Stabilization of the Misfit Layer Compound $(\mathrm{PbS}{)}_{1.13}{\mathrm{TaS}}_2$ by Metal Cross Substitution

Photoemission microspectroscopy on the layered misfit compound $(\mathrm{PbS}{)}_{1.13}{\mathrm{TaS}}_2$ provides direct evidence for Ta substitution into PbS layers as well as for Pb substitution into ${\mathrm{TaS}}_2$ layers. This metal cross substitution alters the charge balance between alternating layers and can explain the remarkable stability of $(\mathrm{PbS}{)}_{1.13}{\mathrm{TaS}}_2$ and, possibly, of analogous misfit compounds. It is suggested that even formally stoichiometric misfit compounds can be stabilized by this mechanism.

Figure 1

Crystal structure of the misfit layer compound $(\mathrm{PbS}{)}_{1.13}{\mathrm{TaS}}_2$. The large yellow (light gray) spheres represent sulfur atoms of both subsystems, the smaller blue (dark gray) spheres correspond to the lead atoms of the PbS subsystem, and the small gray spheres represent the tantalum atoms of the ${\mathrm{TaS}}_2$ subsystem (left: side view, right: front view, top: top view). Commensurate and incommensurate directions are indicated. Possible interlayer bonding mechanisms (a)–(d) are described in the text.

Figure 2

Surface area of $(\mathrm{PbS}{)}_{1.13}{\mathrm{TaS}}_2$, as imaged by photoemission microscopy from (a) the Pb $5d_{5/2}$ core level ($h\nu =73.1\mathrm{eV}$), (b) the Ta $5d$ valence band ($E-E_F\approx 0.5\mathrm{eV}$, $h\nu =43.9\mathrm{eV}$), (c) the Ta $4f_{5/2}$ core level ($h\nu =116.5\mathrm{eV}$), and (d) the Cs $4d_{5/2}$ core level after 3 min. of Cs deposition ($h\nu =116.5\mathrm{eV}$).

Figure 3

Pb $5d$ and Ta $4f$ core level spectra of $(\mathrm{PbS}{)}_{1.13}{\mathrm{TaS}}_2$, taken at BL31/MAX-lab with high spatial and moderate energy resolution from (a) a PbS domain ($h\nu =116.5\mathrm{eV}$) and (g) a ${\mathrm{TaS}}_2$ domain ($h\nu =73.2\mathrm{eV}$). Experimental data (dots) and fit results (solid lines) are plotted. The decomposition of the spectra into different components is indicated. Components originating from cross-substituted atoms are highlighted in red (dark gray). A Shirley background has been subtracted. (b)–(f) Corresponding spectra taken at BW3/HASYLAB with higher energy and moderate spatial resolution from the transition region between PbS and ${\mathrm{TaS}}_2$ domains of a different sample ($h\nu =116.5\mathrm{eV}$). (h) Core level spectrum measured from the same PbS domain as in (a) after 3 min. of Cs deposition ($h\nu =116.5\mathrm{eV}$).