探究（LaPr）CoO3 钙钛矿中低自旋态和高自旋态的不相干混合：关注结构相变。

Department of Physics, Indian Institute of Technology, Guwahati, Assam 781039, India.

J Phys Condens Matter. 2023 Jun 19;35(37). doi: 10.1088/1361-648X/acdb22.

We report the mixed valence and intermediate spin-state (IS) transitions in Pr substituted LaCoOperovskites in the form of bulk and nanostructures. Various compositions () of LaPrCoO(0 ⩽⩽ 0.9) were synthesized using the sol-gel process under moderate heat treatment conditions (600 °C). The structural analysis of these compounds reveals a phase crossover from the monoclinic phase (space group, s.g.:) to an orthorhombic one (s.g.:), and a rhombohedral phase (s.g.:) to an orthorhombic one (s.g.:) in the bulk and nanostructures, respectively, for the composition range 0 ⩽⩽ 0.6. Such a structural transformation remarkably reduces the Jahn-Teller distortion factor Δ: 0.374 → 0.0016 signifying the dominant role of the IS state (= 1) of trivalent Co ions in the investigated system. Magnetization measurements reveal the ferromagnetic (FM) nature of bulk LaCoOalong with a weak antiferromagnetic (AFM) component coexisting with an FM component. This coexistence results in a weak loop-asymmetry (zero-field exchange-bias effect ∼134 Oe) at low temperatures. Here the FM ordering occurs due to the double-exchange interaction (∼ 11.25 K) between the tetravalent and trivalent Co ions. Significant decrease in the ordering temperatures was noticed in the nanostructures (∼ 50 K) as compared to the bulk counterpart (∼90 K) due to the finite size/surface effects in the pristine compound. However, the incorporation of Pr leads to the development of a strong AFM component (/∼ 18.2 K) and enhances the ordering temperatures (∼145 K for= 0.9) with negligible FM correlations in both bulk and nanostructures of LaPrCoOdue to the dominant super-exchange interaction: Co‒O‒Co. Further evidence of the incoherent mixture of low-spin (LS) and high-spin (HS) states comes from the-measurements which yields a saturation magnetization of∼ 275 emu mol(under the limit of 1/→ 0) consistent with the theoretical value of 279 emu molcorresponding to the spin admixture: 65% LS + 10% IS of trivalent Co along with 25% of LS Coin the bulk pristine compound. A similar analysis yields: Co[30% LS + 20% IS] + Co[50% of LS] for the nanostructures of LaCoOyet the Pr substitution decreases the spin admixture configuration. The Kubelka-Munk analysis of the optical absorbance results in a significant decrease in the optical energy band gap (:1.86 → 1.80 eV) with the incorporation of Pr in LaCoOwhich corroborates the above results.

我们报告了在 LaCoO 钙钛矿的体相和纳米结构中 Pr 取代的混合价态和中间自旋态（IS）转变。使用溶胶-凝胶法在中等热处理条件（600°C）下合成了各种组成（）的 LaPrCoO（0 ⩽ ⩽ 0.9）。这些化合物的结构分析表明，在体相和纳米结构中，从单斜相（空间群，s.g.：）到正交相（s.g.：），以及从菱面体相（s.g.：）到正交相（s.g.：）的相转变，对于 0 ⩽ ⩽ 0.6 的组成范围。这种结构转变显著降低了 Jahn-Teller 畸变因子 Δ：0.374 → 0.0016，表明在研究体系中三价 Co 离子的 IS 态（= 1）占主导地位。磁化测量表明，LaCoO 体相具有铁磁性（FM）性质，同时存在与 FM 分量共存的弱反铁磁（AFM）分量。这种共存导致低温下出现弱环路不对称（零场交换偏置效应约为 134 Oe）。这里的 FM 有序是由于四价和三价 Co 离子之间的双交换相互作用（∼ 11.25 K）引起的。与体相相比（∼90 K），纳米结构中观察到有序温度显著降低（∼50 K），这是由于原始化合物中的有限尺寸/表面效应所致。然而，Pr 的掺入导致强 AFM 分量的发展（/∼ 18.2 K），并增强了有序温度（对于= 0.9，约为 145 K），同时在 LaPrCoO 的体相和纳米结构中几乎没有 FM 相关性，这是由于主导的超交换相互作用：Co-O-Co。来自-measurements 的进一步证据表明，低自旋（LS）和高自旋（HS）态的不连续混合存在，这导致饱和磁化强度为∼ 275 emu mol（在 1/→ 0 的极限下），与 279 emu mol 的理论值一致，对应于自旋混合：三价 Co 的 65% LS + 10% IS 以及体相原始化合物中 25%的 LS Co。类似的分析表明：LaCoO 纳米结构中的 Co[30% LS + 20% IS] + Co[50% LS]，但 Pr 取代降低了自旋混合配置。Kubelka-Munk 对光吸收的分析导致光学能隙（：1.86 → 1.80 eV）的显著减小，这与 LaCoO 中 Pr 的掺入相符，这证实了上述结果。