Elastic properties of ammonium oxalate hydrate, ammonium hydrogen oxalate hemihydrate, and ammonium tetroxalate dihydrate

The elastic constants of the two orthorhombic crystals (NH₄)₂C₂O₄. H₂O and (NH₄)HC₂O₄.½H₂O were determined by diffraction of light by ultrasonic waves. These constants and those of the triclinic (NH₄)H₃(C₂O₄)₂.2H₂O previously measured are correlated with the crystal structure. All three crystals show extreme elastic anisotropy which may be explained by the layer-like packing of the oxalate groups. The minimum elastic stiffness is observed normal to the C₂O₄ planes, whereas the direction of the maximum elastic stiffness may be correlated with the course of the periodic bond chains. It is shown that hydrogen bonds make a considerable contribution to the elastic properties of crystals. Correlations between elastic behaviour, morphology, and optical properties are discussed.