Low‐frequency Raman scattering from vitreous and molten B2O3

Realizing Persistent Zero Area Compressibility over a Wide Pressure Range in Cu2 GeO4 by Microscopic Orthogonal-Braiding Strategy

Zero area compressibility (ZAC) is an extremely rare mechanical response that exhibits an invariant two-dimensional size under hydrostatic pressure. All known ZAC materials are constructed from units in two dimensions as a whole. Here, we propose another strategy to obtain the ZAC by microscopically orthogonal-braiding one-dimensional zero compressibility strips. Accordingly, ZAC is identified in a copper-based compound with a planar [CuO4 ] unit, Cu2 GeO4 , that possesses an area compressibility as low as 1.58(26) TPa-1 over a wide pressure range from ≈0 GPa to 21.22 GPa. Based on our structural analysis, the subtle counterbalance between the shrinkage of [CuO4 ] and the expansion effect from the increase in the [CuO4 ]-[CuO4 ] dihedral angle attributes to the ZAC response. High-pressure Raman spectroscopy, in combination with first-principles calculations, shows that the electron transfer from in-plane bonding dx 2 -y 2 to out-of-plane nonbonding dz 2 orbitals within copper atoms causes the counterintuitive extension of the [CuO4 ]-[CuO4 ] dihedral angle under pressure. Our study provides an understanding on the pressure-induced structural evolution of copper-based oxides at an electronic level and facilitates a new avenue for the exploration of high-dimensional anomalous mechanical materials.

Zinc borate glasses: properties, structure and modelling of the composition-dependence of borate speciation

The short-range order of binary zinc borate glasses, xZnO-(1-x)B₂O₃, has been quantitatively described as a function of ZnO content over the entire glass forming range for the first time, to the best of our knowledge. Multiple spectroscopic techniques (¹¹B NMR, Raman, infrared) reveal detailed structural information regarding borate speciation and network connectivity, and a new model for quantifying the molar fractions of short-range order units is proposed. A consistent thermal history dependence for the fraction of tetrahedral boron (N₄) is well accounted for by the proposed model. The model predicts density within 0.1% of experimental values and N₄ to within 1% of NMR values. The intermediate character of four-coordinated zinc in borate glasses of this series is evident by the far infrared profiles and the glass transition temperature behavior, which decreases non-monotonically with increasing ZnO content.

Many-body effects at the origin of structural transitions in B2O3

The structural properties of glassy diboron trioxide, g-B₂O₃, are investigated from ambient to high pressure conditions using two types of atomic force-field models that account for many-body effects. These models are parameterized by a dipole- and force-fitting procedure of reference datasets created via first-principles calculations on a series of configurations. The predictions of the models are tested against experimental data, where particular attention is paid to the structural transitions in g-B₂O₃ that involve changes to both the short- and medium-range order. The models outperform those previously devised, where improvement originates from the incorporation of two key physical ingredients, namely, (i) the polarizability of the oxide ion and (ii) the ability of an oxide ion to change both size and shape in response to its coordination environment. The results highlight the importance of many-body effects for accurately modeling this challenging system.

A molecular interpretation of vitreous boron oxide dynamics

The mobility of vitreous boron oxide is studied by molecular dynamics simulation. A polarization model that incorporates induced dipoles arising both from charges and from other induced dipoles on atoms with nonzero polarizability is used to simulate boron oxide glass at various temperatures above the glass transition temperature. Particle mobility is investigated through the calculation of the self-intermediate scattering function and the mean-squared displacement. The calculations clearly reveal a two-step relaxation with a plateau at intermediate times for all investigated temperatures. With respect to atomic species, boron atoms are less mobile than oxygen atoms at all temperatures within the plateau region. Through analyzing particle trajectories, it is revealed that BO(3) groups move as one unit and follow each other in a stringlike manner. Three connected BO(3) groups comprise a six-membered boroxol ring, which is shown to move in a collective manner, requiring the simultaneous movement of all ring atoms. The boroxol ring is observed to be confined, or caged, during the plateau region, and jumps to a new location at longer times. This observation is linked to the concept of strong versus fragile glass formers and the potential energy landscape. In addition to the caging feature, an overshoot or dip occurs in the plateau regions of the mean-squared displacement and self-intermediate scattering functions respectively. These features are followed by a ringing pattern, previously associated with finite size effects in other strong glass formers, which persist for the duration of the plateau region. Both features are shown to be consistent with the bending of atomic "cages" from the plane of the boroxol ring, and arise due to the displacement of atoms from local minimum energy configurations.