436 research outputs found
The Effect of Symmetry Lowering on the Dielectric Response of
We use first-principles density functional theory calculations to investigate
the dielectric response of BaZrO perovskite. A previous study [Arkbarzadeh
{\em et al.} Phys. Rev. B {\bf 72}, 205104 (2005)] reported a disagreement
between experimental and theoretical low temperature dielectric constant
for the high symmetry BaZrO structure. We show that a fully
relaxed 40-atom BaZrO structure exhibits O octahedral tilting, and
that agrees with experiment. The change in from
high-symmetry to low-symmetry structure is due to increased phonon frequencies
as well as decreased mode effective charges.Comment: 4 pages, 2 figure
Structure and elastic properties of Mg(OH) from density functional theory
The structure, lattice dynamics and mechanical properties of the magnesium
hydroxide have been investigated with static density functional theory
calculations as well as \it {ab initio} molecular dynamics. The hypothesis of a
superstructure existing in the lattice formed by the hydrogen atoms has been
tested. The elastic constants of the material have been calculated with static
deformations approach and are in fair agreement with the experimental data. The
hydrogen subsystem structure exhibits signs of disordered behaviour while
maintaining correlations between angular positions of neighbouring atoms. We
establish that the essential angular correlations between hydrogen positions
are maintained to the temperature of at least 150 K and show that they are well
described by a physically motivated probabilistic model. The rotational degree
of freedom appears to be decoupled from the lattice directions above 30K
Commensurate-Incommensurate Magnetic Phase Transition in Magnetoelectric Single Crystal LiNiPO
Neutron scattering studies of single-crystal LiNiPO reveal a spontaneous
first-order commensurate-incommensurate magnetic phase transition. Short- and
long-range incommensurate phases are intermediate between the high temperature
paramagnetic and the low temperature antiferromagnetic phases. The modulated
structure has a predominant antiferromagnetic component, giving rise to
satellite peaks in the vicinity of the fundamental antiferromagnetic Bragg
reflection, and a ferromagnetic component giving rise to peaks at small
momentum-transfers around the origin at . The wavelength of the
modulated magnetic structure varies continuously with temperature. It is argued
that the incommensurate short- and long-range phases are due to
spin-dimensionality crossover from a continuous to the discrete Ising state.
These observations explain the anomalous first-order transition seen in the
magnetoelectric effect of this system
Local structural origins of the distinct electronic properties of Nb-substituted SrTiO and BaTi
Near or less than 10% Nb substitution on the Ti site in perovskite SrTiO
results in metallic behavior, in contrast to what is seen in BaTiO. Given
the nearly identical structure and electron counts of the two materials, the
distinct ground states for low substitution have been a long-standing puzzle.
Here we find from neutron studies of average and local structure, the subtle
yet critical difference that we believe underpins the distinct electronic
properties in these fascinating materials. While
SrTiNb_O possesses a distorted non-cubic structure at 15
K, the BO octahedra in the structure are regular.
