Structural, magnetic and dielectric properties of La2-xCaxNiO4+δ (x=0, 0 1, 0 2, 0 3)

The synthesis structural magnetic and dielectric properties of a new type of high permittivity materials La2-xCaxNiO4+delta (x=0 0 1 0 2 0 3) (abbreviated as LCNs) were reported The samples were prepared through conventional solid state reaction route Detailed structural information was retrieved by Rietveld refinement normalized bond length and bond valence was calculated to investigate the compression/dilation effect of bonds and atoms in unit cell It can be found all samples belong to K2NiF4 structure with space group I4/mmm Doping of Ca in La2NiO4+delta shrinks the unit cell and makes the structure tend to become instable Three types of (La Ca)-O bonds and two kinds of Ni-O bonds exist in LCNs Along c axis there are alternately compressed (La Ca)O-9 dodecahedra and lengthened NiO6 octahedra Room temperature magnetic measurements show that the materials are paramagnetic and Ca doping can improve the spontaneous magnetization Furthermore all samples have colossal values of the dielectric constant (a) at frequencies lower than 1 kHz Interestingly La1 8Ca0 2NiO4+delta maintains its high permittivity at frequencies up to 1 MHz while La1 7Ca0 3NiO4+delta has the lowest dielectric loss (tan delta) Calcium doping can effectively enhance epsilon and inhibit tan delta The distortion of (La Ca)O-9 dodecahedra can well explain their dielectric properties (C) 2010 Elsevier B V All rights reserved