<< We have done Rietveld refinement on GaN powder obtained by grinding single
crystals and the results were the following:
a=3.1843(5) c=5.1746(8)
Stan
[EMAIL PROTECTED] >>
I extrapolated a value of 5.1847(5)A on a freestanding (presumably high
purity), [00l] textured GaN film using MoKa1/Ka2 rad. Thus, my value happens
to be identical with that reported in PDF # 50-0792 which is 5.185A. The
actual reference cited in the PDF lists a value of 5.1855(2)A.
But I have a question: Why does the PDF not adopt the experimental or
measurement uncertainties found in the reference it lists? The lattice
spacings found in the reference are exactly those found in the PDF. Further,
the PDF lists spacings <1A with a precision of 1 part in 100,000 (which the
authors never did) while the c-parameter is given only with a precision of 1
in 1000. Are the lattice parameters in the PDF reported as 'accurate'
parameters? If so, a measurement uncertainty should be associated with such a
quantity. On the other hand, if the parameters are derived by statistical
methods (such as refinement) then they are precise values. Anybody knows what
PDF had in mind?
L. Keller
Thank you for all the responses on GaN.
