Calcium hexaboride: Difference between revisions

| verifiedrevid = 408859223

| Section1 = {{Chembox Identifiers

| CASNo = 12007-99-7

| PubChem = 16212529

| EINECS = 234-525-3

| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}

| SMILES = [B-]123B45B16[B-]47B52B376.[Ca+2]

| StdInChI_Ref = {{stdinchicite|correct|chemspider}}

| StdInChI=1S/B6.Ca/c1-2-3(1)5(1)4(1,2)6(2,3)5;/q-2;+2

| InChI=1/B6.Ca/c1-2-3(1)5(1)4(1,2)6(2,3)5;/q-2;+2

| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}

| Section2 = {{Chembox Properties

| Formula = CaB₆

| MolarMass = 104.94 g/mol

| Appearance = black powder

| Density = 2.45 g/cm³

| MeltingPt = 2235°C /2770 K

| BoilingPt =

| Solubility = insoluble

| Section3 = {{Chembox Structure

| CrystalStruct = Cubic

| SpaceGroup = P''m<span style="text-decoration: overline">3</span>m'' ; O_h<ref name = "Turrell"/>

| Section7 = {{Chembox Hazards

| MainHazards =

| FlashPt =

| Autoignition =

}}

'''Calcium hexaboride''' (sometimes '''calcium boride''') is a compound of [[calcium]] and [[boron]] with the chemical formula CaB₆. It is an important material due to its high [[electrical conductivity]], [[hardness]], chemical stability, and [[melting point]]. It is a black, lustrous, chemically inert powder with a low density. It has the cubic structure typical for metal hexaborides, with octahedral units of 6 boron atoms combined with calcium atoms.<ref>{{cite book| title = Boron and Refractory Borides| author = Matkovich, V. I.| publisher = Springer-Verlag| location = Berlin| year = 1977| isbn = 038708181X}}</ref> CaB₆ and [[lanthanum]] doped CaB₆ show weak [[ferromagnetism|ferromagnetic]] properties, which is a remarkable fact because calcium and boron are neither magnetic, nor have inner 3d or 4f electronic shells, which are usually required for ferromagnetism.

CaB₆ has been investigated in the past due to a variety of peculiar physical properties, such as [[superconductivity]], valence fluctuation and [[Kondo effect]]s.<ref name = kondo>{{cite journal| author= J. Etourneau and P. Hagenmuller| title = Structure and physical features of the rare-earth borides| doi = 10.1080/13642818508240625 | journal = Phil. Mag. B | volume = 52| issue= 3 |page = 589 | year = 1985}}.</ref> However, the most remarkable property of CaB₆ is its ferromagnetism. It occurs at unexpectedly high temperature (600 K) and with low magnetic moment (below 0.07 [[Bohr magneton|<math>\mu_\mathrm{B}</math>]] per atom). The origin of this high temperature ferromagnetism is the ferromagnetic phase of a dilute electron gas, linkage to the presumed excitonic state in calcium boride, or external impurities on the surface of the sample. The impurities might include [[Iron]] and [[Nickel]] probably coming from boron in sample preparation.

<ref name = "Young">{{cite journal| title =High-temperature weak ferromagnetism in a low-density free-electron gas| author= Young, D.P ''et al.''| journal = Nature | year = 1999 | volume = 397| issue =6718 | pages = 412–414| doi = 10.1038/17081}}</ref>

CaB₆ is insoluble in H₂O, MeOH (methanol), and EtOH (ethanol) and dissolves slowly in acids.<ref name ="Calcium">{{cite book| title = Calcium boride – Dictionary of Inorganic Compounds | journal = University Press|location = Cambridge| year = 1992| volume = 1}}</ref> Its microhardness is 27 GPa, [[Knoop hardness]] is 2600&nbsp;kg/mm²), [[Young modulus]] is 379 GPa, and electrical resistivity is greater than 2·10¹⁰ Ω·m for pure crystals.<ref name ="Borides"/><ref name = pyr/> CaB₆ is a semiconductor with an energy gap estimated as 1.0 eV. The low, semi-metallic conductivity of many CaB₆ samples can be explained by unintentional doping due to impurities and possible non-stoichiometry.<ref>{{cite journal| title = Electronic Band Structure and Fermi Surface of CaB6 Studied by Angle-Resolved Photoemission Spectroscopy| author = S. Souma ''et al.''| journal = Phys. Rev. Lett.| volume = 90| issue = 2| year = 2003| page = 027202| doi =10.1103/PhysRevLett.90.027202| pmid=12570575| bibcode=2003PhRvL..90b7202S}}</ref>

