Strategies in structure determination from powder data

2.2.1- High quality pattern

Accuracy of ± 0.02° 2-theta.

Minimal FWHM = 0.14 to 0.20° 2-theta may suffice.

Minimal FWHM lowered to 0.08-0.04° 2-theta with correct set of slits.

Kalpha-1 is better but Kalpha-1,2 works too.

Counting step should be as low as 0.02-0.01° 2-theta.

All this is possible on a conventional X-ray powder diffractometer.

Synchrotron can do far better with FWHMs as low as 0.008° 2-theta.

Neutron powder diffractometers cannot offer comparable performances.
Minimal FWHMs are near of 0.12 or 0.20 or even 0.30° 2-theta depending on the instrument

Two techniques are useful in the objective of collecting perfect data for indexing purposes :

a- A standard is mixed with your sample for calibration.
b- Auto-calibration.

According to the technique selected, the high quality pattern which will produce data for indexation can be realized (on a standard in-laboratory instrument) :

from 5 to 80° 2-theta (or more if you wish) if a standard material for calibration is mixed with your sample,
from 5 to130 or even to 160° 2-theta if the sample is pure for a final pattern. Low counting step, high counting time.