Departamento de Ciencias de la Tierra y Física de la Materia Condensada, Universidad de Cantabria, Avenida de los Castros s/n, 39005 Santander, Spain.
Inorg Chem. 2013 Jun 17;52(12):6923-33. doi: 10.1021/ic400105z. Epub 2013 May 31.
Many relevant properties (including superconductivity and colossal magnetoresistance) of layered materials containing Cu(2+), Ag(2+), or Mn(3+) ions are commonly related to the Jahn-Teller instability. Along this line, the properties of the CuF6(4-) complex in the K2ZnF4 layered perovskite have recently been analyzed using a parametrized Jahn-Teller model with an imposed strain [Reinen, D. Inorg. Chem.2012, 51, 4458]. Here, we present results of ab initio periodic supercell and cluster calculations on K2ZnF4:Cu(2+), showing unequivocally that the actual origin of the unusual compressed geometry of the CuF6(4-) complex along the crystal c axis in that tetragonal lattice is due to the presence of an electric field due to the crystal surrounding the impurity. Our calculations closely reproduce the experimental optical spectrum. The calculated values of the equilibrium equatorial and axial Cu(2+)-F(-) distances are, respectively, R(ax) = 193 pm and R(eq) = 204 pm, and so the calculated distortion R(ax) - R(eq) = 11 pm is three times smaller than the estimated through the parametrized Jahn-Teller model. As a salient feature, we find that if the CuF6(4-) complex would assume a perfect octahedral geometry (R(ax) = R(eq) = 203 pm) the antibonding a(1g)(∼3z(2) - r(2)) orbital is placed above b(1g)(∼x(2) - y(2)) with a transition energy E((2)A(1g) → (2)B(1g)) = 0.34 eV. This surprising fact stresses that about half the experimental value E((2)A(1g) → (2)B(1g)) = 0.70 eV is not due to the small shortening of the axial Cu(2+)-F(-) distance, but it comes from the electric field, E(R)(r), created by the rest of the lattice ions on the CuF6(4-) complex. This internal field, displaying tetragonal symmetry, is thus responsible for the compressed geometry in K2ZnF4:Cu(2+) and the lack of symmetry breaking behind the ligand relaxation. Moreover, we show that the electronic energy gain in this process comes from bonding orbitals and not from antibonding ones. The present results underline the key role played by ab initio calculations for unveiling all the complexity behind the properties of the model system K2ZnF4:Cu(2+), opening at the same time a window for improving our knowledge on d(9), d(7), or d(4) ions in other layered compounds.
许多含有 Cu(2+)、Ag(2+) 或 Mn(3+) 离子的层状材料的相关性质（包括超导性和庞磁电阻）通常与 Jahn-Teller 不稳定性有关。沿着这条线，最近使用带有应变的参数化 Jahn-Teller 模型对 K2ZnF4 层状钙钛矿中的 CuF6(4-) 配合物的性质进行了分析[Reinen，Inorg. Chem.2012, 51, 4458]。在这里，我们展示了对 K2ZnF4:Cu(2+) 的 ab initio 周期性超晶胞和团簇计算的结果，明确表明该四方晶格中 CuF6(4-) 配合物沿晶体 c 轴的异常压缩几何形状的实际起源是由于杂质周围晶体产生的电场。我们的计算非常接近实验的光学光谱。平衡赤道和轴向 Cu(2+)-F(-) 距离的计算值分别为 R(ax) = 193 pm 和 R(eq) = 204 pm，因此计算出的畸变 R(ax) - R(eq) = 11 pm 比通过参数化 Jahn-Teller 模型估计的值小三倍。一个显著的特点是，如果 CuF6(4-) 配合物假设为完美的八面体几何形状（R(ax) = R(eq) = 203 pm），则反键 a(1g)(∼3z(2) - r(2))轨道位于 b(1g)(∼x(2) - y(2))轨道之上，跃迁能 E((2)A(1g) → (2)B(1g)) = 0.34 eV。这个令人惊讶的事实表明，实验值 E((2)A(1g) → (2)B(1g)) = 0.70 eV 的一半左右不是由于轴向 Cu(2+)-F(-) 距离的缩短，而是来自于晶格离子对 CuF6(4-) 配合物的电场 E(R)(r)。这种具有四方对称性的内电场负责 K2ZnF4:Cu(2+) 中的压缩几何形状和配体弛豫后没有对称性破坏。此外，我们表明，该过程中的电子能量增益来自成键轨道，而不是反键轨道。这些结果强调了从头算计算在揭示 K2ZnF4:Cu(2+) 模型系统性质背后的所有复杂性方面所起的关键作用，同时也为我们在其他层状化合物中了解 d(9)、d(7) 或 d(4) 离子提供了一个窗口。
