A series of lanthanide and yttrium carboxyethylphosphonates has been hydrothermally prepared (200 degrees C, 4 days) from the 1:1 mixture of carboxyethylphosphonic acid and the metal chlorides. The crystal structure of the praesodymium compound Pr(III)[O(3)P(CH(2))(2)CO(2)] has been determined ab initio from X-ray powder diffraction data and refined by the Rietveld method. The compound crystallizes in the monoclinic space group P2(1)/m (No. 11) with cell parameters at 20 degrees C a = 8.3617(2) Å, b = 7.1899(2) Å, c = 5.4586(2) Å, beta = 103.784(2) degrees, and Z = 2. The final agreement factors converged to the values R(p) = 0.139, R(wp) = 0.177, Bragg R = 0.123, R(F) = 0.055, and chi(2) = 3.50. The hybrid framework consists of inorganic Pr/O/P/C layers connected by organic groups, with an interlayer spacing of approximately 8.36 Å. The praesodymium atoms are 7-fold coordinated in this pillared layered structure. Isostructural compounds were prepared for yttrium and the entire series of the lanthanide elements.