【作者】 韩宪越；

【导师】 田永君； 柳忠元；

【作者基本信息】 燕山大学 ， 材料物理与化学， 2011， 博士

【摘要】 硼化物陶瓷由于其高的熔点和化学稳定性及良好的耐高能粒子轰击性等,通常被应用作为具有优良性能的特殊功能材料。由于MgB₂超导电性的发现,碱土金属硼化物受到了人们极大地关注,而钙-硼体系化合物作为碱土金属硼化物的重要一员,自然也引起众多科学家的研究热情。在常压Ca-B相图中,仅有立方结构CaB6一种化合物。尽管有报道指出可能存在与稀土元素四硼化物结构一致的CaB₄化合物,但一直没在实验上成功合成出这种晶体。Schmitt等人采用固态反应法制备了CaB_4-xC_x化合物（0< x <5%）,这种晶体的合成与碳的掺杂有关,使用不同的原料,改变反应条件都未能得到纯CaB₄化合物。我们小组采用高温高压的方法成功合成了纯的CaB₄单晶体,研究表明CaB₄是一种高压下的稳定相。研究成果丰富了Ca-B体系的相形成规律,并为获得新型硼化物提供了有效的新途径。本文对CaB₄晶体的形成规律和物理性质作了系统性研究。研究结果表明在CaB₄晶体形成过程中,硼首先以固液反应方式溶入液态钙中,然后与钙反应形成CaB₄晶体析出。当反应温度较低时,合成的晶体多为细长棒状,获得的晶体数量相对较少。当反应温度较高时,溶入的硼量增多,进而能获得尺寸较大的块状晶体,且晶体形成的产率较高。实验获得的CaB₄晶体密度为2.62（±0.03） g/cm3。采用电感耦合等离子体（ICP）法测定CaB₄晶体中元素含量比为Ca∶B = 1∶4.1。CaB₄晶体电性能测试发现,其电阻率随着温度的变化规律符合金属性导电机制,其剩余电阻为ρ（2 K） = 6.7μ（?）cm,霍尔测量表明晶体的多数载流子是电子。应用德拜和爱因斯坦两个模型,对CaB₄晶体的热容和电阻率随温度变化规律进行了表征。晶体的磁性测量表明纯净CaB₄晶体为顺磁性。通过对喇曼实验偏光配置,测量到了四条散射谱。对CaB₄晶体进行Mn元素掺杂研究表明:采用高温高压方法制备Ca-Mn合金能有效抑制钙的挥发和氧化。通过对掺杂晶体喇曼散射测量发现,A1g和Eg模式的高频部分的散射峰位移较为明显,位移量与合金中的掺杂元素的量有关系,并且由掺杂Mn量大的合金合成的晶体电阻率小,晶体的导电性比由掺杂量少的合金合成晶体的好,可以判定晶体内部有掺杂元素进入。通过第一性原理计算研究了CaB₄晶体的能带结构、光学常数、弹性常数、声子谱等。指出了CaB₄的晶格振动中的两类红外吸收激活振动模式（A2u和Eu）,四类喇曼散射激活振动模式(A_1g、B_1g、B_2g和E_g)。通过第一性原理研究了掺杂CaB₄晶体的能带和自旋极化。更多还原

【Abstract】 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.更多还原