14 research outputs found
Origin of the high piezoelectric response in PbZr(1-x)TixO3
High resolution x-ray powder diffraction measurements on poled PbZr(1-x)TixO3
(PZT) ceramic samples close to the rhombohedral-tetragonal phase boundary (the
so-called morphotropic phase boundary, MPB) have shown that for both
rhombohedral and tetragonal compositions, the piezoelectric elongation of the
unit cell does not occur along the polar directions but along those directions
associated with the monoclinic distortion. This work provides the first direct
evidence for the origin of the very high piezoelectricity in PZT.Comment: 4 pages, 4 EPS figures embedded. More specific title and abstract. To
appear in Phys. Rev. Let
Heterovalent and A-atom effects in A(B'B'')O3 perovskite alloys
Using first-principles supercell calculations, we have investigated
energetic, structural and dielectric properties of three different A(B'B'')O_3
perovskite alloys: Ba(Zn_{1/3}Nb_{2/3})O_3 (BZN), Pb(Zn_{1/3}Nb_{2/3})O_3
(PZN), and Pb(Zr_{1/3}Ti_{2/3})O_3 (PZT). In the homovalent alloy PZT, the
energetics are found to be mainly driven by atomic relaxations. In the
heterovalent alloys BZN and PZN, however, electrostatic interactions among B'
and B'' atoms are found to be very important. These electrostatic interactions
are responsible for the stabilization of the observed compositional long-range
order in BZN. On the other hand, cell relaxations and the formation of short
Pb--O bonds could lead to a destabilization of the same ordered structure in
PZN. Finally, comparing the dielectric properties of homovalent and
heterovalent alloys, the most dramatic difference arises in connection with the
effective charges of the B' atom. We find that the effective charge of Zr in
PZT is anomalous, while in BZN and PZN the effective charge of Zn is close to
its nominal ionic value.Comment: 7 pages, two-column style with 2 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#lb_he
Stability of the monoclinic phase in the ferroelectric perovskite PbZr(1-x)TixO3
Recent structural studies of ferroelectric PbZr(1-x)TixO3 (PZT) with x= 0.48,
have revealed a new monoclinic phase in the vicinity of the morphotropic phase
boundary (MPB), previously regarded as the the boundary separating the
rhombohedral and tetragonal regions of the PZT phase diagram. In the present
paper, the stability region of all three phases has been established from high
resolution synchrotron x-ray powder diffraction measurements on a series of
highly homogeneous samples with 0.42 <=x<= 0.52. At 20K the monoclinic phase is
stable in the range 0.46 <=x<= 0.51, and this range narrows as the temperature
is increased. A first-order phase transition from tetragonal to rhombohedral
symmetry is observed only for x= 0.45. The MPB, therefore, corresponds not to
the tetragonal-rhombohedral phase boundary, but instead to the boundary between
the tetragonal and monoclinic phases for 0.46 <=x<= 0.51. This result provides
important insight into the close relationship between the monoclinic phase and
the striking piezoelectric properties of PZT; in particular, investigations of
poled samples have shown that the monoclinic distortion is the origin of the
unusually high piezoelectric response of PZT.Comment: REVTeX file, 7 figures embedde
Ab initio linear response and frozen phonons for the relaxor PMN (PbMg1/3Nb2/3O3)
We report first principles density functional studies using plane wave basis
sets and pseudopotentials and all electron linear augmented plane wave (LAPW)
of the relative stability of various ferroelectric and antiferroelectric
supercells of PMN for 1:2 chemical ordering along [111] and [001]. We used
linear response with density functional perturbation theory (DFPT) as
implemented in the code ABINIT to compute the Born effective charges,
electronic dielectric tensors, long wavelength phonon frequencies and LO-TO
splittings. The polar response is different for supercells ordered along [111]
and [001]. Several polar phonon modes show significant coupling with the
macroscopic electric field giving giant LO-TO splittings. For [111] ordering, a
polar transverse optic (TO) mode with E symmetry is found to be unstable in the
ferroelectric P3m1 structure and the ground state is found to be triclinic.
Multiple phonon instabilities of polar modes and their mode couplings provide
the pathway for polarization rotation. The Born effective charges in PMN are
highly anisotropic and this anisotropy contributes to the observed huge
electromechanical coupling in PMN solid solutions.Comment: 34 pages, 6 figures. to appear in PR