⁵⁹Co²⁺ ENDOR in the Tetragonal Phase of CsCaCl₃

Abstract

EPR and ENDOR investigations of electron-nuclear interactions of doped paramagnetic ions in low-symmetry phases of crystals with structural phase transitions (PT) provide a deeper understanding of the PT nature. The Co2+ ion, due to the spin-orbit character of its ground state, is a magnetic label allowing to study slight changes of the crystal field occurring during PT.The present note is concerned with the study of the influence of the lowtemperature structural PT consequences on the hyperfine (HFI) , quadrupole (QI) , and pseudonuclear Zeeman interactions of the 5 9~0 2 + ion in CsCaC13 crystal at T = 4 K.The structural PT in CsCaC13 crystal (Oh++D4h / I / , To = 98 K / 2 / ) is characterized by the rotation of anionic octahedra around one of their fourfold axes by the angle @ and the simultaneous lattice deformation along the rotational axis. According to the data on ligand HFI and QI in CsCaC13:Co2+ crystal /3/ the local order parameter at T = 4 K is CD = 1.8'. The estimates of the tetragonal deformation by means of X-ray diffraction 121 at T = 4 K lead to the relation of the crystal lattice constants c / a = 1.006(2).The Co2'(3d7) ion EPR spectrum for HI1 C4 consisted of two groups of hyperfine lines evidencing the formation of a two-domain structure after PT. Numerical diagonalizaton of the matrix of the tetragonal spin Hamiltonian enabled to describe the angular dependence of the EPR transitions and led to the following values of interaction parameters : 1 18 -gl = 4*48(1) ' g,, = 4.33(1) s A = 6.87(5)mT . 1 A = 6.33(5)mTY II 59C0 ENDOR was observed for all hyperfine EPR transitions in the frequency range 160 to 450 MHz, the ENDOR linewidth had the value of ~1 . 5 MHz. The angular dependence of the ENDOR frequencies on the direction of the constant magnetic field was investigated in the (010) crystal plane. The spin Hamiltonian describing the interactions of electron and nuclear magnetic moments of the "Co2+ l ) Sibirskii Trakt 10/7, SU-420029 Kazan, USSR. 11 physica (b)