Editing Americium (section)

==Physical properties==
[[File:Closest packing ABAC.png|thumb|Double-hexagonal close packing with the layer sequence ABAC in the crystal structure of α-americium (A: green, B: blue, C: red)]]

In the [[periodic table]], americium is located to the right of plutonium, to the left of curium, and below the lanthanide [[europium]], with which it shares many physical and chemical properties. Americium is a highly radioactive element. When freshly prepared, it has a silvery-white metallic lustre, but then slowly tarnishes in air. With a density of 12&nbsp;g/cm<sup>3</sup>, americium is less dense than both curium (13.52&nbsp;g/cm<sup>3</sup>) and plutonium (19.8&nbsp;g/cm<sup>3</sup>); but has a higher density than europium (5.264&nbsp;g/cm<sup>3</sup>)—mostly because of its higher atomic mass. Americium is relatively soft and easily deformable and has a significantly lower [[bulk modulus]] than the actinides before it: Th, Pa, U, Np and Pu.<ref name="pressure" /> Its melting point of 1173&nbsp;°C is significantly higher than that of plutonium (639&nbsp;°C) and europium (826&nbsp;°C), but lower than for curium (1340&nbsp;°C).<ref name="AM_METALL2">{{cite journal|last1=Wade|first1=W.|title=Preparation and some properties of americium metal|journal=Journal of Inorganic and Nuclear Chemistry|volume=29|page=2577|date=1967|doi=10.1016/0022-1902(67)80183-0|issue=10|last2=Wolf|first2=T.|s2cid=98370243 }}</ref><ref name="AM_METALL4" />

At ambient conditions, americium is present in its most stable α form which has a [[Hexagonal crystal system|hexagonal crystal symmetry]], and a [[space group]] P6<sub>3</sub>/mmc with cell parameters ''a''&nbsp;= 346.8&nbsp;[[picometer|pm]] and ''c''&nbsp;= 1124&nbsp;pm, and four atoms per [[unit cell]]. The crystal consists of a double-[[hexagonal close packing]] with the layer sequence ABAC and so is isotypic with α-lanthanum and several actinides such as α-curium.<ref name="Gmelin" /><ref name = "AM_METALL4">{{cite journal|last1=McWhan|first1=D. B.|last2=Cunningham|first2=B. B.|last3=Wallmann|first3=J. C.|title=Crystal structure, thermal expansion and melting point of americium metal|journal=Journal of Inorganic and Nuclear Chemistry|volume=24|page=1025|date=1962|doi=10.1016/0022-1902(62)80246-2|issue=9}}</ref> The crystal structure of americium changes with pressure and temperature. When compressed at room temperature to 5 GPa, α-Am transforms to the β modification, which has a [[face-centered cubic]] (''fcc'') symmetry, space group Fm{{overline|3}}m and lattice constant ''a''&nbsp;= 489&nbsp;pm. This ''fcc'' structure is equivalent to the closest packing with the sequence ABC.<ref name="Gmelin" /><ref name = "AM_METALL4" /> Upon further compression to 23 GPa, americium transforms to an [[orthorhombic]] γ-Am structure similar to that of α-uranium. There are no further transitions observed up to 52 GPa, except for an appearance of a monoclinic phase at pressures between 10 and 15 GPa.<ref name="pressure">{{cite journal|last1=Benedict|first1=U.|title=Study of actinide metals and actinide compounds under high pressures|journal=Journal of the Less Common Metals|volume=100|page=153|date=1984|doi=10.1016/0022-5088(84)90061-4}}</ref> There is no consistency on the status of this phase in the literature, which also sometimes lists the α, β and γ phases as I, II and III. The β-γ transition is accompanied by a 6% decrease in the crystal volume; although theory also predicts a significant volume change for the α-β transition, it is not observed experimentally. The pressure of the α-β transition decreases with increasing temperature, and when α-americium is heated at ambient pressure, at 770&nbsp;°C it changes into an ''fcc'' phase which is different from β-Am, and at 1075&nbsp;°C it converts to a [[body-centered cubic]] structure. The pressure-temperature phase diagram of americium is thus rather similar to those of lanthanum, [[praseodymium]] and [[neodymium]].<ref>{{cite book| last1= Young |first1= D. A. | title = Phase diagrams of the elements| url = https://books.google.com/books?id=F2HVYh6wLBcC&pg=PA226| date = 1991| publisher = University of California Press| isbn = 978-0-520-91148-2| page = 226 }}</ref>

