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Calcium hexaboride (CaB6) is a compound of calcium and boron in which the coordination number of the calcium is 18. 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.  This compound is continually being researched and new information about the compound is still being discovered. 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.
RBx have been investigated due to a variety of peculiar phycical properties, such as superconductivity, valence fluctuation and Kondo effects, anomalous magnetism, and narrow gap semiconducting properties.  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.
Additional recommended knowledge
Longuet-Higgins and Roberts first calculated the energy bandstructure of calcium hexaboride with a semiempirical molecular orbital in 1954.  The discovery of ferromagnetism in lightly doped calcium hexaboride was in 1999 by Young and his associates. 
Calcium hexaboride is insoluble in H2O, MeOH (methanol), and EtOH (ethanol) and dissolves slowly in acids. 
The microhardness is 27 GPa, ΔHf = -120 kJ/mol, and the resistivity is greater than 200 Ωm X 108. 
The magnetic moment is less than or equal to 0.07μBper atom.
The Curie Temperature is ~ 600 K. 
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. 
It is a semiconductor with an energy gap of 0.48eV and a band gap of about 1.0eV. 
Ferromagnetic order (FM) have been oberved at unexpectedly high termperature 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. 
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.  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.  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 B62- 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. 
Space Group: Pm3m = Oh 
43Ca NMR data contains δpeak at -56.0 ppm and δiso at -41.3 ppm where δiso 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 A1g, Eg, and T2g respectively. 
Observed Vibrational Frequencies cm-1 : 1270(strong) from A1g stretch, 1154 (med.) and 1125(shoulder) from Eg stretch, 526, 520, 485, and 470 from F1g rotation, 775 (strong) and 762 (shoulder) from F2g bend, 1125 (strong) and 1095 (weak)from F1u bend, 330 and 250 from F1u translation, and 880 (med.) and 779 from F2u bend. 
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:
CaO + 3 B2O3 + 10 Mg → CaB6 + 10 MgO
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. 
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Calcium_hexaboride". A list of authors is available in Wikipedia.|