research papers 1512 doi:10.1107/S1600576714014642 J. Appl. Cryst. (2014). 47, 1512–1519 Journal of Applied Crystallography ISSN 1600-5767 Received 19 December 2013 Accepted 21 June 2014 # 2014 International Union of Crystallography X-ray absorption spectroscopy studies on the carbothermal reduction reaction products of 3 mol% yttria-stabilized zirconia A. Sondhi, a O. Okobiah, a S. Chattopadhyay, b,c T. Shibata, b,c T. W. Scharf a * and R. F. Reidy a a Materials Science and Engineering, and Institute for Science and Engineering Simulation (ISES), University of North Texas, 1155 Union Circle #305310, Denton, TX 76203-5017, USA, b CSRRI-IIT, MRCAT, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA, and c Physics Department, Advanced Materials Group, Illinois Institute of Technology, Chicago, Illinois 60616, USA. Correspondence e-mail: scharf@unt.edu Extended X-ray absorption spectroscopy (EXAFS) at the Zr K edge has been used to determine changes in various bond lengths in 3 mol% yttria-stabilized zirconia (YSZ) during zirconium carbide (ZrC) formation. The principal objective of this study was to determine if ZrC formation at the YSZ/carbon interface alters the zirconia structure. A mixed-phase sample (YSZ and graphite) was carbothermally reduced to form ZrC. X-ray diffraction phase quantification by Rietveld analysis confirmed the formation of 50% ZrC in the analyzed sample volume. EXAFS data of ZrC and YSZ powders and a sintered YSZ pellet (96.7% density) were used as standards to compare with the carbothermally reduced sample. Ab inito calculations using these spectra quantified various Zr—O, Zr—C and Zr—Zr bond distances in the system. Best fit results revealed Zr—O I (tetragonal), Zr—O (monoclinic), Zr—Zr (tetra- gonal) and Zr—Zr (monoclinic) bond length values of 2.10, 2.25, 3.65 and 3.52 A ˚ , respectively, in the YSZ powder, Zr—O I (tetragonal) and Zr—Zr (tetragonal) bond length values of 2.12 and 3.62 A ˚ , respectively, in the sintered pellet, and Zr—C and Zr—Zr bond lengths of 2.32 and 3.33 A ˚ , respectively, in the ZrC powder. Similar fitting procedures were carried out on the carbothermally reduced pellet, with measured Zr—O, Zr—Zr (of YSZ), Zr— C and Zr—Zr (of ZrC) bond lengths of 2.13, 3.62, 2.36 and 3.33 A ˚ , respectively. These bond lengths indicate that the formation of ZrC in the YSZ matrix does not influence the local structure when compared to pure standards. Therefore, carbothermal reduction does not induce any apparent strain or thermally induced effects on the first and second coordination shells of Zr as measured by the X-ray absorption spectra of the carbothermally reduced sample. Interest- ingly, the results indicated that sintering of the YSZ powder into pellets did not result in any significant change in the Zr—O and Zr—Zr distances for tetragonal YSZ. 1. Introduction Thermal protection of carbon–carbon composites (CCCs) in extreme environments often requires the addition of protec- tive layers. Yttria-stabilized zirconia (YSZ) is a robust thermal barrier coating that will react with the underlying carbon to form zirconium carbide (ZrC) at high temperatures. ZrC may enhance thermal protection of the CCCs; however, it is unclear if this in situ reaction creates localized strains or structural changes at the YSZ/CCC interface. Therefore, changes in the local structure of YSZ and ZrC before and after carbothermal reduction are the main focus of this paper. Zirconia (ZrO 2 ) is an important ceramic material recog- nized for its unique mechanical, ionic and thermal properties. The three polymorphs of zirconia occur at successive temperature ranges: monoclinic (below 1443 K) (Smith & Newkirk, 1965; McCullough & Trueblood, 1959), tetragonal (1443–2643 K) (Teufer, 1962) and cubic (2643–2953 K) (Smith & Cline, 1962). Ceria, magnesia, calcia and yttria are well known stabilizers of various phases of zirconia to room temperature (Subbarao, 1981; Stubican & Hellmann, 1981). Yttria (Y 2 O 3 )-stabilized zirconia has found a broad range of applications from biomedical implants (Helmer & Driskell, 1969; Christel et al., 1988) and oxygen sensors (Zechnall et al., 1973; Engh & Wallman, 1977; Dueker et al., 1975; Heyne, 1976; Beekmans & Heyne, 1970) to thermal barrier coatings on jet engine turbine blades (Stecura, 1977). Carbothermal reduc-