==Structural Information==

The crystal structure of calcium hexaboride is a cubic lattice with calcium at the cell centre and compact, regular octahedra of boron atoms linked at the vertices by B-B bonds to give a three-dimensional boron network.<ref name ="Calcium"/> Each calcium has 24 nearest-neighbor boron atoms<ref name = "Turrell">{{cite journal| author = Yahia,S.;Turrell, S.; Turrell, G.; Mercurio, J.P.| journal = J. Mol. Struct.| year = 1990 | volume = 224| issue = 1-2 | pages = 303–312| title = Infrared and Raman spectra of hexaborides: force-field calculations, and isotopic effects| doi =10.1016/0022-2860(90)87025-S}}</ref> The calcium atoms are arranged in simple cubic packing so that there are holes between groups of eight calcium atoms situated at the vertices of a cube.<ref name = "Wells">{{cite book| author = Wells, A.F. | title = Structural Inorganic Chemistry| publisher = Clarendon Press| location = Oxford| year = 1984 | pages = 1055–1056| isbn = 0198551258}}</ref> The simple cubic structure is expanded by the introduction of the octahedral B₆ groups and the structure is a CsCl-like packing of the calcium and hexaboride groups.<ref name = "Wells"/> Another way of describing calcium hexaboride is as having a metal and a B₆^2- octahedral polymeric anions in a CsCl-type structure were the Calcium atoms occupy the Cs sites and the B₆ octahedra in the Cl sites.<ref name ="Smith">{{cite journal| author = Zhongijie, L.; Smith, M.E.; Sowrey, F.E.; Newport, R.J.| journal = Physical Review B| title = Probing the local structural environment of calcium by natural-abundance solid-state 43Ca NMR| year = 2004| volume = 69| issue = 22| page = 224107| doi = 10.1103/PhysRevB.69.224107}}</ref> The Ca-B bond length is 3.05 Å and the B-B bond length is 1.7 Å.<ref name = "Wells"/>

Raman Data: Calcium hexaboride has three Raman peaks at 754.3, 1121.8, and 1246.9&nbsp;cm⁻¹ due to the active modes A_1g, E_g, and T_2g respectively.<ref name = "Turrell"/>

Observed Vibrational Frequencies cm⁻¹ : 1270(strong) from A_1g stretch, 1154 (med.) and 1125(shoulder) from E_g stretch, 526, 520, 485, and 470 from F_1g rotation, 775 (strong) and 762 (shoulder) from F_2g bend, 1125 (strong) and 1095 (weak)from F_1u bend, 330 and 250 from F_1u translation, and 880 (med.) and 779 from F_2u bend.<ref name = "Turrell"/>

*One of the main reactions for industrial production is:<ref name="Borides">{{cite book| chapter = Borides:Solid State Chemistry| title = Encyclopedia of Inorganic Chemistry | publisher = John Wiley & Sons| location = West Sussex, England| year = 1994 | volume = 1}}</ref>

:CaO + 3 B₂O₃ + 10 Mg → CaB₆ + 10 MgO

*Direct reaction of calcium or [[calcium oxide]] and boron at 1000°C;

*Reacting Ca(OH)₂ with boron in vacuum at about 1700 °C ([[carbothermal reduction]]);<ref name=otani/>

*Reacting [[calcium carbonate]] with [[boron carbide]] in vacuum at above 1400 °C (carbothermal reduction)

*Reacting of CaO and H₃BO₃ and Mg to 1100°C.<ref name ="Calcium"/>

*Low-temperature (500 °C) synthesis

results in relatively poor quality material.<ref name = "Shi">{{cite journal| author = Shi, L. ''et al.''| journal = Chem. Lett. | title = Low Temperature Synthesis and Characterization of Cubic CaB6 Ultrafine Powders| year =2003 | volume = 32| issue = 10| page =958| doi = 10.1246/cl.2003.958}}</ref>

* To produce pure CaB₆ single crystals, e.g., for use as cathode material, the thus obtained CaB₆ powder is further recrystallized and purified with the [[zone melting]] technique. The typical growth rate is 30&nbsp;cm/h and crystal size ~1x10 cm.<ref name=otani>{{cite journal| title = Preparation of CaB6 crystals by the floating zone method| journal = Journal of Crystal Growth| volume = 192| issue = 1-2| year = 1998| pages = 346–349| author = S. Otani| doi = 10.1016/S0022-0248(98)00444-8| bibcode=1998JCrGr.192..346O}}</ref>

*Single-crystal CaB₆ [[Nanowires]] (diameter 15-40&nbsp;nm, length 1-10 micrometres) can be obtained by pyrolysis of [[diborane]] (B₂H₆) over calcium oxide (CaO) powders at 860-900 °C, in presence of Ni catalyst.<ref name = pyr>{{cite journal| author = Terry T. Xu, Jian-Guo Zheng, Alan W. Nicholls, Sasha Stankovich, Richard D. Piner, and Rodney S. Ruoff| journal = Nano Lett. | volume = 4| issue = 10| year =2004 | pages =2051–2055| doi =10.1021/nl0486620| title =Single-Crystal Calcium Hexaboride Nanowires: Synthesis and Characterization}}</ref>

Calcium hexaboride is used in the manufacturing of boron-[[alloy]]ed [[steel]] <ref name ="Calcium"/> and as a [[deoxidation]] agent in production of [[oxygen-free copper]]. The latter results in higher conductivity than conventionally phosphorus-deoxidized copper owing to the low solubility of boron in copper.<ref name ="Borides"/> CaB₆ can also serve as a high temperature material, surface protection, [[abrasive]]s, tools, and wear resistant material.

CaB₆ is highly conductive, has low work function, and thus can be used as a cathode material. When used at elevated temperature, calcium hexaboride will oxidize degrading its properties and shortening its usable lifespan.<ref>{{cite book| title = Organic light-emitting materials and devices| author = Zhigang R. Li, Hong Meng| publisher = CRC Press| year = 2006| isbn= 157444574X| page = 516}}</ref>

Calcium hexaboride is irritating to the eyes, skin, and respiratory system. This product should be handled with proper protective eyeware and clothing. Never put calcium hexaboride down the drain or add water to it.

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[[ar:سداسي بوريد الكالسيوم]]