[Wien] How to input the structure file for the Mo2N structure

[L. D. Marks]

This is a rather fundamental problem. In x-ray (electron, neutron)
diffraction when only the Bragg spots are measured, this corresponds to a
statistical average structure which can be modelled by partial occupancies
of certain sites. There may be long- or short-range ordering of these
sites, which would appear at non-Bragg locations (diffuse scattering).

To model this, you would have to try and construct a supercell with some
appropriate random distribution of the sites, and (rigorously) average
over several supercells with different configurations. There might be some
approximations that others could suggest, but I believe that this is a
non-trivial problem without a simple solution (unlike a calculation of a
simple fcc or bcc material).

On Tue, 10 Aug 2004, yanming Ma wrote:

> Dear Yushan,
>
> Thanks for your reply. I understand what you
> addressed. Actually, I clearly know the real structure
> for Mo2N. It has pm-3m space group. My problem is how
> to locate the N atom in the Mo2N.
> The initial structure for Mo is BCC, the N atoms are
> doped into the intersitial site of Mo. The thing is N
> only located in one intersitial site inside one plane
> of Mo. Another intersitial site inside the plane is
> vacant. But according to the symmetry operation, there
> should be a N atom in this site. I was puzzled.
>
> Can anybody help me out?
>
> Thanks
>
> Yanming
>
>
> --- Yushan Wang <wys at UDel.Edu> wrote:
>
> > hi, guy,
> > please read the following phrase:
> >
> > A partially disordered structure has been proposed
> > for gamma-Mo2N on
> > electron diffraction data. its cubic pattern
> > cooresponds to a bimolecular
> > cell, having a0 = 4.165 (anst.).
> > The four Mo atoms are considered to be in a
> > face-centered array: 0,0,0;
> > F.C., with some of the face-centering positions
> > empty to correspond to a
> > certain departure from the stoichiometric
> > composition. One nitrogen atom
> > is said to be in 1/2, 1/2, 1/2, the other
> > statistically distributed over
> > the positions 0,0,1/2; 0,1/2,0; and 1/2,0,0.
> >
> >
> > good luck.
> >
> > yushan
> >
> > On Mon, 9 Aug 2004, yanming Ma wrote:
> >
> > > Dear Wien2k users,
> > >
> > > I have a big trouble to input the atomic positions
> > for
> > > the Mo2N structure. I couldn't find any reference
> > > which shows the atomic wyckoff position for Mo2N
> > > structure. Please help.
> > > I will highly appreciate your help.
> > >
> > > Thanks in advance.
> > >
> > > Yanming
> > >
> > > =====
> > > Yanming Ma Ph.D
> > > Steacie Institute for Molecular Sciences,
> > > National Research Councils of Canada,
> > > Ottawa, Ontario
> > > K1A 0R6
> > > Canada
> > >
> > >
> > >
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>
>
> =====
> Yanming Ma Ph.D
> Steacie Institute for Molecular Sciences,
> National Research Councils of Canada,
> Ottawa, Ontario
> K1A 0R6
> Canada
>
>
>
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-----------------------------------------------
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2225 N Campus Drive
Northwestern University
Evanston, IL 60201, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
mailto:L-marks at northwestern.edu
http://www.numis.northwestern.edu
-----------------------------------------------