arXiv subjects

D. E. Cox

Publications and source records attributed to D. E. Cox.

18 recordsLinked to original sources

arXiv2005

Bridging Phases at the Morphotropic Boundaries of Lead-Oxide Solid Solutions

Ceramic solid solutions of PbZr(1-x)TixO3 (PZT) with compositions of about x= 0.50 are well-known for their extraordinarily large piezoelectric responses. The latter are highly anisotropic, and it was recently shown that, for the rhombohedral compositions (x< 0.5), the piezoelectric coefficients were largest away from the polar direction, contrary to common belief. Shortly afterwards a low-symmetry monoclinic phase was observed by synchrotron x-ray diffraction at around x=0.50. Similar behavior and features are also present in a number of related lead-based strongly-piezoelectric single crystals, such as PMN-PT, PZN-PT, and PSN-PT, with piezoelectric coeficients of about 2500 pm/V, the highest values recorded to date. Recent experimental and theoretical work has greatly improved our understanding of these technologically-important systems, but there are still some open questions. In this review we try to summarize the most recent progress in the field.

cond-mat.mtrl-sci↗

arXiv2004

High Pressure Study of Structural Phase Transitions and Superconductivity in La1.48Nd0.4Sr0.12CuO4

We have determined the crystal structures and superconducting transition temperatures of La1.48Nd0.4Sr0.12CuO4 under nearly hydrostatic pressures in diamond anvil cells to 5.0 GPa and 19.0 GPa, respectively. Synchrotron x-ray powder diffraction measurements were used to establish the pressure-temperature structural phase diagram. Under pressure the superconducting transition temperature increases rapidly from Tc = 3 K to a maximum value of 22 K at 5 GPa, a pressure slightly greater than that required to stabilize the undistorted I4/mmm structure in the superconducting state. Increasing the pressure further to 19 GPa leads to a decrease in Tc to ~12 K. These results are discussed in relation to earlier high pressure measurements for similar materials.

cond-mat.supr-con↗

arXiv2004

Low-temperature phases in Pb(Zr0.52Ti0.48)O3: A neutron powder diffraction study

A neutron powder diffraction study has been carried out on Pb(Zr0.52Ti0.48)O3 in order to resolve an ongoing controversy about the nature of the low-temperature structure of this strongly-piezoelectric and technologically-important material. The results of a detailed and systematic Rietveld analysis at 20 K are consistent with the coexistence of two monoclinic phases having space groups Cm and Ic respectively, in the approximate ratio 4:1, and thus support the findings of a recent electron diffraction study by Noheda et al. [Phys. Rev. B 66, 060103 (2002)]. The results are compared to those of two recent conflicting neutron powder diffraction studies of materials of the same nominal composition by Hatch et al. [Phys. Rev. B 65, 212101 (2002)] and Frantti et al. [Phys. Rev. B 66, 064108 (2002)].

cond-mat.mtrl-sci↗

arXiv2002

Phase diagram of the ferroelectric-relaxor (1-x)PbMg(1/3)Nb(2/3)O3-xPbTiO3

Synchrotron x-ray powder diffraction measurements have been performed on unpoled ceramic samples of (1-x)PbMg(1/3)Nb(2/3)O3-xPbTiO3 (PMN-xPT) with 30%<= x<= 39% as a function of temperature around the morphotropic phase boundary (MPB), which is the line separating the rhombohedral and tetragonal phases in the phase diagram. The experiments have revealed very interesting features previously unknown in this or related systems. The sharp and well-defined diffraction profiles observed at high and intermediate temperatures in the cubic and tetragonal phases, respectively, are in contrast to the broad features encountered at low temperatures. These peculiar characteristics, which are associated with the monoclinic phase of MC-type previously reported by Kiat et al and Singh et al., can only be interpreted as multiple coexisting structures with MC as the major component. An analysis of the diffraction profiles has allowed us to properly characterize the PMN-xPT phase diagram and to determine the stability region of the monoclinic phase, which extends from x= 31% to x= 37% at 20 K. The complex lansdcape of observed phases points to an energy balance between the different PMN-xPT phases which is intrinsically much more delicate than that of related systems such as PbZr(1-x)TixO3 or (1-x)PbZn(1/3)Nb(1/3)O3-xPbTiO3. These observations are in good accord with an optical study of x= 33% by Xu et al., who observed monoclinic domains with several different polar directions coexisting with rhombohedral domains, in the same single crystal.

