The linear electro-optic (Pockels) effect has been measured in the organic molecular crystal hexamethyl-enetetramine. Crystals were grown from water, alcohol, and chloroform solutions and from the vapor. Electro-optic coefficients computed from Sénarmont-compensator data range from 0.71 × 10-10 cm V-1 to 0.8 × 10-10 cm V-1 or, equivalently, in terms of half-wave voltage: V1/2λ= 85±5 kV. Crystal strain is not responsible for the much wider variation reported in the literature. Electric-field inhomogeneities are considered a more likely cause.
ROBERT W. LEE, "Linear Electro-Optic (Pockels) Effect in Hexamethylenetetramine: Influence of Crystal Strain," J. Opt. Soc. Am. 59, 1574-1580 (1969)