Editing Protactinium (section)

===Oxides and oxygen-containing salts===
Protactinium oxides are known for the metal oxidation states +2, +4, and +5. The most stable is the white pentoxide [[Protactinium(V) oxide|Pa<sub>2</sub>O<sub>5</sub>]], which can be produced by igniting protactinium(V) hydroxide in [[air]] at a temperature of 500&nbsp;°C.<ref name="g1268">[[#Greenwood|Greenwood]], p. 1268</ref> Its crystal structure is cubic, and the chemical composition is often non-stoichiometric, described as PaO<sub>2.25</sub>. Another phase of this oxide with orthorhombic symmetry has also been reported.<ref name="pao2" /><ref name="pacl4b" /> The black dioxide [[Protactinium(IV) oxide|PaO<sub>2</sub>]] is obtained from the pentoxide by reducing it at 1550&nbsp;°C with hydrogen. It is not readily soluble in either dilute or concentrated [[nitric acid|nitric]], [[hydrochloric acid|hydrochloric]], or [[sulfuric acid]], but easily dissolves in [[hydrofluoric acid]].<ref name="pao2" /> The dioxide can be converted back to pentoxide by heating in oxygen-containing atmosphere to 1100&nbsp;°C.<ref name="pacl4b">{{cite journal|last1=Elson|first1=R.|last2=Fried|first2=Sherman|last3=Sellers|first3=Philip|last4=Zachariasen|first4=W. H.|title=The tetravalent and pentavalent states of protactinium|journal=[[Journal of the American Chemical Society]]|volume=72|pages=5791|date=1950|doi=10.1021/ja01168a547|issue=12|bibcode=1950JAChS..72.5791E }}</ref> The monoxide PaO has only been observed as a thin coating on protactinium metal, but not in an isolated bulk form.<ref name="pao2" />

Protactinium forms mixed binary oxides with various metals. With alkali metals ''A'', the crystals have a chemical formula APaO<sub>3</sub> and [[perovskite structure]]; A<sub>3</sub>PaO<sub>4</sub> and distorted rock-salt structure; or A<sub>7</sub>PaO<sub>6</sub>, where oxygen atoms form a hexagonal close-packed lattice. In all of these materials, the protactinium ions are octahedrally coordinated.<ref name="g1269">[[#Greenwood|Greenwood]], p. 1269</ref><ref>{{cite journal|doi=10.1107/S056774087100284X|last1=Iyer|first1=P. N.|date=1971|pages=731|volume=27|journal=[[Acta Crystallographica B]]|last2=Smith|first2=A. J.|title=Double oxides containing niobium, tantalum or protactinium. IV. Further systems involving alkali metals|issue=4|bibcode=1971AcCrB..27..731I }}</ref> The pentoxide Pa<sub>2</sub>O<sub>5</sub> combines with rare-earth metal oxides R<sub>2</sub>O<sub>3</sub> to form various nonstoichiometric mixed-oxides, also of perovskite structure.<ref>{{cite journal|last1=Iyer|first1=P. N.|last2=Smith|first2=A. J.|title=Double oxides containing niobium, tantalum, or protactinium. III. Systems involving the rare earths|journal=[[Acta Crystallographica]]|volume=23|pages=740|date=1967|doi=10.1107/S0365110X67003639|issue=5|bibcode=1967AcCry..23..740I }}</ref>

Protactinium oxides are [[Basic oxide|basic]]; they easily convert to hydroxides and can form various salts, such as [[sulfate]]s, [[phosphate]]s, [[nitrate]]s, etc. The nitrate is usually white but can be brown due to [[radiolysis|radiolytic]] decomposition. Heating the nitrate in air at 400&nbsp;°C converts it to the white protactinium pentoxide.<ref name="target" /> The polytrioxophosphate Pa(PO<sub>3</sub>)<sub>4</sub> can be produced by reacting the difluoride sulfate PaF<sub>2</sub>SO<sub>4</sub> with [[phosphoric acid]] (H<sub>3</sub>PO<sub>4</sub>) under an inert atmosphere. Heating the product to about 900&nbsp;°C eliminates the reaction by-products, which include [[hydrofluoric acid]], [[sulfur trioxide]], and phosphoric anhydride. Heating it to higher temperatures in an inert atmosphere decomposes Pa(PO<sub>3</sub>)<sub>4</sub> into the diphosphate PaP<sub>2</sub>O<sub>7</sub>, which is analogous to diphosphates of other actinides. In the diphosphate, the PO<sub>3</sub> groups form pyramids of C<sub>2v</sub> symmetry. Heating PaP<sub>2</sub>O<sub>7</sub> in air to 1400&nbsp;°C decomposes it into the pentoxides of phosphorus and protactinium.<ref name="papo3" />