As with many other actinides, self-damage of the crystal structure due to alpha-particle irradiation is intrinsic to americium. It is especially noticeable at low temperatures, where the mobility of the produced [[Interstitial defect|structure defects]] is relatively low, by broadening of [[X-ray diffraction]] peaks. This effect makes somewhat uncertain the temperature of americium and some of its properties, such as electrical [[resistivity]].<ref>{{cite journal|last1=Benedict|first1=U.|last2=Dufour|first2=C.|title=Low temperature lattice expansion of americium dioxide|journal=Physica B+C|volume=102|issue=1|page=303|date=1980|doi=10.1016/0378-4363(80)90178-3|bibcode = 1980PhyBC.102..303B }}</ref> So for americium-241, the resistivity at 4.2 K increases with time from about 2&nbsp;μOhm·cm to 10&nbsp;μOhm·cm after 40 hours, and saturates at about 16&nbsp;μOhm·cm after 140 hours. This effect is less pronounced at room temperature, due to annihilation of radiation defects; also heating to room temperature the sample which was kept for hours at low temperatures restores its resistivity. In fresh samples, the resistivity gradually increases with temperature from about 2 μOhm·cm at [[liquid helium]] to 69&nbsp;μOhm·cm at room temperature; this behavior is similar to that of neptunium, uranium, thorium and [[protactinium]], but is different from plutonium and curium which show a rapid rise up to 60&nbsp;K followed by saturation. The room temperature value for americium is lower than that of neptunium, plutonium and curium, but higher than for uranium, thorium and protactinium.<ref name="res" />

Americium is [[paramagnetic]] in a wide temperature range, from that of [[liquid helium]], to room temperature and above. This behavior is markedly different from that of its neighbor curium which exhibits antiferromagnetic transition at 52&nbsp;K.<ref>{{cite journal|last1=Kanellakopulos|first1=B.|title=The magnetic susceptibility of Americium and curium metal|journal=Solid State Communications|volume=17|page=713|date=1975|doi=10.1016/0038-1098(75)90392-0|issue=6|bibcode = 1975SSCom..17..713K|last2=Blaise|first2=A.|last3=Fournier|first3=J. M.|last4=Müller|first4=W. }}</ref> The [[thermal expansion]] coefficient of americium is slightly anisotropic and amounts to {{val|7.5e-6|0.2|u=/°C}} along the shorter ''a'' axis and {{val|6.2e-6|0.4|u=/°C}} for the longer ''c'' hexagonal axis.<ref name = "AM_METALL4" /> The [[enthalpy of dissolution]] of americium metal in [[hydrochloric acid]] at standard conditions is {{val|−620.6|1.3|u=kJ/mol}}, from which the [[standard enthalpy change of formation]] (Δ<sub>f</sub>''H''°) of aqueous Am<sup>3+</sup> ion is {{val|−621.2|2.0|u=kJ/mol}}. The [[standard potential]] Am<sup>3+</sup>/Am<sup>0</sup> is {{val|−2.08|0.01|u=V}}.<ref>{{cite journal|last1=Mondal|first1=J. U.|last2=Raschella|first2=D. L.|last3=Haire|first3=R. G.|last4=Petereson|first4=J. R.|title=The enthalpy of solution of 243Am metal and the standard enthalpy of formation of Am3+(aq)|journal=Thermochimica Acta|volume=116|page=235|date=1987|doi=10.1016/0040-6031(87)88183-2}}</ref>