Investigations On The Growth And The Physical Properties Of Novel CAB₄ Compound

Borides were usually applied with as special function materials because of its high melting point, chemical stability and hardness. Alkaline earth metal borides are greatly concerned reaserchers attention since the discovery of superconductivity in MgB2. Calcium - boron system as a key member of the alkaline earth metal boron compounds attrced researcher'attention. In the binary phase diagram of Ca-B at the ambient pressure, only CaB6 can be found. Although some articles point out that there should be a calcium tetraboride like the rare earth boride of CeB4, any reports have not been found up to now. Schmitt et. has obtained the compound of CaB_4-xC_x (0 < x < 5%) by solid state reaction. The crystals were in the reference to the doped carbon element; no pure CaB₄ crystal was obtained even by changed the reaction materials at the atbient pressure. We have successfully synthesized CaB₄ crystals under High-Temperature High-Pressure (HPHT) condition, making up the P-T phase diagram of CaB₄, and pointing ou a potential synthesis method for the CaB₂ crystal.Investigations on the physical properties of the novel crystal CaB₄ were carried out in this paper. Experiments shows that in HPHT condition, boron atoms were dissolved into the Ca liquid then forms CaB₄ crystal. Needle like crystals were obtained when the reaction temperature was low in the synthesize experiment and quantity were much small. While, granular crystals were obtained when the reaction temperature was higher, and a large number of crystal can be obtained. Density of crystals was 2.62(±0.03) g/cm3. The average element composition ratio in the crystal were Ca:B=1:4.1 by the inductively coupled plasma (ICP) analysis.The resistivity of CaB₄ single crystal decreases as the temperature decreases, approaching to the residual resistivityρ(2 K) = 6.7μ(?)cm, shows a was metallic conductance property. Hall coefficient measurement shows that the majority carrier is electrons. The resistance and heat capacity of CaB₄ crystal can be described by the Debye and Einstein combined models. Measurement shows that pure CaB₄ crystal is paramagnetism. Four Raman scattering modes were obtained by using different light polarization configurations. Doping experiments show that the alloy precursors prepared by High-Temperature High-Pressure method are valid, the volatilization and oxidation can be effectively suppressed. Raman shift of A_1g and Eg modes at high frequency parts were clearly observed for the Mn-doped CaB₄ crystals, electric conductivity for Mn-doped crystal becomes higher than that for the pure CaB₄ crystal, which indicating that Mn element is successfully doped in the CaB₄ crystal.The band structure, optical constants, elastic constants, phonon spectrum were calculated by first principles calculatioin. Infrared ablsorption (A2u, Eu) and Raman scattering (A_1g, B_1g, B_2g, E_g) active vibrations modes were given. The band structure and spin polarization were calculated by first principles calculatioin.