The first magnetic 2D material discovered, monolayer (ML) CrI3, is particularly fascinating due to its ground state ferromagnetism. However, because ML materials are difficult to probe experimentally, much remains unresolved about ML CrI3’s structural, electronic, and magnetic properties. Here, we leverage Density Functional Theory (DFT) and high-accuracy Diffusion Monte Carlo (DMC) simulations to predict lattice parameters, magnetic moments, and spin–phonon and spin–lattice coupling of ML CrI3. We exploit a recently developed surrogate Hessian DMC line search technique to determine CrI3’s ML geometry with DMC accuracy, yielding lattice parameters in good agreement with recently published STM measurements—an accomplishment given the ∼10% variability in previous DFT-derived estimates depending upon the functional. Strikingly, we find that previous DFT predictions of ML CrI3’s magnetic spin moments are correct on average across a unit cell but miss critical local spatial fluctuations in the spin density revealed by more accurate DMC. DMC predicts that magnetic moments in ML CrI3 are 3.62 μB per chromium and −0.145 μB per iodine, both larger than previous DFT predictions. The large disparate moments together with the large spin–orbit coupling of CrI3’s I-p orbital suggest a ligand superexchange-dominated magnetic anisotropy in ML CrI3, corroborating recent observations of magnons in its 2D limit. We also find that ML CrI3 exhibits a substantial spin–phonon coupling of ∼3.32 cm−1. Our work, thus, establishes many of ML CrI3’s key properties, while also continuing to demonstrate the pivotal role that DMC can assume in the study of magnetic and other 2D materials.

Amorphous uranium oxides are known to arise via industrial processes relevant to the nuclear fuel cycle yet evade rigorous structural characterization. A promising approach is to develop statistical relationships between uranium–oxygen coordination environments and thermodynamic stability from which general statements about the likelihood of observing particular U–O arrangements can be made. The number of known crystalline uranium oxides is insufficient to build statistical relationships. We have developed a database of theoretical compounds using genetic algorithms with the density functional theory as a foundation to analyze coordination geometries in the uranium–oxygen phase space. We draw fundamental insights into the nature of uranium–oxygen interactions by correlating total energy with the coordination environment. The most stable configuration of U cations with O anions is in an environment with coordination numbers 5–8 in a cubic configuration. Higher and lower coordination numbers are observed only in metastable phases. General trends are observable in the relationships between the coordination number, density, and uranium fraction in each structure. The new insight into uranium coordination enabled by these analyses is foundational for further studies into the characteristic properties of individual uranium oxide materials and for elucidation of potential oxidation pathways for uranium metal.

Ionic diffusion coefficients are important parameters to model mass transport processes in many high temperature water systems, such as nuclear and fossil-fueled power stations and hydrothermal geochemical systems. This work is a critical assessment of the molar conductivity data for aqueous potassium chloride which is directly related to diffusion coefficients. The literature includes more than 550 experimental data sets measured from temperatures T = 273 to 1073 K, pressures p = 0.1 to 1200 MPa, and water densities ρw = 85 to 1000 kg·m–3. In some cases, the measurements were reanalyzed with modern conductivity equations to yield more accurate limiting molar conductivity data, Λ°KCl. The selected Λ°KCl data were split into single-ion conductivities (λ°) for K+ and Cl– using transference number extrapolations. Simple empirical functions of the solvent viscosity and density were derived that can reproduce the data from T = 288 to 923.15 K, p = 0.1 to 407 MPa, and ρw = 450 to 1000 kg·m–3 to less than the estimated uncertainties. A revised equation to express the temperature and density dependence of KCl ion-pair formation constant, K A, based on flow conductivity measurements (T = 491 to 873 K, p = 2.25 to 300 MPa, and ρw = 160 to 852.45 kg·m–3), is also reported. This study recommends the use of potassium chloride as a chemical standard for high temperature conductivity experiments along with the Fuoss–Hsia–Fernández–Prini (FHFP) equation and the fitted parameters for Λ°KCl and K A reported here to verify the accuracy of hydrothermal conductivity measurements.