The structural phase transitions of aminoguanidinium(1+) dihydrogen phosphate—study of crystal structures, vibrational spectra and thermal behavior

Aminoguanidinium(1+) dihydrogen phosphate was prepared by crystallization from aqueous solution. On the basis of the results of DSC measurements, X-ray structural analysis was carried out at temperatures of 160, 215 and 293K for three aminoguanidinium(1+) dihydrogen phosphate phases (I|>222K|P1¯(2) |Z=2|non-ferroic |melting point 408K; II |201–222K|P1¯(2) |Z=2|non-ferroic|-; III |<201K|P1¯(2)|Z=4|non-ferroic|-). The triclinic unit cell dimensions (a=6.8220(2), b=7.1000(2), c=7.4500(2)Å, α=86.925(2)°, β=80.731(2)°, γ=79.630(2)°, V=350.21(2)Å3—phase I) are similar for all three structural phases with the exception of phase III, where doubling of the c-axis length leads to an increase in the volume to 692.34(3)Å3. The crystal structure of all three modifications consists of parallel layers of dihydrogen phosphate anions that are interconnected by aminoguanidinium(1+) cations through hydrogen bonds of the N–H…O type. The planar aminoguanidinium(1+) cations are oriented almost parallel to each other and are perpendicular to the anion layers. The primary differences amongst phases I, II and III lie in the location of the H atom in the short O–H…O bonds connecting the dihydrogen phosphate anions in layers. The FTIR and FT Raman spectra of natural and deuterated compounds were recorded and interpreted. The FTIR spectra were studied down to a temperature of 90K.