Berkelium was the fifth transuranium element discovered after neptunium, plutonium, curium and americium. These are actinides other than uranium and plutonium and include neptunium, americium and curium. This criterion includes plutonium, uranium, americium, and neptunium among others. Technetium, promethium, astatine, neptunium, and plutonium were discovered through synthesis before being found in nature. Natural deposits of plutonium and neptunium are produced by neutron capture in uranium ore. Examples include gallium, ytterbium, bismuth, mercury and neptunium. Examples include gallium, ytterbium, bismuth and neptunium. Similar species exist for neptunium and plutonium. In addition, the dioxides of cerium, plutonium and neptunium have the same structures. In their experiments bombarding uranium with deuterons, they observed the creation of neptunium, element 93. The oxides and hydroxides of neptunium are closely related to its ions. Many of the resulting compounds also have an equivalent compound that substitutes uranium for neptunium. Two neptunium phosphide compounds have been reported, NpP and NpP. Ions of neptunium are prone to hydrolysis and formation of coordination compounds. Three neptunium arsenide compounds have been prepared, NpAs, NpAs, and NpAs.