The potential lead‐free piezoelectric material sodium bismuth titanate, Na0.5Bi0.5TiO3, was investigated by means of high‐resolution single‐crystal X‐ray diffractometry. The splitting of Bragg peaks observed in the high‐resolution reciprocal‐space maps suggests that the average structure of Na0.5Bi0.5TiO3 has lower than rhombohedral symmetry. This observation is contrary to the commonly adopted model, which has followed from many previous analyses of neutron and X‐ray powder diffraction data.
Piezoelectrics are materials that linearly deform in response to an applied electric field. As a fundamental prerequisite, piezoelectric materials must have a noncentrosymmetric crystal structure. For more than a century, this has remained a major obstacle for finding piezoelectric materials. We circumvented this limitation by breaking the crystallographic symmetry and inducing large and sustainable piezoelectric effects in centrosymmetric materials by the electric field–induced rearrangement of oxygen vacancies. Our results show the generation of extraordinarily large piezoelectric responses [with piezoelectric strain coefficients (
d
33
) of ~200,000 picometers per volt at millihertz frequencies] in cubic fluorite gadolinium-doped CeO
2−
x
films, which are two orders of magnitude larger than the responses observed in the presently best-known lead-based piezoelectric relaxor–ferroelectric oxide at kilohertz frequencies. These findings provide opportunities to design piezoelectric materials from environmentally friendly centrosymmetric ones.
It has been well documented that single-crystal stable-temperature (ST)-cut quartz substrates can horizontally align single-walled carbon nanotubes (SWNTs) along the x direction ([100]) during catalyzed chemical vapor deposition. It has been suggested that alignment is due to either surface-guided growth along step edges and/or lattice directions, although recent atomic force microscopy (AFM) evidence suggests the latter is more appropriate [Liu, et al. J. Am. Chem. Soc. 2008, 130, 5428]. In light of this we explore the growth mechanism on single-crystal quartz in more detail, focusing particularly on the crystallography of the alignment surface and the effects of annealing treatments. X-ray diffraction (XRD) studies enable us to identify, for the first time, the crystallographic plane of ST-cut quartz as (011 j 1), with a surface normal lying close to [121], which is 42.5°from the y axis ([010]). For this plane, formulation of both crystallographic orientation and polar diagrams, the latter which reveals the magnitude of the longitudinal piezoelectric effect, indicate a high degree of correlation between the preferential SWNT growth direction and both the atomic arrangement and piezoelectric effect, strongly supporting a crystal plane driven alignment process. This is further supported by the absence of steps on the surface, observed topographically using AFM. Further evidence for crystal plane controlled alignment comes from field emission scanning electron microscopy (FE-SEM) and AFM observations of bending of the SWNTs away from the preferred alignment direction at defined angles. These angles are in agreement with the Si-O-Si atomic bond angles in quartz. Additionally, surfaces which are shown by XRD to have either minimal distortion in the lattice or surface misorientation caused by, e.g., tilts, cracks, etc., are those that are found to align the SWNTs best. Improvement in surface crystallinity can be achieved by annealing in air at high temperature; optimal annealing conditions of 950°C in air for 30 mins were determined, in agreement with XRD data and FE-SEM images.
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