Machine learning the screening factor in the soft bond valence approach for rapid crystal structure estimation

The development of novel functional ceramics is critically important for several applications, including the design of better electrochemical batteries and fuel cells, in particular solid oxide fuel cells. Computational prescreening and selection of such materials can help discover novel materials but is also challenging due to the high cost of electronic structure calculations which would be needed to compute the structures and properties of interest such as the material's stability and ion diffusion properties. The soft bond valence (SoftBV) approach is attractive for rapid prescreening among multiple compositions and structures, but the simplicity of the approximation can make the results inaccurate. In this study, we explore the possibility of enhancing the accuracy of the SoftBV approach when estimating crystal structures by adapting the parameters of the approximation to the chemical composition. Specifically, on the examples of perovskite- and spinel-type oxides that have been proposed as promising solid-state ionic conductors, the screening factor - an independent parameter of the SoftBV approximation - is modeled using linear and non-linear methods as a function of descriptors of the chemical composition. We find that making the screening factor a function of composition can noticeably improve the ability of the SoftBV approximation to correctly model structures, in particular new, putative crystal structures whose structural parameters are yet unknown. We also analyze the relative importance of nonlinearity and coupling in improving the model and find that while the quality of the model is improved by including nonlinearity, coupling is relatively unimportant. While using a neural network showed practically no improvement over linear regression, the recently proposed GPR-NN method that is a hybrid between a single hidden layer neural network and kernel regression showed substantial improvement, enabling the prediction of structural parameters of new ceramics with accuracy on the order of 1%. Machine learning of the screening factor in the SoftBV approximation as a function of chemical composition was used to improve the accuracy of structure estimation with SoftBV to help rapid prescreening of ceramic materials.