Niobium and some niobium alloys are physiologically inert and hypoallergenic. Niobium and niobium carbide dissolves in steel at very high temperatures. Niobium tetraargide, NbAr probably has the argon atoms arranged in a square around the niobium. The characteristics of niobium electrolytic capacitors and tantalum electrolytic capacitors are roughly comparable. All unstable isotopes of molybdenum decay into isotopes of zirconium, niobium, technetium, and ruthenium. The ore contains such rare metals as beryllium, lithium, tantalum, and niobium. Hafnium is used in alloys with iron, titanium, niobium, tantalum, and other metals. Minerals that contain niobium often also contain tantalum. All unstable isotopes of molybdenum decay into isotopes of niobium, technetium, and ruthenium. Other tunicates similarly concentrate niobium and tantalum. Columbite and tantalite are ores used to produce the elements niobium and tantalum. Natural rutile may contain up to 10% iron and significant amounts of niobium and tantalum. Some superalloys used for special applications contain hafnium in combination with niobium, titanium, or tungsten. Nanoparticles of lithium niobate and niobium pentoxide can be produced at low temperature.