Powder X-ray Diffractometer: Theory and Lab

A: 0.10° is the threshold here. Zero offset is an additive angular error in 2θ, not scaled by d-spacing or wavelength. At 28° 2θ, the diffractometer geometry doesn’t attenuate it. Back-of-envelope: whatever offset you dial in shows up nearly one-for-one in peak position, especially at low-to-mid angles. That’s why even small zero errors matter during phase ID.

A: 15° is the boundary that trips people. Bragg’s law uses θ, not 2θ. d = λ / (2 sinθ) = 1.5406 / (2·sin15°). Sin15° ≈ 0.259, giving d ≈ 2.98 Å. Using 30° instead doubles the sine and collapses d by mistake, a common lab error when you’re moving fast.

A: Plate-like particles are the trigger here. Side-loading disrupts alignment by filling laterally rather than normal to the surface. High-pressure pressing drives plates to lie flat. Grease adds amorphous background and can dissolve organics. Single crystal mounts defeat the whole point of powder averaging.

A: 640 is the number that matters. NIST SRM 640 silicon has a certified lattice parameter and narrow peaks, making it suitable for angle calibration. Alumina is phase-variable by grade. NaCl hydrates and shifts. Glassy carbon is used for background checks, not angle accuracy.