Neutron Diffraction Analysis of a Single-Crystal Structure of Rb0.51Tl0.49H2PO4

A crystal structure analysis of the mixed crystal Rb0.51Tl0.49H2PO4 was conducted at ambient temperature by neutron diffraction analysis using a high-quality single crystal grown from an aqueous solution. RbH2PO4 (RDP) and TlH2PO4 (TDP) exhibit clearly different crystal structures from each other, although Rb+ and Tl+ have almost the same ionic radii. The crystal structure of the title compound, i.e., Rb0.51Tl0.49H2PO4, is isotypic to TlH2PO4, having partially disordered H atoms in strong symmetric 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 considered to have an effect on the stabilization of the two-dimensional network of hydrogen-bonded PO4 groups. Although the difference in chemical composition is very subtle, in contrast to the previous case of Rb0.46Tl0.54H2PO4, the distribution of the lone-pair electrons seems to change and the hydrogen bond angle, especially for disordered hydrogen atoms, also shows significant changes.