Calcium hexaboride

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IUPACName = Calcium hexaboride
OtherNames = Calcium boride
Section1 = Chembox Identifiers
CASNo = 12007-99-7
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Section2 = Chembox Properties
Formula = CaB₆
MolarMass = 104.94 g/mol
Appearance =
Density = 2.45 g/cm³
MeltingPt = 2235°C /2770 K
BoilingPt =
Solubility = insoluble
Section3 = Chembox Hazards
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Calcium hexaboride (CaB₆) is a compound of calcium and boron in which the coordination number of the calcium is 18. Zhongijie, L.; Smith, M.E.; Sowrey, F.E.; Newport, R.J.; "Physical Review B" 2004, "69", 224107-1-224107-7. ] Calcium hexaboride is also referred to as calcium boride. It in an important material due to its large electrical conductivity, moderate hardness, high chemical stability, and high melting point. Shi, L.; Gu, Y.; Chen, L; Yang, Z.; Ma, J.; Qian,Y. "Chem. Lett." 2003, "32,"958-959 ] Zheng, S.Q.; Min, G.H.; Zou, Z.D.; Tatsuyama, C. "Mater. Lett." 2004, "58," 2586-2589 ] It is a black lustrous powder, with a low density and a high melting point. It is a chemically inert compound that has the typical metal hexaboride structure, with octahedral units of 6 boron atoms combined with calcium atoms. CaB₆ and lanthanum doped CaB₆ has been found to have weak ferromagnetic properties.

RB_x have been investigated due to a variety of peculiar physical properties, such as superconductivity, valence fluctuation and Kondo effects, anomalous magnetism, and narrow gap semiconducting properties. Baranovskiy, A.E.; Grechnew, G.E.; Logosha, A.V.; Svechkarew, I.V.; Fillippov, V.B.; Shitsevalova, N. Yu.; Zogal, O.J.; Eriksson, O."Phys. Stat. Sol." 2006, "3," 229-232. ] Typical samples of CaB₆ are non-stoichiometric, i.e. the ratio of boron to calcium is not exactly 6:1, for example the ultra-fine powders of CaB₆ produced by some researchers had B:Ca ratio of 5.91:1. [ Liang Shi, Yunle Gu, Luyang Chen, Zeheng Yang, Jianhua Ma and Yitai Qian Chemistry Letters Vol. 32 (2003), 10, 958 doi:10.1246/cl.2003.958 "Low Temperature Synthesis and Characterization of Cubic CaB6 Ultrafine Powders"]

History

Longuet-Higgins and Roberts first calculated the energy bandstructure of calcium hexaboride with a semiempirical molecular orbital in 1954. Hasegawa, A.; Yanase, A. "J. Phys. C: Solid State Phys," 1979, "12," 5431-5440. ] The discovery of ferromagnetism in lightly doped calcium hexaboride was in 1999 by Young and his associates. Lee, B.; Wang, L. "Appl. Phys. Lett." 2005, "87," 262509-1 - 262509-3. ] Young, D.P; Hall, D.; Torelli, M.E.; Fisk, Z.; Sarrao, J.L.; Thompson, J.D.; Ott, H.R.; Oseroff, S.B.; Goodrich, R.G.; Zysler, R. "Nature" 1999, "397," 412-414. ]

Chemical Properties

Calcium hexaboride is insoluble in H₂O, MeOH (methanol), and EtOH (ethanol) and dissolves slowly in acids.Calcium boride."Dictionary of Inorganic Compounds"; University Press: Cambridge, 1992; Vol. 1. ]

The microhardness is 27 GPa, ΔH_f = -120 kJ/mol, and the resistivity is greater than 200 Ωm X 10⁸.

The magnetic moment is less than or equal to 0.07μ_Bper atom.

