Temperature dependence of lattice constants of Rb3H(SeO4)2, a 'zero-dimensional' h-bonded crystal

In view of the interest in the 'zero-dimensional' H-bond network and low transition temperature in M 3 H(XO 4 ) 2 -type crystals, lattice constants of Rb 3 H(SeO 4 ) 2 have been measured in a temperature range between room temperature and 20 K using an automatic X-ray 4-circle diffractometer with a closed-cycle refrigerator in order to confirm whether it does not exhibit any phase transitions. With lowering temperature, a-, b-, and c-axes in monoclinic cell showed a monotonous decrease, while β decreased very little, with no remarkable changes suggesting phase transitions. These results raise a fundamental problem, i.e. why Rb 3 H(SeO 4 ) 2 does not exhibit phase transition although the crystallographically symmetric H-bond of 2.514(7) Å lies in a range where disordering of the protons will almost certainly occur.