J S Forrester

The giant piezoelectric effect: electric field induced monoclinic phase or piezoelectric distortion of the rhombohedral parent?

Lead zinc niobate–lead titanate (PZN–PT) single crystals show very large piezoelectric strains for electric fields applied along the unit cell edges e.g. [001]R. It has been widely reported that this effect is caused by an electric field induced phase transition from rhombohedral (R3m) to monoclinic (Cm or Pm) symmetry in an essentially continuous manner. Group theoretical analysis using the computer program ISOTROPY indicates phase transitions between R3m and Cm (or Pm) must be discontinuous under Landau theory. An analysis of the symmetry of a strained unit cell in R3m and a simple expansion of the piezoelectric strain equation indicate that the piezoelectric distortion due to an electric field along a cell edge in rhombohedral perovskite-based ferroelectrics is intrinsically monoclinic (Cm), even for infinitesimal electric fields. PZN–PT crystals have up to nine times the elastic compliance of other piezoelectric perovskites and it might be expected that the piezoelectric strains are also very large. A field induced phase transition is therefore indistinguishable from the piezoelectric distortion and is neither sufficient nor necessary to understand the large piezoelectric response of PZN–PT.

Ferroelastic switching in a soft lead zirconate titanate

Abstract Poled and unpoled ferroelectric lead zirconate titanate (“soft” PZT) samples were subjected to incremental compressive stresses up to 270 MPa to examine the mechanisms by which this type of ceramic responds to large static loads. Data indicate abundant ferroelastic switching leading to substantial non-linear deformation in the region 10–60 MPa. The ferroelastic switching is time dependent leading to creep under DC or static loading conditions. The creep is transient in nature and modelled reasonably well by an Andrade power law, initially increasing with increasing stress, however above 30 MPa, exhaustion of the creep mechanism reduces the creep rate. The creep exponent is 0.16±0.03 at most stresses. Above 60 MPa, the ceramic responds as a linear elastic solid; however, there is evidence that the elastic constants change at applied stresses of greater than 90 MPa. No reverse switching occurred during unloading causing the final state of the ceramic to be mechanically poled transverse to the stress axis.

Rhombohedral to cubic phase transition in the relaxor ferroelectric PZN

Abstract The structure and phase transition in lead zinc niobate (PZN) were studiedusing very high resolution powder neutron diffraction between 4.2 and 450 K.The structure is unequivocally rhombohedral in space group R 3 m with a =4 . 06048Aand˚ α = 89 . 8693 ◦ at 4.2 K. Nolowsymmetryphases were observed,nor was an octahedral tiltingtransitionto R 3 c as occurs insome parallel systems(e.g. PZT). Within the rhombohedral structure, large average ion displacementswere found relative to the rather small spontaneous strains. On heating, atransition to a cubic perovskite occurs, with a critical temperature of∼384 K. 1. Introduction There has been considerable technological interest in relaxor ferroelectric materials inthe Pb ( Zn 1 / 3 Nb 2 / 3 ) O 3 –PbTiO 3 (PZN–PT) system due to their excellent piezoelectricproperties [1, 2]. PZN–PT single crystals show the largest piezoelectric coefficient ( d 33 ∼2500 pC N −1 )andpiezoelectric strains (up to 1.7%) of any practical piezoelectric material [3].In common with many ferroelectrics, PZN–PT materials have distorted perovskite structures,in this case with multiple occupancy of the

A Study of the Platinum/Alumina Interface

Single crystal sapphire in one of four orientations (0001), (11 2 0), (1102) and (1010) was bonded to polycrystalline platinum foil at 1330oC under light load 20 N (0.8 MPa) in argon. The different orientations were used as a means to model individual grains within a polycrystal without the uncertain influence of grain boundaries and glass-forming contamination (e.g. SiO2). Partial bonding was observed in all cases. Optical microscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to study the nature of the bond which appears to be physical rather than chemical, in agreement with previous work.

The phase transition sequence in the relaxor ferroelectric PZN-8% PT

Crystal structures and phase transitions in the giant piezoelectric effect material lead zinc niobate–8% lead titanate (PZN–8% PT) between 4.2 and 455 K have been determined using very high resolution powder neutron diffraction. The structure at 4.2 K is monoclinic (Cm). On heating, the monoclinic phase transforms first into tetragonal (P4mm) and then cubic (Pm3m) structures via first-order transitions with wide two-phase regions.