Dielectric properties associated with structural phase transitions observed in tetramethylammonium salt of o-phenylenebis(squaric acid)

Temperature and frequency dependence of the dielectric permittivity of a mono-tetramethylammonium (TMA) salt of a novel squaric acid derivative, o-phenylenebis(squaric acid) (o-PBSQ), was investigated by ac dielectric measurement. The origin of the anomaly observed around 300 K and 100 K was discussed based on the crystal structure analysis at 350, 179, and 112 K. The space group changes from P42/n (350 K) to P21 (112 K) through P21/n (179 K); o-PBSQ forms a dimer through two ionic hydrogen bonds in three crystal structures. The dielectric behavior around 300 K is originated from the ion polarization because the structural phase transition around 300 K is characterized by the translational shifts of the o-PBSQ dimer and TMA. The anomaly around 100 K may be derived from the proton transfer along the two ionic hydrogen bonds of the o-PBSQ dimer because the transition temperature exhibited a distinct deuteration effect.