Structural relaxation in tetrahedrally coordinated Co2+ along the gahnite-Co-aluminate spinel solid solution
Structural relaxation in tetrahedrally coordinated Co2+ along the gahnite-Co-aluminate spinel solid solution
复制标题
四面体配位 Co2 沿尖晶石-钴-铝酸盐尖晶石固溶体的结构弛豫
DOI：
10.2138/am.2012.4093
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发表时间：
2012
期刊：
影响因子：
--
通讯作者：
M. Dondi
中科院分区：
文献类型：
--
作者：
M. Ardit;G. Cruciani;M. Dondi
摘要采用X射线衍射和电子吸收光谱相结合的方法研究了锌铝尖晶石（ZnAl 2 O 4）-钴铝酸盐（CoAl 2 O 4）界面上钴离子沿着的结构弛豫。通过固相反应（1300 °C缓慢冷却）获得单相尖晶石样品（Zn 1-yCoyAl 2 O 4，y = 0，0.25，0.5，0.75和1apfu）。钴的掺入引起晶胞参数（a）的线性增加，伴随着反转参数的增加（高达0.07），使得在八面体位置中Co 2+对Al 3+的取代在第一近似下是晶格膨胀的原因。然而，仔细考虑的T-O距离突出的作用所发挥的增强的共价度的Zn-O键。的光学光谱的特征在于发生的电子跃迁的Co2+四面体协调的影响下，一个强大的自旋-轨道耦合，导致三倍分裂的自旋允许带。进一步的复杂性源于四重态和二重态的混合（导致自旋禁带的一致强度增益）和电子振动效应（产生强烈的边带）。随着钴含量的增加，晶体场强从4187到4131 cm-1，而Racah B参数在744-751 cm-1（C = 3375 cm-1）范围内。为了实现局部Co-O距离的可靠估计，四面体距离演化被重铸以消除反转度的影响。通过这种方式，获得了低至ε = 0.47的弛豫系数，即，显著小于其它尖晶石体系的文献数据。锌铝尖晶石-Co-铝酸盐连接似乎受到Zn 2+对四面体位点的强烈偏好的约束，在该四面体位点中可以发挥其增强的共价性，限制四面体和八面体位点之间的阳离子交换以及晶格灵活性。
Abstract The structural relaxation around the Co2+ ion along the gahnite (ZnAl2O4)-Co-aluminate (CoAl2O4) join was investigated by a combined X-ray diffraction (XRD) and electronic absorption spectroscopy (EAS) approach. Monophasic spinel samples (Zn1-yCoyAl2O4 with y = 0, 0.25, 0.5, 0.75, and 1 apfu) were obtained through solid-state reaction (1300 °C with slow cooling). The cobalt incorporation induces a linear increase of the unit-cell parameter (a) accompanied by an increasing inversion parameter (up to 0.07) so that the Co2+ for Al3+ substitution in the octahedral site is, at a first approximation, the cause of the lattice expansion. However, a careful consideration of T-O distances highlights the role played by an enhanced covalence degree of Zn-O bonds. The optical spectra are characterized by the occurrence of electronic transitions of Co2+ in tetrahedral coordination affected by a strong spin-orbit coupling, causing a threefold splitting of spin-allowed bands. Further complications stem from mixing of quadruplet and doublet states (leading to a consistent intensity gain of spin-forbidden bands) and vibronic effects (producing intense sidebands). Crystal field strength goes from 4187 to 4131 cm-1 with increasing cobalt amount, while the Racah B parameter is in the 744-751 cm-1 range (C ∼3375 cm-1). To achieve a reliable estimation of the local Co-O distance, the tetrahedral distance evolution was recast to eliminate the effects of the inversion degree. By this way, a relaxation coefficient as low as ε = 0.47 was obtained, i.e., significantly smaller than literature data for other spinel systems. The gahnite-Co-aluminate join seems to be constrained by the strong preference of Zn2+ for the tetrahedral site in which its enhanced covalency can be exerted, limiting the cation exchange between tetrahedral and octahedral sites as well as the lattice flexibility.