Pressure-induced phase transitions in L-alanine,revisited

The effect of pressure on L-alanine has been studied by X-ray powder diffraction (up to 12.3 GPa), single-crystal X-ray diffraction (up to 6 GPa), Raman spectroscopy and optical microscopy (up to 6.4 GPa). No structural phase transitions in L-alanine have been observed. At about 1.5-2 GPa (close to the pressure of the previously reported phase transition into a tetragonal phase, 2.3 GPa [1], [2]), the cell parameters a and b become accidentally equal to each other, but without a change in the space group symmetry. Neither could be a polymorphic transformation into a monoclinic phase at about 9 GPa confirmed, the changes in the cell parameters till the highest measured pressures being continuous and the cell metrics remaining orthorhombic. Our Raman experiments confirmed the effects described previously for the spectra of L-alanine [1], but we have shown that the spectral changes are continuous and are not related to structural phase transitions. During a slow decompression of L-alanine single crystal (in a methanol-ethanol 4:1 mixture as a pressure-transmitting liquid) from about 6 GPa, a new phase crystallized in the diamond anvil cell in the pressure range between 3-5 GPa, which dissolved at about 2 GPa. This previously unknown phase was characterized by powder X-ray diffraction and Raman spectroscopy and seems to be a solvate L-alanine. The study was supported by a grant from RFBR (09-03- 00451), a BRHE grant RUX0-008-NO-06) and a FASI Contract (№ 02.740.11.5102). [1] Teixeira, A.M.R., Freire, P.T.C., Moreno, A.J.D., Sasaki, J.M., Ayala, A.P., Mendes Filho, J., Melo, F.E.A. Solid State Commun., 116 (7), 405. [2] Olsen, J.S., Gerward, L., Freire, P.T.C., Mendes Filho, J., Melo, F.E.A., Souza Filho, A.G, J. Phys. Chem. Solids, 2008, 69 (7), 1641.