3 research outputs found
ShakeNBreak: Navigating the defect configurational landscape
Point defects are present in all crystalline solids, controlling the properties and performance of
most functional materials, including thermoelectrics, photovoltaics and catalysts. However, the
standard modelling approach, based on local optimisation of a defect placed on a known crystal
site, can miss the true ground state structure. This structure may lie within a local minimum
of the potential energy surface (PES), trapping a gradient-based optimisation algorithm in a
metastable arrangement and thus yielding incorrect defect structures that compromise predicted
properties (Mosquera-Lois & Kavanagh, 2021). As such, an efficient way to explore the defect
energy landscape and identify low-energy structures is required
Identifying the ground state structures of point defects in solids
Point defects are a universal feature of crystalline materials. Their
identification is often addressed by combining experimental measurements with
theoretical models. The standard approach of simulating defects is, however,
prone to missing the ground state atomic configurations associated with
energy-lowering reconstructions from the idealised crystallographic
environment. Missed ground states compromise the accuracy of calculated
properties. To address this issue, we report an approach to efficiently
navigate the defect configurational landscape using targeted bond distortions
and rattling. Application of our workflow to a range of materials (,
, , , , , , anatase-) reveals symmetry breaking in each host crystal that
is not found via conventional local minimisation techniques. The point defect
distortions are classified by the associated physico-chemical factors. We
demonstrate the impact of these defect distortions on derived properties,
including formation energies, concentrations and charge transition levels. Our
work presents a step forward for quantitative modelling of imperfect solids
Imperfections are not 0 K: free energy of point defects in crystals
Defects determine many important properties and applications of materials,
ranging from doping in semiconductors, to conductivity in mixed
ionic-electronic conductors used in batteries, to active sites in catalysts.
The theoretical description of defect formation in crystals has evolved
substantially over the past century. Advances in supercomputing hardware, and
the integration of new computational techniques such as machine learning,
provide an opportunity to model longer length and time-scales than previously
possible. In this Tutorial Review, we cover the description of free energies
for defect formation at finite temperatures, including configurational
(structural, electronic, spin) and vibrational terms. We discuss challenges in
accounting for metastable defect configurations, progress such as machine
learning force fields and thermodynamic integration to directly access entropic
contributions, and bottlenecks in going beyond the dilute limit of defect
formation. Such developments are necessary to support a new era of accurate
defect predictions in computational materials chemistry