Structural strain and competition between charge density wave and superconductivity in $\mathrm{La}\left(\mathrm{Fe},\mathrm{Mn}\right)\mathrm{As}\left({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}\right)$ compounds

Selected members of the $\mathrm{La}\left(\mathrm{Fe},\mathrm{Mn}\right)\mathrm{As}\left({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}\right)$ system were analyzed using high-resolution synchrotron x-ray powder diffraction. The tetragonal to orthorhombic structural transition is progressively recovered in the optimally electron-doped $\mathrm{LaFeAs}\left({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}\right)$ phase by very light Mn substitution; at the same time, superconductivity is suppressed whereas magnetic ordering is restored. Distinct incommensurate satellite peaks develop within different thermal ranges and mark the occurrence of charge density waves characterized by distinct propagation wave vectors, as well as multiple incommensurate structural transitions; in particular, some of them arise in conjunction with the structural transformation process, disappearing after the completion of the dissymmetrization. The thermal evolution of satellite reflections observed at $\mathbf{Q}\sim 1.93{\AA }^{–1}$ indicates a strong competition between the charge density waves and the superconductive state. A phase diagram of the $\mathrm{La}\left(\mathrm{Fe},\mathrm{Mn}\right)\mathrm{As}\left({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}\right)$ system is drawn on the basis of the structural, magnetic, and electronic properties of the analyzed samples.

Figure 1

Evolution of the cell parameters as a function of Mn content in $\mathrm{La}\left({\mathrm{Fe}}_{1–x}{\mathrm{Mn}}_x\right)\mathrm{As}\left({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}\right)$ samples at 295 K.

Figure 2

Distortion patterns of the LaFeAsO crystal structure as observed for different combinations of F and Mn substitution (arrows represent atomic displacements).

Figure 3

Thermal evolution of the tetragonal anisotropic strain parameters obtained by Rietveld refinement for the samples $x=0.0020$ (on the left) and 0.0010 (on the right); lines are guides to the eye; β represents the integral breadth of the diffraction line profile. The insets show the corresponding tensor surfaces calculated at selected temperatures.

Figure 4

Portions of full-pattern synchrotron x-ray Rietveld refinement plots for $x=0.0010$ at 10 K showing the tetragonal 220 and orthorhombic 400 + 040 diffraction lines fitted with different structural models.

Figure 5

Williamson-Hall plot obtained after Rietveld refinement for the sample with $x=0.0010$ at $T=250\mathrm{K}$ (β represents the integral breadth of the diffraction line profile measuring the broadening produced by structural strain).

Figure 6

Thermal expansion of the sample $x=0.0020$, selected as representative; the hatched line represents the region where the tetragonal to orthorhombic structural transition takes place; the solid line represents the best fit to a second-order Grüneisen approximation,

Figure 7

Enlarged view of the thermodiffractograms for the samples with $x=0.0010$ and 0.0020 showing the thermal dependence of the intensity of the satellite peaks located at $\mathbf{Q}\sim 1.93$, ∼ 1.975, and $\sim 2.00{\AA }^{–1}$ (arrows).

Figure 8

Thermal evolution of the satellite peak at $\mathbf{Q}\sim 2.36{\AA }^{–1}$ in samples with $x=0.0010$ (on the left) and $x=0.0020$ (on the right).

Figure 9

Comparison between the thermal evolutions of the satellite peak at $\mathbf{Q}\sim 2.36{\AA }^{–1}$ and the tetragonal 220 diffraction line splitting into the orthorhombic 400+040 reflections in samples with $x=0.0050$ and $x=0.0075$.

Figure 10

Tentative phase diagram of the $\mathrm{La}\left({\mathrm{Fe}}_{1–x}{\mathrm{Mn}}_x\right)\mathrm{As}\left({\mathrm{O}}_{0.89}{\mathrm{F}}_{0.11}\right)$ system in the inspected compositional range; the magnetic phase field is drawn for both a magnetic volume fraction equal to 50% (sparse pattern) and 100% (dense pattern) (see text for details); temperatures at which incommensurate peaks are observed are also indicated (△: CDW at $\mathbf{Q}\sim 1.93{\AA }^{–1}$; $◯$: CDW at $\mathbf{Q}\sim 1.97{\AA }^{–1}$; □: CDW at $\mathbf{Q}\sim 2.00{\AA }^{–1}$; $•$: CDW at $\mathbf{Q}\sim 2.36{\AA }^{–1}$).