Reliability of second-harmonic generation in crystalline powders at optical frequencies as a test of absence of centrosymmetry

The detection of centre of symmetry is a problem of great importance in the determination of crystal structures by X-ray methods, and not always solvable by these methods alone. Generation of a second harmonic at optical frequency is governed by coefficients which are zero for all centrosymmetric point groups and 432, 422, 622 groups and non-zero for all other non-centrosymmetric point groups. Results are reported of second-harmonic tests on 90 crystalline powders. The wide range of applicability makes the second-harmonic generation test rather attractive and more advantageous than other physical methods.