Armel, you hit the point: I do generally know the structure type, but I
don't know the exact position of the elements in the unit-cell, if you mean
that by "structure".
Take an example: You make a perovskite A2BB'O6. You know, it is a distorted
perovskite of the GdFeO3-type. Additionally, you have (partial or total)
cation ordering (teh B's), thus your space group is P 1 21/N 1 (to be
precise with all those cell choices in monoclinic). But La and all oxygens
are not on totally fixed positions, thus they obey free parameters.
Profile matching works quite well, but if you have a rather poorly
crystallized sample and the distorsion is not very important, you will
result with nearly identical R-values for the orthorhombic Pnma, the space
group of the disordered or pure GeFeO3-type, respectively. You could even
index the whole pattern in cubic primitive (fortunately, there is logic to
exclude this possibility; you do not find extinction rules, but any CUBIC
superstructure of the perovskite type would be F-centered).
Seems to be a simple problem, huh? But I assure, that you will hardly
arrive to make the difference between cubic, orthorhombic and monoclinic
with nothing but powder-profile fitting. The moment, you take the
intensities into account, you will arrive (but even then, the difference
between orthorhombic and monclinic is of about 5% in Rwp, roughly).
Carsten
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