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+δ (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+δ 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)O9 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 (ɛ) at frequencies lower than 1 kHz. Interestingly, La1.8Ca0.2NiO4+δ maintains its high permittivity at frequencies up to 1 MHz while La1.7Ca0.3NiO4+δ has the lowest dielectric loss (tan δ). Calcium doping can effectively enhance ɛ and inhibit tan δ. The distortion of (La,Ca)O9 dodecahedra can well explain their dielectric properties.

Research highlights▶ Detailed structural information of La2−xCaxNiO4+δ (x = 0–0.3) was retrieved by Rietveld refinement. ▶ Normalized bond length and bond valence were calculated to investigate the compression/dilation effects of bonds and atoms in unit cell. ▶ Structure–property relationship was established to explain physical properties. ▶ Structural disorder or excessive oxygen brought by calcium doping might play an active role in enhancement of magnetic properties. ▶ The distortion of (La,Ca)O9 dodecahedra can well explain dielectric properties of prepared samples.