Editing Neptunium (section)

===Physical===
Neptunium is a [[hardness (materials science)|hard]], silvery, [[ductility|ductile]], [[radioactivity|radioactive]] actinide [[metal]]. In the [[periodic table]], it is located to the right of the actinide [[uranium]], to the left of the actinide [[plutonium]] and below the [[lanthanide]] [[promethium]].<ref name="Yoshida718" /> Neptunium is a hard metal, having a bulk modulus of 118&nbsp;[[pascal (unit)|GPa]], comparable to that of [[manganese]].<ref>{{cite journal |last1=Dabos |first1=S. |last2=Dufour |first2=C. |last3=Benedict |first3=U. |last4=Pagès |first4=M. |date=1987 |title=Bulk modulus and P–V relationship up to 52&nbsp;GPa of neptunium metal at room temperature |journal=Journal of Magnetism and Magnetic Materials |volume=63–64 |pages=661–3 |doi=10.1016/0304-8853(87)90697-4|bibcode = 1987JMMM...63..661D }}</ref> Neptunium metal is similar to uranium in terms of physical workability. When exposed to air at normal temperatures, it forms a thin oxide layer. This reaction proceeds more rapidly as the temperature increases.<ref name="Yoshida718" /> Neptunium melts at {{Nowrap|639 ± 3 °C}}: this low [[melting point]], a property the metal shares with the neighboring element plutonium (which has melting point 639.4&nbsp;°C), is due to the [[orbital hybridization|hybridization]] of the 5f and 6d orbitals and the formation of directional bonds in the metal.<ref name="Yu. D. Tretyakov" /> The [[boiling point]] of neptunium is not empirically known and the usually given value of 4174&nbsp;°C is extrapolated from the [[vapor pressure]] of the element. If accurate, this would give neptunium the largest liquid range of any element (3535&nbsp;K passes between its [[Melting point|melting]] and [[boiling point]]s).<ref name="Yoshida718" /><ref name="Gray" />

Neptunium is found in at least three [[allotrope]]s.<ref name="CRC" /> Some claims of a fourth allotrope have been made, but they are so far not proven.<ref name="Yoshida718">Yoshida et al., p. 718.</ref> This multiplicity of allotropes is common among the [[actinide]]s. The [[crystal structure]]s of neptunium, [[protactinium]], uranium, and plutonium do not have clear analogs among the [[lanthanide]]s and are more similar to those of the 3d&nbsp;[[transition metal]]s.<ref name="Yu. D. Tretyakov">{{cite book|editor=Yu. D. Tretyakov|title = Non-organic chemistry in three volumes| place =Moscow|publisher = Academy|date = 2007|volume = 3|series = Chemistry of transition elements|isbn = 978-5-7695-2533-9}}</ref>

{| class="wikitable" style="margin:auto; text-align:center;"
|+ Allotropes of neptunium<ref name="Arblaster 2018" /><ref name="Yoshida718" /><ref name = "alo">{{cite journal | last1 = Lee | first1 = J. | last2 = Mardon | first2 = P. | last3 = Pearce | first3 = J. | last4 = Hall | first4 = R. | title = Some physical properties of neptunium metal II: A study of the allotropic transformations in neptunium | journal = Journal of Physics and Chemistry of Solids | volume = 11 | pages = 177–181 | date = 1959 | doi = 10.1016/0022-3697(59)90211-2 | issue = 3–4|bibcode = 1959JPCS...11..177L }}</ref>
|-
!Allotrope
!α (measured at 20&nbsp;°C)
!β (measured at 313&nbsp;°C)
!γ (measured at 600&nbsp;°C)
|-
!Transition temperature<ref name="Arblaster 2018" />
|(α→β) 280&nbsp;°C
|(β→γ) 576&nbsp;°C
|(γ→liquid) 639&nbsp;°C
|-
!Crystal structure
|[[Orthorhombic]]
|[[Tetragonal]]
|[[Body-centered cubic]]
|-
![[Pearson symbol]]
|oP8
|tP4
|cI2
|-
![[Space group]]
|''Pnma''
|''P42<sub>1</sub>2''
|''Im{{overline|3}}m''
|-
!Density (g/cm<sup>3</sup>, <sup>237</sup>Np)<ref name="Arblaster 2018" />
|20.45
|19.36
|18.08
|-
![[Lattice parameter]]s ([[picometer|pm]])<ref name="Arblaster 2018" />
|''a'' = 472.3<br/>''b'' = 488.7<br/>''c'' = 666.3
|''a'' = 489.5<br/>''c'' = 338.9
|''a'' = 351.8
|}

