Abstract
Geometry of the four-circle diffractometry in the fixed φ mode was reexamined to revise control programs of the diffractometer for intensity measurements with high-pressure diamond-anvil cells. The benefit of the φ=0 mode has been explained on the basis of the fact that the sum of αp (the angle between the incident X-rays and the radial symmetry axis of the diamond-anvils) and αd (the angle between the diffracted X-rays and the radial symmetry axis of the anvils) becomes minimum. In the present studies, it was revealed that not only the sum of αp and αd, but αp and αd themselves are minimum when φ=0 or π. This elucidates the above benefit more easily and simply. Importance of the φ=π mode measurements was pointed out.