The Curie Temperature is ~ 600 K. Helms, Z.M.; Sen, P.; Mitas, L. "Electronic Structure and Origin of Ferromagnetism in CaB6" 2005 Los Alamos National Laboratory. ]

The ferromagnetic transition temperature of calcium hexaboride is around 900 K which is high for a molecule without partially filled d- or f-shell orbitals. Mean, B.J.; Lee, K.H.; Kang, K.H.; Lee, Moohee; Rhee, J.S.; Cho, B.K.; "Physica B" 2005, "359-361", 1204-1206. ]

It is a semiconductor with an energy gap of 0.48eV and a band gap of about 1.0eV.

Ferromagnetic order (FM) have been observed at unexpectedly high temperature in La doped alkaline-earth hexaboride compounds including calcium hexaboride. The origin of this high temperature FM is the FM phase of a dilute electron gas, linkage to the presumed excitonic state in calcium boride, or external impurities on the surface of the sample. In fact, the impurities include Iron and Nickel probably coming from boron in sample preparation.

tructural Information

The crystal structure of calcium hexaboride is a cubic lattice with calcium at the cell centre and compact, regular octahedral of boron atoms linked at the vertices by B-B bonds to give a 3-D boron network. Each calcium has 24 nearest-neighbor boron atoms. Yahia,S.;Turrell, S.; Turrell, G.; Mercurio, J.P."J. Mol. Struct." 1990, "224," 303-312. ] 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. Wells, A.F. "Structural Inorganic Chemistry"; Clarendon Press, Oxford, 1984 pp. 1055-1056. ] 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. 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. The Ca-B bond length is 3.05 Å and the B-B bond length is 1.7 Å. The predicted lattice constant varies between 4.08 and 4.105 Å while it has been experimentally measured from 4.145 to 4.153 Å.

Crystal Structure:Cubic

Space Group: P"m3m"

Point Group: O_h

⁴³Ca NMR data contains δ_peak at -56.0 ppm and δ_iso at -41.3 ppm where δ_iso is taken as peak max +0.85 width, the negative shift is due to the high coordination number.

Raman Data: Calcium hexaboride has three Raman peaks at 754.3, 1121.8, and 1246.9 cm^-1 due to the active modes A_1g, E_g, and T_2g respectively.

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.

Preparation

Methods of producing CaB₆ powder include high temperature reactions of: calcium and boron at 1000°C, Ca(OH)₂ and boron in vacuum at about 1700°C, calcium carbonate, CaCO₃ and boron, B₄C in vacuum above 1600°C, B with Calcium or calcium oxide (CaO) at 800-1800°C, or from CaO and H₃BO₃ and Mg at 1100°C. One of the main reactions for industrial production is: Borides:Solid State Chemistry "Encyclopedia of Inorganic Chemistry"; John Wiley & Sons; West Sussex, England, 1994; Vol 1 ] :CaO + 3 B₂O₃ + 10 Mg → CaB₆ + 10 MgO

Uses

Carbon hexaboride is used in the manufacturing of boron-alloyed steel. Also, calcium hexaboride is used as a deoxidation agent in production of oxygen-free copper, which results in higher conductivity than conventionally phosphorus-deoxidized copper owing to the low solubility of boron in copper. Also, it can serve as a high temperature material, surface protection, abrasives, tools, and wear resistant material.

When used at elevated temperature, calcium hexaboride will oxidize degrading its properties and shortening its usable lifespan.

Precautions

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.

Harardous decomposition products: Boron oxides, avoid strong oxidizing agents.

uppliers

Sigma Aldrich [http://www.sigmaaldrich.com/catalog/search/ProductDetail/ALDRICH/394785]

ee also

Boride

Calcium

General references

* Terry T. Xu, Jian-Guo Zheng, Alan W. Nicholls, Sasha Stankovich, Richard D. Piner, and Rodney S. Ruoff, Nano Lett., Vol. 4, No. 10, 2004 2051-2055 doi 10.1021/nl0486620, "Single-Crystal Calcium Hexaboride Nanowires: Synthesis and Characterization"
* ESK chemicals "Calcium hexaboride" Preliminary product information [http://www.esk.com/Uploads/PFL_calciumhexaboride_20mesh_e.pdf]
*Greenwood&Earnshaw
* Crystal Lattice Structures [http://cst-www.nrl.navy.mil/lattice/]

References