Pressure and temperature variations of the spin-Hamiltonian parameters and electron paramagnetic resonance (EPR) linewidths of non-central Jahn–Teller [CuF4F4]6− complexes in SrF2 crystal were studied by continuous-wave EPR. It was found that the static spin-Hamiltonian parameters, found at T = 85 K and at normal pressure (g∥ = 2.491, g⊥ = 2.083, a∥ = 360, a⊥ = 26, Ax′′ = 96, Ay′′ = 99, Az′′ = 403 and βexp = 17°), are slightly changed with hydrostatic pressure and, at T = 85 K and P = 550 MPa, become equal to g∥ = 2.489, g⊥ = 2.083, a∥ = 348, a⊥ = 27, Ax′′ = 99, Ay′′ = 102, Az′′ = 406 and βexp = 20° (a and A values in megahertz, x′′-, y′′-and z′′-axes are eigenvectors of the super-hyperfine tensor A, βexp is the experimental value of the angle between the C4 symmetry axis of the complex and the x′′-axis). With increasing temperature the well-resolved EPR spectrum of the complex is transformed continuously into a single broad line both at normal pressure and at a hydrostatic pressure of 550 MPa. But in the first case the coalescence point corresponds to 220–230 K while in the second case it is 195–205 K. Treatment using the linear combination of atomic orbitals representation of molecular orbitals (LCAO MO) model was performed to establish some relations between variations of the spin-Hamiltonian parameters and pressure-induced changes in the molecular structure of the complex. To get some additional information about the molecular structure of the complex and variations of its structural parameters with pressure, treatment using the rigid-ion model was performed. Experimental and theoretical results are discussed in the framework of the Jahn–Teller model of the complex.

The Depression of the Dynamic Jahn-Teller Effect BY R. L. DAVIDOVICH (b) A.M. ZIATDINOV (a), M. M. ZARIPOV (a), YU.V. YABLOKOV (a), and The phenomena connected with the Jahn-Teller distortion of complexes in the presence of degenerate electronic states result in consequences important in solid state physics (1, 2) . The study of transitions between the potential minima of Jahn-

The investigation of a Yb3+ trigonal centre in KMgF3 was carried out by means of ENDOR. Expressions for the frequencies for ENDOR transitions in trigonally distorted complexes for arbitrary magnetic field orientation with respect to the crystallographic axes were obtained. A structural model of the centre was determined. The parameters of interaction of Yb3+ with F ions of the nearest environment were also determined. A method to determine the absolute signs of the crystal field parameters B43 and B63 was suggested. The microscopic calculation of crystal lattice deformation in KMgF3 near a paramagnetic impurity was carried out. The microscopic analysis of transferred hyperfine interaction parameters for an ion in a non-S state and having a local non-cubic symmetry, taking into account the effects of the spin polarisation of the outer filled 5s and 5p shells of the rare-earth ion, has been carried out for the first time.

The investigation of Er3+ (4f11) ions in a KZnF3 crystal is carried out by means of electron-nuclear double resonance. The parameters of the interaction of Er3+ with F- ions of the nearest environment are determined. The microscopic calculation of these parameters is carried out. Also, the microscopic calculation of the pseudo-Zeeman interaction parameters is carried out for the first time.