Shoemaker Daniel P, Li Jun, Seshadri Ram
Materials Department, Materials Research Laboratory, University of California, Santa Barbara, California 93106, USA. 93106Received March 17, 2009; E-mail:
J Am Chem Soc. 2009 Aug 19;131(32):11450-7. doi: 10.1021/ja902096h.
At first sight, the quenched tetragonal spinel CuMn(2)O(4) can be formulated with Cu(2+) and Mn(3+), implying that the tetrahedral site is Jahn-Teller (JT)-active Cu(2+) and the octahedral site is JT-active Mn(3+). High-resolution, high-momentum-transfer neutron scattering analysis suggests that the sample has approximately 30% inversion: Mn on the tetrahedral Cu site with compensating Cu on the octahedral site. Reverse Monte Carlo (RMC) analysis of the pair distribution function allows details of metal-oxygen connectivity to be probed in a manner that is significantly on the local rather than the average scale. Bond valence analysis of the RMC supercell reveals that both JT ions disproportionate to higher and lower valence states as a means of avoiding their JT tendency, particularly on the tetrahedral site. The occurrence of Cu(3+) in particular is suggested for the first time and is supported by X-ray photoelectron spectroscopy data. The bimodal distribution of O-Cu-O bond angles at the tetrahedral site (distinct from what is seen for O-Mn-O bond angles) further reveals a hidden distinction between sites previously considered to be equivalent. Application of total scattering techniques originally developed for highly disordered materials permits the examination of nanoscale crystalline structure with elemental specificity that is not available in traditional reciprocal-space analysis.
乍一看，猝灭的四方尖晶石CuMn₂O₄可以用Cu²⁺和Mn³⁺来表示，这意味着四面体位置是具有 Jahn-Teller（JT）活性的Cu²⁺，八面体位置是具有JT活性的Mn³⁺。高分辨率、高动量转移中子散射分析表明，该样品约有30%的反位：四面体Cu位置上有Mn，八面体位置上有补偿性的Cu。对配位数函数进行反向蒙特卡罗（RMC）分析，可以以一种主要基于局部而非平均尺度的方式探究金属-氧连接的细节。对RMC超晶胞进行键价分析表明，两个JT离子都发生歧化反应，形成更高和更低的价态，以此来避免其JT倾向，特别是在四面体位置上。首次提出了Cu³⁺的存在，并得到了X射线光电子能谱数据的支持。四面体位置上O-Cu-O键角的双峰分布（与O-Mn-O键角不同）进一步揭示了以前认为等效的位置之间隐藏的差异。最初为高度无序材料开发的全散射技术的应用，使得能够以传统倒易空间分析中无法获得的元素特异性来研究纳米级晶体结构。
