title: High-frequency electron paramagnetic resonance analysis of the oxidation state and local structure of Ni and Mn ions in Ni,Mn-codoped LiCoO(2).
abstract: High-frequency electron paramagnetic resonance (HF-EPR) spectroscopy was employed to examine the oxidation state and local structure of Ni and Mn ions in Ni,Mn-codoped LiCoO(2). The assignment of EPR signals was based on Mg,Mn-codoped LiCoO(2) and Ni-doped LiCoO(2) used as Mn(4+) and low-spin Ni(3+) EPR references. Complementary information on the oxidation state of transition-metal ions was obtained by solid-state (6,7)Li NMR spectroscopy. For slightly doped oxides (LiCo(1-x)Ni(x)Mn(x)O(2) with x < 0.05), nickel and manganese substitute for cobalt in the CoO(2) layers and are stabilized as Ni(3+) and Mn(4+) ions. The local structure of Mn(4+) ions was determined by modeling of the axial zero-field-splitting parameter in the framework of the Newman superposition model. It has been found that the local trigonal distortion around Mn(4+) is smaller in comparison with that of the host site. To achieve a local compensation of Mn(4+) charge, several defect models are discussed. With an increase in the total dopant content (LiCo(1-x)Ni(x)Mn(x)O(2) and 0.05 <or= x <or= 0.10), a saturation in the Ni(3+) amount (up to x < 0.05) is attained, while the Mn(4+) content increases gradually. Ni(3+) ions are surrounded by Co(3+) ions only in the whole concentration range (0 < x <or= 0.10). The first metal coordination sphere of Mn(4+) ions undergoes a transformation with an increase in the total Ni and Mn contents due to a progressive replacement of Co(3+) by Mn(4+) and Ni(2+) ions. For highly doped oxides (LiCo(1-x)Ni(x)Mn(x)O(2) with x = 0.10), nickel and manganese achieve, with respect to the local charge compensation, their usual oxidation states of 2+ and 4+.
authors.affiliation: Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria. radstoy@svr.igic.bas.bg
authors.forename: R
authors.initials: R
authors.lastname: Stoyanova
authors.forename: A-L
authors.initials: AL
authors.lastname: Barra
authors.forename: M
authors.initials: M
authors.lastname: Yoncheva
authors.forename: E
authors.initials: E
authors.lastname: Zhecheva
authors.forename: E
authors.initials: E
authors.lastname: Shinova
authors.forename: P
authors.initials: P
authors.lastname: Tzvetkova
authors.forename: S
authors.initials: S
authors.lastname: Simova