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*To*:*Multiple recipients of list <coredmg@iucr.org>***Subject**:**Re: proposal for twin terms****From**:**Howard Flack <Howard.Flack@cryst.unige.ch>***Date*:*Wed, 18 Oct 2000 09:38:39 +0100 (BST)**Reply-To*:*coredmg@iucr.org*

Dear David, > I would like to propose as an addition to the Core some items concerning > twinned samples. I agree to this. Here are a few thoughts. > In priciple only a few items would be necessary: > > -twinning matrices One needs to be clear on what these rotation matrices operate. From the analysis of the diffraction pattern, its best to have them operate on hkl. From the point of view of understanding the twin laws and the interrelations of the domain orientations, it is probably better to have them in direct space. One has the choice of providing for the two different ways or being a dictator and insisting that there is one 'best' way. Moreover I just wonder whether one should not include a translation vector so that antiphase domains can be treated in the same formalism. Twinning only deals with rotations but in the treatment of phase transitions, the coset representatives in the decomposition of the high symmetry space group with respect to the low symmetry spae group will also give information on the translations. > -volume fractions I think I would prefer to call them mass fractions (or perhaps twin fractions or perhaps mole fractions) and that the twinning matrices and the mass fractions need to be in a loop where both items must be present. As far as I know most least-squares programmes work by assuming that the model structure is that of the first component (with an assumed twinning matrix of identity) whose mass fraction is constrained to be (1 - sum of all the other refined mass fractions). This means that authors do not specifically report the mass fraction of the basic component which is in general the largest one. Should one then insist that the mass fractions of all components including the one with the identity matrix as twin matrix be reported. It goes without saying that the mass fraction must be reported with a standard uncertainty. As the sum of the mass fractions is contrained to be equal to 1.0, the covariance terms are large, and important in the calculation of derived quantities. Should CIF provide a mechanism for reporting the full variance-covariance matrix of the mass fractions? I only know of one calculation (See section 8 of Absolute Structure and Absolute Configuration. H. D. Flack and G. Bernardinelli (1999). Acta Cryst. A55, 908-915. http://www.iucr.org/cgi-bin/details/actaa/v55/p908 - BTW that form of the URL is a general way of finding papers in the IUCr journals from the 'ordinary' literature reference) where one makes linear combinations of twin fractions in order to calculate something of physical significance. However in that particular case, CIF has already the necessary data items for reporting the quantity and its su. > I do not know if some > additional items (perhaps pointers to indicate the contribution of each > domain to a given intensity) are required to cover with generality this > topic. Very very tricky to get something that is general and useable at the same time. Even in the case of 'merohedral' twinning (there are '' because one has to be absolutely sure what one means by this term) space group Pa-3 is unique. For the 0kl (and hk0, h0l) reflections there is the condition for reflection k = 2n with no corresponding condition on l. This means that in the crystal twinned by a two-fold axis along [011], one gets reflections with observable intensity from the twin domain overlapping space group absent reflections from the basic component. How should one index such reflections? Split reflections are the same kettle of fish. In section 1 of Absolute Structure and Absolute Configuration. H. D. Flack and G. Bernardinelli (1999). Acta Cryst. A55, 908-915. http://www.iucr.org/cgi-bin/details/actaa/v55/p908, there is a glossary of terms. I would like to see these included in the coreCIF dictionnary. Comments please. N.B. What I am asking is for a breakaway in the use of CIF. CIF data names will have to be provided for the items in my glossary but presumably nobody will ever use them in a real live CIF. They should however be used in the definition of other data items within the coreCIF dictionnary. Actually I did not make it to the CODATA 2000 conference in Baveno near Stresa in Italy this week. Due to the numerous murderous landslides and terrible flooding in the Alps both on the Italian and Swiss sides, there was no way for me to get to Baveno. I sent an e-mail and then a fax to the conference hotel in Baveno to say I was having trouble with travelling. When I phoned to say I could not make it, the line was down! I wonder whether Brian is really there and having a nice time swimming in the Lago Maggiore? As a result of my need to search through numerous sources of information to try and find out what was going on, I now have an even clearer idea about running a valid web information service. E.g. 24 hours after the road over the Simplon pass was blocked by a landslide, the Swiss TCS (automobile club) web site was still saying it was open. 24 hours later, all one could get out of it was that the server was overloaded. Its all full of Java stuff that flashes false messages at you all the time. Useless. The Swiss railways put up special messages every two hours, but their route planer still offered me on the 'timetable for today' a journey over a stretch of railway that was closed. The Italian railway web site continued unabated as though nothing had happened. The only thing working was their web site. Best wishes, Howard -- Howard Flack http://www.unige.ch/crystal/ahdf/Howard.Flack.html Laboratoire de Cristallographie Phone: 41 (22) 702 62 49 24 quai Ernest-Ansermet mailto:Howard.Flack@cryst.unige.ch CH-1211 Geneva 4, Switzerland Fax: 41 (22) 702 61 08

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