Magnetic structure of the high-density single-valentegJahn-Teller systemLaMn7O12

Abstract

We studied the structural and physical properties of powder samples of the high-pressure ${\text{LaMn}}_{7}{\text{O}}_{12}$ compound by means of neutron diffraction, dc magnetization, specific-heat, and dc electrical resistivity measurements. Our structural refinement shows that ${\text{LaMn}}_{7}{\text{O}}_{12}$ is the counterpart with quadruple perovskite structure of the well-known simple-perovskite ${\text{LaMnO}}_{3}$ because both compounds are single-valent ${\text{Mn}}^{3+}$ systems sharing a similar pseudocubic network of buckled corner-sharing ${\text{MnO}}_{6}$ octahedra distorted by the Jahn-Teller (JT) effect. Besides this similarity, ${\text{LaMn}}_{7}{\text{O}}_{12}$ exhibits the following structural differences: (i) a monoclinic $I2/m$, instead of orthorhombic $Pnma$, structure; (ii) a much larger buckling of the ${\text{MnO}}_{6}$ octahedra, corresponding to a Mn-O-Mn bond angle as small as $\ensuremath{\psi}=180\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}\ensuremath{\phi}=136\ifmmode^\circ\else\textdegree\fi{}$. At ${T}_{N,B}=78\text{ }\text{K}$, the neutron data show evidence of an antiferromagnetic (AFM) structure of the octahedral ${\text{Mn}}^{3+}$ ions ($B$ sites), which consists of ferromagnetically coupled antiferromagnetic $ac$ planes. This structure, commonly referred to as $C$ type, markedly differs from the $A$-type structure of ${\text{LaMnO}}_{3}$, which consists of antiferromagnetically coupled ferromagnetic $ac$ planes. We argue that this difference may be due not only to the different buckling, because the magnetic superexchange interaction is sensitive to $\ensuremath{\phi}$, but also to the existence of two distinct JT distorted $B$ sites, a characteristic feature of the quadruple perovskite structure not found in ${\text{LaMnO}}_{3}$. A further feature of this structure is the presence of a pseudocubic sublattice of JT ${\text{Mn}}^{3+}$ ions with square coordination (${A}^{\ensuremath{'}}$ site) in addition to the $B$-site sublattice. At ${T}_{N,{A}^{\ensuremath{'}}}=21\text{ }\text{K}$, the ${A}^{\ensuremath{'}}$ sublattice of ${\text{LaMn}}_{7}{\text{O}}_{12}$ is found to form a second AFM structure consisting of AFM coupled ferromagnetic planes. This additional ordering appears to occur independently of that of the $B$ sublattice. Finally, we observed a thermally activated insulating behavior of the resistivity similar to that previously reported for ${\text{LaMnO}}_{3}$, which confirms the hypothesis that the superexchange interaction is predominant in single-valent systems while the double-exchange interaction is relevant only in mixed-valence or disordered systems.