Editing Samarium (section)

===Borides===
[[Sintering]] powders of samarium oxide and boron, in a vacuum, yields a powder containing several samarium boride phases; the ratio between these phases can be controlled through the mixing proportion.<ref name="smb6b" /> The powder can be converted into larger crystals of samarium borides using [[Electric arc furnace|arc melting]] or [[zone melting]] techniques, relying on the different melting/crystallization temperature of SmB<sub>6</sub> (2580&nbsp;°C), SmB<sub>4</sub> (about 2300&nbsp;°C) and SmB<sub>66</sub> (2150&nbsp;°C). All these materials are hard, brittle, dark-gray solids with the hardness increasing with the boron content.<ref name="smb6" /> Samarium diboride is too volatile to be produced with these methods and requires high pressure (about 65&nbsp;kbar) and low temperatures between 1140 and 1240&nbsp;°C to stabilize its growth. Increasing the temperature results in the preferential formation of SmB<sub>6</sub>.<ref name="smb2" />

====Samarium hexaboride====
{{Main|Samarium hexaboride}}

Samarium hexaboride is a typical intermediate-valence compound where samarium is present both as Sm<sup>2+</sup> and Sm<sup>3+</sup> ions in a 3:7 ratio.<ref name="smb6b">{{cite journal|last1=Nickerson|first1=J.|last2=White|first2=R.|last3=Lee|first3=K.|last4=Bachmann|first4=R.|last5=Geballe|first5=T.|last6=Hull|first6=G.|title=Physical Properties of SmB<sub>6</sub> |journal=Physical Review B|volume=3|page=2030|date=1971|doi=10.1103/PhysRevB.3.2030|issue=6|bibcode=1971PhRvB...3.2030N }}</ref> It belongs to a class of [[Kondo insulator]]s; at temperatures above 50&nbsp;K, its properties are typical of a Kondo metal, with metallic electrical conductivity characterized by strong electron scattering, whereas at lower temperatures, it behaves as a non-magnetic insulator with a narrow [[band gap]] of about 4–14&nbsp;[[electronvolt|meV]].<ref>{{cite journal |doi=10.1103/PhysRevB.52.R14308 |pmid=9980746 |last1=Nyhus |date=1995 |first1=P. |pages=14308–14311 |volume=52|last2=Cooper|journal=Physical Review B|first2=S.|last3=Fisk|first3=Z.|author4-link=John Sarrao |last4=Sarrao |first4=J. |title=Light scattering from gap excitations and bound states in SmB<sub>6</sub> |issue=20|bibcode=1995PhRvB..5214308N }}</ref> The cooling-induced metal-insulator transition in SmB<sub>6</sub> is accompanied by a sharp increase in the [[thermal conductivity]], peaking at about 15&nbsp;K. The reason for this increase is that electrons themselves do not contribute to the thermal conductivity at low temperatures, which is dominated by [[phonon]]s, but the decrease in electron concentration reduces the rate of electron-phonon scattering.<ref>{{cite journal |last1=Sera |first1=M. |last2=Kobayashi |first2=S. |last3=Hiroi |first3=M.|last4=Kobayashi|first4=N.|last5=Kunii|first5=S.|title=Thermal conductivity of RB<sub>6</sub> (R=Ce, Pr, Nd, Sm, Gd) single crystals |journal=Physical Review B |volume=54 |date=1996 |doi=10.1103/PhysRevB.54.R5207 |pmid=9986570|issue=8 |pages=R5207–R5210|bibcode=1996PhRvB..54.5207S }}</ref>