Structural transitions of 4:1 methanol–ethanol mixture and silicone oil under high pressure

A 4:1(volume ratio)methanol–ethanol(ME)mixture and silicone oil are two of the most widely used liquid pressure-transmitting media(PTM)in high-pressure studies.Their hydrostatic limits have been extensively studied using various methods;however,the evolution of the atomic structures associated with their emerging nonhydrostaticity remains unclear.Here,we monitor their structures as functions of pressure up to∼30 GPa at room temperature using in situ high-pressure synchrotron x-ray diffraction(XRD),optical micro-Raman spectroscopy,and ruby fluorescence spectroscopy in a diamond anvil cell.No crystallization is observed for either PTM.The pressure dependence of the principal diffraction peak position and width indicates the existence of a glass transition in the 4:1MEmixture at∼12 GPa and in the silicone oil at∼3 GPa,beyond which a pressure gradient emerges and grows quickly with pressure.There may be another liquid-to-liquid transition in the 4:1 ME mixture at∼5 GPa and two more glass-to-glass transitions in the silicone oil at∼10 GPa and∼16 GPa.By contrast,Raman signals only show peak weakening and broadening for typical structural disordering,and Raman spectroscopy seems to be less sensitive than XRD in catching these structural transitions related to hydrostaticity variations in both PTM.These results uncover rich pressure-induced transitions in the two PTM and clarify their effects on hydrostaticity with direct structural evidence.The high-pressure XRD and Raman data on the two PTM obtained in this work could also be helpful in distinguishing between signals from samples and those from PTM in future high-pressure experiments.

Crystal and electronic structure of a quasi-two-dimensional semiconductor Mg_(3)Si_(2)Te_(6)

We report the synthesis and characterization of a Si-based ternary semiconductor Mg_(3)Si_(2)Te_(6),which exhibits a quasitwo-dimensional structure,where the trigonal Mg_(3)Si_(2)Te_(6)layers are separated by Mg ions.Ultraviolet-visible absorption spectroscopy and density functional theory calculations were performed to investigate the electronic structure.The experimentally determined direct band gap is 1.39 eV,consistent with the value of the density function theory calculations.Our results reveal that Mg_(3)Si_(2)Te_(6)is a direct gap semiconductor,which is a potential candidate for near-infrared optoelectronic devices.

Structure and magnetic properties of the S=3/2 zigzag spin chain antiferromagnet BaCoTe_(2)O_(7)

We report a study of the structure and magnetic properties of the S=3/2 zigzag spin chain compound BaCoTe_(2)O_(7).Neutron diffraction measurements show that it crystallizes in the noncentrosymmetric space group Ama2 with a canted↑↑↓↓spin structure along the quasi-one-dimensional zigzag chain and a moment size of 1.89(2)μBat 2 K.Both magnetic susceptibility and specific heat measurements yield an antiferromagnetic phase transition at TN=6.2 K.A negative Curie-Weiss temperature,ΘCW=-74.7(2) K,and an empirical frustration parameter,f=|ΘCW|/TN≈12,are obtained by fitting the magnetic susceptibility,indicating antiferromagnetic interactions and strong magnetic frustration.From ultraviolet-visible absorption spectroscopy and first-principles calculations,an indirect band gap of 2.68(2) eV is determined.We propose that the canted zigzag spin chain of BaCoTe_(2)O_(7) may produce a change in the polarization via the exchange-striction mechanism.