Large miscibility gap in the Ba(MnxFe1−x)2As2system

Abstract

The compounds BaMn${}_{2}$As${}_{2}$ and BaFe${}_{2}$As${}_{2}$ both crystallize in the body-centered-tetragonal ThCr${}_{2}$Si${}_{2}$-type (122-type) structure at room temperature but exhibit quite different unit cell volumes and very different magnetic and electronic transport properties. Evidently reflecting these disparities, we have discovered a large miscibility gap in the system Ba(Mn${}_{x}$Fe${}_{1\ensuremath{-}x}$)${}_{2}$As${}_{2}$. Rietveld refinements of powder x-ray diffraction (XRD) measurements on samples slow-cooled from 1000 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C to room temperature (RT) reveal a two-phase mixture of BaMn${}_{2}$As${}_{2}$ and $\mathrm{Ba}({\mathrm{Mn}}_{0.12}{\mathrm{Fe}}_{0.88}){}_{2}{\mathrm{As}}_{2}$ phases together with impurity phases for $x=0.2$, 0.4, 0.5, 0.6, and 0.8. We infer that there exists a miscibility gap in this system at 300 K with composition limits $0.12\ensuremath{\lesssim}x\ensuremath{\lesssim}1$. For samples quenched from 1000 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C to 77 K, the refinements of RT XRD data indicate that the miscibility gap at RT narrows at 1000 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C to $0.2\ensuremath{\lesssim}x\ensuremath{\lesssim}0.8$. Samples with $x=0.4$, 0.5, and 0.6 quenched from 1100 to 1400 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C to 77 K contain a single 122-type phase together with significant amounts of Fe${}_{1\ensuremath{-}x}$Mn${}_{x}$As and FeAs${}_{2}$ impurity phases. These results indicate that the system is not a pseudobinary system over the whole composition range and that the 122-type phase has a significant homogeneity range at these temperatures. Magnetic susceptibility $\ensuremath{\chi}$, electrical resistivity $\ensuremath{\rho}$, and heat capacity measurements versus temperature $T$ of the single-phase quenched polycrystalline samples with $x=0.2$ and 0.8 are reported. We also report attempts to grow single crystals of the substituted compounds Ba(Mn${}_{1\ensuremath{-}x}{T}_{x}$)${}_{2}$As${}_{2}$ ($T$ $=$ Cr, Fe, Co, Ni, Cu, Ru, Rh, Pd, Re, and Pt) and BaMn${}_{2}$(As${}_{1\ensuremath{-}x}$Sb${}_{x}$)${}_{2}$ out of Sn flux. Energy-dispersive x-ray analyses show that most of these elements do not substitute into the lattice in amounts greater than 0.5%. However, concentrations of 4.4%, $~10$% and 2.6% were achieved for Cr, Fe, and Sb substitutions, respectively, and $\ensuremath{\chi}(T)$ and $\ensuremath{\rho}(T)$ data for these crystals are presented.