As with oxides, protactinium forms mixed halides with alkali metals. An alternative reaction is between protactinium pentachloride and hydrogen bromide or thionyl bromide. "uranium-X" (later established to be protactinium). Oxyhalides and oxysulfides of protactinium are known. Also a harder neutron spectrum helps to achieve acceptable breeding without protactinium isolation. Protactinium forms borohydrides which include Pa(BH). The estimated shear modulus of protactinium is similar to that of titanium. In all these materials, protactinium ions are octahedrally coordinated. The protactinium removal step is not required per se for a LFTR. Protactinium forms mixed binary oxides with various metals. Other known protactinium ions include , , PaF, , , and . Protactinium was possibly isolated in 1900 by William Crookes. For ease of pronunciation, the name was shortened to "protactinium" by the IUPAC in 1949.