Poorly crystalline rhabdophane-(Yb or Lu) was synthesized by precipitation from mixed aqueous solutions of Ln(NO3)(3) (Ln = Yb or Lu), (NH4)(2)HPO4, and citric acid with the mixing mole ratios of (NH4)(2)HPO4/Ln(NO3)(3) = 4 and citric acid/Ln(NO3)(3) = 5 to 30, at pH 7 and 90 degreesC for 7 to 30 days. However, unknown XRD peaks and C-O bond FTIR absorption peaks were observed for the precipitates. These peaks disappeared with increasing heating temperature up to 400 degreesC in air, and single phase rhabdophane-(Yb or Lu) was obtained when heated at 400 to 800 degreesC in air. Chemical formulas of the single phase rhabdophane were YbPO4.0.4H(2)O and LuPO4.0.5H(2)O, respectively. The water corresponding to 0.4H(2)O (or 0.5H(2)O) was zeolitic water. The lattice constants are a = 0.676 and c = 0.626 nm for Yb, and a = 0.674 and c = 0.630 nm for Lu. Rhabdophane-(Yb or Lu) changed into the xenotime structure above 860 degreesC, which was stable even at 1800 degreesC in air.