On the pressure dependence of the crystal structure of squaric acid (H₂C₄O₄)

The room-temperature crystal structure of squaric acid (H₂SQ), H₂C₄O₄, has been determined by X-ray diffraction at 2.75 GPa; and the lattice parameters have been measured at a number of pressures between 0.93 and 4.70 GPa. Under these conditions, H₂SQ is above the monoclinic-to-tetragonal transition temperature, T_c, in its tetragonal phase. The principal structural changes with pressure are shown to be compression of 2R, the length of the O-H-O bonds connecting the C₄O₄ groups, and a reduction of the separation between the planar H₂C₄O₄ sheets so formed. The C₄O₄ groups maintain their size and shape, but rotate a little in a sense that tends to reduce the compression of 2R. The form of 2R(P) is used to re-examine the relationship between the changes in 2R and T_c observed on deuteration of H₂SQ.