Dependence of dielectric properties of manganese-doped strontium titanate ceramics on sintering atmosphere

It is shown in this paper that the microscopic mechanism of the dielectric relaxation in Mn-doped strontium titanate (SrTiO₃-ST) ceramics is associated with the off-center displacement of Mn_Sr^{2 +} ions. This was accomplished by studying the dielectric properties and electron spin resonance spectroscopy in combination with X-ray powder diffractometry and scanning electron microscopy techniques in Sr_1-xMn_xTiO₃ ceramic samples sintered in different atmospheres (air, oxygen and nitrogen) at 1500 °C. First, it is shown that manganese is incorporated into the perovskite lattice of ST, preferably as Mn²⁺ at Sr sites. However, a small amount of Mn⁴⁺ at Ti sites is also observed when fired in air or oxygen flow. The concentration of Mn_Ti^{4 +} is the highest for sintering in oxygen, but firing in a reducing atmosphere (nitrogen) results solely in the incorporation of Mn²⁺ at Sr sites. Correspondingly, the dielectric relaxation observed in Sr_1-xMn_xTiO₃ markedly increases in intensity and slightly shifts towards a higher temperature for ceramics sintered in nitrogen compared with those fired in air or oxygen. All these facts are consistent with a suggestion that the off-center displacement of Mn²⁺ ions at the Sr sites of the highly polarizable ST lattice is the source of the observed relaxation behavior.