Comparative studies of the structural, magnetic, and optical properties of the sol–gel synthesized Ln_0.67Ca_0.33MnO₃ (Ln = La, Pr, Nd, and Sm) nanoparticles were carried out focusing on the effect of the A-site average cation size ⟨r_A⟩. Rietveld refinements of x-ray diffraction data demonstrate all nanoparticles crystallize in an orthorhombic crystal structure (Pnma space group). Their unit cell volumes and the Mn–O–Mn bond angle decreased with reducing ⟨r_A⟩, whereas the Mn–O bond length increased. The morphologies of nanoparticles evolved from spherical to irregular shapes, and their single-crystalline nature was confirmed by HRTEM images. Infrared spectra identified the stretching mode of the Mn–O bond near 600 cm⁻¹, and the softening of this phonon mode as reducing ⟨r_A⟩ is ascribed to the elongation of the Mn–O bond length. X-ray photoelectron spectroscopy reveals the mixed Mn³⁺ and Mn⁴⁺ cations with a content ratio of Mn³⁺/Mn⁴⁺ = 2:1, divalent Ca cations, and trivalent rare earth Ln cations in all nanoparticles and oxygen element existing as lattice oxygen and chemically absorbed oxygen. The La_0.67Ca_0.33MnO₃ nanoparticles exhibited ferromagnetic behavior, whereas Ln_0.67Ca_0.33MnO₃ (Ln = Pr, Nd, and Sm) nanoparticles displayed antiferromagnetic behavior and strong exchange bias effect. Temperature dependence of dc magnetizations suggests the spin-glass behavior established in the La_0.67Ca_0.33MnO₃ nanoparticles, while magnetic cluster-glass behavior formed in the Ln_0.67Ca_0.33MnO₃ (Ln = Pr, Nd, and Sm) nanoparticles, in which the charge-ordered and antiferromagnetic phases were completely suppressed. Electronic bandgaps of the nanoparticles were about 1.55 eV–1.66 eV, which was ascribed to the electronic charge transfer between two e_g bands of the Mn cation with up-spins and down-spins separated by Hund’s coupling energy.

中文翻译：

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溶胶-凝胶法合成钙钛矿Ln0.67Ca0.33MnO3（Ln = La，Pr，Nd和Sm）锰矿纳米颗粒的结构，磁性和光学性质的比较研究

溶胶-凝胶合成的Ln _0.67 Ca _0.33 MnO ₃（Ln = La，Pr，Nd和Sm）纳米粒子的结构，磁性和光学性质的比较研究着眼于A位平均阳离子的影响大小⟨ ř_甲⟩。X射线衍射数据的Rietveld改进表明，所有纳米颗粒均以正交晶体结构（Pnma空间群）结晶。随着unit r _A的减小，它们的晶胞体积和Mn–O–Mn键角减小⟩，而Mn–O键长增加。纳米颗粒的形态从球形演变为不规则形状，并通过HRTEM图像证实了其单晶性质。红外光谱识别为靠近600厘米中的Mn-O键的伸缩模式^-1，并且该声子模式，作为减少软化⟨ ř_甲⟩归因于将Mn-O键长的伸长。X射线光电子能谱显示，在所有存在的纳米粒子和氧元素中，Mn ³⁺ / Mn ⁴⁺ = 2：1的Mn ³⁺和Mn ⁴⁺混合比，二价Ca阳离子和三价稀土Ln阳离子作为晶格中的氧和化学吸收的氧。La _0.67Ca _0.33 MnO ₃纳米颗粒表现出铁磁行为，而Ln _0.67 Ca _0.33 MnO ₃（Ln = Pr，Nd和Sm）纳米颗粒表现出反铁磁行为和强烈的交换偏置作用。直流磁化强度的温度依赖性表明，在La _0.67 Ca _0.33 MnO ₃纳米颗粒中建立了自旋玻璃行为，而在Ln _0.67 Ca _0.33 MnO _3中形成了磁簇玻璃行为。（Ln = Pr，Nd和Sm）纳米粒子，其中电荷序相和反铁磁相被完全抑制。纳米粒子的电子带隙约为1.55 eV–1.66 eV，这归因于Mn阳离子的两个e _g带之间的电荷转移，其中上旋和下旋被Hund耦合能分开。