Organic acids under pressure: elastic properties, negative mechanical phenomena and pressure induced phase transitions in the lactic, maleic, succinic and citric acids

Abstract

A detailed study of the behavior under pressure of four important organic acids using the first-principles solid-state methodology is presented. These organic acids are the L-(+)-lactic, maleic, succinic and citric acids. The citric acid monohydrate is also investigated. The computed crystal structures and associated X-ray diffraction patterns are in very good agreement with their experimental counterparts. The elastic tensors of these materials are determined using the finite deformation method and the mechanical stability of their structures is studied. A set of relevant elastic properties is obtained in terms of the computed elastic tensors. This set includes the bulk, shear and Young moduli, the Poisson's ratio, the ductility, hardness and anisotropy indices and the bulk modulus pressure derivatives. In the solid state, these organic acids are shown to be stable, relatively weak and very anisotropic materials and, with the exception of citric acid monohydrate, all of them exhibit the negative Poisson's ratio (NPR) phenomenon. The deformation of the crystal structures under isotropic pressures and anisotropic stresses is then evaluated and analyzed. The organic acids considered display the negative linear compressibility (NLC) phenomenon in several narrow pressure ranges. The presence of large NLC effects in these materials is mainly related to the onset of pressure induced phase transitions or sudden structural rearrangements. The lactic acid exhibits a large NLC effect under the effect of isotropic pressures due to the proximity of a pressure induced phase transition occurring at a pressure of P ∼ 1.0 GPa. The maleic acid shows NLC under isotropic pressures of the order of P ∼ 1.1 GPa but no phase transition is observed. Under anisotropic stresses directed along the minimum Poisson's ratio direction, maleic acid also shows a large NLC effect at small external pressures of P ∼ 0.1 GPa. Succinic acid displays small NLC effects in several pressure ranges under isotropic pressures but large NLC values under anisotropic stresses directed along the minimum NPR direction because it undergoes a pressure induced phase transition near P ∼ 1.5 GPa. Finally, the citric acid shows small NLC values for negative isotropic pressures near −0.5 GPa and a large NLC effect under low anisotropic stresses (P ∼ 0.1 GPa) which is accompanied by the breaking of one intramolecular hydrogen bond present in this material at zero pressure.