Neutron diffraction study of a Rb0.5TI0.5H2PO4 single crystal

Crystal structure analysis of a mixed crystal Rb1-x,TlxH2PO4 (x = 0.5) was performed at room temperature by neutron diffraction on a high-quality single crystal grown from aqueous solution. RbH2PO4 (RDP) and TlH2PO4 (TDP) show distinct crystal structures despite the same ionic radii of Rb and T1. The crystal structure of Rb0.5Tl0.5H2PO4 is isotypic to TDP showing disordered H atoms in the O center dot center dot center dot H center dot center dot center dot 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, Rb0.5Tl0.5H2PO4 shows no phase transition at low temperatures despite the disordered character, with a strengthening of two hydrogen bonds as indicated by shorter O center dot center dot center dot O distances and greater O center dot center dot center dot H center dot center dot center dot O bond angles.