Understanding the structure and properties of refractory oxides is critical for high temperature applications. In this work, a combined experimental and simulation approach uses an automated closed loop via an active learner, which is initialized by x-ray and neutron diffraction measurements, and sequentially improves a machine-learning model until the experimentally predetermined phase space is covered. A multiphase potential is generated for a canonical example of the archetypal refractory oxide, ${\mathrm{HfO}}_2$, by drawing a minimum number of training configurations from room temperature to the liquid state at $\sim 2900°\mathrm{C}$. The method significantly reduces model development time and human effort.

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 HfO_6,7 polyhedra resembling that observed in the monoclinic phase.

The structural, elastic, electronic, phonon and thermodynamic properties of uranium nitride (UN) have been systematically studied by density functional theory (DFT) calculations. The calculated electronic band structure shows that UN is a metallic phase. The ground state structural and elastic properties predicted by DFT agree well with experiments. The thermodynamic properties of UN are studied by quasiharmonic approximation by including both lattice vibrational and thermal electronic contributions to free energies. The calculated enthalpy, entropy, Gibbs energy and heat capacity show an excellent agreement with experimental results. The thermal electronic contribution due to 5f electrons of U is found to be critical to describe the free energy of UN due to its metallic character.