Local structure and excitations in systems with CuF64− units: lack of Jahn–Teller effect in the low symmetry compound Na2CuF4,Physical Chemistry Chemical Physics

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Local structure and excitations in systems with CuF64− units: lack of Jahn–Teller effect in the low symmetry compound Na2CuF4
Physical Chemistry Chemical Physics ( IF 2.9 ) Pub Date : 2020/03/09 , DOI: 10.1039/c9cp06843k
I. Sanchez-Movellan _{1,

2,

3,

4} , J. A. Aramburu _{1,

2,

3,

4} , M. Moreno _{1,

2,

3,

4}

Affiliation

This work attempts to unveil the similarities and differences between Jahn–Teller (JT) and non-JT systems involving CuF₆⁴⁻ units. For achieving this goal, we firstly explore Na₂CuF₄ and NaF:Cu²⁺ systems through first principles calculations and pay particular attention to the links between JT and non-JT systems looking at the electronic density of the hole. The results on Na₂CuF₄ in the monoclinic P2₁/c space group and also in the parent Pbam structure reveal that the local geometry can be understood as an initial tetragonally compressed CuF₆⁴⁻ unit, followed by an additional orthorhombic instability that excludes the JT effect as the origin. Although the present results on NaF:Cu²⁺ underpin an elongated equilibrium geometry such as that measured for Cu²⁺ ions in the cubic perovskite KZnF₃, the force constant for NaF:Cu²⁺ is half that for KZnF₃:Cu²⁺. This crucial fact is direct proof of the elastic decoupling of CuF₆⁴⁻ from the NaF lattice leading to a JT energy, E_JT, which is twice that found for KZnF₃:Cu²⁺. However, both systems have practically the same linear electron–vibration coupling constant, V_1e, a relevant fact whose origin is discussed. The final aim of this work concerns the influence of tetragonal and orthorhombic distortions as well as the internal electric field on the A_1g–B_1g energy gap, Δ, of a variety of systems with CuF₆⁴⁻ complexes. Interestingly, it is shown that compounds with orthorhombic instability and an internal electric field can have a Δ value comparable to the JT system NaF:Cu²⁺. Accordingly, explanations for optical spectra of transition metal compounds based on simple parameterized models can be meaningless. The present study shows that properties displayed by d⁹ compounds in low symmetry lattices can hardly stem from a static JT effect.

中文翻译：

具有CuF64−单元的系统中的局部结构和激发：低对称化合物Na2CuF4中缺乏Jahn–Teller效应

这项工作试图揭示Jahn–Teller（JT）与涉及CuF ₆⁴⁻单元的非JT系统之间的异同。为了实现此目标，我们首先通过第一性原理计算来探索Na ₂ CuF ₄和NaF：Cu ²⁺系统，并特别关注JT和非JT系统之间的联系，着眼于空穴的电子密度。Na ₂ CuF ₄在单斜晶P 2 ₁ / c空间群以及母体Pbam结构中的结果表明，局部几何形状可以理解为初始四方压缩的CuF ₆⁴⁻单位，其后是附加的正交各向异性不稳定性，将JT效应排除在原点之外。尽管目前关于NaF：Cu^2+的结果支撑了细长的平衡几何结构，例如在立方钙钛矿KZnF_3中对Cu²⁺离子的测量，但NaF：Cu²⁺的力常数是KZnF₃：Cu²⁺的力常数的一半。。这个至关重要的事实直接证明了CuF₆⁴⁻与NaF晶格的弹性解耦导致产生JT能量E _JT，这是KZnF₃：Cu^2+的两倍。。但是，两个系统实际上都具有相同的线性电子振动耦合常数V _1e，这是一个有关其起源的相关事实。这项工作的最终目的涉及四方和正交畸变以及内部电场对具有CuF ₆⁴⁻配合物的各种系统的A _1g –B _1g能隙Δ的影响。有趣的是，已证明具有正交各向异性和内部电场的化合物的Δ值可与JT系统NaF：Cu ²⁺相媲美。。因此，基于简单的参数化模型对过渡金属化合物光谱的解释可能毫无意义。本研究表明，d ⁹化合物在低对称晶格中显示的性质几乎不能源自静态JT效应。

更新日期：2020-04-15

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