cond-mat.mtrl-sci↗

arXiv2002

Electric-field induced phase transitions in rhombohedral PZN-xPT

High-energy x-ray diffraction experiments peformed on rhombohedral (1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3(PZN-xPT) crystals with x= 4.5 and 8% show that an electric field applied along the [001] direction induces the tetragonal phase, as proposed by Park and Shrout. Our experiments reveal that in 4.5PT such phase change occurs via a third phase with monoclinic symmetry, MA, which is observed at intermediate field values. This is in agreement with first-principles calculations by Fu and Cohen predicting the rotation of the polarization between the rhombohedral and tetragonal phases in this material. A different polarization path between the rhombohedral and tetragonal phases, through a second monoclinic phase, MC, has been previously reported in 8PT. The microscopic characterization of these crystals allows us to explain the ultra-high macroscopic strain observed in PZN-x%PT under an electric field. Moreover, some unusual scattering profiles displayed by exceptionally good crystals, are experimental evidence of the high anharmonicities and near-degeneracy of the different phases in these extremely deformable materials.

cond-mat.mtrl-sci↗

arXiv2001

Phase diagram of the relaxor ferroelectric (1-x)PbZn(1/3)Nb(2/3)O3-xPbTiO3

Recently, a new orthorhombic phase has been discovered in the ferroelectric system Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-xPT) for x= 9%, and for x= 8% after the application of an electric field. In the present work, synchrotron x-ray measurements have been extended to higher concentrations, 10% <= x <=15%. The orthorhombic phase was observed for x= 10%, but, surprisingly, for x>= 11% only a tetragonal phase was found down to 20 K. The orthorhombic phase thus exists only in a narrow concentration range with near vertical phase boundaries on both sides. This orthorhombic symmetry (MC-type) is in contrast to the monoclinic MA-type symmetry recently identified at low temperatures in the PbZr(1-x)TixO3 (PZT) system over a triangle-shaped region of the phase diagram in the range x= 0.46-0.52. To further characterize this relaxor-type system neutron inelastic scattering measurements have also been performed on a crystal of PZN-xPT with x= 15%. The anomalous soft-phonon behaviour (''waterfall'' effect) previously observed for x= 0% and 8% is clearly observed for the 15% crystal, which indicates that the presence of polar nanoregions extends to large values of x.

cond-mat.mtrl-sci↗

arXiv2001

Low symmetry phases in piezoelectric systems: PZN-xPT single crystal and powder

In this review we address some of the experimental challenges found in the x-ray diffraction characterization of the low-symmetry phases recently discovered in the PZT, PZN-PT and PMN-PT piezoelectric systems around their morphotropic phase boundaries. In particular, we discuss the need for special care in the preparation of powder samples made from single crystals in order to avoid degradation and peak-broadening effects.

cond-mat.mtrl-sci↗

arXiv2001

Universal Phase Diagram for High-Piezoelectric Perovskite Systems

Strong piezoelectricity in the perovskite-type PbZr(1-x)TixO3 (PZT) and Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) systems is generally associated with the existence of a morphotropic phase boundary (MPB) separating regions with rhombohedral and tetragonal symmetry. An x-ray study of PZN-9%PT has revealed the presence of a new orthorhombic phase at the MPB, and a near-vertical boundary between the rhombohedral and orthorhombic phases, similar to that found for PZT between the rhombohedral and monoclinic phases. We discuss the results in the light of a recent theoretical paper by Vanderbilt and Cohen, which attributes these low-symmetry phases to the high anharmonicity in these oxide systems.

cond-mat.mtrl-sci↗

arXiv2000

Polarization rotation via a monoclinic phase in the piezoelectric 92%PbZn1/3Nb2/3O3-8%PbTiO3

