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Objective:

The goal of the ACS-PRF funded project was to prepare novel thermoelectric materials via chemical fusion of semimetallic RESb/REBi and insulating RE₂O₃ binaries (RE is a rare-earth element). The proposed research also focused on new layered oxycarbides with a general formula (RESb)_n(REOC_0.25) or (REBi)_n(REOC_0.25). The idea was to structurally combine the LnSb/LnBi and LnOC_0.25 phases with different properties in order to obtain "electron-crystal phonon-glass" materials with improved thermoelectric performance (Figure 1).

Figure 1. Formation of (LnSb)_n(LnOC_0.25) from LnSb, an "electron crystal", and LnOC_0.25, a "phonon glass".

Results:

1) RE₃SbO₃ and RE₈Sb_3-d O₈ antimonide oxides.

In the search of high-temperature thermoelectric materials, two families of novel narrow band-gap semiconducting suboxides with the RE₃SbO₃ and RE₈Sb_3-d O₈ compositions (RE = La, Sm, Gd, Ho) have been discovered (Figure 2). Their synthesis was motivated by attempts to open a band gap in the semimetallic RESb binaries through a chemical fusion of RESb and corresponding insulating RE₂O₃. Temperatures of 1350^oC or higher are required to obtain these phases. Both RE₃SbO₃ and RE₈Sb_3-d O₈ adopt new monoclinic structures with the C2/m space group and feature similar REO frameworks composed of "RE₄O" tetrahedral units. In both structures, the Sb atoms occupy the empty channels within the REO sublattice. High-purity bulk Sm and Ho samples were prepared and subjected to electrical resistivity measurements. Both the RE₃SbO₃ and RE₈Sb_3-d O₈ (RE = Sm, Ho) phases exhibit a semiconductor-type electric behavior. While a small band gap in RE₃SbO₃ results from the separation of the valence and conduction bands, a band gap in RE₈Sb_3-d O₈ appears to result from the Anderson localization of electrons.

Discovery of RE₃SbO₃ and RE₈Sb_3-d O₈ opens a new chapter in the chemistry of rare-earth suboxides, which was barely explored until now.

Figure 2. Structure of RE₃SbO₃ and RE₈Sb_3-d O₃

2) Layred (RESb)_n(REOC_0.25) antimonide oxycarbides

We were also able to produce layered oxycarbides (HoSb)_1.25(HoOC_0.25) and (DySb)_2.5(DyOC_0.25). Their structures consist of two and four layers of HoSb and DySb and one layer of HoOC_0.25 and DyOC_0.25, respectively, stacked along the z direction (Figure 3). To our knowledge, such composite structures are unique. No physical properties were measured as only small single crystals were obtained.

These results provide a solid evidence for the existence of novel layered suboxides with a general formula (LnPn)_n(LnOC_0.25).

Figure 3. Structures of (DySb)_2.5(DyOC_0.25) and (HoSb)_1.25(HoOC_0.25).