research focuses on semiconducting transition-metal dichalcogenides (for example, MoS2), recently discovered unexpected properties of WTe2 are provoking strong interest in semimetallic transition metal dichalcogenides featuring large magnetoresistance, pressure-driven superconductivity and Weyl semimetal states. We investigate the sister compound of WTe2, MoTe2, predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that bulk MoTe2 exhibits superconductivity with a transition temperature of 0.10 K. Application of external pressure dramatically enhances the transition temperature up to maximum value of 8.2 K at 11.7 GPa. The observed dome-shaped superconductivity phase diagram provides insights into the interplay between superconductivity and topological physics.

Temperature-pressure phase diagram of a D2O-D2 system at pressures to 1.8 kbar.

Using a volumetric technique, the deuterium solubility, X, in heavy water (L), low-pressure hexagonal ice (I h), and high-pressure cubic clathrate ice (sII) is studied at deuterium pressures up to 1.8 kbar and temperatures from -40 to +5 degrees C. The triple point of the L + I(h) + sII equilibrium is located at P = 1.07(3) kbar and T = -4.5(8) degrees C. The molar ratios D2/D2O of phases at the triple point are X(L) = 0.020(5), X(Ih) = 0.012(5), and X(sII) = 0.207(5).