Skills and Expertise
Material CharacterizationMaterialsX-ray DiffractionSynthesisXRD AnalysisCrystal StructureCoordination ChemistryDSCPowder X-ray DiffractionTGAInorganic SynthesisX-ray CrystallographyCrystallographyDiffractionStructure DeterminationPowder DiffractionSolid State CharacterizationX-Ray ScatteringPerovskitesSynchrotron X-Ray DiffractionStructural ChemistryStructure AnalysisNeutron DiffractionSolid State SynthesisNeutron ScatteringApplied X-ray CrystallographyRietveld MethodBallmillingXRPDFullprof
Research Item (8)
- Apr 2018
Sodium bismuthate is a commercially available, inexpensive, non-toxic and very potent inorganic oxidant and photocatalyst. It is one of the important reagents for oxidative separation of Am³⁺ from the chemically similar lanthanide ions, for its recovery or safe disposal from reprocessed nuclear fuel. While the structure of NaBiO3 has been described from powder and neutron diffraction; the structure of NaBiO3·XH2O, the manufactured form of sodium bismuthate, is currently unknown. Herein, we describe the structure of NaBiO3·XH2O (X=3) using pair distribution function (PDF) analysis of X-ray total scattering data. In our proposed structure model, NaBiO3·3H2O is similar to NaBiO3, but with turbostratic disorder in the stacking direction of the alternating Bi-O and Na-O layers. We propose locations for the lattice water, and its role in creating turbostratic disorder. We also used PDF to describe the structural evolution of sodium bismuthate upon exposure to nitric acid, the conditions employed in for nuclear fuel reprocessing. We supported the proposed model for pristine NaBiO3·3H2O and its acidified derivatives by a variety of techniques including thermogravimetry, infrared spectrometry, powder X-ray diffraction (PXRD), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). By employing both surface and bulk techniques, we hypothesize that the bismuth reduced to Bi³⁺ upon aqueous acid exposure remains in the lattice, rather than completely dissolving and/or depositing on the surface, as prior suggested. Using pretreated acidified sodium bismuthate samples, we delineated the effects of acid strength vs. bismuthate structure/composition on Ce³⁺ to Ce⁴⁺ oxidation efficacy.
- Mar 2018
Two new perovskites, (Bi0.5A0.5)(Sc0.5Nb0.5)O3 where A = Na⁺ and K⁺, have been made phase pure for the first time. The synthesis procedure that prevented formation of pyrochlore impurities is described. B-site ordering in A = K⁺ is successfully achieved by prolonged heating. Combined Rietveld refinement on synchrotron X-ray diffraction and neutron diffraction data is used to determine the complex average structure of these materials. A = Na⁺ crystallizes in the tetragonal P4/mbm space group and has disordered aoaoc⁺ rotations of BO6 octahedra. A = K⁺ crystallizes in the cubic Pm-3m space group, but shows local A-site displacements in the  direction. Both materials are highly disordered, but illustrate the potential of (A)NbO3-BiScO3 solid solutions as useful piezoelectrics or relaxors.
Structure-property relationships were determined for the family of 3-layer Aurivillius materials Bi2Sr(A)TiNb2O12 (A = Ca²⁺, Sr²⁺, Ba²⁺). X-ray and neutron diffraction along with selected area electron diffraction indicate that Bi2SrBaTiNb2O12 crystallizes in the non-polar I4/mmm space group whereas the polar B2cb space group best describes Bi2SrCaTiNb2O12 and Bi2Sr2TiNb2O12. Despite the different space groups, all three compositions show relaxor behav-ior as evidenced through P(E) and dielectric measurements. These relaxor properties are derived from the extensive amount of disorder in each composition that is found at every cationic crystallographic site and do not depend on the space group. This disorder is so extensive that it disrupts the ferroelectric properties allowed by symmetry in the B2cb space group. This work demonstrates the important role of cation substitution and site disorder in these 3-layered Aurivillius materials and its significant effect on both ferroelectric and dielectric properties.
- May 2017
Hard carbon is the leading candidate anode for commercialization of Na-ion batteries. Hard carbon has a unique local atomic structure, which is composed of nanodomains of layered rumpled sheets that have short-range local order resembling graphene within each layer, but complete disorder along the c-axis between layers. A primary challenge holding back the development of Na-ion batteries is that a complete understanding of the structure–capacity correlations of Na-ion storage in hard carbon has remained elusive. This article presents two key discoveries: first, the characteristics of hard carbons structure can be modified systematically by heteroatom doping, and second, that these structural changes greatly affect Na-ion storage properties, which reveals the mechanisms for Na storage in hard carbon. Specifically, via P or S doping, the interlayer spacing is dilated, which extends the low-voltage plateau capacity, while increasing the defect concentrations with P or B doping leads to higher sloping sodiation capacity. The combined experimental studies and first principles calculations reveal that it is the Na-ion-defect binding that corresponds to the sloping capacity, while the Na intercalation between graphenic layers causes the low-potential plateau capacity. The understanding suggests a new design principle of hard carbon anode: more reversibly binding defects and dilated turbostratic domains, given that the specific surface area is maintained low.
- Jul 2016
The capacity of hard carbon anodes in Na-ion batteries rarely reaches values beyond 300 mAh/g. We report that doping POx into local structures of hard carbon increases its reversible capacity from 283 to 359 mAh/g. We confirm that the doped POx is redox inactive by X-ray adsorption near edge structure measurements, thus not contributing to the higher capacity. We observe two significant changes of hard carbon’s local structures caused by doping. First, the (002) d-spacing inside the turbostratic nanodomains is increased, revealed by both laboratory and synchrotron X-ray diffraction. Second, doping turns turbostratic nanodomains more defective along ab planes, indicated by neutron total scattering and the associated pair distribution function studies. The local structural changes of hard carbon are correlated to the higher capacity, where both the plateau and slope regions in the potential profiles are enhanced. Our study demonstrates that Na-ion storage in hard carbon heavily depends on carbon local structures, where such structures, despite being disordered, can be tuned toward unusually high capacities.
- Aug 2015
Non-graphitizable carbon, also known as hard carbon, is considered one of the most promising anodes for the emerging Na-ion batteries. The current mechanistic understanding of Na-ion storage in hard carbon is based on the 'card-house' model first raised in the early 2000's. This model describes that Na-ion insertion occurs first through intercalation between graphene sheets in turbostratic nanodomains, followed by Na filling of the pores in the carbon structure. We testified this model by tuning the sizes of turbostratic nanodomains but found the opposite trend. Moreover, the results revealed a correlation between the structural defects and Na-ion storage. Based on our experimental data, we propose an alternative three-phase model for sodiation of hard carbon that consists of Na-ion storage at defect sites, through intercalation and lastly by pore-filling.