Evidence for breaking of the center of symmetry in zero-dimensional H-bonded quantum paraelectric Rb3H(SeO4)2

We show by means of single-crystal neutron-scattering that the space group of the quantum paraelectric Rb3H(SeO4)2 is not centrosymmetric A2/a as accepted so far, but noncentrosymmetric A2. The intensities of the reflections forbidden for the space group A2/a, (h 0 l): h≠2n, decrease with decreasing temperature. The refinement of the structure gave a systematically smaller discrepancy values in A2 than in A2/a. This makes it necessary to modify the commonly accepted fact that the space group is A2/a for most members of M3H(XO4)2-type crystals. The symmetry breaking is mainly due to the atoms directly connected to the O–H⋯O hydrogen-bond system, other parts being substantially unchanged. The possible origin of the temperature dependence of the intensities of the symmetry-breaking reflections is discussed.