Protactinium

Isotopes

Twenty-nine radioisotopes of protactinium have been discovered, the most stable being ²³¹Pa with a half-life of 32,760 years, ²³³Pa with a half-life of 27 days, and ²³⁰Pa with a half-life of 17.4 days. All of the remaining isotopes have half-lives shorter than 1.6 days, and the majority of these have half-lives less than 1.8 seconds. Protactinium also has two nuclear isomers, ²¹⁷ᵐPa (half-life 1.2 milliseconds) and ²³⁴ᵐPa (half-life 1.17 minutes).
The primary decay mode for isotopes of protactinium lighter than (and including) the most stable isotope ²³¹Pa (i.e., ²¹²Pa to ²³¹Pa) is alpha decay and the primary mode for the heavier isotopes (i.e., ²³²Pa to ²⁴⁰Pa) is beta decay. The primary decay products of isotopes of protactinium lighter than (and including) ²³¹Pa are actinium isotopes and the primary decay products for the heavier isotopes of protactinium are uranium isotopes.

Occurrence

Protactinium is one of the rarest and most expensive naturally occurring elements. It is found in the form of two isotopes – ²³¹Pa and ²³⁴Pa, with the isotope ²³⁴Pa occurring in two different energy states. Nearly all natural protactinium is protactinium-231. It is an alpha emitter and is formed by the decay of uranium-235, whereas the beta radiating protactinium-234 is produced as a result of uranium-238 decay. Nearly all uranium-238 (99.8%) decays first to the shorter-lived ²³⁴ᵐPa isomer.
Protactinium occurs in uraninite (pitchblende) at concentrations of about 0.3-3 parts ²³¹Pa per million parts (ppm) of ore. Whereas the usual content is closer to 0.3 ppm (e.g. in Jáchymov, Czech Republic), some ores from the Democratic Republic of the Congo have about 3 ppm. Protactinium is homogeneously dispersed in most natural materials and in water, but at much lower concentrations on the order of one part per trillion, that corresponds to the radioactivity of 0.1 picocuries (pCi)/g. There is about 500 times more protactinium in sandy soil particles than in water, even the water present in the same sample of soil. Much higher ratios of 2,000 and above are measured in loam soils and clays, such as bentonite.

In nuclear reactors

Two major protactinium isotopes, ²³¹Pa and ²³³Pa, are produced from thorium in nuclear reactors; both are undesirable and are usually removed, thereby adding complexity to the reactor design and operation. In particular, ²³²Th via (n,2n) reactions produces ²³¹Th which quickly (half-life 25.5 hours) decays to ²³¹Pa. The last isotope, while not a transuranic waste, has a long half-life of 32,760 years and is a major contributor to the long term radiotoxicity of spent nuclear fuel.
Protactinium-233 is formed upon neutron capture by ²³²Th. It further either decays to uranium-233 or captures another neutron and converts into the non-fissile uranium-234. ²³³Pa has a relatively long half-life of 27 days and high cross section for neutron capture (the so-called "neutron poison"). Thus instead of rapidly decaying to the useful ²³³U, a significant fraction of ²³³Pa converts to non-fissile isotopes and consumes neutrons, degrading the reactor efficiency. To avoid this, ²³³Pa is extracted from the active zone of thorium molten salt reactors, during their operation, so that it only decays to ²³³U. This is achieved using several meters tall columns of molten bismuth with lithium dissolved in it. In a simplified scenario, lithium selectively reduces protactinium salts to protactinium metal which is then extracted from the molten-salt cycle, and bismuth is merely a carrier. It is chosen because of its low melting point (271 °C), low vapor pressure, good solubility for lithium and actinides, and immiscibility with molten halides.

Physical and chemical properties

Protactinium is an actinide which is positioned in the periodic table to the left of uranium and to the right of thorium, and many of its physical properties are intermediate between those two actinides. So, protactinium is more dense and rigid than thorium but is lighter than uranium, and its melting point is lower than that of thorium and higher than that of uranium. The thermal expansion, electrical and thermal conductivities of these three elements are comparable and are typical of post-transition metals. The estimated shear modulus of protactinium is similar to that of titanium. Protactinium is a metal with silvery-gray luster that is preserved for some time in air. Protactinium easily reacts with oxygen, water vapor and acids, but not with alkalis.
At room temperature, protactinium crystallizes in the body-centered tetragonal structure which can be regarded as distorted body-centered cubic lattice; this structure does not change upon compression up to 53 GPa. The structure changes to face-centered cubic (fcc) upon cooling from high temperature, at about 1200 °C. The thermal expansion coefficient of the tetragonal phase between room temperature and 700 °C is 9.9×10⁻⁶/°C.
Protactinium is paramagnetic and no magnetic transitions are known for it at any temperature. It becomes superconductive at temperatures below 1.4 K. Protactinium tetrachloride is paramagnetic at room temperature but turns ferromagnetic upon cooling to 182 K.
Protactinium exists in two major oxidation states, +4 and +5, both in solids and solutions, and the +3 and +2 states were observed in some solid phases. As the electron configuration of the neutral atom is [Rn]5f²6d¹7s², the +5 oxidation state corresponds to the low-energy (and thus favored) 5f⁰ configuration. Both +4 and +5 states easily form hydroxides in water with the predominant ions being Pa(OH)³⁺, Pa(OH)²⁺
₂, Pa(OH)⁺
₃ and Pa(OH)₄, all colorless. Other known protactinium ions include PaCl²⁺
₂, PaSO²⁺
₄, PaF³⁺, PaF²⁺
₂, PaF⁻
₆, PaF²⁻
₇ and PaF³⁻
₈.

