National Synchrotron Radiation Research Center , 101 Hsin Ann Road, Hsinchu 30076, Taiwan, R.O.C.
J Am Chem Soc. 2014 Jan 29;136(4):1514-9. doi: 10.1021/ja4114006. Epub 2014 Jan 15.
The complex metal oxide SrCo0.5Ru0.5O(3-δ) possesses a slightly distorted perovskite crystal structure. Its insulating nature infers a well-defined charge distribution, and the six-fold coordinated transition metals have the oxidation states +5 for ruthenium and +3 for cobalt as observed by X-ray spectroscopy. We have discovered that Co(3+) ion is purely high-spin at room temperature, which is unique for a Co(3+) in an octahedral oxygen surrounding. We attribute this to the crystal field interaction being weaker than the Hund's-rule exchange due to a relatively large mean Co-O distances of 1.98(2) Å, as obtained by EXAFS and X-ray diffraction experiments. A gradual high-to-low spin state transition is completed by applying high hydrostatic pressure of up to 40 GPa. Across this spin state transition, the Co Kβ emission spectra can be fully explained by a weighted sum of the high-spin and low-spin spectra. Thereby is the much debated intermediate spin state of Co(3+) absent in this material. These results allow us to draw an energy diagram depicting relative stabilities of the high-, intermediate-, and low-spin states as functions of the metal-oxygen bond length for a Co(3+) ion in an octahedral coordination.
具有略微扭曲钙钛矿晶体结构的复杂金属氧化物 SrCo0.5Ru0.5O(3-δ)。其绝缘性质推断出明确的电荷分布，并且六配位过渡金属具有+5 的氧化态（如 X 射线光谱所观察到的）为钌和+3 的钴。我们发现 Co(3+)离子在室温下纯粹是高自旋，这对于八面体氧配位中的 Co(3+)是独特的。我们将其归因于晶体场相互作用比 Hund 规则交换弱，因为通过 EXAFS 和 X 射线衍射实验获得的平均 Co-O 距离为 1.98(2) Å。通过施加高达 40 GPa 的高压静压，可以完成从高自旋到低自旋状态的逐步转变。在这个自旋态转变过程中，Co Kβ发射光谱可以通过高自旋和低自旋光谱的加权和来完全解释。因此，在这种材料中不存在 Co(3+)的中间自旋态，这是一个备受争议的问题。这些结果使我们能够绘制一个能量图，该图描绘了在八面体配位中 Co(3+)离子的金属-氧键长的函数下，高、中、低自旋态的相对稳定性。
