Martensitic transformations in single crystals of 1,20-eicosanedioic acid, HOOC(CH2)18COOH

A martensitic transformation is found in single crystals of 1, 20-eicosanedioic acid. It has been studied by optical microscopy and X-ray diffraction, using diamond anvil cells and a cold stage. The new phase has a triclinic crystal structure. The transformation from the monoclinic (ordinary form) to the triclinic phase occurs mostly between 5 and 10kbar on applying hydrostatic pressure at room temperature, and between-50 and-70°C on cooling at 1 atm (the transformation pressures and temperatures varied from crystal to crystal). The reverse transformation occurs at about room temperature and 1 atm in both cases. The lattice constants of the triclinic phase at room temperature are a =27·9, b = 5·56, c = 814 Å, α = 79·3°, β=113·9°, γ= 116·4° at 1 atm and a = 27·8, fc = 5·37, c = 7-7Å, a = 79°, β=115°, γ=117° at 7.7kbar. From the lattice constants and from optical microscopy, the transformation is shown to proceed by simple shear. The shear plane and shear direction are (01 l) m , [100] m or (0l 1 ) m , [ 1 00] m for the transformation and (0l 1 ), [100], for the reverse transformation. The magnitude of shear is 0-61; each {011 } m or (01 1 ), plane slips relative to the others by 2-4-2.6 Å along the chain axis.