Theoretical study of fluorine doping in layered LaOBiS2 -type compounds

Abstract

We theoretically investigate the fluorine doping in LaOBiS 2-type quaternary compounds (LaOBiS 2 , NdOBiS 2 , LaOBiSe 2 , and LaOSbSe 2), which are promising candidates of thermoelectric and superconducting materials. These compounds possess a layered structure comprising blocking LnO (Ln = La, Nd, etc.) layers and conducting PnCh 2 (Pn = Bi, Sb; Ch = S, Se) layers. Their carrier concentration is generally tuned via substitutional doping of F atoms in the O site for improving the thermoelectric performance or the superconductivity; however, the tunability of the electrical properties via F doping strongly depends on constituent elements. In order to elucidate the difference, we theoretically examine the electronic and structural properties of these F-doped systems using first-principles calculation. Our results show that the monoclinic distortion of the mother compound, which is closely related to the Pn element, can drastically decrease the capability of F doping. Replacement of the Ln atom from La to Nd in LnOBiS 2 makes F doping difficult, which is consistent with experimental observation. We also find that the tetragonal structure is gradually stabilized by F-doping for all the systems investigated in this study. Our results will be important knowledge for controlling the electrical properties of LaOBiS 2-type compounds both as thermoelectric and superconducting materials.

First-principles study of defect formation energies in LaO$X$S$_2$ ($X=$ Sb, Bi)

“…Its charge state is basically F O • (i.e., q = +1) when the chemical potential lies in the band gap, which is naturally understood by considering that O 2− is replaced with F − This is consistent with many experimental studies of LaOBiS 2 where fluorine is doped into the crystal to introduce electron carriers 2 . The defect formation energy of F O is higher in P n = Sb, which is consistent with the previous theoretical study 24 . Even so, it is noteworthy that the defect formation energy of F O is negative both for LaOSbS 2 and for LaOBiS 2 In fact, an experimental study found that lattice parameters are changed and the electrical conductivity is to some extent increased by fluorine doping for LaOSbSe 2 20 , which suggests that fluorine was successfully doped into the crystal in experiments.…”

“…Figure 1 shows the crystal structure of LaOBiS 2 . Also for Sb compounds, this space group was experimentally reported for Ce(O,F)SbS was theoretically suggested to be stable for LaOP nS 2 (P n = As, Sb, Bi) 41 and LaOSbSe 2 24 . The space group is not necessarily common in LnOP nCh 2 compounds, because several BiS 2 compounds, such as CeOBiS 2 38 , and also LnOSbSe 2 (Ln = La, Ce) 20 were experimentally reported to have the tetragonal P 4/nmm space group.…”

“…Because our recent theoretical study predicts that high thermoelectric performance can be realized in P n = As, Sb compounds 23 , efficient control of the carrier concentration in Sb compounds is of great importance. In fact, our previous theoretical calculation showed that the stability of the fluorine substitutional doping is lower in P n = Sb than in P n = Bi 24 . However, because that study focuses on the relative stability between Sb and Bi compounds and assumes a simple fluorine-rich limit using a relatively small supercell, it is still unclear whether the fluorine substitutional doping is stable even in Sb compounds.…”

Investigation of Superconducting Properties and Possible Nematic Superconductivity in Self‐Doped BiCh2‐Based Superconductor CeOBiS1.7Se0.3

“…[ 13 ] The trend in which electron carrier doping suppresses the structural transition and stabilizes the tetragonal structure is commonly observed in a REOBiCh 2 ‐type structure, [ 10 ] and a theoretical calculation also suggested the stability of the tetragonal structure after electron doping. [ 36 ] The expected amount of electron carriers doped in CeOBiS 1.7 Se 0.3 is 0.25 per Bi because of the Ce valence of 3.25. Having considered the amount of doped electrons in the present phase and structural trend of the REOBiCh 2 ‐type compounds, we consider that the low‐temperature structure is tetragonal.…”

Structural Phase Diagram of LaO1−xFxBiSSe: Suppression of the Structural Phase Transition by Partial F Substitutions

“…It has been known that carrier doping via partial element substitution suppresses the structural transition and stabilizes tetragonal structure in a REOBiCh 2 -type structure [3]. Additionally, theoretical analysis on the stability of the crystal structure as a function of carrier concentration suggested that the tetragonal structure is more stabilized than the monoclinic one in electron-doped (F-substituted) LaOBiS 2 [27].…”