The crystal and magnetic structures of newly synthesized ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ $(0\mathrm{}<~x<~1)$ and ${\mathrm{Sr}}_{1-y}{\mathrm{Ba}}_y{\mathrm{MnO}}_3$ $(y<~0.2)$ 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 $\mathrm{Pm}3\mathrm{}¯m$ to tetragonal $I4/mcm$ (at $x\sim 0.3)$ to orthorhombic Pbnm at $x\sim \mathrm{0.4.}$ In agreement with neutron-diffraction data, resistive and magnetic measurements show that the samples are antiferromagnetic with Néel temperatures $T_N,$ varying from 233 to ∼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 θ. The behavior of $T_N$ can be satisfactorily described as a function of $〈{\cos }^2\theta 〉$ related to the superexchange interaction integral, and ${\sigma }^2,$ 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.

• Received 10 April 2001

DOI:https://doi.org/10.1103/PhysRevB.64.134412