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Bonding Charge Density in SrTiO3 under an Electric Field Measured by Quantitative Convergent Beam Electron Diffraction

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The precise measurement of charge density when influenced by an electric field is clearly of interest in the understanding of the electrical properties of dielectrics. Considerable practical difficulties have prevented the production of experimental results that could test theoretical calculations of such distortion of charge density. This comment applies to both X-ray and electron diffraction charge density measurements. With X-rays, due to the requirement to use a perfect crystal when an electric field is applied, severe extinction prevents access to the charge density sensitive, low order region of reciprocal space. With electron diffraction, the application of a sufficiently strong electric field while simultaneously cooling the sample is a combination not available in commercial specimen holders. In the present work, modification of an old design of the Gatan 636 double tilting cooling holder has overcome this limitation. The near zone axis technique of Quantitative Convergent Beam Electron Diffraction, QCBED , as detailed in [1], was used to measure the low order structure factors of SrTiO3 with zero field applied. Some 150 diffraction patterns were recorded over a range of zone axes, accelerating voltages and temperatures between -144C and room temperature. The experiment is planned to repeated with a field of 1 to 12 V/micron applied in the 001 direction, limiting patterns to orientations near the 100 zone axis. The zero field data is a reference against which the field data may be measured as a perturbation. Also, two prior measurements of charge density at zero field, which differ substantially from each other [2] [3], were available for comparison. Upon the application of a field in the 001 direction, the 010 mirror line of symmetry in SrTiO3, available in CBED patterns near the 100 zone, should disappear, the crystal symmetry being lowered from Pm3m to P4mm. Calculated CBED patterns for the above geometry and with an applied electric field have been prepared, using an Independent Atom Model (IAM) for electric field influenced atoms and DFT calculated atom positions. The Fourier coefficients found were substituted into the JEMS program and the calculated CBED patterns show that a considerable asymmetry should be observed. DFT calculations of the effect have so far proved to be intractable. Experimentally, samples to which an electric field can be applied have also been found very difficult to prepare.

[1] P N H Nakashima et al, Science 2011, 331, 1583.

[2] J Friis et al, Acta Cryst., 2004, A60 , 402.

[3] W Jauch & M Reehuis, Acta Cryst., 2005, A61 , 411.

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