Dean Samuel Keeble

Revised structural phase diagram of (Ba0.7Ca0.3TiO3)-(BaZr0.2Ti0.8O3)

The temperature-composition phase diagram of barium calcium titanate zirconate (x(Ba0.7Ca0.3TiO3)-(1 − x)(BaZr0.2Ti0.8O3); BCTZ) has been reinvestigated using high-resolution synchrotron x-ray powder diffraction. Contrary to previous reports of an unusual rhombohedral-tetragonal phase transition in this system, we have observed an intermediate orthorhombic phase, isostructural to that present in the parent phase, BaTiO3, and we identify the previously assigned T-R transition as a T-O transition. We also observe the O-R transition coalescing with the previously observed triple point, forming a phase convergence region. The implication of the orthorhombic phase in reconciling the exceptional piezoelectric properties with the surrounding phase diagram is discussed.The temperature-composition phase diagram of barium calcium titanate zirconate (x(Ba{sub 0.7}Ca{sub 0.3}TiO{sub 3})-(1 - x)(BaZr{sub 0.2}Ti{sub 0.8}O{sub 3}); BCTZ) has been reinvestigated using high-resolution synchrotron x-ray powder diffraction. Contrary to previous reports of an unusual rhombohedral-tetragonal phase transition in this system, we have observed an intermediate orthorhombic phase, isostructural to that present in the parent phase, BaTiO{sub 3}, and we identify the previously assigned T-R transition as a T-O transition. We also observe the O-R transition coalescing with the previously observed triple point, forming a phase convergence region. The implication of the orthorhombic phase in reconciling the exceptional piezoelectric properties with the surrounding phase diagram is discussed.

The missing boundary in the phase diagram of PbZr1−xTixO3

PbZr(1-x)Ti(x)O3 (PZT) is one of the most important and widely used piezoelectric materials. The study of its local and average structures is of fundamental importance in understanding the origin of its high-performance piezoelectricity. Pair distribution function analysis and Rietveld refinement have been carried out to study both the short- and long-range order in the Zr-rich rhombohedral region of the PZT phase diagram. The nature of the monoclinic phase across the Zr-rich and morphotropic phase boundary area of PZT is clarified. Evidence is found that long-range average rhombohedral and both long- and short-range monoclinic regions coexist at all compositions. In addition, a boundary between a monoclinic (M(A)) structure and another monoclinic (M(B)) structure has been found. The general advantage of a particular monoclinic distortion (M(A)) for high piezoactivity is discussed from a spatial structural model of susceptibility to stress and electric field, which is applicable across the wide field of perovskite materials science.

Exploring high temperature templating in non-planar phthalocyanine/copper iodide (111) bilayers

Elevated substrate temperature growth of phthalocyanine thin films is known to influence film morphology and increase crystallinity. Structural templating offers another method through which the structure of phthalocyanine films can be controlled. Here we combine the use of copper iodide (CuI) and elevated substrate temperatures and investigate their effect on the growth of a non-planar phthalocyanine system. Employing X-ray diffraction and atomic force microscopy we present detailed surface and crystal structure information. Vanadyl phthalocyanine (VOPc) is shown to adopt an edge-on orientation on CuI at ambient substrate temperatures, a behaviour in stark contrast to that of previously studied planar phthalocyanine molecules. Elevated substrate temperature is shown to result in changes in the surface morphology and structure demonstrating the versatility of the system. The crystal structure of VOPc was redetermined and used to infer the molecular orientation of the various VOPc/CuI bilayer structures.

Structural and magnetic investigations of single-crystalline neodymium zirconate pyrochlore Nd 2 Zr 2 O 7

We report structural and magnetic properties studies of large high-quality single crystals of the frustrated magnet Nd2Zr2O7. Powder x-ray diffraction analysis confirms that Nd2Zr2O7 adopts the pyrochlore structure. Room-temperature x-ray diffraction and time-of-flight neutron-scattering experiments show that the crystals are stoichiometric in composition with no measurable site disorder. The temperature dependence of the magnetic susceptibility shows no magnetic ordering at temperatures down to 0.5 K. Fits to the magnetic susceptibility data using a Curie-Weiss law reveal a ferromagnetic coupling between the Nd moments. Magnetization versus field measurements show a local Ising anisotropy along the axes of the Nd3+ ions in the ground state. Specific heat versus temperature measurements in zero applied magnetic field indicate the presence of a thermal anomaly below T ~ 7 K, but no evidence of magnetic ordering is observed down to 0.5 K. The experimental temperature dependence of the single-crystal bulk dc susceptibility and isothermal magnetization are analyzed using crystal field theory and the crystal field parameters and exchange coupling constants determined.

