6 research outputs found
Structurally Constrained Evolutionary Algorithm for the Discovery and Design of Metastable Phases
Metastable materials are abundant in nature and technology, showcasing
remarkable properties that inspire innovative materials design. However,
traditional crystal structure prediction methods, which rely solely on
energetic factors to determine a structure's fitness, are not suitable for
predicting the vast number of potentially synthesizable phases that represent a
local minimum corresponding to a state in thermodynamic equilibrium. Here, we
present a new approach for the prediction of metastable phases with specific
structural features, and interface this method with the XtalOpt evolutionary
algorithm. Our method relies on structural features that include the local
crystalline order (e.g., the coordination number or chemical environment), and
symmetry (e.g., Bravais lattice and space group) to filter the parent pool of
an evolutionary crystal structure search. The effectiveness of this approach is
benchmarked on three known metastable systems: XeN, with a two-dimensional
polymeric nitrogen sublattice, brookite TiO, and a high pressure BaH
phase that was recently characterized. Additionally, a newly predicted
metastable melaminate salt, -1 WCN, was found to possess an
energy that is lower than two phases proposed in a recent computational study.
The method presented here could help in identifying the structures of compounds
that have already been synthesized, and developing new synthesis targets with
desired properties
Structure, Stability and Superconductivity of N-doped Lutetium Hydrides at kbar Pressures
The structure of the material responsible for the room temperature and near
ambient pressure superconductivity reported in an N-doped lutetium hydride
[Nature, 615, 244 (2023)] has not been conclusively determined. Herein, density
functional theory calculations are performed in an attempt to uncover what it
might be. Guided by a range of strategies including crystal structure
prediction and modifications of existing structure types, we present an array
of Lu-N-H phases that are dynamically stable at experimentally relevant
pressures. Although none of the structures found are thermodynamically stable,
and none are expected to remain superconducting above 17 K at 10 kbar, a number
of metallic compounds with fcc Lu lattices -- as suggested by the experimental
X-ray diffraction measurements of the majority phase -- are identified. The
system whose calculated equation of states matches best with that measured for
the majority phase is fluorite-type LuH2, whose 10 kbar superconducting
critical temperature was estimated to be 0.09 K using the Allen-Dynes modified
McMillan equation.Comment: 11 pages, 8 figure
Structurally Constrained Evolutionary Algorithm for the Discovery and Design of Metastable Phases
Metastable materials are abundant in nature and technology, showcasing remarkable properties that inspire innovative materials design. However, traditional crystal structure prediction methods, which rely solely on energetic factors to determine a structure's fitness, are not suitable for predicting the vast number of potentially synthesizable phases that represent a local minimum corresponding to a state in thermodynamic equilibrium. Here, we present a new approach for the prediction of metastable phases with specific structural features, and interface this method with the XTALOPT evolutionary algorithm. Our method relies on structural features that include the local crystalline order (e.g., the coordination number or chemical environment), and symmetry (e.g., Bravais lattice and space group) to filter the parent pool of an evolutionary crystal structure search. The effectiveness of this approach is benchmarked on three known metastable systems: XeN8 , with a two-dimensional polymeric nitrogen sublattice, brookite TiO2 , and a high pressure BaH 4 phase that was recently characterized. Additionally, a newly predicted metastable melaminate salt, P-1 WC3N6 , was found to possess an energy that is lower than two phases proposed in a recent computational study. The method presented here could help in identifying the structures of compounds that have already been synthesized, and developing new synthesis targets with desired properties
Substitution Patterns Understood through Chemical Pressure Analysis: Atom/Dumbbell and Ru/Co Ordering in Derivatives of YCo<sub>5</sub>
Interstitials,
mixed occupancy, and partial substitution of one
geometrical motif for another are frequently encountered in the structure
refinements of intermetallic compounds as disorder or the formation
of superstructures. In this article, we illustrate how such phenomena
can serve as mechanisms for chemical pressure (CP) release in variants
of the CaCu<sub>5</sub> type. We begin by comparing the density functional
theory CP schemes of YCo<sub>5</sub>, an f-element free analogue of
the permanent magnet SmCo<sub>5</sub>, and its superstructure variant
Y<sub>2</sub>Co<sub>17</sub> = [Y<sub>2</sub>(Co<sub>2</sub>)<sub>1</sub>]ÂCo<sub>15</sub> (Th<sub>2</sub>Zn<sub>17</sub>-type) in which
one-third of the Y atoms are replaced by Co<sub>2</sub> dumbbells.
The CP scheme of the original YCo<sub>5</sub> structure reveals intensely
anisotropic pressures acting on the Y atoms (similar to CP schemes
of other CaCu<sub>5</sub>-type phases). The Y atoms experience large
negative pressures along the length of the hexagonal channels they
occupy while being simultaneously squeezed by the channel walls. Moving
to the Y<sub>2</sub>Co<sub>17</sub> structure provides significant
relief to this CP scheme: the inserted Co<sub>2</sub> pairs densify
the atomic packing along the hexagonal channels while providing space
for the bulging of the walls to better accommodate the remaining Y
atoms. This Y/Co<sub>2</sub> substitution pattern thus yields a much
smoother CP scheme, but residual pressures remain. The experimental
relevance of these remaining stresses is investigated through a structural
refinement of a Ru-substituted variant of Y<sub>2</sub>Co<sub>17</sub> using single crystal X-ray diffraction. A comparison of the Y<sub>2</sub>Co<sub>17</sub> CP scheme with the observed Ru/Co ordering
reveals that Ru preferentially substitutes for Co atoms whose net
CPs are most negative, in accord with the larger size of the Ru atoms.
These results hint that a wider variety of elemental site preferences
may be understandable from the viewpoint of CP relief
Twists and Puckers: Tuning Crystal Chemistry in the La(Au<sub><i>x</i></sub>Ge<sub>1–<i>x</i></sub>)<sub>2</sub> Compositional Series
The
physical properties of solid-state materials are closely tied
to their crystal structure, yet our understanding of how competing
structural arrangements energetically compare is limited. In this
work, we explore how small differences in composition affect the structure
in the La(AuxGe1–x)2 series of compounds, which comprises four unique
structure types between LaGe2 and LaAu2. This
family includes the previously unknown AlB2-type compound
with stoichiometry La(Au0.375Ge0.625)2 as well as La(Au0.25Ge0.75)2, an
intergrowth of the AlB2 and ThSi2 structure
types. We then study the chemical forces driving the structure changes
and use phonon band structure calculations and DFT-Chemical Pressure
to evaluate atomic-size effects. These calculations show that the
parent AlB2 structure type is disfavored in Au-rich compounds
due to soft atomic motions along the c axis. The
instability of AlB2-type LaAuGe is confirmed by the presence
of imaginary modes in the phonon band structure that correspond to
a “puckering” of the hexagonal AlB2-type
lattice, resulting in the experimentally observed LiGaGe structure
type. The impact of size effects is less clear for Au-poor compositions;
instead, twisting the AlB2 structure type to form the ThSi2 type opens a pseudogap at the Fermi level in the electronic
density of states. This investigation demonstrates how crystal structure
in solid-state materials can be compositionally tuned based on balancing
size and electronics when multiple structure types are in close thermodynamic
competition