Phonon-assisted diffusion in bcc phase of titanium and zirconium from first-principles

Diffusion is the underlying mechanism for many complicated materials phenomena, and understanding it is basic to the discovery of novel materials with desired physical and mechanical properties. Certain groups of solid phases, such as the bcc phase of IIIB and IVB metals and their alloys, which are only stable when they reach high enough temperatures and experience anharmonic vibration entropic effects, exhibit "anomalously fast diffusion". However, the underlying reason for the observed extraordinary fast diffusion is poorly understood and due to the existence of harmonic vibration instabilities in these phases the standard models fail to predict their diffusivity. Here, we indicate that the anharmonic phonon-phonon coupling effects can accurately describe the anomalously large macroscopic diffusion coefficients in the bcc phase of IVB metals, and therefore yield a new understanding of the underlying mechanism for diffusion in these phases. We utilize temperature-dependent phonon analysis by combining ab initio molecular dynamics with lattice dynamics calculations to provide a new approach to use the transition state theory beyond the harmonic approximation. We validate the diffusivity predictions for the bcc phase of titanium and zirconium with available experimental measurements, while we show that predictions based on harmonic transition state theory severely underestimates diffusivity in these phases.Comment: 11 pages, 5 figures, supplemental material of 18 pages with 11 figure

Deep Learning for Predicting the Formation Energy and Synthesizability of Crystalline Materials

Predicting the stability and properties of hypothetical crystal structures is critical to accelerating materials discovery. This work introduces a neural network model that harnesses deep convolutional neural networks (CNNs) to analyze crystal voxel representations (CVRs) and predict a crystal's synthesizability and formation energy. CVRs represent crystals as 3D images, allowing CNNs to extract intricate structural motifs and make detailed predictions. Advanced CNN architectures, such as those with skip-connections, further enhance the model's capabilities. Moreover, rotational data augmentation enables the CNN to overcome limitations in handling symmetries and helps in the generalization of the predictions. The model achieves high performance, proficiently differentiating synthesizable from anomalous crystals and achieving a 0.046 eV/atom error for formation energy, on par with the state-of-the-art machine-learned predictive models for formation energy. Overall, this work demonstrates image representation learning for materials in computational materials science, where leading-edge CNNs analyze CVRs to enable precise crystal structure predictions and accelerate the discovery of novel materials

Understanding the role of anharmonic phonons in diffusion of bcc metal

Diffusion in high-temperature bcc phase of IIIB-IVB metals such as Zr, Ti, and their alloys is observed to be orders of magnitude higher than bcc metals of group VB-VIB, including Cr, Mo, and W. The underlying reason for this higher diffusion is still poorly understood. To explain this observation, we compare the first-principles-calculated parameters of monovancy-mediated diffusion between bcc Ti, Zr, and dilute Zr- Sn alloys and bcc Cr, Mo, and W. Our results indicate that strongly anharmonic vibrations promote both the vacancy concentration and the diffusive jump rate in bcc IVB metals and can explain their markedly faster diffusion compared to bcc VIB metals. Additionally, we provide an efficient approach to calculate diffusive jump rates according to the transition state theory (TST). The use of standard harmonic TST is impractical in bcc IIIB/IVB metals due to the existence of ill-defined harmonic phonons, and most studies use classical or ab initio molecular dynamics for direct simulation of diffusive jumps. Here, instead, we use a stochastically-sampled temperature-dependent phonon analysis within the transition state theory to study diffusive jumps without the need of direct molecular dynamics simulations. We validate our first-principles diffusion coefficient predictions with available experimental measurements and explain the underlying reasons for the promotion of diffusion in bcc IVB metals/alloys compared to bcc VIB metals.Comment: 8 figures, 1 table, 5 supplementary figures, 1 supplementary table, 1 supplementary not