The O K-edge x-ray Raman scattering (XRS), Brillouin scattering and diffraction studies on silica glass at high pressure have been elucidated in a unified manner using model structures obtained from First-Principles molecular dynamics calculations. This study provides a comprehensive understanding on how the structure is related to the physical and electronic properties. The origin of the “two peak” pattern in the XRS is found to be the result of increased packing of oxygen near the Si and is not a specific sign for sixfold coordination. The compression mechanism involving the presence of 5- and 6-fold coordinated silicon is confirmed. A slight increase in the silicon-oxygen coordination higher than six was found to accompany the increase in the acoustic wave velocity near 140 GPa.

When a material is heated, generally, it dilates. Here, we find a general trend that the average distance between a center atom and atoms in the first nearest-neighbor shell contracts for several metallic melts upon heating. Using synchrotron X-ray diffraction technique and molecular dynamics simulations, we elucidate that this anomaly is caused by the redistribution of polyhedral clusters affected by temperature. In metallic melts, the high-coordinated polyhedra are inclined to evolve into low-coordinated ones with increasing temperature. As the coordination number decreases, the average atomic distance between a center atom and atoms in the first shell of polyhedral clusters is reduced. This phenomenon is a ubiquitous feature for metallic melts consisting of varioussized polyhedra. This finding sheds light on the understanding of atomic structures and thermal behavior of disordered materials and will trigger more experimental and theoretical studies of liquids, amorphous alloys, glasses, and casting temperature effect on solidification process of crystalline materials.metal liquids | bond lengths | contraction T he study of metallic liquid structure is of importance because it is a fundamental issue in materials science and condensedmatter physics due to its critical role in understanding the processes of melting, solidification, and glass transition (1-6). Progress has been achieved in recent years both experimentally (7-24) and theoretically (13,(25)(26)(27)(28)(29)(30)(31)(32)(33). It is widely accepted that metallic liquids are composed of atomic clusters (7-33). However, how these clusters evolve upon external effects (e.g., temperature and pressure) still remains unclear (7-37). Generally, materials undergo thermal expansion and average atomic distance in the first shell increase upon heating. Here, we report a contraction of average atomic distance between a center atom and atoms in the first shell for metallic Al, Zn, Sn, In, Cu, Ni, Ag, and Au melts during heating. The thermal behaviors of metallic melts (pure elements and alloys) have been intensely studied, whereas the anomalous behavior of average atomic distance between a center atom and atoms in the first shell in liquids was usually ignored or not systematically evaluated (7-9, 21, 24). The anomalous behavior is focused upon and systematically investigated here by applying the state-of-the-art advanced synchrotron radiation-based experimental techniques and theoretical methods. Fig. 1 shows the pair correlation function g(r) at different temperatures for Al and Zn obtained by in situ high-temperature X-ray diffraction (XRD). Similar results for Sn and In metallic melts were also obtained in Fig. S1. The g(r) was obtained by Fourier transformation of the structure factor S(q) data, which reveals the average probabilities for finding atoms at a distance r for a given atom. In crystalline phases, atoms are located in discrete shells. However, in disordered structures they usually exhibit a broad distribution. The peak shape at various r values in g...

ZnO 2 nanoparticles have been synthesized by an organometallic precursor method. The structure, structural stability, and magnetic and optical properties of ZnO 2 nanoparticles have been investigated by experiments and first-principles calculations. It is found that ZnO 2 nanoparticles decompose into ZnO at about 230°C and is stable up to 36 GPa at ambient temperature. The cubic ZnO 2 phase has a bulk modulus of B 0 ) 174 GPa at zero pressure. Nanocrystalline ZnO 2 material is an indirect semiconductor with an energy gap of about 4.5 eV and paramagnetic down to 5 K.