Structural phase diagram of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Mn}{\mathrm{O}}_{3+\delta }$: Relationship to magnetic and transport properties

The structural properties of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{Mn}{\mathrm{O}}_{3+\delta }$ have been studied using neutron powder diffraction as a function of both Sr doping ($0<~x<~0.225$) and oxygen partial pressure during synthesis [2.1×${10}^{-4\mathrm{}}$ atm≤P(${\mathrm{O}}_2$)≤1 atm]. A structural phase diagram constructed as a function of these parameters has a rhombohedral phase ($R\overline{3}c$), an orthorhombic phase ($\mathrm{Pbnm}$), and a monoclinic phase ($\frac{P{2}_1}{c}$). For a given $x$, decreasing P(${\mathrm{O}}_2$) yields smaller cation vacancy concentrations. At low temperature, the $R$ phase is ferromagnetic, while the $M$ phase is antiferromagnetic. The $O$ phase is ferromagnetic for $x>~0.125$, and the ferromagnetism is independent of the $O-R$ phase transition that coincides with the transition from nonmetal to metal. Transport measurements made between 20 and 350 K show that $O$ and $M$ samples are nonmetallic ($\frac{d\rho }{\mathrm{dT}}<0\mathrm{}$), while the $R$ samples exhibit a temperature-dependent nonmetal-to-metal transition at temperatures close to the Curie temperature. Magnetoresistance (MR) is observed in all three phases. The largest value at 9 T, found in the orthorhombic and monoclinic samples, is of similar order ($\Delta \rho ={\rho }_0-{\rho }_{9\mathrm{T}}\sim {10}^4$ Ωcm) to that reported for ${10}^6$% colossal MR powder samples. However, our lower sintering temperatures result in large ${\rho }_{9\mathrm{T}}$ values that yield $\frac{\Delta \rho }{{\rho }_{9\mathrm{T}}}\sim 230\%$.