BaTiNbO on the other hand shows evidence for local
cation off-centering whilst retaining a cubic structure.Comment: 4 pages, 5 figure
First-principles study of orthorhombic CdTiO3 perovskite
In this work we perform an ab-initio study of CdTiO3 perovskite in its
orthorhombic phase using FLAPW method. Our calculations help to decide between
the different cristallographic structures proposed for this perovskite from
X-Ray measurements. We compute the electric field gradient tensor (EFG) at Cd
site and obtain excellent agreement with available experimental information
from a perturbed angular correlation (PAC) experiment. We study EFG under an
isotropic change of volume and show that in this case the widely used "point
charge model approximation" to determine EFG works quite well.Comment: 4 pages, 1 figure. Accepted in Physical Review
High pressure phases in highly piezoelectric Pb(Zr0.52Ti0.48)O3
Two novel room-temperature phase transitions are observed, via synchrotron
x-ray diffraction and Raman spectroscopy, in the Pb(Zr0.52Ti0.48)O3 alloy under
hydrostatic pressures up to 16 GPa. A monoclinic (M)-to-rhombohedral (R1) phase
transition takes place around 2-3 GPa, while this R1 phase transforms into
another rhombohedral phase, R2, at about 6-7 GPa. First-principles calculations
assign the R3m and R3c symmetry to R1 and R2, respectively, and reveal that R2
acts as a pressure-induced structural bridge between the polar R3m and a
predicted antiferrodistortive R-3c phase.Comment: REVTeX, 4 pages with 3 figures embedded. Figs 1 and 3 in colo
Characterization of Ordering in A-Site Deficient Perovskite Ca1-xLa2x/3TiO3 Using STEM/EELS
The vacancy ordering behavior of an A-site deficient perovskite system, Ca1-xLa2x/3TiO3, was studied using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), with the aim of determining the role of A-site composition changes. At low La content (x = 0.2), adopting Pbnm symmetry, there was no indication of long-range ordering. Domains, with clear boundaries, were observed in bright-field (BF) imaging, but were not immediately visible in the corresponding high-angle annular dark-field (HAADF) image. These boundaries, with the aid of displacement maps from A-site cations in the HAADF signal, are shown to be tilt boundaries. At the La-rich end of the composition (x = 0.9), adopting Cmmm symmetry, long-range ordering of vacancies and La3+ ions was observed, with alternating La-rich and La-poor layers on (001)p planes, creating a double perovskite lattice along the c axis. These highly ordered domains can be found isolated within a random distribution of vacancies/La3+, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, these highly ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries. The occurrence and characteristics of these ordered structures are discussed and compared with similar perovskite systems
Evidence for and phases in the morphotropic phase boundary region of : A Rietveld study
We present here the results of the room temperature dielectric constant
measurements and Rietveld analysis of the powder x-ray diffraction data on
(PMN-PT) in the composition range
to show that the morphotropic phase boundary (MPB)
region contains two monoclinic phases with space groups Cm (or type) and
Pm (or type) stable in the composition ranges and
, respectively. The structure of PMN-PT in the
composition ranges 0.26, and is found to be
rhombohedral (R3m) and tetragonal (P4mm), respectively. These results are
compared with the predictions of Vanderbilt & Cohen's theory.Comment: 20 pages, 11 pdf figure
Antiferromagnetism in the magnetoelectric effect single crystal LiMnPO
Elastic and inelastic neutron scattering studies reveal details of the
antiferromagnetic tansition and intriguing spin-dynamics in the
magneto-electric effect single crystal LiMnPO. The elastic scattering
studies confirm the system is antiferromagnetic (AFM) below =33.75 K with
local magnetic moments (Mn; ) that are aligned along the
crystallographic a-axis. The spin-wave dispersion curves propagating along the
three principal axes, determined by inelastic scattering, are adequately
modeled in the linear spin-wave framework assuming a spin-Hamiltonian that is
parameterized by inter- and in-plane nearest- and next-nearest-neighbor
interactions, and by easy-plane anisotropy. The temperature dependence of the
spin dynamics makes this an excellent model many-body spin system to address
the question of the relationship between spin-wave excitations and the order
parameter
Spin Dynamics in the Magnetoelectric Effect LiCoPO Compound
Inelastic neutron scattering (INS) experiments were performed to investigate
the spin dynamics in magnetoelectric effect (ME) LiCoPO single crystals.
Weak dispersion was detected in the magnetic excitation spectra along the three
principal crystallographic axes measured around the (0 1 0) magnetic
reflection. Analysis of the data using linear spin-wave theory indicate that
single-ion anisotropy in LiCoPO is as important as the strongest
nearest-neighbor exchange coupling. Our results suggest that Co
single-ion anisotropy plays an important role in the spin dynamics of
LiCoPO and must be taken into account in understanding its physical
properties. High resolution INS measurements reveal an anomalous low energy
excitation that we hypothesize may be related to the magnetoelectric effect of
LiCoPO.Comment: 6 pages, 5 figures, accepted for publication in Phys. Rev.
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