This work systematically investigates the thermodynamic stability of SiaOb(M)cCdstructures derived from polymeric precursors incorporating metal fillers: Ta, Nb, and Hf, at 1200 and 1500°C. Structural characterization of the polymer derived ceramics (PDCs) employs X‐ray diffraction, Fourier transform infrared spectroscopy, and X‐ray photoelectron spectroscopy. Enthalpies of formation relative to crystalline components (metal oxide, silica, silicon carbide, and graphite) are obtained from thermodynamic measurements by high temperature oxide melt solution calorimetry. The enthalpies of formation (∆H°f, comp) of Ta‐1200, Hf‐1200, Nb‐1200, Ta‐1500, Hf‐1500, and Nb‐1500 specimens are −137.82 ± 9.72, −256.31 ± 8.97, −82.80 ± 9.82, −182.80 ± 7.85, −292.54 ± 9.38, −224.98 ± 9.60 kJ/mol, respectively. Overall incorporation of Hf results in most thermodynamically stable structures at all synthesis temperatures. SiaOb(M)cCdspecimens employing Nb fillers undergo the most stable structural evolution in this temperature range. The results indicate strong thermodynamic drive for carbothermal reduction of metal oxide domains. Incorporation of Ta provides the greatest stabilization of SiO3C mixed bonding environments. Ultimately, the choice of metal filler influences composition, structural evolution, and thermodynamic stability in PDCs.
The discovery of another monoclinic polymorph in the niobium trisulfide system expands the structural possibilities for quasi-1D transition metal trichalcogenide materials. We describe here NbS3-VI prepared by chemical vapor transport (CVT) using ammonium chloride as the transport agent rather than the typical iodine or excess chalcogen. This example establishes precedent for transport agent control over CVT product polymorphism, thereby opening an alternative avenue for structural engineering. The single crystal x-ray diffraction structure of NbS3-VI shows that this polymorph unexpectedly incorporates features of both NbS3-IV and NbS3-V; specifically, NbS3-VI contains corrugated chains with paired Nb–Nb and uniform chains with unpaired, equidistant Nb centers. We also use single crystal x-ray diffraction to compare NbS3-VI with (Nb0.6Ti0.4)S3, which contains solely uniform chains with slightly shorter metal–metal distances than those of uniform NbS3-VI chains.
more » « less- PAR ID:
- 10366506
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Applied Physics Letters
- Volume:
- 120
- Issue:
- 17
- ISSN:
- 0003-6951
- Page Range / eLocation ID:
- Article No. 173103
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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