## Abstract

Despite its relevance, the microscopic origin of the energy barrier, *B*, between the compressed and elongated geometries of Jahn–Teller (JT) systems is not well understood yet because of a lack of quantitative data about its various contributions. Seeking to clear up this matter, we have carried out both periodic and cluster ab initio calculations on the model system NaCl:Ni^{+}. This system is particularly puzzling because, according to experimental data, its barrier is much smaller than that for other d^{9} and d^{7} ions in similar lattices. All calculations performed on the model system lead, in fact, to values |*B*| ≤ 160 cm^{–1}, which are certainly smaller than *B* = 500 cm^{–1} derived for NaCl:M^{2+} (M = Ag, Rh) or *B* = 1024 cm^{–1} obtained for KCl:Ag^{2+}. As a salient feature, analysis of calculations carried out as a function of the *Q*_{θ} (3*z*^{2} – *r*^{2}) coordinate unveils the microscopic origin of the barrier. It is quantitatively proven that the elongated geometry observed for NaCl:Ni^{+} is due to the 3d–4s vibronic admixture, which is slightly larger than the anharmonicity in the e_{g} JT mode that favors a compressed geometry. The existence of these two competing mechanisms explains the low value of *B* for the model system, contrary to cases where the complex formed by d^{9} or d^{7} ions is elastically decoupled from the host lattice. Although the magnitude of *B* for NaCl:Ni^{+} is particularly small, the tunneling splitting, 3Γ, is estimated to be below 9 cm^{–1}, thus explaining why the coherence is easily destroyed by random strains and thus a static JT effect is observed experimentally. As a main conclusion, the barrier in JT systems cannot be understood neglecting the tiny changes of the electronic density involved in small distortions. The present calculations reasonably explain the experimental **g** tensor of NaCl:Ni^{+}, pointing out that the d–d transitions in NiCl_{6}^{5–} are much smaller than those for CuCl_{6}^{4–} and the optical electronegativity of Ni^{+} is only around 1.

Original language | English |
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Journal | Inorganic Chemistry |

Volume | 52 |

Issue number | 16 |

Pages (from-to) | 9338-9348 |

ISSN | 0020-1669 |

DOIs | |

Publication status | Published - 2013 |