L-asparagine monohydrate at high temperatures: transformations and negative thermal expansion. A study by thermal analysis and synchrotron X-ray diffraction

In the present work, the crystal of L-asparagine monohydrate (C₄H₈N₂O₃.H₂O) was studied at high temperatures by thermal analysis and synchrotron X-ray diffraction. Two structural transformations (dehydration and decomposition) are briefly discussed. L-asparagine monohydrate exhibits anisotropic thermal expansion, as it is more intense along the b-axis. This crystal has an even more interesting effect, a negative thermal expansion along the a-axis in the range of 311–358 K. Thermal expansions were analyzed from the anisotropic atomic displacement parameters using the rigid body model. The behavior of the translation and libration tensors indicate positive expansions along the hydrogen bonds that connect the molecular chains in the b x c plane, as well as a negative thermal expansion along the hydrogen bond transverse to the this plane.