Shu Guo-Jiun, Wu Pei-Chieh, Chou F C
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology Taipei 10608 Taiwan
Institute of Mineral Resources Engineering, National Taipei University of Technology Taipei 10608 Taiwan.
RSC Adv. 2020 Nov 26;10(70):43117-43128. doi: 10.1039/d0ra09675j. eCollection 2020 Nov 23.
Based on an integrated study of magnetic susceptibility, specific heat, and thermal expansion of single-crystal LaCoO free from cobalt and oxygen vacancies, two narrow spin gaps are identified before and after the phonon softening of gap size Δ ∼ 0.5 meV in a CoO-octahedral crystal electric field (CEF) and the thermally activated spin gap ∼ 25 meV, respectively. Significant excitation of Co spins from a low-spin (LS) to a high-spin (HS) state is confirmed by the thermal activation behavior of spin susceptibility of energy gap ∼ 25 meV, which follows a two-level Boltzmann distribution to saturate at a level of 50% LS/50% HS statistically above ∼200 K, without the inclusion of a postulated intermediate spin (IS) state. A threefold increase in the thermal expansion; coefficient () across the same temperature range as that of thermally activated HS population growth is identified, which implies the non-trivial spin-orbit-phonon coupling caused the bond length of Co-O fluctuation and the local lattice distortion. The unusually narrow gap of Δ ∼ 0.5 meV for the CoO octahedral CEF between e-t indicates a more isotropic negative charge distribution within the octahedral CEF environment, which is verified by the Electron Energy Loss Spectroscopy (EELS) study to show nontrivial La-O covalency.
基于对不含钴和氧空位的单晶LaCoO的磁化率、比热和热膨胀的综合研究，在CoO八面体晶体电场（CEF）中，在声子软化前后分别识别出两个窄自旋能隙，其能隙大小Δ ∼ 0.5 meV，以及热激活自旋能隙 ∼ 25 meV。能隙 ∼ 25 meV的自旋磁化率的热激活行为证实了Co自旋从低自旋（LS）态到高自旋（HS）态的显著激发，在统计上高于 ∼200 K时，其遵循双能级玻尔兹曼分布，在50% LS/50% HS水平达到饱和，且不包含假定的中间自旋（IS）态。在与热激活HS态数量增长相同的温度范围内，热膨胀系数（）增加了三倍，这意味着非平凡的自旋 - 轨道 - 声子耦合导致了Co - O键长的波动和局部晶格畸变。对于CoO八面体CEF，e - t之间异常窄的能隙Δ ∼ 0.5 meV表明在八面体CEF环境中负电荷分布更具各向同性，电子能量损失谱（EELS）研究证实了这一点，显示出非平凡的La - O共价性。
