A Genuine Jahn–Teller System with Compressed Geometry and Quantum Effects Originating from Zero‐Point Motion

Abstract

First-principle calculations together with analysis of the experimental data found for 3d(9) and 3d(7) ions in cubic oxides proved that the center found in irradiated CaO:Ni(2+) corresponds to Ni(+) under a static Jahn-Teller effect displaying a compressed equilibrium geometry. It was also shown that the anomalous positive g∥ shift (g∥ -g0 =0.065) measured at T=20 K obeys the superposition of the |3 z(2) -r(2) ⟩ and |x(2) -y(2) ⟩ states driven by quantum effects associated with the zero-point motion, a mechanism first put forward by O'Brien for static Jahn-Teller systems and later extended by Ham to the dynamic Jahn-Teller case. To our knowledge, this is the first genuine Jahn-Teller system (i.e. in which exact degeneracy exists at the high-symmetry configuration) exhibiting a compressed equilibrium geometry for which large quantum effects allow experimental observation of the effect predicted by O'Brien. Analysis of the calculated energy barriers for different Jahn-Teller systems allowed us to explain the origin of the compressed geometry observed for CaO:Ni(+) .

New Ideas for Understanding the Structure and Magnetism in AgF2: Prediction of Ferroelasticity

“…As shown on Figure 8, the lowest energy value in the η <0 branch appears at η L =−0.10 Å involving an energy barrier per single complex equal to B =88 meV above the absolute minimum at η L =0.16 Å. This situation is thus akin to that found for d 9 impurities in cubic crystals under a static JT effect [39,52,55] . In these cases, with the known exception of CaO : Ni + , [53,54] the equilibrium geometry corresponds to an elongated octahedron while the compressed geometry ( η <0) is located at an energy above the minimum equal to B =28 meV for NaF : Ag 2+ or B =62 meV for NaCl : Ag 2+ [11] …”

“…This geometry is usually observed for d 9 impurities in cubic lattices and sixfold coordination[ 39 , 53 ] with the exception of CaO : Ni + , [54] a matter discussed in ref. [55] . A geometry close to the elongated one in octahedral symmetry has also been observed when the host lattice is trigonal such as for ZnSiF 6 ⋅ 6 H 2 O : Cu 2+[47–49] or CdCl 2 : M 2+ (M=Cu, Ag).…”

“…This situation is thus akin to that found for d 9 impurities in cubic crystals under a static JT effect. [ 39 , 52 , 55 ] In these cases, with the known exception of CaO : Ni + ,[ 53 , 54 ] the equilibrium geometry corresponds to an elongated octahedron while the compressed geometry ( η <0) is located at an energy above the minimum equal to B =28 meV for NaF : Ag 2+ or B =62 meV for NaCl : Ag 2+ . [11] …”

Insight into Compounds with Cu(H2O)62+ Units: New Ideas for Understanding Cu2+ in Tutton Salts

“…In such systems, the observed equilibrium geometry is tetragonal and not orthorhombic. [30][31][32]59,60,67,68,[70][71][72]75,76 (3) The JTE in an octahedral complex is easily destroyed by tetragonal perturbations leading to energy changes comparable to the energy barrier, B, usually in the range 0.01−0.1 eV. 60 Good examples of this situation are K 2 ZnF 4 :Cu 2+ and Ba 2 ZnF 6 :Cu 2+ where the absence of a JTE has been demonstrated.…”

Jahn–Teller and Non-Jahn–Teller Systems Involving CuF64– Units: Role of the Internal Electric Field in Ba2ZnF6:Cu2+ and Other Insulating Systems

“…The main factors favoring the compressed geometry in a static JT system are discussed in Refs. 17,33,49 .…”