Oxides and oxygen-containing salts

Protactinium oxides are known for the metal oxidation states +2, +4 and +5. The most stable is white pentoxide Pa₂O₅, which can be produced by igniting protactinium(V) hydroxide in air at a temperature of 500 °C. Its crystal structure is cubic, and the chemical composition is often non-stoichiometric, described as PaO2.25. Another phase of this oxide with orthorhombic symmetry has also been reported. The black dioxide PaO₂ is obtained from the pentoxide by reducing it at 1550 °C with hydrogen. It is not readily soluble in either dilute or concentrated nitric, hydrochloric or sulfuric acids, but easily dissolves in hydrofluoric acid. The dioxide can be converted back to pentoxide by heating in oxygen-containing atmosphere to 1100 °C. The monoxide PaO has only been observed as a thin coating on protactinium metal, but not in an isolated bulk form.
Protactinium forms mixed binary oxides with various metals. With alkali metals A, the crystals have a chemical formula APaO₃ and perovskite structure, or A₃PaO₄ and distorted rock-salt structure, or A₇PaO₆ where oxygen atoms form a hexagonal close-packed lattice. In all these materials, protactinium ions are octahedrally coordinated. The pentoxide Pa₂O₅ combines with rare-earth metal oxides R₂O₃ to form various nonstoichiometric mixed-oxides, also of perovskite structure.
Protactinium oxides are basic; they easily convert to hydroxides and can form various salts, such as sulfates, phosphates, nitrates, etc. The nitrate is usually white but can be brown due to radiolytic decomposition. Heating the nitrate in air at 400 °C converts it to the white protactinium pentoxide. The polytrioxophosphate Pa(PO₃)₄ can be produced by reacting difluoride sulfate PaF₂SO₄ with phosphoric acid (H₃PO₄) under inert gas atmosphere. Heating the product to about 900 °C eliminates the reaction by-products such as hydrofluoric acid, sulfur trioxide and phosphoric anhydride. Heating to higher temperatures in an inert atmosphere decomposes Pa(PO₃)₄ into the diphosphate PaP₂O₇, which is analogous to diphosphates of other actinides. In the diphosphate, the PO₃ groups form pyramids of C₂ᵥ symmetry. Heating PaP₂O₇ in air to 1400 °C decomposes it into the pentoxides of phosphorus and protactinium.

Halides

Protactinium(V) fluoride forms white crystals where protactinium ions are arranged in pentagonal bipyramids and coordinated by 7 other ions. The coordination is the same in protactinium(V) chloride, but the color is yellow. The coordination changes to octahedral in the brown protactinium(V) bromide and is unknown for protactinium(V) iodide. The protactinium coordination in all its tetrahalides is 8, but the arrangement is square antiprismatic in protactinium(IV) fluoride and dodecahedral in the chloride and bromide. Brown-colored protactinium(III) iodide has been reported where protactinium ions are 8-coordinated in a bicapped trigonal prismatic arrangement.
Protactinium(V) fluoride and protactinium(V) chloride have a polymeric structure of monoclinic symmetry. There, within one polymeric chain, all the halide atoms lie in one graphite-like plane and form planar pentagons around the protactinium ions. The coordination 7 of protactinium originates from the 5 halide atoms and two bonds to protactinium atoms belonging to the nearby chains. These compounds easily hydrolyze in water. The pentachloride melts at 300 °C and sublimates at even lower temperatures.
Protactinium(V) fluoride can be prepared by reacting protactinium oxide with either bromine pentafluoride or bromine trifluoride at about 600 °C, and protactinium(IV) fluoride is obtained from the oxide and a mixture of hydrogen and hydrogen fluoride at 600 °C; a large excess of hydrogen is required to remove atmospheric oxygen leaks into the reaction.
Protactinium(V) chloride is prepared by reacting protactinium oxide with carbon tetrachloride at temperature of 200–300 °C. The by-products (such as PaOCl₃) are removed by fractional sublimation. Reduction of protactinium(V) chloride with hydrogen at about 800 °C yields protactinium(IV) chloride – a yellow-green solid which sublimes in vacuum at 400 °C; it can also be obtained directly from protactinium dioxide by treating it with carbon tetrachloride at 400 °C.
Protactinium bromides are produced by the action of aluminium bromide, hydrogen bromide, carbon tetrabromide or a mixture of hydrogen bromide and thionyl bromide on protactinium oxide. An alternative reaction is between protactinium pentachloride and hydrogen bromide or thionyl bromide. Protactinium(V) bromide has two similar monoclinic forms, one is obtained by sublimation at 400–410 °C and another by sublimation at slightly lower temperature of 390–400 °C.
Protactinium iodides result from the oxides and aluminium iodide or ammonium iodide heated to 600 °C. Protactinium(III) iodide was obtained by heating protactinium(V) iodide in vacuum. As with oxides, protactinium forms mixed halides with alkali metals. Among those, most remarkable is Na₃PaF₈ where protactinium ion is symmetrically surrounded by 8 F⁻ ions which form a nearly perfect cube.
More complex protactinium fluorides are also known such as Pa₂F₉ and ternary fluorides of the types MPaF₆ (M = Li, Na, K, Rb, Cs or NH₄), M₂PaF₇ (M = K, Rb, Cs or NH₄) and M₃PaF₈ (M = Li, Na, Rb, Cs), all being white crystalline solids. The MPaF₆ formula can be represented as a combination of MF and PaF₅. These compounds can be obtained by evaporating a hydrofluoric acid solution containing these both complexes. For the small alkali cations like Na, the crystal structure is tetragonal, whereas it lowers to orthorphombic for larger cations K⁺, Rb⁺, Cs⁺ or NH₄⁺. A similar variation was observed for the M₂PaF₇ fluorides, namely the crystal symmetry was dependent on the cation and differed for Cs₂PaF₇ and M₂PaF₇ (M = K, Rb or NH₄).