[[File:Phase diagram of neptunium (1975).png|thumb|upright=1.1|left|Phase diagram of neptunium]]
α-neptunium takes on an [[orthorhombic]] structure, resembling a highly distorted body-centered cubic structure.<ref name="Lemire">Lemire, R. J. et al.,''Chemical Thermodynamics of Neptunium and Plutonium'', Elsevier, Amsterdam, 2001.</ref><ref>{{cite web|url=http://cst-www.nrl.navy.mil/lattice/struk/a_c.html|title=Crystal Lattice Structures: The αNp (Ac) Structure|publisher=United States Naval Research Laboratory Center for Computational Materials Science|access-date=2013-10-16|url-status=dead|archive-url=https://web.archive.org/web/20121002050018/http://cst-www.nrl.navy.mil/lattice/struk/a_c.html|archive-date=2012-10-02}}</ref> Each neptunium atom is coordinated to four others and the Np–Np bond lengths are 260&nbsp;pm.<ref name="Yoshida719">Yoshida et al., p. 719.</ref> It is the densest of all the actinides and the fifth-densest of all naturally occurring elements, behind only [[rhenium]], [[platinum]], [[iridium]], and [[osmium]].<ref name="Gray">Theodore Gray. ''The Elements''. Page 215.</ref> α-neptunium has [[semimetal]]lic properties, such as strong [[covalent bond]]ing and a high [[electrical resistivity and conductivity|electrical resistivity]], and its metallic physical properties are closer to those of the [[metalloid]]s than the true metals. Some allotropes of the other actinides also exhibit similar behaviour, though to a lesser degree.<ref>Hindman J. C. 1968, "Neptunium", in C. A. Hampel (ed.), ''The encyclopedia of the chemical elements'', Reinhold, New York, pp. 434.</ref><ref>{{cite journal | last1 = Dunlap | first1 = B. D. | last2 = Brodsky | first2 = M. B. | last3 = Shenoy | first3 = G. K. | last4 = Kalvius | first4 = G. M. | date = 1970 | title = Hyperfine interactions and anisotropic lattice vibrations of <sup>237</sup>Np in α-Np metal | journal = Physical Review B | volume = 1 | issue = 1| pages = 44–46 | doi = 10.1103/PhysRevB.1.44 | bibcode = 1970PhRvB...1...44D }}</ref> The densities of different isotopes of neptunium in the alpha phase are expected to be observably different: α-<sup>235</sup>Np should have density 20.303&nbsp;g/cm<sup>3</sup>; α-<sup>236</sup>Np, density 20.389&nbsp;g/cm<sup>3</sup>; α-<sup>237</sup>Np, density 20.476&nbsp;g/cm<sup>3</sup>.<ref name="critical">{{cite web|publisher = Institut de Radioprotection et de Sûreté Nucléaire|title = Evaluation of nuclear criticality safety data and limits for actinides in transport|page = 15|url = http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf|access-date=2010-12-20 }}</ref>

β-neptunium takes on a distorted tetragonal close-packed structure. Four atoms of neptunium make up a unit cell, and the Np–Np bond lengths are 276&nbsp;pm.<ref name="Yoshida719" /> γ-neptunium has a [[body-centered cubic]] structure and has Np–Np bond length of 297&nbsp;pm. The γ form becomes less stable with increased pressure, though the melting point of neptunium also increases with pressure.<ref name="Yoshida719" /> The β-Np/γ-Np/liquid [[triple point]] occurs at 725&nbsp;°C and 3200&nbsp;[[pascal (unit)|MPa]].<ref name="Yoshida719" /><ref>{{cite journal |last1=Stephens |first1=D. R. |date=1966 |title=Phase diagram and compressibility of neptunium |journal=Journal of Physics |volume=27 |issue=8 |pages=1201–4 |doi=10.1016/0022-3697(66)90002-3|bibcode = 1966JPCS...27.1201S }}</ref>

====Alloys====
Due to the presence of valence 5f electrons, neptunium and its alloys exhibit a very interesting magnetic behavior, like many other actinides. These can range from the itinerant band-like character characteristic of the [[transition metal]]s to the local moment behavior typical of [[scandium]], [[yttrium]], and the [[lanthanide]]s. This stems from 5f-orbital hybridization with the orbitals of the metal [[ligand]]s, and the fact that the 5f orbital is [[relativistic effects|relativistically]] destabilized and extends outwards.<ref name="Yoshida720">Yoshida et al., pp. 719–20.</ref> For example, pure neptunium is [[paramagnetic]], Np[[aluminium|Al]]<sub>3</sub> is [[ferromagnetic]], Np[[germanium|Ge]]<sub>3</sub> has no magnetic ordering, and Np[[tin|Sn]]<sub>3</sub> may be a [[heavy fermion material]].<ref name="Yoshida720" /> Investigations are underway regarding alloys of neptunium with uranium, [[americium]], [[plutonium]], [[zirconium]], and [[iron]], so as to recycle long-lived waste isotopes such as neptunium-237 into shorter-lived isotopes more useful as nuclear fuel.<ref name="Yoshida720" />

One neptunium-based [[superconductivity|superconductor]] alloy has been discovered with formula Np[[palladium|Pd]]<sub>5</sub>Al<sub>2</sub>. This occurrence in neptunium compounds is somewhat surprising because they often exhibit strong magnetism, which usually destroys superconductivity. The alloy has a tetragonal structure with a superconductivity transition temperature of −268.3&nbsp;°C (4.9&nbsp;K).<ref name="lanl" /><ref>{{cite journal |author=T. D. Matsuda |author2= Y. Hagal |author3= D. Aoki |author4= H. Sakai |author5= Y. Homma |author6= N. Tateiwa|author7=E. Yamamoto |author8=Y. Onuki |date=2009 |title=Transport properties of neptunium superconductor NpPd<sub>5</sub>Al<sub>2</sub> |journal=Journal of Physics: Conference Series |volume=150 |issue=4 | pages=042119 |doi=10.1088/1742-6596/150/4/042119|bibcode = 2009JPhCS.150d2119M |doi-access=free }}</ref>