Texture

The idea behind the texture method is illustrated for a two-dimensional case in the figure below, where the diffraction pattern of an "ideal" powder with randomly oriented crystallites is shown together with that of a textured powder sample. In both cases, the corresponding single-crystal pattern has been superimposed.

Because not all crystallite orientations are equally represented in the textured sample, the reflections are concentrated in certain regions of reciprocal space, so by measuring the diffraction pattern along different radial directions (e.g. by orienting the sample differently in the X-ray beam), additional intensity information can be extracted. For example, the four reflections marked with arrows in the figure above overlap in the normal powder diffraction pattern, but can be separated in the pattern from the textured sample. The sample orientations yielding the desired information depend upon how the crystallites are oriented in the sample.

The data can be collected in either reflection or transmission geometry (see below). In both cases, synchrotron radiation (or at least an intense highly collimated X-ray source) is needed. The reflection mode experiment requires 3 days of synchrotron beamtime per sample, extreme corrections to the data for higher tilt angles, and a large homogeneously textured specimen. The transmission mode experiment (using an area detector) allows the beamtime to be reduced to ca 6h per sample, involves no tilt correction, and requires only a very small sample, but these advantages are gained at the expense of resolution. These initial experiments were conducted on the Swiss-Norwegian Beamlines at the European Synchrotron Radiation Facility in Grenoble.

In an attempt to improve the resolution of the transmission geometry setup, the experiment has been adapted to accommodate the one-dimensional Si microstrip detector on the powder diffractometer on the Materials Science Beamline at the Swiss Light Source. This setup has allowed the resolution of the data to be improved both in 2θ (from 0-35° to 0-60°) and in peak width (from ca 0.06°2θ to ca 0.03°2θ) with an acceptable increase in the amount of beamtime required (ca 12h per sample).

Several structures have now been solved using this texture approach, but the most impressive one is that of the high-silica extra-large-pore zeolite UTD-1F, with 117 atoms in the asymmetric unit:

Supported by the Swiss National Science Foundation.

References:

T. Wessels, Ch. Baerlocher and L.B. McCusker, "Single-crystal-like diffraction data from polycrystalline materials", Science (1999), 284, 477-479

T. Wessels, Ch. Baerlocher, L.B. McCusker and E.J. Creyghton, "An ordered form of the extra-large-pore zeolite UTD-1: synthesis and structure analysis from powder diffraction data", J. Am. Chem. Soc. (1999), 121, 6242-6247

Ch. Baerlocher, L.B. McCusker, S. Prokic and T. Wessels, "Exploiting texture to estimate the relative intensities of overlapping reflections", Z. Kristallogr. (2004), 219, 803-812

J. Grässlin, L.B. McCusker, Ch. Baerlocher, F. Gozzo, B. Schmitt and L. Lutterotti, "Advances in exploiting preferred orientation in the structure analysis of polycrystalline materials" (2013) J. Appl. Crystallogr. 46, 173-180