Neutron diffraction study of a Rb_0.5Tl_0.5H ₂PO₄ single crystal

Crystal structure analysis of a mixed crystal Rb_1-xTl _xH₂PO₄ (x = 0.5) was performed at room temperature by neutron diffraction on a high-quality single crystal grown from aqueous solution. RbH₂PO₄ (RDP) and TlH₂PO ₄ (TDP) show distinct crystal structures despite the same ionic radii of Rb and Tl. The crystal structure of Rb_0.5Tl_0.5H ₂PO₄ is isotypic to TDP showing disordered H atoms in the O⋯H⋯O hydrogen bonds. The lone-pair electrons of Tl⁺ ions are believed to stabilize the two-dimensional network of hydrogen bonded PO₄-groups. In contrast to the case of TDP, Rb_0.5Tl _0.5H₂PO₄ shows no phase transition at low temperatures despite the disordered character, with a strengthening of two hydrogen bonds as indicated by shorter O⋯O distances and greater O⋯H⋯O bond angles.