40 research outputs found
Strain controlled oxygen vacancy formation and ordering in CaMnO
We use first-principles calculations to investigate the stability of
bi-axially strained \textit{Pnma} perovskite CaMnO towards the formation of
oxygen vacancies. Our motivation is provided by promising indications that
novel material properties can be engineered by application of strain through
coherent heteroepitaxy in thin films. While it is usually assumed that such
epitaxial strain is accommodated primarily by changes in intrinsic lattice
constants, point defect formation is also a likely strain relaxation mechanism.
This is particularly true at the large strain magnitudes (4%) which
first-principles calculations often suggest are required to induce new
functionalities. We find a strong dependence of oxygen vacancy defect formation
energy on strain, with tensile strain lowering the formation energy consistent
with the increasing molar volume with increasing oxygen deficiency. In
addition, we find that strain differentiates the formation energy for different
lattice sites, suggesting its use as a route to engineering vacancy ordering in
epitaxial thin films.Comment: 7 pages, 7 figure
Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites
The improper ferroelectricity in YMnO and other related multiferroic
hexagonal manganites are known to cause topologically protected ferroelectric
domains that give rise to rich and diverse physical phenomena. The local
structure and structural coherence across the ferroelectric transition,
however, were previously not well understood. Here we reveal the evolution of
the local structure with temperature in YMnO using neutron total scattering
techniques, and interpret them with the help of first-principles calculations.
The results show that, at room temperature, the local and average structures
are consistent with the established ferroelectric symmetry. On
heating, both local and average structural analyses show striking anomalies
from K up to the Curie temperature consistent with increasing
fluctuations of the order parameter angle. These fluctuations result in an
unusual local symmetry lowering into a \textit{continuum of structures} on
heating. This local symmetry breaking persists into the high-symmetry non-polar
phase, constituting an unconventional type of order-disorder transition.Comment: 10 pages, 5 figure
Charged domain walls in improper ferroelectric hexagonal manganites and gallates
Ferroelectric domain walls are attracting broad attention as atomic-scale
switches, diodes and mobile wires for next-generation nanoelectronics. Charged
domain walls in improper ferroelectrics are particularly interesting as they
offer multifunctional properties and an inherent stability not found in proper
ferroelectrics. Here we study the energetics and structure of charged walls in
improper ferroelectric YMnO, InMnO and YGaO by first principles
calculations and phenomenological modeling. Positively and negatively charged
walls are asymmetric in terms of local structure and width, reflecting that
polarization is not the driving force for domain formation. The wall width
scales with the amplitude of the primary structural order parameter and the
coupling strength to the polarization. We introduce general rules for how to
engineer - and -type domain wall conductivity based on the domain size,
polarization and electronic band gap. This opens the possibility of fine-tuning
the local transport properties and design --junctions for domain
wall-based nano-circuitry.Comment: 10 pages, 6 figures, Supp. Info. available on reques
Coupling and competition between ferroelectricity, magnetism, strain, and oxygen vacancies in AMnO₃ perovskites
We use first-principles calculations based on density functional theory to investigate the interplay between oxygen vacancies, A-site cation size/tolerance factor, epitaxial strain, ferroelectricity, and magnetism in the perovskite manganite series, AMnO3 (A = Ca2+, Sr2+, Ba2+). We find that, as expected, increasing the volume through either chemical pressure or tensile strain generally lowers the formation energy of neutral oxygen vacancies consistent with their established tendency to expand the lattice. Increased volume also favors polar distortions, both because competing rotations of the oxygen octahedra are suppressed and because Coulomb repulsion associated with cation off-centering is reduced. Interestingly, the presence of ferroelectric polarization favors ferromagnetic (FM) over antiferromagnetic (AFM) ordering due to suppressed AFM superexchange as the polar distortion bends the Mn–O–Mn bond angles away from the optimal 180°. Intriguingly, we find that polar distortions compete with the formation of oxygen vacancies, which have a higher formation energy in the polar phases; conversely the presence of oxygen vacancies suppresses the onset of polarization. In contrast, oxygen vacancy formation energies are lower for FM than AFM orderings of the same structure type. Our findings suggest a rich and complex phase diagram, in which defect chemistry, polarization, structure, and magnetism can be modified using chemical potential, stress or pressure, and electric or magnetic fields
