Relationship between structural parameters and the Néel temperature inSr1−xCaxMnO3(0<~x<~1)andSr1−yBayMnO3(y<~0.2)

Abstract

The crystal and magnetic structures of newly synthesized ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ $(0l~xl~1)$ and ${\mathrm{Sr}}_{1\ensuremath{-}y}{\mathrm{Ba}}_{y}{\mathrm{MnO}}_{3}$ $(yl~0.2)$ perovskite materials (of the $\mathrm{AB}{\mathrm{O}}_{3}$ type) were investigated using neutron and synchrotron x-ray powder diffraction at temperatures between 10 and 530 K. Upon decreasing the size of the A-site ion (or the unit-cell size) a series of structural transitions appears at room temperature from cubic $\mathrm{Pm}3\ifmmode\bar\else\textasciimacron\fi{}m$ to tetragonal $I4/mcm$ (at $x\ensuremath{\sim}0.3)$ to orthorhombic Pbnm at $x\ensuremath{\sim}0.4.$ In agreement with neutron-diffraction data, resistive and magnetic measurements show that the samples are antiferromagnetic with N\'eel temperatures ${T}_{N},$ varying from 233 to \ensuremath{\sim}125 K and from 233 to 212 K by increasing the Ca and Ba contents, respectively. The observed variation of ${T}_{N}$ cannot be solely explained by the changes of the unit-cell size or the average Mn-O-Mn bond angle \ensuremath{\theta}. The behavior of ${T}_{N}$ can be satisfactorily described as a function of $〈{\mathrm{cos}}^{2}\ensuremath{\theta}〉$ related to the superexchange interaction integral, and ${\ensuremath{\sigma}}^{2},$ the A-site ionic size variance [L. M. Rodriguez-Martinez and J. P. Attfield, Phys. Rev. B 54, R15 622 (1996)] regardless of the structural symmetry of the perovskite material.