A method for forming a metal oxide includes a step of adding citrate acid to a salt-containing solution to produce a precursor solution. Characteristically, the salt-containing solution includes a neodymium salt or an americium salt. A chilled solution of hexamethylenetetramine (HMTA) and urea is added to a chilled precursor solution to form a citrate gel. Advantageously, citrate gel can be produced with an R-value of 0.5 to 3, wherein the R-value is a molar ratio of HMTA to Nd. The citrate gel is then heated to form the metal oxide of neodymium or americium.