Other inorganic compounds

Oxyhalides and oxysulfides of protactinium are known. PaOBr₃ has a monoclinic structure composed of double-chain units where protactinium has coordination 7 and is arranged into pentagonal bipyramids. The chains are interconnected through oxygen and bromine atoms, and each oxygen atom is related to three protactinium atoms. PaOS is a light-yellow non-volatile solid with a cubic crystal lattice isostructural to that of other actinide oxysulfides. It is obtained by reacting protactinium(V) chloride with a mixture of hydrogen sulfide and carbon disulfide at 900 °C.
In hydrides and nitrides, protactinium has a low oxidation state of about +3. The hydride is obtained by direct action of hydrogen on the metal at 250 °C, and the nitride is a product of ammonia and protactinium tetrachloride or pentachloride. This bright yellow solid is stable to heating to 800 °C in vacuum. Protactinium carbide PaC is formed by reduction of protactinium tetrafluoride with barium in a carbon crucible at a temperature of about 1400 °C. Protactinium forms borohydrides which include Pa(BH₄)₄. It has an unusual polymeric structure with helical chains where the protactinium atom has coordination number of 12 and is surrounded by six BH₄⁻ ions.

Organometallic compounds

Protactinium(IV) forms a tetrahedral complex tetrakis(cyclopentadienyl)protactinium(IV) (or Pa(C₅H₅)₄) with four cyclopentadienyl rings, which can be synthesized by reacting protactinium(IV) chloride with molten Be(C₅H₅)₂. One ring can be substituted with a halide atom. Another organometallic complex is golden-yellow bis(π-cyclooctatetraene) protactinium, or protactinocene, Pa(C₈H₈)₂, which is analogous in structure to uranocene. There, the metal atom is sandwiched between two cyclooctatetraene ligands. Similar to uranocene, it can be prepared by reacting protactinium tetrachloride with dipotassium cyclooctatetraenide, K₂C₈H₈, in tetrahydrofuran.

Applications

Although protactinium is located in the periodic table between uranium and thorium, which both have numerous applications, there are currently no uses for protactinium outside scientific research owing to its scarcity, high radioactivity, and high toxicity.
Protactinium-231 arises from the decay of uranium-235 formed in nuclear reactors, and by the reaction ²³²Th + n → ²³¹Th + 2n and subsequent beta decay. It was once thought to be able to support a nuclear chain reaction, which could in principle be used to build nuclear weapons: the physicist Walter Seifritz once estimated the associated critical mass as 750±180 kg. However, the possibility of criticality of ²³¹Pa has been ruled out since then.
With the advent of highly sensitive mass spectrometers, an application of ²³¹Pa as a tracer in geology and paleoceanography has become possible. So, the ratio of protactinium-231 to thorium-230 is used for radiometric dating of sediments which are up to 175,000 years old and in modeling of the formation of minerals. In particular, its evaluation in oceanic sediments allowed to reconstruct the movements of North Atlantic water bodies during the last melting of Ice Age glaciers. Some of the protactinium-related dating variations rely on the analysis of the relative concentrations for several long-living members of the uranium decay chain – uranium, protactinium, and thorium, for example. These elements have 6, 5 and 4 valence electrons and thus favor +6, +5 and +4 oxidation states, respectively, and show different physical and chemical properties. So, thorium and protactinium, but not uranium compounds are poorly soluble in aqueous solutions, and precipitate into sediments; the precipitation rate is faster for thorium than for protactinium. Besides, the concentration analysis for both protactinium-231 (half-life 32,760 years) and thorium-230 (half-life 75,380 years) allows to improve the accuracy compared to when only one isotope is measured; this double-isotope method is also weakly sensitive to inhomogeneities in the spatial distribution of the isotopes and to variations in their precipitation rate.