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arXiv 2202.10409cond-mat.str-elcond-mat.mtrl-sci
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双钙钛矿Sr $_{2-x}$ La $_x$ CoNbO $_6$的各向异性热膨胀和Jahn-Teller效应的中子衍射观察

Observation of anisotropic thermal expansion and Jahn-Teller effect in double perovskites Sr$_{2-x}$La$_x$CoNbO$_6$ using neutron diffraction

Ajay Kumar, Anil Jain, S. M. Yusuf, R. S. Dhaka

摘要:

利用变温中子粉末衍射(NPD)研究了双钙钛矿结构Sr $_{2-x}$ La $_x$ CoNbO $_6$($x=$ 0.4和0.6)的结构变化和磁相互作用。在$x=$ 0.4和0.6样品之间观察到从四斜晶系($I4/m$)到单斜晶系($P2_1/n$)的结构相变。有趣的是,晶胞参数的温度演变遵循Grüneisen近似,并且分析表明$x=$ 0.4样品的各向同性热膨胀,而$x=$ 0.6样品显示各向异性,其中沿$c$轴的热膨胀显著偏离Grüneisen函数。我们在CoO $_6$中观察到$z$ -out Jahn-Teller畸变,并因此在相邻的NbO $_6$八面体中观察到$z$ -in畸变。随着La取代的增加,八面体畸变程度的降低是显而易见的,从B位原子周围的局部畸变参数$Δ$的显着减少。

英文摘要:

We use temperature dependent neutron powder diffraction (NPD) to investigate the structural changes and magnetic interactions in double perovskite Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0.4 and 0.6). A structural phase transition from tetragonal ($I4/m$) to monoclinic ($P2_1/n$) is observed between $x=$ 0.4 and 0.6 samples. Interestingly, temperature evolution of the unit cell parameters follow the Grüneisen approximation, and the analysis suggest the isotropic thermal expansion in case of the $x=$ 0.4 sample, whereas the $x=$ 0.6 sample shows the anisotropy where the thermal expansion along the $c$-axis significantly deviates from the Grüneisen function. We observe the $z$-out Jahn-Teller distortion in the CoO$_6$ and consequently $z$-in distortion in the adjacent NbO$_6$ octahedra. With increase in the La substitution, a decrease in the degree of octahedral distortion is evident from the significant reduction in the local distortion parameter $Δ$ around the B-site atoms.

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