Structural changes at the semiconductor-insulator phase transition in the single-layered perovskiteLa0.5Sr1.5MnO4

Abstract

The semiconductor-insulator phase transition of the single-layer manganite La 0.5 Sr 1.5 MnO 4 has been studied by means of high-resolution synchrotron x-ray powder diffraction and resonant x-ray scattering at the Mn K edge. We conclude that a concomitant structural transition from tetragonal I4/mmm to orthorhombic Cmcm phases drives this electronic transition. A detailed symmetry-mode analysis reveals that condensation of three soft modes-2 (B 2u ), X + 1 (B 2u ) and X + 1 (A)-acting on the oxygen atoms accounts for the structural transformation. The 2 mode leads to a pseudo Jahn-Teller distortion (in the orthorhombic bc plane only) on one Mn site (Mn1), whereas the two X + 1 modes produce an overall contraction of the other Mn site (Mn2) and expansion of the Mn1 one. The X + 1 modes are responsible for the tetragonal superlattice (1/2,1/2,0)-type reflections in agreement with a checkerboard ordering of two different Mn sites. A strong enhancement of the scattered intensity has been observed for these superlattice reflections close to the Mn K edge, which could be ascribed to some degree of charge disproportion between the two Mn sites of about 0.15 electrons. We also found that the local geometrical anisotropy of the Mn1 atoms and its ordering originated by the condensed 2 mode alone perfectly explains the resonant scattering of forbidden (1/4,1/4,0)-type reflections without invoking any orbital ordering.

Strong local lattice instability in hexagonal ferritesRFe2O4(R= Lu,Y,Yb) revealed by x-ray absorption spe

“…Summarizing, previous RXS studies have shown that charge segregation exists in the low-temperature CO phase, but it is impossible to determine the value of this charge segregation without a quantitative analysis including the structure factor of the exact CO sequence. Similar preliminary studies were shown in the past for manganites [55] that were later amended [56,57]. The occurrence of integer valence segregation in mixed-valence transition-metal oxides seems to be an exceptional fact, since the charge disproportionation associated with their CO transitions are generally small fractions of one electron [58].…”

Correlating quasiparticle excitations with quantum femtosecond magnetism in photoexcited nonequilibrium states of insulating antiferromagnetic manganites

“…The only energy dispersion comes from electron motion along a single chain with FM spins [9,53]. For large E JT , this k x -dispersion is also small, due to the energy barrier E JT between the alternating JT-distorted and undistorted sites observed experimentally [20,44,52]. Therefore, the electron charge density is mostly localized in all three directions for large E JT .…”

Probing a crystal's short‐range structure and local orbitals by Resonant X‐ray Diffraction methods

“…In 2011, Herrero‐Martìn et al . investigated the phase transition of La 0.5 Sr 1.5 MnO 4 from tetragonal I 4 /mmm to an orthorhombic phase while lowering temperature, accompanied by a semiconductor‐insulator transition. Using DAFS scans at h /2 h /2 l and h /4 h /4 l superstructure reflections of the tetragonal phase ( h odd and l even) as well as their azimuthal dependence they were able to confirm the transition to space group Cmcm involving three active modes, which cause a shift of the oxygen atoms.…”