For example consider the pyrochlore structure A2B2O6O'. We can modify the host as AA'BB'O6O'. Currently, I am using DIAMOND software. In it we can distinguish the two oxygen atoms as they are occupying different crystal sites (here 48f and 8b). But all A-cations are going to crystal site 16d. I am not able to distinguish different A-site cations in the output diagram, so I can't predict the second nearest neighbors.
If there is no ordering of cations on the A-site this is simply a statistical problem. You cannot show this in DIAMOND, but you can work out what the second nearest neighbours are, simply from the knowledge of fractional occupancies of the cations on the A-site.
eg if A-site is 0.5 M and 0.5 M' then the average A site will have 50% M and 50% M'. Simply use this to work out the next nearest neighbours.
If the cations are ordered, you will have lower symmetry and then you can distinguish between them.
Consider CaLaSnNbO6O'. Here all cations have occupancies 0.5.calcium can have Sn or Nb as the neighbors.then which one have greater probabilities?is it possible to distinguish Ca&La, Sn&Nb in the output crystal structure diagram?
If the structure is truly disordered, then each calcium will have an equal number of Sn and Nb neighbours. For calcium, 50% of the time its neighbour will be Sn and 50% of the time it will be Nb.
You can change the colour of the Ca&La atoms so they are different from Sn&Nb:
With diamond, choose the 'picture' tab and then 'atomic designs'. In this menu change the colour of Sn&Nb for example.
16d or 16c refer to two Wyckoff positions characterizing a general or special positions of a specific space group. Each space group has its own set of Wyckoff positions i.e. they are specific to each space group. Therefore, in order to help you in your question, you need to specify the corresponding space group. Each Wyckoff position has its own site symmetry group and the answer to your question depends on the space group.
Int.Tables or Equivalent No.: 227
Hermann-Mauguin Symbol: F d -3 m
Hall Symbol: -F_4vw_2vw_3
Schoenflies Symbol: Oh^7
Point Group: m-3m
Laue Group: m-3m
Crystal System: cubic
Centrosymmetric: yes
both special postions have the same site symmetry (3barm). However, the 16 equivalent positions of 16d are shifted by 1/2 1/2 1/2 regarding to the equivalent positions 16c. Assuming that all the other atoms atoms keep their positions, putting 16 A atoms in c or d should change the coordination
Back to your original question, if a specific site has been refined with a statistical distribution with identical weight (or not) of two or more atomic types, there is no way that you can attribute a fraction of the 16 c or d positions to a specific atom type.
In general, the refinement of a complex structure like AA'BB'O6O' in classical crystallography is sometimes a first approximation. You need a better X-ray resolution (e.g. synchrotron) to observe may be a modulation of the structure (superstructure). In this case, you might distinguish atoms which were statistically distributed in the model with lower resolution.
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