Tuning the Thermoelectric Performance of CaMnO3-Based Ceramics by Controlled Exsolution and Microstructuring
The thermoelectric properties of CaMnO3-δ/CaMn2O4 composites were tuned via microstructuring and compositional adjustment. Single-phase rock-salt-structured CaO-MnO materials with Ca:Mn ratios larger than unity were produced in reducing atmosphere and subsequently densified by spark plasma sintering in vacuum. Annealing in air at 1340 °C between 1 and 24 h activated redox-driven exsolution and resulted in a variation in microstructure and CaMnO3-δ materials with 10 and 15 vol % CaMn2O4, respectively. The nature of the CaMnO3-δ/CaMn2O4 grain boundary was analyzed by transmission electron microscopy on short- and long-term annealed samples, and a sharp interface with no secondary phase formation was indicated in both cases. This was further complemented by density functional theory (DFT) calculations, which confirmed that the CaMnO3-δ indeed is a line compound. DFT calculations predict segregation of oxygen vacancies from the bulk of CaMnO3-δ to the interface between CaMnO3-δ and CaMn2O4, resulting in an enhanced electronic conductivity of the CaMnO3-δ phase. Samples with 15 vol % CaMn2O4 annealed for 24 h reached the highest electrical conductivity of 73 S·cm-1 at 900 °C. The lowest thermal conductivity was obtained for composites with 10 vol % CaMn2O4 annealed for 8 h, reaching 0.56 W·m-1K-1 at 700 °C. However, the highest thermoelectric figure-of-merit, zT, was obtained for samples with 15 vol % CaMn2O4 reaching 0.11 at temperatures between 800 and 900 °C, due to the enhanced power factor above 700 °C. This work represents an approach to boost the thermoelectric performance of CaMnO3-δ based composites
Observation of Antiferroelectric Domain Walls in a Uniaxial Hyperferroelectric
Ferroelectric domain walls are a rich source of emergent electronic
properties and unusual polar order. Recent studies showed that the
configuration of ferroelectric walls can go well beyond the conventional
Ising-type structure. N\'eel-, Bloch-, and vortex-like polar patterns have been
observed, displaying strong similarities with the spin textures at magnetic
domain walls. Here, we report the discovery of antiferroelectric domain walls
in the uniaxial ferroelectric PbGeO. We resolve highly
mobile domain walls with an alternating displacement of Pb atoms, resulting in
a cyclic 180 flip of dipole direction within the wall. Density
functional theory calculations reveal that PbGeO is
hyperferroelectric, allowing the system to overcome the depolarization fields
that usually suppress antiparallel ordering of dipoles along the longitudinal
direction. Interestingly, the antiferroelectric walls observed under the
electron beam are energetically more costly than basic head-to-head or
tail-to-tail walls. The results suggest a new type of excited domain-wall
state, expanding previous studies on ferroelectric domain walls into the realm
of antiferroic phenomena
Electrical half-wave rectification at ferroelectric domain walls
Ferroelectric domain walls represent multifunctional 2D-elements with great
potential for novel device paradigms at the nanoscale. Improper ferroelectrics
display particularly promising types of domain walls, which, due to their
unique robustness, are the ideal template for imposing specific electronic
behavior. Chemical doping, for instance, induces p- or n-type characteristics
and electric fields reversibly switch between resistive and conductive
domain-wall states. Here, we demonstrate diode-like conversion of
alternating-current (AC) into direct-current (DC) output based on neutral
180 domain walls in improper ferroelectric ErMnO. By combining
scanning probe and dielectric spectroscopy, we show that the rectification
occurs for frequencies at which the domain walls are fixed to their equilibrium
position. The practical frequency regime and magnitude of the output is
controlled by the bulk conductivity. Using density functional theory we
attribute the transport behavior at the neutral walls to an accumulation of
oxygen defects. Our study reveals domain walls acting as 2D half-wave
rectifiers, extending domain-wall-based nanoelectronic applications into the
realm of AC technology