These elements are vanadium, rubidium, indium, lanthanum, europium, rhenium and lutetium. Monazite is an important ore for thorium, lanthanum, and cerium. Three group 3 elements occur naturally&colon; scandium, yttrium, and either lanthanum or lutetium. All the trivalent lanthanide ions, except lanthanum and lutetium, have unpaired f electrons. It is always preceded by cerium, lanthanum and usually also by neodymium. This row would begin with a trivalent lanthanum, a teravalent cerium, and a pentavalent didymium. Examples include&colon; rubidium, barium, uranium, and lanthanum. Examples are lithium niobate (LiNbO) and lanthanum niobate (LaNbO). Samarium can also be obtained by reducing its oxide with lanthanum. They are commonly lanthanum and actinium, and less often lutetium and lawrencium. Scandia stabilized zirconia (ScSZ), ceria based electrolytes or lanthanum gallate materials. They are called lanthanides because the elements in the series are chemically similar to lanthanum. It is the most common of the lanthanides, followed by neodymium, lanthanum, and praseodymium. The metals that formed these oxides were thus named "lanthanum" and "didymium". Lanthanum and actinium are commonly depicted as the remaining group 3 members.