Low temperature ESR spectra of nickel — doped NaCl crystals

Origin of Small Barriers in Jahn–Teller Systems: Quantifying the Role of 3d–4s Hybridization in the Model System NaCl:Ni+

“…Such values are thus not far from the experimental figures. 15 With regard to the covalency, if we simply write the normalized wave function of the antibonding b 1g *(∼x 2 −y 2 ) orbital as…”

“…Concerning the barrier, B, a rough estimation of this quantity for NaCl:Ni + can be obtained from temperature T t = 35 K at which the static spectrum starts to disappear 15 in conjunction a Δ 0 , Δ 1 , and Δ 2 mean the energies of the transitions from respectively a 1g *(∼3z 2 −r 2 ), b 2g *(∼xy), and e g *(∼xz;yz) antibonding orbitals to the b 1g *(∼x 2 −y 2 ) one. The results obtained for DFT cluster calculations on NaCl:Ni + using BP86 (R eq = 2.563 Å; R ax = 2.873 Å) and PW1PW (R eq = 2.560 Å; R ax = 2.845 Å) functionals are both reported and compared to experimental values for D 4h CuCl 6 4−62,63 and AgCl 6 4− complexes.…”

“…However, this static spectrum starts to disappear at T t = 35 K in the X band, while an isotropic spectrum with g = 2.40 is already observed at 55 K. 15 Thus, both the low value of T t and the high value of g ∥ − g ⊥ strongly suggest that the barrier between two equivalent distortions is certainly small because B is roughly proportional 9,35 to T t /L(g ∥ − g ⊥ ). For comparison, in NaCl:Ag 2+ , where g ∥ = 2.198 and g ⊥ = 2.041, 16,10 T t = 95 K, while the calculated B value is equal 35 to 500 cm −1 .…”

“…19 The influence of the 3d−4s mixing on the equilibrium geometry of Cu 2+ complexes has previously been suggested by several authors. 24−28 Although Ni + is an unusual oxidation state for nickel, such a valence has been stabilized in lattices like LiF, 29 15 have undoubtedly proven the formation of Ni + impurities without irradiating the samples. Such impurities are likely formed through chemical reactions at a temperature of ∼1100 K during the purification process of the molten salt.…”

“…Such impurities are likely formed through chemical reactions at a temperature of ∼1100 K during the purification process of the molten salt. 15 This is the reason why the Ni + ions formed in this way easily replace a host Na + cation in the NaCl lattice without having a close vacancy. Thus, at variance with what happens for LiF:Ni + , 29 KMgF 3 :Ni + , 31 or CsCaF 3 :Ni + , 32 only one Ni + center is formed in the samples of NaCl:Ni + explored by Shengelaya et al 15 This is thus an important reason for choosing NaCl:Ni + among the different JT systems involving Ni + .…”