Crystal growth and physical properties of monoclinic L‐arginine hydrochloride monohydrate, C 6 H 14 O 2 N 4 HCl · H 2 O, and L‐arginine hydrobromide monohydrate, C 6 H 14 O 2 N 4 HBr · H 2 O

Single crystals of optical quality of L‐arginine HCl · H 2 O and L‐arginine HBr · H 2 O with dimensions up to 80 × 60 × 30 mm have been grown from aqueous solutions by controlled evaporation at about 310 K. The isotypic crystals exhibit a quite similar behaviour with respect to morphological, pyroelectric, dielectric, optical, thermal expansion, electrostrictive, electrooptic, elastic, thermoelastic and non‐linear optical properties. A distinct anisotropy of the longitudinal elastic stiffness is observed with a maximum along the direction of the alignment of the arginine ions. Nearly in the same direction a minimum of thermal expansion, dielectric constant and index of refraction is found. The pyroelectric, electrostrictive, electrooptic and non‐linear‐optical effects are of only mediocre magnitude except the electrostrictive constant d 211 which surmounts the longitudinal electrostrictive constant d 111 of alpha‐quartz by a factor three.