Hi,
I think the difference between the lattice
parameter of single crystal and powder is
coming from the misoriented surface of the
crystal.
To detect this rotate the crystal in its own plane
360 degrees with a convenable step and plot the
lattice parameter "a" versus the number of steps,
you'll get a sin type curve. The midle between
a_max and a_min is the true lattice parameter.
If both, the powder and crystal are stress free
the two values should coincide.
Danut
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On Thu, 11 Mar 1999, Charles Witham wrote:
> I recently encountered this with Y2O3. The literature results from single
> crystal measurements show a lattice parameter of 10.604 Å while powder
> diffraction results show 10.607 Å. I am using Y2O3 to calibrate the
> twotheta response of my cps-120.
>
> Originally I thought I should use 10.604 Å to calculate my peak positions
> because I thought single crystal would be more, and i hesitate before i say
> this, accurate? reliable? If I calibrate my equipment with the single
> crystal values, does this imply that I will calculate values in line with
> single crystal measurements from subsequent powder diffraction measurements
> with that calibration?
>
> Can someone explain why single crystal experiments would generate different
> cell parameters than powder diffraction? Or maybe offer a reference that
> talks about it?
>
> thanks,
>
> chuck
>
> >Do note the differences that occur between cell parameters
> >determined form single crystal and powder diffraction data.
> >
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>
> Charles Witham
>
> Graduate Research Assistant, Materials Science
> California Institute of Technology
> Mail Stop 138-78 [EMAIL PROTECTED]
> Pasadena, CA 91125 (626) 395-3627
> fax 795-6132
> JPL lab (818) 393-4209
>
>
>
