β‑Mn-Type Co_8+<i>x</i>Zn_{12–<i>x</i>} as a Defect Cubic Laves Phase: Site Preferences, Magnetism, and Electronic Structure

The results of crystallographic analysis, magnetic characterization, and theoretical assessment of β-Mn-type Co–Zn intermetallics prepared using high-temperature methods are presented. These β-Mn Co–Zn phases crystallize in the space group P4132 [Pearson symbol cP20; a = 6.3555(7)–6.3220(7)], and their stoichiometry may be expressed as Co8+xZn12–x [1.7(2) < x < 2.2(2)]. According to a combination of single-crystal X-ray diffraction, neutron powder diffraction, and scanning electron microscopy, atomic site occupancies establish clear preferences for Co atoms in the 8c sites and Zn atoms in the 12d sites, with all additional Co atoms replacing some Zn atoms, a result that can be rationalized by electronic structure calculations. Magnetic measurements and neutron powder diffraction of an equimolar Co:Zn sample confirm ferromagnetism in this phase with a Curie temperature of ∼420 K. Neutron powder diffraction and electronic structure calculations using the local spin density approximation indicate that the spontaneous magnetization of this phase arises exclusively from local moments at the Co atoms. Inspection of the atomic arrangements of Co8+xZn12–x reveals that the β-Mn aristotype may be derived from an ordered defect, cubic Laves phase (MgCu2-type) structure. Structural optimization procedures using the Vienna ab initio simulation package (VASP) and starting from the undistorted, defect Laves phase structure achieved energy minimization at the observed β-Mn structure type, a result that offers greater insight into the β-Mn structure type and establishes a closer relationship with the corresponding α-Mn structure (cI58).本文报道了采用高温法制备的β-Mn型钴锌（Co–Zn）金属间化合物（intermetallics）的晶体学分析（crystallographic analysis）、磁学表征（magnetic characterization）及理论评估（theoretical assessment）结果。该类β-Mn型Co-Zn相的晶体空间群（space group）为P4₁3₂[皮尔逊符号（Pearson symbol）cP20；晶格常数a=6.3555(7)~6.3220(7)]，其化学计量比（stoichiometry）可表示为Co₈₊ₓZn₁₂₋ₓ[1.7(2)<x<2.2(2)]。结合单晶X射线衍射（single-crystal X-ray diffraction）、中子粉末衍射（neutron powder diffraction）与扫描电子显微镜（scanning electron microscopy）的表征结果，原子占位率（atomic site occupancies）分析表明Co原子优先占据8c晶位，Zn原子优先占据12d晶位，其余额外的Co原子会取代部分Zn原子，这一结论可通过电子结构计算得到合理解释。对等摩尔比Co:Zn样品的磁学测量与中子粉末衍射结果证实，该相具有铁磁性（ferromagnetism），居里温度（Curie temperature）约为420 K。采用局域自旋密度近似（local spin density approximation）的中子粉末衍射与电子结构计算结果表明，该相的自发磁化完全源于Co原子的局域磁矩。对Co₈₊ₓZn₁₂₋ₓ的原子排布进行分析可知，β-Mn型原型结构（aristotype）可由有序缺陷立方拉夫斯相（Laves phase）（MgCu₂型）结构衍生得到。以无畸变的缺陷拉夫斯相结构为初始结构，采用维也纳从头算模拟包（Vienna ab initio simulation package, VASP）进行结构优化，最终在观测到的β-Mn型结构类型下实现了能量最小化，这一结果加深了对β-Mn型结构类型的理解，并明确了其与对应α-Mn结构（cI58）之间的紧密关联。