<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>Single crystals of the title compounds are prepared from mixtures of M<sub>2</sub>CO<sub>3</sub> (M: Rb, Cs), CdO, and H<sub>3</sub>BO<sub>3</sub> at ratios of 1:1:10 (Pt crucible, 300, 600, and 850 °C, 12 h each).</p> </abstract>