20 research outputs found
Localized Basis for Effective Lattice Hamiltonians: Lattice Wannier Functions
A systematic method is presented for constructing effective Hamiltonians for
general phonon-related structural transitions. The key feature is the
application of group theoretical methods to identify the subspace in which the
effective Hamiltonian acts and construct for it localized basis vectors, which
are the analogue of electronic Wannier functions. The results of the symmetry
analysis for the perovskite, rocksalt, fluorite and A15 structures and the
forms of effective Hamiltonians for the ferroelectric transition in
and , the oxygen-octahedron rotation transition in , the
Jahn-Teller instability in and the
antiferroelectric transition in are discussed. For the oxygen-
octahedron rotation transition in , this method provides an
alternative to the rotational variable approach which is well behaved
throughout the Brillouin zone. The parameters appearing in the Wannier basis
vectors and in the effective Hamiltonian, given by the corresponding invariant
energy expansion, can be obtained for individual materials using first-
principles density-functional-theory total energy and linear response
techniques, or any technique that can reliably calculate force constants and
distortion energies. A practical approach to the determination of these
parameters is presented and the application to ferroelectric
discussed.Comment: extensive revisions in presentation, 32 pages, Revtex, 7 Postscript
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Effect of resistivity ratio on energy storage and dielectric relaxation properties of 0–3 dielectric composites
Below the Hall–Petch Limit in Nanocrystalline Ceramics
Reducing
the grain size of metals and ceramics can significantly
increase strength and hardness, a phenomenon described by the Hall–Petch
relationship. The many studies on the Hall–Petch relationship
in metals reveal that when the grain size is reduced to tens of nanometers,
this relationship breaks down. However, experimental data for nanocrystalline
ceramics are scarce, and the existence of a breakdown is controversial.
Here we show the Hall–Petch breakdown in nanocrystalline ceramics
by performing indentation studies on fully dense nanocrystalline ceramics
fabricated with grain sizes ranging from 3.6 to 37.5 nm. A maximum
hardness occurs at a grain size of 18.4 nm, and a negative (or inverse)
Hall–Petch relationship reduces the hardness as the grain size
is decreased to around 5 nm. At the smallest grain sizes, the hardness
plateaus and becomes insensitive to grain size change. Strain rate
studies show that the primary mechanism behind the breakdown, negative,
and plateau behavior is not diffusion-based. We find that a decrease
in density and an increase in dissipative energy below the breakdown
correlate with increasing grain boundary volume fraction as the grain
size is reduced. The behavior below the breakdown is consistent with
structural changes, such as increasing triple-junction volume fraction.
Grain- and indent-size-dependent fracture behavior further supports
local structural changes that corroborate current theories of nanocrack
formation at triple junctions. The synergistic grain size dependencies
of hardness, elasticity, energy dissipation, and nanostructure of
nanocrystalline ceramics point to an opportunity to use the grain
size to tune the strength and dissipative properties