X-ray and neutron scattering

Abstract:

PbZr1-x Tix O3 (PZT) ceramics made by mixed-oxide and sol-gel routes within the rhombohedral region of the phase diagram have been prepared, and their structures determined by Rietveld refinement using high-resolution time-of-flight neutron diffraction. Different structural models have been used but the best fits have been found using a mixture of rhombohedral R3c (C 3v 6) and monoclinic Cm (Cs3) across the whole rhombohedral range with the proportion of monoclinic phase increasing continuously with composition x toward the morphotropic phase boundary. Some differences are found between the ceramics prepared by the two routes, especially with respect to the diffraction peak widths, suggesting that the precise structural arrangements in these materials depend on the method of preparation. The results show that PZT ceramics made in this region always consist of mixed phases and this accounts for the fact that no phase boundary has been discovered in the PZT phase diagram between the monoclinic and rhombohedral phases. © 2009 The American Physical Society.

Abstract:

The phase diagram of lead-free piezoelectric Kx Na1-x NbO3 has been studied, with particular focus on the proposed morphotropic phase boundaries, by powder and single crystal x-ray diffraction. The tilt system and cation displacement has been mapped out as a function of temperature and composition, highlighting changes in the oxygen octahedra at x=0.2 and x=0.4 at room temperature. The orthorhombic to monoclinic boundary at x=0.5 has been investigated, with a subtle change in the structure observed. The conclusion is that Kx Na1-x NbO3 does not display a morphotropic phase boundary comparable with that in lead zirconate titanate, and that the most significant structural change as a function of composition occurs at x=0.2 because of the change of the tilt system. © 2009 American Institute of Physics.

Abstract:

A dense amorphous form of silica was prepared at high pressure from the highly compressible, siliceous zeolite, silicalite-1-F. Reverse Monte Carlo modeling of total X-ray scattering data shows that the structure of this novel amorphous form of SiO(2) recovered under ambient conditions is distinct from vitreous SiO(2) and retains the basic framework topology (i.e., chemical bonds) of the starting crystalline zeolite. This material is, however, amorphous over the different length scales probed by Raman and X-ray scattering due to strong geometrical distortions. This is thus an example of new topologically ordered, amorphous material with a different intermediate-range structure, a lower entropy with respect to a standard glass, and distinct physical and mechanical properties, eventually approaching those of an "ordered" or "perfect" glass. The same process in more complex aluminosilicate zeolites will, in addition, lead to an amorphous material which conserves the framework topology and chemical order of the crystal. The large volume collapse in this material may also be of considerable interest for new applications in shock wave absorption.

Abstract:

Coherent lattice vibrations are excited and probed with pulses of 10 fs duration in LaMnO(3). The measured frequencies correspond to those of Jahn-Teller stretching and of out-of phase rotations of the oxygen octahedra. Surprisingly, the amplitude and damping rate of both modes exhibit a sharp discontinuity at the Néel temperature, highlighting nontrivial coupling between light, lattice, and magnetic structure. We explain this effect by applying the Goodenough-Kanamori rules to the excited state of LaMnO(3), and note that charge transfer can invert the sign of the semicovalent exchange interaction, which in turn perturbs the equilibrium bond lengths.