The origin of ultrahigh piezoelectricity in the relaxor ferroelectric PbZn1/3Nb2/3O3-PbTiO3 was studied with an electric field applied along the [001] direction. The zero-field rhombohedral R phase starts to follow the direct polarization path to tetragonal symmetry via an intermediate monoclinic M phase, but then jumps irreversibly to an alternate path involving a different type of monoclinic distortion. Details of the structure and domain configuration of this novel phase are described. This result suggests that there is a nearby R-M phase boundary as found in the Pb(Ti,Zr)O3 system.

cond-mat.mtrl-sci↗

arXiv2000

Vertical Boundary at x ~ 0.11 in the Structural Phase Diagram of the La1-xSrxMnO3 System (0.08 <= x <= 0.125)

The structural phase diagram of the La1-xSrxMnO3 system in the compositional range 0.08 <= x <= 0.125 has been investigated by high-resolution synchrotron x-ray powder diffraction techniques between 20-600 K. Recent studies have reported that there is an unusual rentrant-type phase transition in this range involving an abrupt change in lattice parameters but no change in the crystal symmetry, which remains orthorhombic Pbnm. The transition to the reentrant phase is from a ferromagnetic metallic to a ferromagnetic insulating phase with some unusual properties. Our results demonstrate that for samples with x =0.11-0.125 there exist two lower-symmetry structural regions having monoclinic and triclinic symmetry respectively. There is a sharp first-order transition from the monoclinic to the triclinic phase coinciding with the transition to the ferromagnetic insulating phase, and an abrupt crossover from the orthorhombic Pbnm region with a near-vertical phase boundary just below x = 0.11. The new phases indicate the presence of some novel type of orbital ordering unlike that found in LaMnO3.

cond-mat.str-el↗

arXiv2000

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.

cond-mat.mtrl-sci↗

arXiv2000

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.

cond-mat.mtrl-sci↗

arXiv2000

The monoclinic phase in PZT: new light on morphotropic phase boundaries

A summary of the work recently carried out on the morphotropic phase boundary (MPB) of PZT is presented. By means of x-ray powder diffraction on ceramic samples of excellent quality, the MPB has been successfully characterized by changing temperature in a series of closely spaced compositions. As a result, an unexpected monoclinic phase has been found to exist in between the well-known tetragonal and rhombohedral PZT phases. A detailed structural analysis, together with the investigation of the field effect in this region of compositions, have led to an important advance in understanding the mechanisms responsible for the physical properties of PZT as well as other piezoelectric materials with similar morphotropic phase boundaries.

cond-mat.mtrl-sci↗

arXiv1999

A tetragonal-to-monoclinic phase transition in a ferroelectric perovskite: the structure of PbZr(0.52)Ti(0.48)O3

The perovskite-like ferroelectric system PbZr(1-x)Ti(x)O3 (PZT) has a nearly vertical morphotropic phase boundary (MPB) around x=0.45-0.50. Recent synchrotron x-ray powder diffraction measurements by Noheda et al. [Appl. Phys. Lett. 74, 2059 (1999)] have revealed a new monoclinic phase between the previously-established tetragonal and rhombohedral regions. In the present work we describe a Rietveld analysis of the detailed structure of the tetragonal and monoclinic PZT phases on a sample with x= 0.48 for which the lattice parameters are respectively: at= 4.044 A, ct= 4.138 A, at 325 K, and am= 5.721 A, bm= 5.708 A, cm= 4.138 A, beta= 90.496 deg., at 20K. In the tetragonal phase the shifts of the atoms along the polar [001] direction are similar to those in PbTiO3 but the refinement indicates that there are, in addition, local disordered shifts of the Pb atoms of ~0.2 A perpendicular to the polar axis.. The monoclinic structure can be viewed as a condensation along one of the <110> directions of the local displacements present in the tetragonal phase. It equally well corresponds to a freezing-out of the local displacements along one of the <100> directions recently reported by Corker et al.[J. Phys. Condens. Matter 10, 6251 (1998)] for rhombohedral PZT. The monoclinic structure therefore provides a microscopic picture of the MPB region in which one of the "locally" monoclinic phases in the "average" rhombohedral or tetragonal structures freezes out, and thus represents a bridge between these two phases.

