I had sent a screen capture of the PDF card. There are two other sources
for the lattice parameter: the editorial file data (AIDS-83) and PDF-2
printed output (also a graphic image, but one not screen-limited). Both
the AIDS-83 file and the printed output for PDF 50-0792 show
c=5.1855(2)A. The screen capture did not report an esd and truncated the c
axis value reported by the author (this is a bug in PCPDFWIN and will be
fixed).
I have attached an Adobe PDF file that shows the printed output (a graphic
file with 13.3KB).
>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
In the editorial process, the data are independently evaluated: we check
the lattice parameter results using least-squares analysis. We don't alter
the d,I pairs given by the author. However, if the cell parameter
least-squares values yield significantly better residuals (e.g., as judged
by the FOM) then the derived lattice parameters may be reported on the PDF
card. In these cases however, we add comment lines for the entry that
alert the PDF user to this substitution. The comments cards are then used
to report the author's cell.
For 50-0792, a quality index code reported an average error of 2-5 parts
per 10^5. In this case, the author's data and esd's were used as submitted
and match the reported journal reference.
>
>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?
500792.pdf
_____________________________________
Dr. John Faber
Principal Scientist
ICDD
12 Campus Boulevard
Newtown Square, PA, 19073
ph : (610)325-9814
fax: (610)325-9823
e-mail: [EMAIL PROTECTED]
also see: http:\\www.icdd.com