Investigation of the depolarisation transition in Bi-based relaxor ferroelectrics

The loss of macroscopic polarisation in relaxor ferroelectric (Na0.8K0.2)½Bi½TiO3 ceramics doped with BiZn½Ti½O3 has been studied by electrical and structural methods. These indicate that the phenomena that are coupled in a displacive phase transition are not necessarily coupled in the depolarisation of Na½Bi½TiO3-based relaxors and a concept of correlated and uncorrelated switching of dipoles within adjacent unit cells is used to explain this. Second harmonic generation performed on poled ceramics during heating yields values of the freezing temperature and shows a broad temperature range of ∼100 °C across which the structure changes from field-induced ferroelectric to an equilibrium-state ergodic relaxor. Electrical poling at room temperature causes poled regions to increase in size by ∼2 orders of magnitude. A model illustrating the main steps in thermal depolarisation is described that does not require a phase transition to take place on a unit cell level.

Controlling templating effects at the organic/inorganic interface using (111) oriented copper iodide

Structural templating of organic semiconductors affords control of out-of-plane film structure and molecular orientation with respect to solid surfaces. Herein we use a prototypical copper iodide (CuI)/planar phthalocyanine system to produce detailed surface and crystal structure information using atomic force microscopy (AFM) and X-ray diffraction (XRD). The out-of-plane structure of the CuI layer was characterised and identified as the (111) plane of single crystal CuI. The dependance of surface morphology and grain size in the CuI (111) templating layer upon substrate temperature was demonstrated. The formation of a thin film of iron phthalocyanine (FePc) on this model layer was characterised at multiple points during growth, changes in the surface morphology were observed, and the crystal structure of the final film was used to infer the molecular orientation therein. These changes were elucidated using the re-determined single crystal structure of FePc which is also presented.

Topology and temperature dependence of the diffuse X-ray scattering in Na0.5Bi0.5TiO3 ferroelectric single crystals

The high-resolution temperature-dependent diffuse X-ray scattering from an Na0.5Bi0.5TiO3 perovskite-based single crystal is measured on the approach to the polymorphic phase transition. The previously unseen topological features of the diffuse scattering and their temperature evolution are reported.

On the tetragonality of the room-temperature ferroelectric phase of barium titanate, BaTiO3

The room-temperature phase of the important ferroelectric material barium titanate, BaTiO3, was re-investigated by single-crystal X-ray diffraction on a sample grown by the top-seeded solution growth method, with the intention of demonstrating once again that the structure has tetragonal symmetry consistent with the space-group assignment P4mm and thus resolving recent controversy in the scientific community and literature [Yoshimura, Kojima, Tokunaga, Tozaki & Koganezawa (2006). Phys. Lett. A, 353, 250-254; Yoshimura, Morioka, Kojima, Tokunaga, Koganezawa & Tozaki (2007). Phys. Lett. A, 367, 394-401]. To this end, the X-ray diffraction pattern of a small (341 mu m(3)) sample of top-seeded solution-grown BaTiO3 was measured using an Oxford Diffraction Gemini CCD diffractometer employing Mo K alpha radiation and an extended 120 mm sample-to-detector distance. More than 10(4) individual diffraction maxima observed out to a maximum resolution of 0.4 angstrom were indexed on two tetragonal lattices. These were identical to within the standard deviations on the lattice parameters and were related to each other by a single rotation of 119.7 degrees about the [11 (1) over bar] direction of the first tetragonal lattice ( the major twin component), although the actual twinning operation that explains the observed diffraction pattern both qualitatively and quantitatively is shown to be conventional 90 degrees twinning by the m[101] operation. Importantly, it is not necessary to invoke either monoclinic symmetry or a coexistence of tetragonal and monoclinic phases to explain the observed diffraction data

Observation of unusual temperature-dependent stripes in LiTaO3 and LiTaxNb1−xO3 crystals with near-zero birefringence

A birefringence imaging microscopy study of LiTaO3 and some mixed LiTa (x) Nb1-x O-3 crystals is reported. The initially congruent LiTaO3 crystals have been subject to vapour transport equilibration in order to increase the Li content and at the same time to reduce the optical birefringence close to zero at room temperature. In both types of crystal, the presence of unusual periodic self-organized stripes, existing over large distances, has been observed, which correlate with the well documented cleavage and twin planes in this material. Furthermore, the stripes become narrower as the temperature is changed away from the zero-birefringence temperature, but become temperature-independent above the ferroelectric-paraelectric phase-transition temperature.

Structural and magnetic properties of single-crystals of the geometrically frustrated zirconium pyrochlore, Pr2Zr2O7

We report the growth of large high quality crystals of the frustrated magnet, Pr2Zr2O7 by the floating zone technique. Several crystals have been produced starting with varying levels of excess of Pr6O11 to compensate for the loss of Pr2O3 during the growth process. X-ray diffraction analysis of the crystals shows that the Pr composition in our crystals, as determined by the Pr site occupancy in the pyrochlore structure, is always slightly less than 1. Fits to the magnetic susceptibility data below 10 K using a Curie–Weiss law reveal an antiferromagnetic coupling between the Pr moments, with an estimated Weiss temperature of Θ W = −2.5 K and an effective moment for the Pr3+ of μ eff = 2.5 μ B. Magnetization versus field measurements show a spin ice-like anisotropy.