Addendum to “Polymorphic phase transition and thermal stability in squaric acid (H2C4O4)” by K.-S. Lee et al. [J. Phys. Chem. Solids 73 (2012) 890]

While a paper mentioned above being published on line, we have become aware of the high-pressure neutron diffraction study of squaric acid (H2C4O4) by C.L. Bull et al. They developed that neutron diffraction experiments could be performed under quasi-hydrostatic conditions to pressures of up to 18 GPa and showed that the tetragonal phase of H2C4O4 was still observed at 14.5 GPa (above the critical pressure of Pc=0.75 GPa at room temperature) beyond the previous pressure limits of 7 GPa. Taking the high-pressure neutron diffraction results into consideration, modified temperature-pressure phase diagram in the paper stated above is reported.