To provide a new compound with pores finely tunable in size so as to take up a specific element and release the specific element taken up in the pores as necessary, a synthesis method of the new compound, and a separation and recovery agent. The new compound represented by the following molecular formula: (NH4)[Ln(C2O4)2(H2O)] wherein Ln represents a lanthanide selected from Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu.