Academic Appointments


  • Social Science Research Scholar, Center for International Security and Cooperation

2021-22 Courses


All Publications


  • Pressure-induced structural modifications of rare-earth hafnate pyrochlore. Journal of physics. Condensed matter : an Institute of Physics journal Turner, K. M., Rittman, D. R., Heymach, R. A., Tracy, C. L., Turner, M. L., Fuentes, A. F., Mao, W. L., Ewing, R. C. 2017; 29 (25): 255401-?

    Abstract

    Complex oxides with the pyrochlore (A2B2O7) and defect-fluorite ((A,B)4O7) structure-types undergo structural transformations under high-pressure. Rare-earth hafnates (A2Hf2O7) form the pyrochlore structure for A  =  La-Tb and the defect-fluorite structure for A  =  Dy-Lu. High-pressure transformations in A2Hf2O7 pyrochlore (A  =  Sm, Eu, Gd) and defect-fluorite (A  =  Dy, Y, Yb) were investigated up to ~50 GPa and characterized by in situ Raman spectroscopy and synchrotron x-ray diffraction (XRD). Raman spectra at ambient pressure revealed that all compositions, including the defect-fluorites, have some pyrochlore-type short-range order. In situ high-pressure synchrotron XRD showed that all of the rare earth hafnates investigated undergo a pressure-induced phase transition to a cotunnite-like (orthorhombic) structure that begins between 18 and 25 GPa. The phase transition to the cotunnite-like structure is not complete at 50 GPa, and upon release of pressure, the hafnates transform to defect-fluorite with an amorphous component. For all compositions, in situ Raman spectroscopy showed that disordering occurs gradually with increasing pressure. Pyrochlore-structured hafnates retain their short-range order to a higher pressure (30 GPa vs.  <10 GPa) than defect-fluorite-structured hafnates. Rare earth hafnates quenched from 50 GPa show Raman spectra consistent with weberite-type structures, as also reported for irradiated rare-earth stannates. The second-order Birch-Murnaghan equation of state fit gives a bulk modulus of ~250 GPa for hafnates with the pyrochlore structure, and ~400 GPa for hafnates with the defect-fluorite structure. Dy2Hf2O7 is intermediate in its response, with some pyrochlore-type ordering, based on Raman spectroscopy and the equation of state, with a bulk modulus of ~300 GPa. As predicted based on the similar ionic radius of Zr(4+) and Hf(4+), rare-earth hafnates show similar behavior to that reported for rare earth zirconates at high pressure.

    View details for DOI 10.1088/1361-648X/aa7148

    View details for PubMedID 28541929

  • High pressure synthesis of a hexagonal close-packed phase of the high-entropy alloy CrMnFeCoNi NATURE COMMUNICATIONS Tracy, C. L., Park, S., Rittman, D. R., Zinkle, S. J., Bei, H., Lang, M., Ewing, R. C., Mao, W. L. 2017; 8

    Abstract

    High-entropy alloys, near-equiatomic solid solutions of five or more elements, represent a new strategy for the design of materials with properties superior to those of conventional alloys. However, their phase space remains constrained, with transition metal high-entropy alloys exhibiting only face- or body-centered cubic structures. Here, we report the high-pressure synthesis of a hexagonal close-packed phase of the prototypical high-entropy alloy CrMnFeCoNi. This martensitic transformation begins at 14 GPa and is attributed to suppression of the local magnetic moments, destabilizing the initial fcc structure. Similar to fcc-to-hcp transformations in Al and the noble gases, the transformation is sluggish, occurring over a range of >40 GPa. However, the behaviour of CrMnFeCoNi is unique in that the hcp phase is retained following decompression to ambient pressure, yielding metastable fcc-hcp mixtures. This demonstrates a means of tuning the structures and properties of high-entropy alloys in a manner not achievable by conventional processing techniques.

    View details for DOI 10.1038/ncomms15634

    View details for Web of Science ID 000401966000001

    View details for PubMedID 28541277

  • Structural response of titanate pyrochlores to swift heavy ion irradiation ACTA MATERIALIA Shamblin, J., Tracy, C. L., Ewing, R. C., Zhang, F., Li, W., Trautmann, C., Lang, M. 2016; 117: 207-215
  • Role of composition, bond covalency, and short-range order in the disordering of stannate pyrochlores by swift heavy ion irradiation PHYSICAL REVIEW B Tracy, C. L., Shamblin, J., Park, S., Zhang, F., Trautmann, C., Lang, M., Ewing, R. C. 2016; 94 (6)
  • Stability of fluorite-type La2Ce2O7 under extreme conditions JOURNAL OF ALLOYS AND COMPOUNDS Zhang, F. X., Tracy, C. L., Lang, M., Ewing, R. C. 2016; 674: 168-173
  • Anisotropic expansion and amorphization of Ga2O3 irradiated with 946 MeV Au ions NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Tracy, C. L., Lang, M., Severin, D., Bender, M., Trautmann, C., Ewing, R. C. 2016; 374: 40-44
  • Probing disorder in isometric pyrochlore and related complex oxides NATURE MATERIALS Shamblin, J., Feygenson, M., Neuefeind, J., Tracy, C. L., Zhang, F., Finkeldei, S., Bosbach, D., Zhou, H., Ewing, R. C., Lang, M. 2016; 15 (5): 507-?

    Abstract

    There has been an increased focus on understanding the energetics of structures with unconventional ordering (for example, correlated disorder that is heterogeneous across different length scales). In particular, compounds with the isometric pyrochlore structure, A2B2O7, can adopt a disordered, isometric fluorite-type structure, (A, B)4O7, under extreme conditions. Despite the importance of the disordering process there exists only a limited understanding of the role of local ordering on the energy landscape. We have used neutron total scattering to show that disordered fluorite (induced intrinsically by composition/stoichiometry or at far-from-equilibrium conditions produced by high-energy radiation) consists of a local orthorhombic structural unit that is repeated by a pseudo-translational symmetry, such that orthorhombic and isometric arrays coexist at different length scales. We also show that inversion in isometric spinel occurs by a similar process. This insight provides a new basis for understanding order-to-disorder transformations important for applications such as plutonium immobilization, fast ion conduction, and thermal barrier coatings.

    View details for DOI 10.1038/NMAT4581

    View details for Web of Science ID 000374763500010

    View details for PubMedID 26928636

  • Reassess New Mexico's nuclear-waste repository NATURE Tracy, C. L., Dustin, M. K., Ewing, R. C. 2016; 529 (7585): 149-151

    View details for Web of Science ID 000368015700013

    View details for PubMedID 26762442

  • Pressure-induced phase transitions of beta-type pyrochlore CsTaWO6 RSC ADVANCES Zhang, F. X., Tracy, C. L., Shamblin, J., Palomares, R. I., Lang, M., Park, S., PARK, C., Tkachev, S., Ewing, R. C. 2016; 6 (97): 94287-94293

    View details for DOI 10.1039/c6ra11185h

    View details for Web of Science ID 000385632400001

  • Phase transformations in Ln(2)O(3) materials irradiated with swift heavy ions PHYSICAL REVIEW B Tracy, C. L., Lang, M., Zhang, F., Trautmann, C., Ewing, R. C. 2015; 92 (17)
  • Response of Gd2Ti2O7 and La2Ti2O7 to swift-heavy ion irradiation and annealing ACTA MATERIALIA Park, S., Lang, M., Tracy, C. L., Zhang, J., Zhang, F., Trautmann, C., Rodriguez, M. D., Kluth, P., Ewing, R. C. 2015; 93: 1-11
  • In situ defect annealing of swift heavy ion irradiated CeO2 and ThO2 using synchrotron X-ray diffraction and a hydrothermal diamond anvil cell JOURNAL OF APPLIED CRYSTALLOGRAPHY Palomares, R. I., Tracy, C. L., Zhang, F., Park, C., Popov, D., Trautmann, C., Ewing, R. C., Lang, M. 2015; 48: 711-717
  • Characterization of ion-induced radiation effects in nuclear materials using synchrotron x-ray techniques JOURNAL OF MATERIALS RESEARCH Lang, M., Tracy, C. L., Palomares, R. I., Zhang, F., Severin, D., Bender, M., Trautmann, C., Park, C., Prakapenka, V. B., Skuratov, V. A., Ewing, R. C. 2015; 30 (9): 1366-1379

