On the temperature dependence of the hydrogen bond order in squaric acid: neutron diffraction studies at four different temperatures1

The crystal structure of squaric acid [3,4-dihydroxy-3-cyclobutene-1,2-dione, H 2 C 4 O 4 , M = 114.1] has been determined from neutron diffraction data at 15 K, T c - 21K, T c - 13 K and T c + 5 K below and above the phase transition at 374 K. The space group of the material is monoclinic P2 1 /m from 15 K and up to T c and tetragonal 14/m above. After corrections for absorption and thermal diffuse scattering, least-squares refinements were carried out with 69 variable parameters in the model below T c and 25 above T c . The analysis of difference vibrational parameters along the C-O internuclear direction strongly suggest that the high-temperature structure has a disordered C 4 O 4 unit. In agreement with previous findings, the protons are distributed on two principal positions 0.53 A apart along the O... O hydrogen bond above T c . In the low-temperature phase, an increasing amount of disorder with increasing temperature is found, particularly in the protonic positions. However, the protons in the two different hydrogen bonds show distinctly different behavior, and hence immediately below T c different degrees of order are found along the x- and z-directions. These structural findings are discussed in the light of the 1-D correlations observed in the material.