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

A crystal structure analysis of the mixed crystal Rb 0.51 Tl 0.49 H 2 PO 4 was conducted at ambient temperature by neutron diffraction analysis using a high-quality single crystal grown from an aqueous solution. RbH 2 PO 4 (RDP) and TlH 2 PO 4 (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., Rb 0.51 Tl 0.49 H 2 PO 4 , is isotypic to TlH 2 PO 4 , having partially disordered H atoms in strong symmetric O...H...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 PO 4 groups. Although the difference in chemical composition is very subtle, in contrast to the previous case of Rb 0.46 Tl 0.54 H 2 PO 4 , the distribution of the lone-pair electrons seems to change and the hydrogen bond angle, especially for disordered hydrogen atoms, also shows significant changes.