The Structure of Liquid and Amorphous Hafnia.

Understanding the atomic structure of amorphous solids is important in predicting and tuning their macroscopic behavior. Here, we use a combination of high-energy X-ray diffraction, neutron diffraction, and molecular dynamics simulations to benchmark the atomic interactions in the high temperature stable liquid and low-density amorphous solid states of hafnia. The diffraction results reveal an average Hf-O coordination number of ~7 exists in both the liquid and amorphous nanoparticle forms studied. The measured pair distribution functions are compared to those generated from several simulation models in the literature. We have also performed ab initio and classical molecular dynamics simulations that show density has a strong effect on the polyhedral connectivity. The liquid shows a broad distribution of Hf-Hf interactions, while the formation of low-density amorphous nanoclusters can reproduce the sharp split peak in the Hf-Hf partial pair distribution function observed in experiment. The agglomeration of amorphous nanoparticles condensed from the gas phase is associated with the formation of both edge-sharing and corner-sharing HfO6,7 polyhedra resembling that observed in the monoclinic phase.

Network topology for the formation of solvated electrons in binary CaO-Al2O3 composition glasses.

Glass formation in the CaO-Al2O3 system represents an important phenomenon because it does not contain typical network-forming cations. We have produced structural models of CaO-Al2O3 glasses using combined density functional theory-reverse Monte Carlo simulations and obtained structures that reproduce experiments (X-ray and neutron diffraction, extended X-ray absorption fine structure) and result in cohesive energies close to the crystalline ground states. The O-Ca and O-Al coordination numbers are similar in the eutectic 64 mol % CaO (64CaO) glass [comparable to 12CaO·7Al2O3 (C12A7)], and the glass structure comprises a topologically disordered cage network with large-sized rings. This topologically disordered network is the signature of the high glass-forming ability of 64CaO glass and high viscosity in the melt. Analysis of the electronic structure reveals that the atomic charges for Al are comparable to those for Ca, and the bond strength of Al-O is stronger than that of Ca-O, indicating that oxygen is more weakly bound by cations in CaO-rich glass. The analysis shows that the lowest unoccupied molecular orbitals occurs in cavity sites, suggesting that the C12A7 electride glass [Kim SW, Shimoyama T, Hosono H (2011) Science 333(6038):71-74] synthesized from a strongly reduced high-temperature melt can host solvated electrons and bipolarons. Calculations of 64CaO glass structures with few subtracted oxygen atoms (additional electrons) confirm this observation. The comparable atomic charges and coordination of the cations promote more efficient elemental mixing, and this is the origin of the extended cage structure and hosted solvated (trapped) electrons in the C12A7 glass.

A molecular dynamics simulation interpretation of neutron and x-ray diffraction measurements on single phase Y(2)O(3)-Al(2)O(3) glasses.

Molecular dynamics simulations and complementary neutron and x-ray diffraction studies have been carried out within the single phase glass forming range of (Y(2)O(3))(x)(Al(2)O(3))((100-x)), for x = 27 and 30. For x = 27, the experimental Al-O and Y-O coordination numbers are found to be 4.9 ± 0.2 and 6.9 ± 0.4 respectively, compared to 4.4 and 6.8 obtained from the simulation. Similar results were found for x = 30. An R-factor analysis showed that the simulation models agreed to within ∼6% of the diffraction data in both cases. The Al-O polyhedra are dominated by fourfold and fivefold species and the Y-O local coordinations are dominated by sixfold, sevenfold and eightfold polyhedra. Analysis of the oxygen environments reveals a large number of combinations, which explains the high entropy of single phase yttrium aluminate glasses and melts. Of these, the largest variation between x = 27 and 30 is found in the number of aluminum oxygen triclusters (oxygens bonded to three Al) and oxygens surrounded by three Y and a single Al. The most abundant connections are between the AlO(x) and YO(y) polyhedra of which 30% are edge shared. The majority of AlO(x)-AlO(x) connections were found to be corner shared.