X-ray and neutron scattering

Abstract:

The relationship between crystal structure and physical properties in the ferroelectric Na0.5Bi0.5TiO3 (NBT) has been of interest for the last two decades. Originally, the average structure was held to be of rhombohedral (R3c) symmetry with a fixed polarization direction. This has undergone a series of revisions, however, based on high-resolution X-ray diffraction, total neutron scattering, and optical and electron microscopy. The recent experimental findings suggest that the true average symmetry is monoclinic (space group Cc), which allows for a rotatable spontaneous polarization. Neither polarization rotation nor its potentially important real role in enhanced piezoelectricity is well understood. The present work describes an in situ investigation of the average monoclinic distortion in NBT by time-resolved single-crystal X-ray diffraction under external electric fields. The study presents a high-resolution inspection of the characteristic diffraction features of the monoclinic distortion – splitting of specific Bragg reflections – and their changes under a cyclic electric field. The results favour a model in which there is direct coupling between the shear monoclinic strain and the polarization rotation. This suggests that the angle of polarization rotation under a sub-coercive electric field could be 30° or more.

Abstract:

In a recent paper [Huang, Gog, Kim, Kasman, Said, Casa, Wieczorek, Hönnicke and Assoufid (2018). J. Appl. Cryst. 51, 140–147], a description of the structure of quartz was given that is incorrect. There is a long history of such errors in publications describing the quartz structure. This was fully and correctly discussed in 1978 [Donnay and Le Page (1978). Acta Cryst. A34, 584–594], and yet these errors still persist. In the present paper the description by Huang et al. is corrected and the seminal work of Donnay and Le Page revisited.

Abstract:

The magnetic properties of CuO encompass several contemporary themes in condensed matter physics, including quantum magnetism, magnetic frustration, magnetically-induced ferroelectricity and orbital currents. Here we report polarized and unpolarized neutron inelastic scattering measurements which provide a comprehensive map of the cooperative spin dynamics in the low temperature antiferromagnetic (AFM) phase of CuO throughout much of the Brillouin zone. At high energies $(E \gtrsim 100\,meV)$ the spectrum displays continuum features consistent with the des Cloizeax--Pearson dispersion for an ideal $S=\frac{1}{2}$ Heisenberg AFM chain. At lower energies the spectrum becomes more three-dimensional, and we find that a linear spin-wave model for a Heisenberg AFM provides a very good description of the data, allowing for an accurate determination of the relevant exchange constants in an effective spin Hamiltonian for CuO. In the high temperature helicoidal phase, there are features in the measured low-energy spectrum that we could not reproduce with a spin-only model. We discuss how these might be associated with the magnetically-induced multiferroic behavior observed in this phase.