Acta Crystallographica Section B

Structural Science

Volume 65, Part 6 (December 2009)

research papers

Acta Cryst. (2009). B65, 676-683 [ doi:10.1107/S0108768109032728 ]

Octahedral tilt twinning and compositional modulation in NaLaMgWO₆

G. King, S. Garcia-Martin and P. M. Woodward

Abstract: A combination of selected-area electron diffraction (SAED), neutron powder diffraction (NPD) and high-resolution transmission electron microscopy (HRTEM) reveals a complex superstructure in the ordered perovskite NaLaMgWO₆. Through indexing of SAED patterns the unit-cell dimensions are found to be 46.8 × 7.8 × 7.9 Å, which corresponds to a 12a_p × 2a_p × 2a_p superstructure of the simple $Pm\overline 3 m$ perovskite unit cell. HRTEM images reveal the formation of an unmistakable stripe contrast that repeats with the same periodicity. Doubling of the b and c axes is brought about by a combination of layered ordering of Na and La, rock-salt ordering of Mg and W, and octahedral tilting. The a axis repeat distance results from a one-dimensional twinning of the octahedral tilts in combination with a compositional modulation. Modeling of the NPD pattern shows that the underlying tilt system is a^-a^-c⁰ with tilt angles of 8° about the a and b axes. The octahedral tilt-twin boundaries run perpendicular to the a axis and are separated by 6a_p. Simulated HRTEM images show that octahedral tilt twinning alone cannot explain the stripes seen in the HRTEM images, rather a compositional modulation involving the A-site cations is necessary to explain the experimental images.

Keywords: twinning; modulation; neutron powder diffraction; electron diffraction; electron microscopy.


	Text file (17.0 kbytes) [ doi:10.1107/S0108768109032728/hw5005sup2.txt ] CIF of model structure

	Text file (10.3 kbytes) [ doi:10.1107/S0108768109032728/hw5005sup3.txt ] Atomic coordinates and displacement parameters of model structure

	Text file (66.2 kbytes) [ doi:10.1107/S0108768109032728/hw5005sup4.txt ] Atomic coordinates and displacement parameters of model structure

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