Two-theta (degree) Figure S1. Experimental X-ray powder pattern (top) and simulated powder pattern (bottom) based on the results from single-crystal X-ray diffraction for Cs 3 EuSi 6 O 15 50 10 20 30 40

A neodymium oxalatophosphonate and its praseodymium analogue, Na[Nd3(H2O)4(C2O4)4(CH3PO3)]·2H2O and Na[Pr3(H2O)4(C2O4)4(CH3PO3)]·2H2O, have been synthesized by a hydrothermal method and characterized by single-crystal X-ray diffraction and thermogravimetric analysis. We studied the photoluminescence spectra of the Nd and Pr compounds as well as the magnetic properties of the Nd compound. The structure consists of layers built of tetramers of edge- and face-sharing NdO9 (or PrO9) polyhedra connected by oxalate and methylphosphonate units, which are linked by zigzag infinite chains of oxalate-bridged NdO9 (or PrO9) polyhedra to form a 3D framework. They are the first examples of lanthanide oxalate-methylphosphonate. The Nd compound displays characteristic emission bands in the near IR region. The χM T value decreases from 1.68 cm3 K/mol Nd at 300 K to 0.659 cm3 K/mol Nd at 2 K. The value at room temperature is close to the theoretical value for a mononuclear Nd3+ species. Luminescence of the Pr compound consists of many emission bands at 480−760 nm. The lifetimes of both compounds are much shorter than the expected values because of the quenching effects of water molecules in the structure. Crystal data for the Nd compound:  triclinic, P1̄ (No. 2), a = 9.0682(2) Å, b = 9.5114(3) Å, c = 14.0066(4) Å, α = 74.975(2)°, β = 86.534(2)°, γ = 82.725(2)°, and Z = 2. Crystal data for the Pr compound are the same as those for the Nd compound, except a = 9.125(1) Å, b = 9.549(1) Å, c = 14.073(2) Å, α = 75.086(2)°, β = 86.611(2)°, and γ = 82.545(2)°.