Reza,
isomorphism is not an absolute term; there are different "degrees of isomorphism". The quantity to look at is the absolute difference of cell axes; for good isomorphism the value should be less than the high resolution limit of the data, divided by 3.
Why? Because then the same high resolution position in reciprocal space has indices that differ by at most 1/3, e.g. if there were a reflection with H=50 in one of the crystals, its non-integer equivalent should be between 50-1/3 and 50+1/3 .
If the difference were larger, e.g absolute difference=resolution, then that would result in indices differing (in that example) by 1. And if that happens, why should one expect that intensities are the same if indices differ by 1? The factor of 1/3 is thus a safeguard against large intensity differences (or in other words, different sampling points of the molecular transform).
HTH,
Kay
On Sun, 10 Jul 2016 14:51:38 +0000, Reza Khayat <[log in to unmask]> wrote:
>Hi,
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>How similar do cell dimensions have to be in order for the x-tals to be considered as isomorphous? Thanks.
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>Best wishes,
>Reza
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>
>Reza Khayat, PhD
>Assistant Professor
>City College of New York
>Department of Chemistry
>New York, NY 10031
>
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