Welsbach separated didymium into praseodymium and neodymium. He wrote his thesis on "Equilibria in the oxide systems of praseodymium and americium". Ac (actinium and acetyl) and Pr (praseodymium and propyl). They came to be known as praseodymium and neodymium. Notable impurities in johnsenite-(Ce) are iron, titanium, niobium, yttrium, potassium, praseodymium, and neodymium. Cerium(IV) oxide has the fluorite structure, similarly to the dioxides of praseodymium and terbium. In addition, larger overabundances are often seen in praseodymium and neodymium. Praseodymium sulfate is stable under standard conditions. It is the most common of the lanthanides, followed by neodymium, lanthanum, and praseodymium. It is the most stable form of praseodymium oxide at ambient temperature and pressure. Praseodymium is the third member of the lanthanide series. Praseodymium always occurs naturally together with the other rare-earth metals. Praseodymium was also accepted as the name of the minor fraction. Praseodymium is a chemical element with the symbol Pr and atomic number 59.