    View details for DOI 10.1557/jmr.2015.6

    View details for Web of Science ID 000355283500016

  • Ultrafast laser and swift heavy ion irradiation: Response of Gd2O3 and ZrO2 to intense electronic excitation APPLIED PHYSICS LETTERS Rittman, D. R., Tracy, C. L., Cusick, A. B., Abere, M. J., Torralva, B., Ewing, R. C., Yalisove, S. M. 2015; 106 (17)

    View details for DOI 10.1063/1.4919720

    View details for Web of Science ID 000353839100029

  • Redox response of actinide materials to highly ionizing radiation NATURE COMMUNICATIONS Tracy, C. L., Lang, M., Pray, J. M., Zhang, F., Popov, D., Park, C., Trautmann, C., Bender, M., Severin, D., Skuratov, V. A., Ewing, R. C. 2015; 6

    Abstract

    Energetic radiation can cause dramatic changes in the physical and chemical properties of actinide materials, degrading their performance in fission-based energy systems. As advanced nuclear fuels and wasteforms are developed, fundamental understanding of the processes controlling radiation damage accumulation is necessary. Here we report oxidation state reduction of actinide and analogue elements caused by high-energy, heavy ion irradiation and demonstrate coupling of this redox behaviour with structural modifications. ThO2, in which thorium is stable only in a tetravalent state, exhibits damage accumulation processes distinct from those of multivalent cation compounds CeO2 (Ce(3+) and Ce(4+)) and UO3 (U(4+), U(5+) and U(6+)). The radiation tolerance of these materials depends on the efficiency of this redox reaction, such that damage can be inhibited by altering grain size and cation valence variability. Thus, the redox behaviour of actinide materials is important for the design of nuclear fuels and the prediction of their performance.

    View details for DOI 10.1038/ncomms7133

    View details for Web of Science ID 000348832300004

    View details for PubMedID 25626111

  • Incorporation of uranium in pyrochlore oxides and pressure-induced phase transitions JOURNAL OF SOLID STATE CHEMISTRY Zhang, F. X., Lang, M., Tracy, C., Ewing, R. C., Gregg, D. J., Lumpkin, G. R. 2014; 219: 49-54
  • Swift heavy ion track formation in Gd2Zr2-xTixO7 pyrochlore: Effect of electronic energy loss NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Lang, M., Toulemonde, M., Zhang, J., Zhang, F., Tracy, C. L., Lian, J., Wang, Z., Weber, W. J., Severin, D., Bender, M., Trautmann, C., Ewing, R. C. 2014; 336: 102-115
  • Swift heavy ion irradiation-induced amorphization of La2Ti2O7 NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Park, S., Lang, M., Tracy, C. L., Zhang, J., Zhang, F., Trautmann, C., Kluth, P., Rodriguez, M. D., Ewing, R. C. 2014; 326: 145-149
  • Defect accumulation in ThO2 irradiated with swift heavy ions NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Tracy, C. L., Pray, J. M., Lang, M., Popov, D., Park, C., Trautmann, C., Ewing, R. C. 2014; 326: 169-173
  • Swift heavy ion-induced phase transformation in Gd2O3 NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS Lang, M., Zhang, F., Zhang, J., Tracy, C. L., Cusick, A. B., VonEhr, J., Chen, Z., Trautmann, C., Ewing, R. C. 2014; 326: 121-125
  • Structural response of A(2)TiO(5) (A = La, Nd, Sm, Gd) to swift heavy ion irradiation ACTA MATERIALIA Tracy, C. L., Lang, M., Zhang, J., Zhang, F., Wang, Z., Ewing, R. C. 2012; 60 (11): 4477-4486