Editing Berkelium (section)

===Allotropes===
At ambient conditions, berkelium assumes its most stable α form which has a [[hexagonal crystal family|hexagonal]] symmetry, [[space group]] ''P6<sub>3</sub>/mmc'', lattice parameters of 341&nbsp;[[picometre|pm]] and 1107&nbsp;pm. The crystal has a double-[[Close-packing of equal spheres|hexagonal close packing]] structure with the layer sequence ABAC and so is [[isostructural|isotypic]] (having a similar structure) with α-lanthanum and α-forms of actinides beyond curium.<ref name = "Peterson" /> This crystal structure changes with pressure and temperature. When compressed at room temperature to 7&nbsp;GPa, α-berkelium transforms to the β modification, which has a [[Cubic crystal system|face-centered cubic]] (''fcc'') symmetry and space group ''Fm{{overline|3}}m''. This transition occurs without change in volume, but the [[enthalpy]] increases by 3.66&nbsp;kJ/mol.{{sfn|Peterson|Hobart|1984|p=44}} Upon further compression to 25&nbsp;GPa, berkelium transforms to an [[Orthorhombic crystal system|orthorhombic]] γ-berkelium structure similar to that of α-uranium. This transition is accompanied by a 12% volume decrease and delocalization of the electrons at the [[electron shell|5f electron shell]].<ref name="pressure2" /> No further phase transitions are observed up to 57 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|pages=153–170|date=1984|doi=10.1016/0022-5088(84)90061-4}}</ref><ref>Young, David A. [https://books.google.com/books?id=F2HVYh6wLBcC&pg=PA228 Phase diagrams of the elements], University of California Press, 1991, {{ISBN|0-520-07483-1}} p. 228</ref>

Upon heating, α-berkelium transforms into another phase with an ''fcc'' lattice (but slightly different from β-berkelium), space group ''Fm{{overline|3}}m'' and the [[lattice constant]] of 500&nbsp;pm; this ''fcc'' structure is equivalent to the closest packing with the sequence ABC. This phase is metastable and will gradually revert to the original α-berkelium phase at [[room temperature]].<ref name="Peterson" /> The temperature of the [[phase transition]] is believed to be quite close to the melting point.<ref name="H&P" /><ref name="Fahey">{{cite journal|last1 = Fahey|first1 = J. A.|last2 = Peterson|first2 = J. R.|last3 = Baybarz|first3 = R. D.|date = 1972|title = Some properties of berkelium metal and the apparent trend toward divalent character in the transcurium actinide metals|journal = Inorg. Nucl. Chem. Lett.|volume = 8|issue = 1|pages = 101–7|doi = 10.1016/0020-1650(72)80092-8}}</ref><ref name="Ward">{{cite journal|last1 = Ward|first1 = John W.|last2 = Kleinschmidt|first2 = Phillip D.|last3 = Haire|first3 = Richard G.|date = 1982|title = Vapor pressure and thermodynamics of Bk-249 metal|journal = J. Chem. Phys.|volume = 77|issue = 3|pages = 1464–68|doi = 10.1063/1.443975|bibcode = 1982JChPh..77.1464W }}</ref>