Julio A. Gonzalo

Tetragonal-to-monoclinic phase transition in a ferroelectric perovskite: The structure of PbZr0.52Ti0.48O3

The perovskitelike ferroelectric system PbZr1-xTixO3 (PZT) has a nearly vertical morphotropic phase boundary (MPB) around x=0.45–0.50. Recent synchrotron x-ray powder diffraction measurements have revealed a 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, β=90.496°, at 20 K. 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 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 directions recently reported 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.

Tetragonal-to-monoclinic phase transition in a ferroelectric perovskite

The perovskitelike ferroelectric system PbZr1-xTixO3 (PZT) has a nearly vertical morphotropic phase boundary (MPB) around x=0.45–0.50. Recent synchrotron x-ray powder diffraction measurements have revealed a 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, β=90.496°, at 20 K. 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 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 directions recently reported 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.

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

A previously unreported ferroelectric phase has been discovered in a highly homogeneous sample of PbZr{sub 0.52}Ti{sub 0.48}O{sub 3} by high-resolution synchrotron x-ray powder diffraction measurements. At ambient temperature the sample has tetragonal symmetry (a{sub t} = 4.037 {angstrom}, c{sub t} = 4.138 {angstrom}), and transforms below {approx} 250 K into a phase which, unexpectedly, has monoclinic symmetry (a{sub m} = 5.717 {angstrom}, b{sub m} = 5.703 {angstrom}, c{sub m} = 4.143 {angstrom}, {beta}= 90.53{sup o}, at 20 K). The intensity data strongly indicate that the polar axis lies in the monoclinic ac plane close to the pseudocubic [111] direction, which would be an example of the species m3m(12)A2Fm predicted on symmetry grounds by Shuvalov.A previously unreported ferroelectric phase has been discovered in a highly homogeneous sample of PbZr{sub 0.52}Ti{sub 0.48}O{sub 3} by high-resolution synchrotron x-ray powder diffraction measurements. At ambient temperature the sample has tetragonal symmetry (a{sub t}=4.037{Angstrom}, c{sub t}=4.138{Angstrom}), and transforms below {approximately}250 K into a phase which, unexpectedly, has monoclinic symmetry (a{sub m}=5.717{Angstrom}, b{sub m}=5.703{Angstrom}, c{sub m}=4.143{Angstrom}, {beta}=90.53{degree}, at 20 K). The intensity data strongly indicate that the polar axis lies in the monoclinic {ital ac} plane close to the pseudocubic [111] direction, which would be an example of the species m3m(12)A2Fm predicted on symmetry grounds by Shuvalov. {copyright} {ital 1999 American Institute of Physics.}

Ferroelectric materials as energy converters

The performance of ferroelectric materials as converters of thermal into electrical energy has been analyzed in terms of their fundamental dielectric and thermal properties. The time dependent operation of a single stage pyro-ferroelectric converter performing a spontaneous cyclic process has been evaluated, leading to expressions for the specific power, current and voltage outputs in terms of the relevant static and transport parameters. It is concluded that ideal efficiencies of the order of 10-15% could be attained if proper materials with relatively large performance factor are developed to be used in multistage converters operating under large ΔTs.

Spectral and thermal properties of quasiphase-matching second-harmonic-generation in Nd3+ :Sr0.6 Ba0.4 (NbO3)2 multiself-frequency-converter nonlinear crystals

Quasiphase-matching second-harmonic-generation experiments have been performed on the Nd3+-doped strontium barium niobate multiself-frequency-converter nonlinear crystals. Optical microscopy has been used to determine the size dispersion of cylindrical ferroelectric domains appearing in as-grown crystals. A broad distribution of domain sizes (ranging between 1.5 and 8 μm) has been obtained. This gives the possibility of broadband second-harmonic generation in the 800–1200 nm range. The dependence of second-harmonic efficiency on crystal temperature has been found to be correlated with the thermal behavior of the spontaneous polarization, so that second-harmonic efficiency is almost temperature independent in the 25–50 °C range.

Dielectric characterization of the phase transitions in Pb1-y/2(Zr1-xTix)1-yNbyO3(0.03≤x≤0.04, 0.02≤y≤0.05)

The Curie temperature and the ideal thermal hysteresis of Pb1−y/2(Zr1−xTix)1−yNbyO3 (0.03 ⩽x⩽0.04, 0.02⩽y⩽0.05) ferroelectric ceramics have been investigated as a function of composition at the FRH–PC phase transition. Experimental results are analyzed using the generalized effective field approach and taking into account the composition dependence of the dimensionless parameters g and h corresponding to the second and third terms in the generalized effective field expansion. With this approach we can describe reasonably well the composition dependence of TC(x,y) at the FRH–PC transition and thermal hysteresis ΔT(x,y) in the narrow range of compositions investigated. The effects of the oxygen octahedra tilt in the dielectric constant at the FRL–FRH transition have also been investigated as a function of composition.

New Features of the Morphotropic Phase Boundary in the Pb(Zr1-xTix)O3 System

Abstract Recently a new monoclinic phase in the Pb(Zr1−xTix)O3 ceramic system has been reported by Noheda et al.[1] for a composition of x=0.48. In this work, samples with Ti contents x=0.47 and 0.50, which are both tetragonal below their Curie point, 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 ∼440K–320K, becoming rhombohedral at around 300 K. No further symmetry change was found down to 20K. 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.

Evolution of the ferroelectric transition character of partially deuterated triglycine selenate

Automatically recorded hysteresis loops and dielectric constant data on pure and partially deuterated triglycine selenate and sulphate are used to characterize the transition evolution from quasi-tricritical to clearly first order character. A quantitative assessment of the transitions displacive degree is made through the first anharmonic contribution to the effective field ( g ), obtained from hysteresis loop data. This characterization is complemented with the assessment by means of a generalized Rhodes-Wohlfarth parameter R = (NkB C /4 P 2 S 0 )1/2 involving the Curie constant C and the saturation spontaneous polarization. P S 0 as done previously by other authors.

Investigation of the character of the phase transitions in Nb doped Zr-rich PZT by pyroelectric and dielectric measurements

We present a study of the character of the phase transitions of the solid solution PbZr1ˇxTixO3. Using the spontaneous polarization curves we determined the mean-field parameters. These parameters show the first order character of the phase transitions for the richer Zr content and its evolution with composition. The FRL‐FRH phase transition presents also first order character for high contents of Zr. The study of the thermal hysteresis, and the jump of the spontaneous polarization, allowed us to investigate this phase transition with composition. # 1999 Elsevier Science Limited. All rights reserved

The effect of uniaxial pressure on the ferroelectric phase transition of TGSe crystals

Isobaric studies of the influence of uniaxial pressure on the transitions of pure TGSe crystals for the three directions are reported. The critical exponents, and , from dielectric constant and hysteresis loop measurements, as well as the coefficients and of the fourth and sixth powers of P in the free-energy expansion under uniaxial pressure were determined. It has been found that uniaxial pressure applied along the -axis keeps the transition near a tricritical point for 0 < X < 200 bar, but that when it is applied along the b- and c-axes it strengthens the second-order phase transition behaviour. The tricritical behaviour of TGSe crystals is discussed and a possible driving mechanism for the change in order of the phase transition is proposed.