Structural phase diagram of La1-xSrxMnO3+ delta : Relationship to magnetic and transport properties.

Abstract

The structural properties of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3+\ensuremath{\delta}}$ 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\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ atm\ensuremath{\le}P(${\mathrm{O}}_{2}$)\ensuremath{\le}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 $O\ensuremath{-}R$ 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 ($\frac{d\ensuremath{\rho}}{\mathrm{dT}}<0$), 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 ($\ensuremath{\Delta}\ensuremath{\rho}={\ensuremath{\rho}}_{0}\ensuremath{-}{\ensuremath{\rho}}_{9\mathrm{T}}\ensuremath{\sim}{10}^{4}$ \ensuremath{\Omega}cm) to that reported for ${10}^{6}$% colossal MR powder samples. However, our lower sintering temperatures result in large ${\ensuremath{\rho}}_{9\mathrm{T}}$ values that yield $\frac{\ensuremath{\Delta}\ensuremath{\rho}}{{\ensuremath{\rho}}_{9\mathrm{T}}}\ensuremath{\sim}230%$.