Phase transition in non-centrosymmetric 2-methyl-5-nitroanilinium dihydrogen phosphate: structural, spectroscopic and optical studies

A reversible phase transition in 2-methyl-5-nitroanilinium dihydrogen phosphate, (H2m5na)H2PO4, has been found and characterized. Lattice parameters of (H2m5na)H2PO4 change continuously upon cooling with smooth inflection point at 196 K. The crystal symmetry lowers from orthorhombic, space group Pca2(1), to monoclinic one, P2(1). Phase transition to the low-temperature phase features a significant movement of the aromatic ring of H2m5na(+) ion assisted by the tilting of a nitro group. Structural modifications of the hydrogen-bonding network in (H2m5na)H2PO4 are identified and described with the use of graph-set descriptors; algebraic equations based on the graph-set descriptors are given. Phase transition was also monitored by temperature-dependent IR spectra, which revealed significant alterations in vibrational characteristics within 1000-950 cm(-1) and 830-780 cm(-1) spectral ranges. Kurtz-Perry test of (H2m5na)H2PO4 shown that relative SHG efficiency is 0.85 that of KDP at room temperature. Low-temperature phase features over four times weaker SHG than room-temperature one.