What are half-order bragg reflections and how can one observe them in epitaxial (00l) thin films? and, how are these half order bragg peaks are correlated to octahedral rotations in perovskites?
Half-order Bragg peaks mean a double-fold periodicity in real space. In perovskites a common source for that is indeed octahedral rotations, that can double the unit cell (i.e. each opposing-ly two rotated octahedra can be seen as one unit cell). For this case it's common to use synchrotron radiation due to the low relative intensity of the half order peaks.
Other sources for half order peaks could be periodic oxygen vacancies, see the Brown-Millerite structure for example.
For further reading on octahedral rotations (someone should come up with a convenient acronym for that already), I recomment the well-written work by May, Rondinelli, Spaldin and co-workers
http://journals.aps.org/prb/abstract/10.1103/PhysRevB.82.014110
arXiv version: http://arxiv.org/pdf/1002.1317.pdf
Are half order reflection synonymous to super structure reflections? Doesn't it mean that one needs to increase the unit cell in order to get integers for Miller indices? In case yes, why it is necessary to introduce a weird explanation for perhaps non-indexable interferences although a well-accepted concept of super- and subgroup relationship covers this problem?
Gert, if I understand your question correctly, it's only a matter of terminology - and your definition sounds as good as any other. It's possible that people from different areas are used to different terminologies.
In the current case, while you can indeed consider this larger unit cell as a super structure, it's often convenient to "think pseudocubic" for epitaxial cubic thin films. While the octahedral rotation is often quite important, it's usually a fine structural detail and it's sometimes convenient to consider it as "mostly cubic" with some deviation or symmetry breaking.
Hi Lior, you are certainly right, but from my (very personal) point of view people might interpret this as different subject, new field of diffraction etc. Only since something seems to be very convenient (for what?) it is not automatically beneficial since often all mathematical relationships, rules and laws cannot be transferred anymore, look different etc. I assume that this is a huge source of misinterpretations and errors, and frankly speaking, often only a missing knowledge about the amazing opportunities given already by classical crystallography. Unfortunately, this part of symmetry relationships is no part of teaching in universities anymore and btw also quite refreshed by the activities of Ulrich Mueller who published these things in International Tables for Crystallography, Vol. A1. Pseudosymmetry is in general underestimated, anyway in which field. And there is always the tendency to make things cubic although they actually aren't, since all calculations are of course easier. Unfortunately the discussion of deviating effects becomes then more fishy and speculative. If one would consequently consider the lower symmetry (with all disadvantages) it would help to keep everything in focus.
Nevertheless, thanks for the confirmation that it is only a terminology problem.
Half order Bragg reflection, if not due to an instrumental effect or some multiple reflections inside the lattice, may only be attributed to the effective presence of a periodicity twice than the planar spacing measured.
May be this is the case of octahedral rotations in perovskites.
I remember the case of ordere InGaP alloy.
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