The impact of morphotropy and polymorphism on electric properties of manganites: the case of Sr0.5Ca0.5Mn1−xVxO3−δ

Abstract

The electron conductors Sr_0.5Ca_0.5Mn_1−xV_xO_3−δ (x = 0.025, 0.05, and 0.1) synthesized via the citrate–nitrate route have been characterized by powder X-ray diffraction at room temperature and at high temperatures in air. At room temperature, the symmetry of unit cells of corresponding morphotropes change from orthorhombic (space group Pbnm) to tetragonal (space group I4/mcm) due to the cooperative Jahn–Teller effect. In samples with x = 0.05 and x = 0.10, the temperature of phase transition from the orthorhombic (space group Pbnm) to tetragonal (space group I4/mcm) polymorph is found to decrease with the increasing vanadium content. The phase transition from tetragonal to cubic (I4/mcm ↔ Pmm) in all samples occur around 673 K. The electrical conductivity (σ) and Seebeck coefficient (S) have been measured in air within 330–1220 K. The increase in absolute values of S with rising temperature is explained employing the model of motion of small polarons, considering the disproportionation of Mn³⁺ ions to Mn²⁺ and Mn⁴⁺ and the spin direction of t_2g-electrons of manganese. The obtained data allow the calculation for the concentration and mobility of charge carriers. The decrease in enthalpy of disproportionation reaction and mobility activation energy is associated with the declining trend of Jahn–Teller distortions following polymorphic transitions with increasing temperature or morphotropic transitions I4/mcm → Pbnm at low temperatures.