On the theory of the structural phase transition in squaric acid

Crystals of the squaric acid C 4 H 2 O 4 (H 2 SQ) and C 4 D 2 O 4 (D 2 SQ) exhibit a structural phase transition at T 0H = 370 K and T 0D = 520 K. The phase transition is associated with the ordering of protons, moving above T 0 in the symmetric double well potential of OHO bonds. The statistic of the phase transition in H 2 SQ and D 2 SQ is investigated in the cluster approximation with account of the tunneling of the protons. It is shown that the phase transition in the model is always of second order and that the increase of the ordering parameter with the decrease of temperature takes place in a large temperature range. The predictions of the theory are compared with experimental data at hand.