38 research outputs found
Insights into cation ordering of double perovskite oxides from machine learning and causal relations
This work investigates the origins of cation ordering of double perovskites
using first-principles theory computations combined with machine learning (ML)
and causal relations. We have considered various oxidation states of A, A', B,
and B' from the family of transition metal ions to construct a diverse
compositional space. A conventional framework employing traditional ML
classification algorithms such as Random Forest (RF) coupled with appropriate
features including geometry-driven and key structural modes leads to highly
accurate prediction (~98%) of A-site cation ordering. We have evaluated the
accuracy of ML models by entailing analyses of decision paths, assignments of
probabilistic confidence bound, and finally introducing a direct non-Gaussian
acyclic structural equation model to investigate causality. Our study suggests
that the structural modes are the most important features for classifying
layered, columnar and rock-salt ordering. For clear layered ordering, the
charge difference between the A and A' is the most important feature which in
turn depends on the B, B' charge separation. Based on the outputs from ML
models, we have designed functional forms with these features to derive energy
differences forming clear layered ordering. The trilinear coupling between
tilt, rotation, and A-site antiferroelectric displacement in Landau free-energy
expansion becomes the necessary condition behind the formation of A-site cation
ordering
AtomAI: A Deep Learning Framework for Analysis of Image and Spectroscopy Data in (Scanning) Transmission Electron Microscopy and Beyond
AtomAI is an open-source software package bridging instrument-specific Python
libraries, deep learning, and simulation tools into a single ecosystem. AtomAI
allows direct applications of the deep convolutional neural networks for atomic
and mesoscopic image segmentation converting image and spectroscopy data into
class-based local descriptors for downstream tasks such as statistical and
graph analysis. For atomically-resolved imaging data, the output is types and
positions of atomic species, with an option for subsequent refinement. AtomAI
further allows the implementation of a broad range of image and spectrum
analysis functions, including invariant variational autoencoders (VAEs). The
latter consists of VAEs with rotational and (optionally) translational
invariance for unsupervised and class-conditioned disentanglement of
categorical and continuous data representations. In addition, AtomAI provides
utilities for mapping structure-property relationships via im2spec and spec2im
type of encoder-decoder models. Finally, AtomAI allows seamless connection to
the first principles modeling with a Python interface, including molecular
dynamics and density functional theory calculations on the inferred atomic
position. While the majority of applications to date were based on atomically
resolved electron microscopy, the flexibility of AtomAI allows straightforward
extension towards the analysis of mesoscopic imaging data once the labels and
feature identification workflows are established/available. The source code and
example notebooks are available at https://github.com/pycroscopy/atomai
Describing condensed matter from atomically resolved imaging data: from structure to generative and causal models
The development of high-resolution imaging methods such as electron and
scanning probe microscopy and atomic probe tomography have provided a wealth of
information on structure and functionalities of solids. The availability of
this data in turn necessitates development of approaches to derive quantitative
physical information, much like the development of scattering methods in the
early XX century which have given one of the most powerful tools in condensed
matter physics arsenal. Here, we argue that this transition requires adapting
classical macroscopic definitions, that can in turn enable fundamentally new
opportunities in understanding physics and chemistry. For example, many
macroscopic definitions such as symmetry can be introduced locally only in a
Bayesian sense, balancing the prior knowledge of materials' physics and
experimental data to yield posterior probability distributions. At the same
time, a wealth of local data allows fundamentally new approaches for the
description of solids based on construction of statistical and physical
generative models, akin to Ginzburg-Landau thermodynamic models. Finally, we
note that availability of observational data opens pathways towards exploring
causal mechanisms underpinning solid structure and functionality
Bridging microscopy with molecular dynamics and quantum simulations: An AtomAI based pipeline
Recent advances in (scanning) transmission electron microscopy have enabled
routine generation of large volumes of high-veracity structural data on 2D and
3D materials, naturally offering the challenge of using these as starting
inputs for atomistic simulations. In this fashion, theory will address
experimentally emerging structures, as opposed to the full range of
theoretically possible atomic configurations. However, this challenge is highly
non-trivial due to the extreme disparity between intrinsic time scales
accessible to modern simulations and microscopy, as well as latencies of
microscopy and simulations per se. Addressing this issue requires as a first
step bridging the instrumental data flow and physics-based simulation
environment, to enable the selection of regions of interest and exploring them
using physical simulations. Here we report the development of the machine
learning workflow that directly bridges the instrument data stream into
Python-based molecular dynamics and density functional theory environments
using pre-trained neural networks to convert imaging data to physical
descriptors. The pathways to ensure the structural stability and compensate for
the observational biases universally present in the data are identified in the
