MALTA Consolider Team, Unidad Asociada ICMA-CSIC, DCITIMAC, Facultad de Ciencias, Universidad de Cantabria, E-39005 Santander, Spain.
J Phys Condens Matter. 2011 Oct 19;23(41):415501. doi: 10.1088/0953-8984/23/41/415501. Epub 2011 Sep 26.
The coordination, the electronic structures and the spin of the ground state of Ni(3+) (3d(7)) and Co(3+) (3d(6)) introduced as impurities in LaAlO(3) are investigated through optical spectroscopy and magnetic measurements. The unusual trivalent valence state in both transition-metal ions was stabilised via a sol-gel process followed by high oxygen pressure treatments. We show that the crystal-field strength at the nearly O(h) transition-metal site in LaAlO(3) locates Ni(3+) and Co(3+) near the spin state crossover, yielding a low-spin ground state in both cases. We analyse how the interplay between the Jahn-Teller (JT) effect and the spin state affects the magnetic moment of the ion and its temperature dependence. The optical spectra reveal a JT effect associated with a low-spin ground state in Ni(3+) and with a thermally populated high-spin low-lying first excited state in Co(3+). The corresponding JT distortions are derived from structural correlations. We conclude that the JT effect is unable to stabilise the intermediate spin state in Co(3+). A low-spin ground state in thermal equilibrium with a high-spin low-lying first excited state is detected in diluted Co(3+)-doped LaAlO(3). These results are compared with those obtained in the parent pure compounds LaNiO(3) and LaCoO(3).
通过光学光谱和磁测量研究了作为杂质引入 LaAlO3 中的 Ni(3+)（3d(7)）和 Co(3+)（3d(6)）的协同作用、电子结构和基态自旋。通过溶胶-凝胶工艺和高压氧处理稳定了两种过渡金属离子异常的三价价态。我们表明，LaAlO3 中近 O(h)过渡金属位点的晶体场强度使 Ni(3+)和 Co(3+)处于自旋交叉附近，导致两种情况下均为低自旋基态。我们分析了 Jahn-Teller（JT）效应和自旋态之间的相互作用如何影响离子的磁矩及其温度依赖性。光学光谱揭示了与 Ni(3+)的低自旋基态和 Co(3+)的热占据的低能高自旋第一激发态相关的 JT 效应。相应的 JT 畸变来自结构相关性。我们得出结论，JT 效应无法稳定 Co(3+)的中间自旋态。在稀释的 Co(3+)-掺杂 LaAlO3 中检测到与低能高自旋第一激发态处于热平衡的低自旋基态。这些结果与在母体纯化合物 LaNiO3 和 LaCoO3 中获得的结果进行了比较。
