Low- and high-temperature electron paramagnetic resonance studies on Cu²⁺-doped monopyrazine zinc sulphate trihydrate single crystal: observation of the Jahn-Teller effect

Abstract

Electron paramagnetic resonance studies on a single crystal of Cu2+-doped monopyrazine zinc sulphate trihydrate (PZST) have been made over an extended temperature range (4.2-375 K). Three physically equivalent, but magnetically inequivalent, Cu2+ complexes have been observed. The spin-Hamiltonian parameters are rigorously evaluated at 334, 295, 77 and 4.2 K, by the method of least-squares fitting, utilising numerical diagonalisation of the spin-Hamiltonian matrix on a digital computer. The principal values of the g- and A-tensors indicate that, in PZST, the Cu2+ ion experiences an octahedral symmetry with orthorhombic distortion. The data are interpreted to conclude the occurrence of both static and dynamic Jahn-Teller effects over the temperature range of investigation, the transition from static to dynamic Jahn-Teller effect occurring at 334+or-1 K.

Aggregate frequency width, nuclear hyperfine coupling and Jahn–Teller effect of Cu2+impurity ion ESR in SrLaAlO4dielectric resonator at 20 millikelvin

“…The temperature was kept at ⩽20 mK to observe the hyperfine multiplets. Rapid broadening of the Cu 2+ ion hyperfine structure was due to the static Jahn-Teller effect [8,9,42] of the nuclear quadruple moment in a lower symmetry due to the paramagnetic ion's site in the elongated CuO 6 octahedral structure [41,49]. The ground state became unstable due to the displacement of the copper nucleus in the centro-symmetric octahedron [49,50].…”

“…At the experiment temperature 20 mK, g-factors along four-fold axes the octahedral symmetry is corresponded to g 3 > g 2 > g 1 with the elongation along z-axis. In such a lower symmetry in the Jahn-Teller stabilization energy of the CuO 6 metal ligand system, an admixture of the ground state | x 2 − y 2 ⟩ with the excited | 3z 2 − r 2 ⟩ state is evident under the condition R = g2−g1 g3−g2 < 1 as implied for the elongation along z-axis 40 . The observed hyperfine structure anisotropy follows the equation no.3 under condition A ∥Cu ≫ P ∥ .…”

“…An angle, φ, is corresponded to a lower symmetry, and the corresponding eigenstates are; ground state 2 which is higher in energy by 4 × (Jahn − Teller energy W JT ) [35,41,15]. Hence, the admixture of the excited state is given by sin 2 ( φ 2 ) for this lower symmetry distortion [42]. From this experiment results, it may be estimated as φ ∼ tan −1 ( 79 132+147+170+211 ) = 6.82 • (figure 3), and which means that the admixture is about 0.36%.…”

“…But the correction term ∼ ( λ ∆ ) 2 is not negligible for the most general case of a rhombic distortions 35,36,39 , which admixes the e g splitted doublet (splitting due to the tetragonal distortion) ground state | x 2 − y 2 ⟩ =| 2 s ⟩ with | 3z 2 − r 2 ⟩ =| 0⟩. For the ground state | 2 s ⟩ in octahedral elongation along z-axis due to the tetragonal distortion, the parallel and perpendicular g-factors anisotropy are δ(g ∥Cu ) = 8 × λ ∆ and δ(g ⊥Cu ) = 2 × λ ∆ respectively (considering crystal field splitting between e g and t 2g in average equal to ∆) 36,40 .…”