Abstract The nature of the non-magnetic to paramagnetic transition of Co3+ oxide LaCoO3 was strongly disputed in the literature for many decades from a low-spin (LS) state below 20 K and to a mixed LS state and high-spin (HS) state or an intermediate-spin (IS) state above 100 K. In this context, the layered perovskite LaSrCoO4 is more favorable for a Jahn-Teller-active IS state because of an elongated distortion, but has been scarcely studied with experimental X-ray spectroscopies as a function of temperature or external pressure. Here, our Co-L2,3 X-ray absorption spectroscopic study indicates a mixture of 40% HS-Co3+ and 60% LS-Co3+ for LaSrCoO4 against 25% HS-Co3+ and 75% LS-Co3+ for LaCoO3 at 300 K and ambient pressure (AP). At 10 K, we observed a sizable magnetic-circular-dichroism signal and a clear HS state of the magnetic Co3+ ion from the Co-L2,3 edge of LaSrCoO4. This result demonstrates that the HS state is already populated in the ground state versus a pure LS ground state in LaCoO3. A quantitative change of quantum number of the spin of the Co3+ ion of LaSrCoO4 as a function of pressure and temperature investigated systematically with Co-Kβ X-ray emission experiments firmly demonstrates not only a mixed state of LS/HS at 300 K and AP but also a presence of the pure LS-Co3+ and HS-Co3+ states only under high pressure and high temperature, respectively.

中文翻译：

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LaSrCoO4 基态中 Co3+ 的不寻常混合自旋态：组合高压和高温研究

摘要 几十年来，Co3+ 氧化物 LaCoO3 从非磁性到顺磁性转变的性质在文献中受到强烈争议，从低于 20 K 的低自旋 (LS) 状态到混合 LS 状态和高自旋 (HS) 状态或高于 100 K 的中间自旋 (IS) 状态。在这种情况下，层状钙钛矿 LaSrCoO4 更适合 Jahn-Teller 活性 IS 状态，因为它具有拉长的畸变，但很少用实验 X 射线光谱进行研究作为温度或外部压力的函数。在这里，我们的 Co-L2,3 X 射线吸收光谱研究表明，在 300 K 和环境温度下，LaSrCoO4 的混合物为 40% HS-Co3+ 和 60% LS-Co3+，而 LaCoO3 为 25% HS-Co3+ 和 75% LS-Co3+压力（AP）。在 10 K 时，我们观察到来自 LaSrCoO4 的 Co-L2,3 边缘的磁性 Co3+ 离子的相当大的磁圆二色性信号和清晰的 HS 状态。该结果表明，与 LaCoO3 中的纯 LS 基态相比，HS 态已经在基态中填充。通过 Co-Kβ X 射线发射实验系统地研究了 LaSrCoO4 的 Co3+ 离子的自旋量子数随压力和温度的变化，这有力地证明了 300 K 和 AP 下的 LS/HS 的混合状态，而且也仅在高压和高温下分别存在纯 LS-Co3+ 和 HS-Co3+ 状态。