196 research outputs found
Atomic motion in solids with dimpled potentials
Polymorphic solids of the same chemical composition can have different atomic
structures; in each polymorph atoms vibrate around a local potential energy
minimum (LPEM). If transformations to other structures have sufficiently high
enthalpy barriers, then each polymorph is either stable or metastable; it is
stationary and does not spontaneously change with time. But what happens, if
those barriers are low? As examples, we consider NiTi shape memory alloy
exhibiting a large elastocaloric effect, and selected elemental solids. We
suggest a model for dynamically polymorphic solids, where multiple LPEMs are
visited by ergodic motion of a single atom. We predict that upon cooling a
dynamically polymorphic phase should undergo a symmetry-breaking first-order
phase transition, accompanied by a finite change of the lattice entropy. We
discuss 3 methods used to calculate phonons in solids with non-harmonic dimpled
atomic potentials, and compare theoretical predictions to experiment.Comment: 11 pages, 12 figure
Coexistence pressure for a martensitic transformation from theory and experiment: revisiting the bcc-hcp transition of iron under pressure
The coexistence pressure of two phases is a well-defined point at fixed
temperature. In experiment, however, due to non-hydrostatic stresses and a
stress-dependent potential energy barrier, different measurements yield
different ranges of pressure with a hysteresis. Accounting for these effects,
we propose an inequality for comparison of the theoretical value to a plurality
of measured intervals. We revisit decades of pressure experiments on the bcc -
hcp transformations in iron, which are sensitive to non-hydrostatic conditions
and sample size. From electronic-structure calculations, we find a bcc - hcp
coexistence pressure of 8.4 GPa. We construct the equation of state for
competing phases under hydrostatic pressure, compare to experiments and other
calculations, and address the observed pressure hysteresis and range of onset
pressures of the nucleating phase.Comment: 8 pages, 1 figure, 202 citation
NiTi shape-memory transformations: minimum-energy pathways between austenite, martensites, and kinetically-limited intermediate states
NiTi is the most used shape-memory alloy, nonetheless, a lack of
understanding remains regarding the associated structures and transitions,
including their barriers. Using a generalized solid-state nudge elastic band
(GSSNEB) method implemented via density-functional theory, we detail the
structural transformations in NiTi relevant to shape memory: those between
body-centered orthorhombic (BCO) groundstate and a newly identified stable
austenite ("glassy" B2-like) structure, including energy barriers (hysteresis)
and intermediate structures (observed as a kinetically limited R-phase), and
between martensite variants (BCO orientations). All results are in good
agreement with available experiment. We contrast the austenite results to those
from the often-assumed, but unstable B2. These high- and low-temperature
structures and structural transformations provide much needed atomic-scale
detail for transitions responsible for NiTi shape-memory effects.Comment: 4 pages, 4 figure
Nudged-elastic band method with two climbing images: finding transition states in complex energy landscapes
The nudged-elastic band (NEB) method is modified with concomitant two
climbing images (C2-NEB) to find a transition state (TS) in complex energy
landscapes, such as those with serpentine minimal energy path (MEP). If a
single climbing image (C1-NEB) successfully finds the TS, C2-NEB finds it with
higher stability and accuracy. However, C2-NEB is suitable for more complex
cases, where C1-NEB misses the TS because the MEP and NEB directions near the
saddle point are different. Generally, C2-NEB not only finds the TS but
guarantees that the climbing images approach it from the opposite sides along
the MEP, and it estimates accuracy from the three images: the highest-energy
one and its climbing neighbors. C2-NEB is suitable for fixed-cell NEB and the
generalized solid-state NEB (SS-NEB).Comment: 3 pages, 4 figure
Anomalous magneto-structural behavior of MnBi explained: a path towards an improved permanent magnet
Low-temperature MnBi (hexagonal NiAs phase) exhibits anomalies in the lattice
constants (a, c) and bulk elastic modulus (B) below 100 K, spin reorientation
and magnetic susceptibility maximum near 90 K, and, importantly for
high-temperature magnetic applications, an increasing coercivity (unique to
MnBi) above 180 K. We calculate the total energy and magneto-anisotropy energy
(MAE) versus (a, c) using DFT+U methods. We reproduce and explain all the above
anomalies. We predict that coercivity and MAE increase due to increasing a,
suggesting means to improve MnBi permanent magnets.Comment: 4 pages, 5 figure
Manipulating the stability of crystallographic and magnetic sub-lattices: A first-order magnetoelastic transformation in transition metal based Laves phase
A first-order magnetoelastic transition (FOMT) is found near the triple point between ferromagnetic, antiferromagnetic and paramagnetic phases in the magneto-chemical phase diagram of (Hf1-xNbx)Fe2 Laves phase system. We show that bringing different magnetic states to the edge of stability, both as a function of the chemical composition and under the influence of external stimuli, such as temperature, pressure and magnetic field, is essential to obtain and control FOMTs. Temperature dependent X-ray diffraction experiments reveal a discontinuity in the lattice parameter a and the unit cell volume without the change in the crystal symmetry at the FOMT. Under applied pressure, the transition temperature drastically shifts downward at a remarkable rate of −122 K/GPa. It is this first-order magnetic transition that leads to a negative thermal expansion (NTE) with average ΔV/(VΔT) ≈ −15 × 10−6 K−1 observed over a 90 K broad temperature range, which is uncommon for magnetoelastic NTE materials. Density functional theory calculations and microstructural analyses demonstrate that the unusual broadness of the FOMT originates from phase separation between ferro- and antiferromagnetic phases, which in turn is rooted in partial segregation of Hf and Nb and a peculiar microstructure. This new understanding of the composition-structure-property relationships in transition metal based Laves phases is an essential step toward a better control and more precise tailoring of rich functionalities in this group of material
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