workflow. This approach is used for a graphene system to reconstruct optimized
geometry and simulate temperature-dependent dynamics including adsorption of Cr
as an ad-atom and graphene healing effects. However, it is universal and can be
used for other material systems
Anomalous Polarization Reversal in Strained Thin Films of CuInPS
Strain-induced transitions of polarization reversal in thin films of a
ferrielectric CuInPS (CIPS) with ideally-conductive electrodes is
explored using the Landau-Ginzburg-Devonshire (LGD) approach with an
eighth-order free energy expansion in polarization powers. Due to multiple
potential wells, the height and position of which are temperature- and
strain-dependent, the energy profiles of CIPS can flatten in the vicinity of
the non-zero polarization states. This behavior differentiates these materials
from classical ferroelectrics with the first or second order
ferroelectric-paraelectric phase transition, for which potential energy
profiles can be shallow or flat near the transition point only, corresponding
to zero spontaneous polarization. Thereby we reveal an unusually strong effect
of the mismatch strain on the out-of-plane polarization reversal, hysteresis
loops shape, dielectric susceptibility, and piezoelectric response of CIPS
films. In particular, by varying the sign of the mismatch strain and its
magnitude in a narrow range, quasi-static hysteresis-less paraelectric curves
can transform into double, triple, and other types of pinched and single
hysteresis loops. The strain effect on the polarization reversal is opposite,
i.e., "anomalous", in comparison with many other ferroelectric films in that
the out-of-plane remanent polarization and coercive field increases strongly
for tensile strains, meanwhile the polarization decreases or vanish for
compressive strains. We explain the effect by "inverted" signs of linear and
nonlinear electrostriction coupling coefficients of CIPS and their strong
temperature dependence. For definite values of temperature and mismatch strain,
the low-frequency hysteresis loops of polarization may exhibit negative slope
in the relatively narrow range of external field amplitude and frequency.Comment: 26 pages, including 8 figures and 1 Appendi
Direct Fabrication of Atomically Defined Pores in MXenes
Controlled fabrication of nanopores in atomically thin two-dimensional
material offers the means to create robust membranes needed for ion transport,
nanofiltration, and DNA sensing. Techniques for creating nanopores have relied
upon either plasma etching or direct irradiation using electrons or ions;
however, aberration-corrected scanning transmission electron microscopy (STEM)
offers the advantage of combining a highly energetic, sub-angstrom sized
electron beam for atomic manipulation along with atomic resolution imaging.
Here, we utilize a method for automated nanopore fabrication with real-time
atomic visualization to enhance our mechanistic understanding of beam-induced
transformations. Additionally, an electron beam simulation technique,
Electron-Beam Simulator (E-BeamSim) was developed to observe the atomic
movements and interactions resulting from electron beam irradiation. Using the
2D MXene Ti3C2Tx, we explore the influence of temperature on nanopore
fabrication by tracking atomic transformation pathways and find that at room
temperature, electron beam irradiation induces random displacement of atoms and
results in a pileup of titanium atoms at the nanopore edge. This pileup was
confirmed and demonstrated in E-BeamSim simulations around the small, milled
area in the MXene monolayer. At elevated temperatures, the surface functional
groups on MXene are effectively removed, and the mobility of atoms increases,
which results in atomic transformations that lead to the selective removal of
atoms layer by layer. Through controllable manufacture using e-beam milling
fabrication, the production and then characterization of the fabricated defects
can be better understood for future work. This work can lead to the development
of defect engineering techniques within functionalized MXene layers.Comment: Experimental and simulations on the electron beam interactions with
MXene monolayers to form nanopores as a function of temperatur
Bending-induced isostructural transitions in ultrathin layers of van der Waals ferrielectrics
Using Landau-Ginzburg-Devonshire (LGD) phenomenological approach we analyze
the bending-induced re-distribution of electric polarization and field, elastic
stresses and strains inside ultrathin layers of van der Waals ferrielectrics.
We consider a CuInP2S6 (CIPS) thin layer with fixed edges and suspended central
part, the bending of which is induced by external forces. The unique aspect of
CIPS is the existence of two ferrielectric states, FI1 and FI2, corresponding
to big and small polarization values, which arise due to the specific four-well
potential of the eighth-order LGD functional. When the CIPS layer is flat, the
single-domain FI1 state is stable in the central part of the layer, and the FI2
states are stable near the fixed edges. With an increase of the layer bending
below the critical value, the sizes of the FI2 states near the fixed edges
decreases, and the size of the FI1 region increases. When the bending exceeds
the critical value, the edge FI2 states disappear being substituted by the FI1
state, but they appear abruptly near the inflection regions and expand as the
bending increases. The bending-induced isostructural FI1-FI2 transition is
specific for the bended van der Waals ferrielectrics described by the eighth
(or higher) order LGD functional with consideration of linear and nonlinear
electrostriction couplings. The isostructural transition, which is revealed in
the vicinity of room temperature, can significantly reduce the coercive voltage
of ferroelectric polarization reversal in CIPS nanoflakes, allowing for the
curvature-engineering control of various flexible nanodevices.Comment: 26 pages, 7 figures and Appendices A-