A neodymium oxalatophosphonate and its praseodymium analogue, Na[Nd-3(H2O)(4)(C2O4)(4)(CH3PO3)]center dot 2H(2)O and Na[Pr-3(H2O)(4)(C2O4)(4)(CH3PO3)]center dot 2H(2)O, have been synthesized by a hydrothermal method and characterized by single-crystal X-r