La3Co29-xFexSi4B10的择优占位、电子结构和晶格振动性质的理论研究

分别用第一性原理及原子间相互作用势对初始模型进行几何优化, 所得稳定结构的晶格参数均与实验值符合较好. 通过第一性原理密度泛函理论, 计算了稳定结构La₃Co_29-xFe_xSi₄B₁₀化合物择优占位情况, 计算结果表明, Fe原子最择优替代Co 原子的2c晶位, 择优占位顺序为2c > 8j1 > 8i2 > 8j2 > 8i3 > 16k > 8i1, 这与实验结果非常符合. Fe原子每次只替代不同晶位的一个Co原子时, La₃Co_29-xFe_xSi₄B₁₀ 体系的晶格常数几乎不变, 磁矩却发生了有趣的变化. 当Fe原子沿着择优顺序依次替代不同晶位的所有Co原子时, 随着La₃Co_29-xFe_xSi₄B₁₀体系中Fe原子含量的增多, 其电子态密度整体向左移动. Fe原子完全替代Co时, 与未掺杂时相比体系的总磁矩增加. 最后, 利用原子间相互作用势进一步预测了La₃Co_29-xFe_xSi₄B₁₀体系的晶格振动及热力学性质. 在低频部分, 振动模式主要由质量较大的Co, Fe和La元素作贡献; 随着掺杂原子Fe的增多, 体系的截止频率先减小后增多, 中频部分由Si元素引起的振动模式减少; B-B强相互作用引起了高频部分的振动模式. 基于声子态密度预测了不同数量Fe掺杂后体系的比热、熵和德拜温度的变化, 当Fe 含量大于Co时, 德拜温度明显升高.

Theoretical studies of the site preference,electronic and lattice vibration properties of La3Co29-xFexSi4B10

In this work, the initial configuration is first optimized by the first principle and interatomic pair potentials separately, the lattice parameters of the stable structure are in good agreement with the experimental values. The site preferences of La₃Co_29-xFe_xSi₄B₁₀ compounds are studied by using the first principle with density function theory method. The calculated results show that the substitution of Fe for Co has a strong preference for the 2c site, and the substitution sequence is 2c > 8j1 > 8i2 > 8j2 > 8i3 > 16k > 8i1, which is in good agreement with the experimental result The lattice parameters of La₃Co_29-xFe_xSi₄B₁₀ system change little, but the magnetic moment changes obviously, when only one Co atom is substituted by Fe atoms each time. We calculate the electronic densities of states and magnetic moments of La₃Co_29-xFe_xSi₄B₁₀ compound when all the Co atoms from different sites are substituted by Fe atoms with the preferential order With the increases of Fe content values in the La₃Co_29-xFe_xSi₄B₁₀, the curves of density of states move leftwards gradually. And the magnetic moment of the La₃Fe₂₉Si₄B₁₀ is larger than that of La₃Co₂₉Si₄B₁₀. Furthermore, the lattice vibrational and thermodynamic properties are predicted by using a series of interatomic pair potentials. The Co, Fe and La atoms contribute to the lower frequency vibrations because of their heavier mass. With the increase of Fe content the cut-off frequencies of La₃Co_29-xFe_xSi₄B₁₀ first decrease and then increase, and the vibration mode induced by Si element decreases in medium frequency. The very strong B-B interaction causes higher frequency vibrations. Furthermore, the specific heat, vibrational entropy and Debye temperature are predicted based on the phonon densities of states of the La₃Co_29-xFe_xSi₄B₁₀ with the different content values of Fe. The Debye temperature rises when the Fe content is bigger than Co content in La₃Co_29-xFe_xSi₄B₁₀compound.