First Principle Study Of CaB₆and Ceramic Preparation

The calcium boride （CaB₆） is one of the alkaline earth metal borides. At present, Theresearchers mainly concentrates to the synthesis of high-purity CaB₆powders at home andabroad, the preparation of block of CaB₆ceramics and the research of mechanics, optics andthermodynamics are less. This article based on the density functional theory first principlecomputational method, from the theoretically we calculate CaB₆of the electronic structureand physical properties of mechanics, optics and thermodynamics, then has prepared theCaB₆ceramics block body material with the vacuum thermo-compression craft and studiesits microscopic structure and mechanics performance.Through carries on the different exchange correlation function to optimize the structureof calcium hexaboride, then use the obtained balance crystal lattice constant compared withthe experimental values and selects the calculation functional is GGA-PBE.The calciumhexaboride energy structure shows that it is the direct band gap semiconductor, the energygap is1.144eV and close with experimental value1.15eV. Population analysis showed thatthe calcium hexaboride has bond two kind of types which the B-B covalent bond and theCa-B ionic. The Ca atoms3s and3p states of the electron density of states sharp and narrowpeaks and shows he strong localization, B atoms in the2s and2p states of the electrondensity of states form intense hybridization shows characteristics of delocalization.The calcium hexaboride elastic constants C₁₁, C₁₂and C₄₄are431.87GPa,7.46GPaand46.47GPa. The calcium hexaboride shear modulus G, the bulk modulus B and theYoung’s modulus E are90.17GPa,148.93GPa and431.62GPa. The CaB₆presents brittlecharacteristics at room temperature according to the G/B=0.61>0.57. The lattice constantdecreases with increasing external hydrostatic pressure. The bulk modulus, shear modulusand Young’s modulus increase with increasing hydrostatic pressure. The toughnessimproved with increasing external hydrostatic pressure.The optical properties of calcium hexaboride calculations show that: the dielectricfunction imaginary component appeared four transition peaks which are located5.07eV,7.02eV,10.57eV and27.62eV place separately; static refractive index n₀=2.88, extinctioncoefficient and absorption coefficient along with energy change situation consistent; theabsorption spectrum has three the biggest absorption peaks which are located11.31eV, 28.09eV and51.28eV place separately, corresponding absorption peak value for3.89×10⁵cm^-1,4.52×10⁵cm^-1and1936.16cm-1; the calcium hexaboride appears three energyloss peak is located8.494eV,16.924eV and29.623eV place separately.The lattice dynamics properties of calcium hexaboride show that: the calciumhexaboride has3acoustic branch and18optical branch in the brillouin zone G point; Batoms has a big contribution to the phonon density of states in high frequency region, thephonon density of states in low frequency region is contributed by Ca and B atoms;Enthalpy, free energy and entropy is consistent with the third law of thermodynamics; In thecase of high-temperature limit, the dulong-petit value of calcium hexaboride is41.697cal/cell.k and debye temperature is about1258k.The Vickers hardness of the prepared calcium hexaboride ceramic block is20.03GPa,which is close with theory predicted value21.79GPa.