cond-mat.mtrl-sci↗

arXiv1999

Charge Ordering and Phase Competition in the Layered Perovskite Lasr2mn2o7

Charge-lattice fluctuations are observed in the layered perovskite manganite LaSr2Mn2O7 by Raman spectroscopy as high as 340 K and with decreasing temperature they become static and form a charge ordered (CO) phase below TCO=210 K. In the static regime, superlattice reflections are observed through neutron and x-ray diffraction with a propagation vector (h+1/4,k-1/4,l). Crystallographic analysis of the CO state demonstrates that the degree of charge and orbital ordering in this manganite is weaker than the charge ordering in three dimensional perovskite manganites. A TN=170K a type-A antiferromagnetism (AF) develops and competes with the charge ordering, that eventually melts below T*=100K. High resolution diffraction measurements suggest that that CO- and AF-states do not coincide within the same region in the material but rather co-exist as separate phases. The transition to type-A antiferromagnetism at lower temperatures is characterized by the competition between these two phases.

cond-mat.mtrl-sci↗

arXiv1999

New Features of the Morphotropic Phase Boundary in the PbZr(1-x)TixO3 System

Recently a new monoclinic phase in the PbZr(1-x)TixO3 ceramic system has been reported by Noheda et al. for the composition x= 0.48. In the present work, samples with Ti contents of x= 0.47 and 0.50, which are both tetragonal below their Curie points, have been investigated. In the sample with x= 0.50, the tetragonal phase was found to transform to a monoclinic phase at about 200 K as the temperature was lowered. The sample with x= 0.47 showed a complicated region of phase coexistence between 440-320 K, becoming rhombohedral at around 300 K. No further symmetry change was found down to 20 K. Dielectric measurements for these two samples are also reported. On the basis of these results, a preliminary phase diagram is presented. Optimum compositional homogeneity is needed to properly characterize the new monoclinic region.

cond-mat.mtrl-sci↗

arXiv1999

A monoclinic ferroelectric phase in the Pb(Zr1-xTix)O3 solid solution

A previously unreported ferroelectric phase has been revealed on a highly homogeneous sample of Pb(Zr0.52Ti0.48)O3 by high-resolution synchrotron x-ray powder diffraction measurements. At ambient temperature the sample has tetragonal symmetry (at= 4.037 A, ct= 4.138 A), and transforms below ~250 K into a phase which, unexpectedly, has monoclinic symmetry (am= 5.717 A, bm= 5.703 A, cm= 4.143 A, beta= 90.53 deg., at 20K). The intensity data strongly indicate that the polar axis lies in the monoclinic ac plane close to the pseudo-cubic [111] direction, which would be an example of the species m3m(12)A2Fm predicted on symmetry grounds by Shuvalov.

cond-mat.mtrl-sci↗

arXiv1998

"Wigner crystal" and "stripe" models for the magnetic and crystallographic superstructures of La0.333Ca0 .667MnO3

The crystallographic (charge-ordered) and magnetic superstructures of La0.333Ca0.667MnO3 were studied by high-resolution synchrotron x-ray and neutron powder diffraction. In the antiferromagnetic structure, which was refined using a non-collinear model, the a lattice parameter is tripled and the c lattice parameter is doubled with respect to the average crystallographic unit cell (Pnma setting). The crystallographic structure below the charge-ordering temperature (TCO ~ 260 K) is characterized by ordering of the dz2 orbitals of the Jahn-Teller-distorted Mn3+O6 octahedra in the orthorhombic ac plane, and the appearance of superlattice peaks in the x-ray patterns corresponding to a tripling of the a axis lattice parameter. The intensities of the superlattice peaks can be accounted for satisfactorily in terms of ordering of the Mn3+ cations in sites as far apart as possible in the ac plane ("Wigner crystal" model) and transverse displacements of the Mn4+O6 octahedra in the c direction. These results are not consistent with a recently proposed model based on transmission electron microscopy (TEM) data in which the Mn3+O6 octahedra are arranged in a stripe pattern ("stripe" model). In particular, the large longitudinal stripe modulation revealed by TEM is not observed, suggesting that the TEM data may not be representative of the bulk sample. Within the framework of the "Wigner crystal" model, the magnetic structure at 1.5 K and the charge-ordered superstructure at 160 K and 1.5 K were refined from the neutron data.

cond-mat.str-el↗