Structural Evolution and Atom Clustering in β-SiAlON: β-Si6−z_{6-z}Alz_zOz_zN8−z_{8-z}

SiAlON ceramics, solid solutions based on the Si$_3$N$_4$ structure, are important, lightweight structural materials with intrinsically high strength, high hardness, and high thermal and chemical stability. Described by the chemical formula β-Si$_{6-z}$Al$_z$O$_z$N$_{8-z}$, from a compositional viewpoint, these materials can be regarded as solid solutions between Si$_3$N$_4$ and Al$_3$O$_3$N. A key aspect of the structural evolution with increasing Al and O ($z$ in the formula) is to understand how these elements are distributed on the β-Si$_3$N$_4$ framework. The average and local structural evolution of highly phase-pure samples of β-Si$_{6-z}$Al$_z$O$_z$N$_{8-z}$ with $z$ = 0.050, 0.075, and 0.125 are studied here, using a combination of X-ray diffraction, NMR studies, and density functional theory calculations. Synchrotron X-ray diffraction establishes sample purity and indicates subtle changes in the average structure with increasing Al content in these compounds. Solid-state magic-angle-spinning $^{27}$Al NMR experiments, coupled with detailed ab initio calculations of NMR spectra of Al in different AlO$_q$N$_{4-q}$ tetrahedra (0 ≤ $q$ ≤ 4), reveal a tendency of Al and O to cluster in these materials. Independently, the calculations suggest an energetic preference for Al-O bond formation, instead of a random distribution, in the β-SiAlON system.C.C. thanks the National Science Foundation for a Graduate Research Fellowship under Grant DGE 1144085. K.J.G. thanks The Winston Churchill Foundation of the United States and the Herchel Smith Scholarship for funding. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE), Office of Science, by Argonne National Laboratory, was supported by the U.S. DOE under Contract DE-AC02-06CH11357. DFT calculations were performed on the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council (U.K.). This work made use of MRL-shared experimental facilities, supported by the MRSEC Program of the NSF under Award DMR 1121053. The MRL is a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org)

Expanding frontiers in materials chemistry and physics with multiple anions

During the last century, inorganic oxide compounds laid foundations for materials synthesis, characterization, and technology translation by adding new functions into devices previously dominated by main-group element semiconductor compounds. Today, compounds with multiple anions beyond the single-oxide ion, such as oxyhalides and oxyhydrides, offer a new materials platform from which superior functionality may arise. Here we review the recent progress, status, and future prospects and challenges facing the development and deployment of mixed-anion compounds, focusing mainly on oxide-derived materials. We devote attention to the crucial roles that multiple anions play during synthesis, characterization, and in the physical properties of these materials. We discuss the opportunities enabled by recent advances in synthetic approaches for design of both local and overall structure, state-of-the-art characterization techniques to distinguish unique structural and chemical states, and chemical/physical properties emerging from the synergy of multiple anions for catalysis, energy conversion, and electronic materials

Correlating Local Compositions and Structures with the Macroscopic Optical Properties of Ce3+-Doped CaSc2O4, an Efficient Green-Emitting Phosphor

The authors thank J. Siewenie for assistance with collection of the neutron powder diffraction data and Dr. Z. Gan for assistance with the 43Ca NMR measurements.International audienceCalcium scandate (CaSc2O4) substituted with small amounts (<1%) of Ce3+ is a recently discovered bright-green-emitting phosphor with favorable light absorption and emission properties and robust temperature stability that make it well suited for solid-state white-lighting applications. Combined analyses of scattering, solid-state nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and photoluminescence measurements establish the compositional and structural origins of the macroscopic optical properties of this phosphor material. Simultaneous refinements of synchrotron X-ray and neutron diffraction data of Ce3+-doped CaSc2O4 enable the average crystal structure to be determined, which is shown to correspond to an exceedingly rigid host structure, as corroborated by density functional theory (DFT) calculations. Such structural rigidity leads to high quantum efficiency, which is optimized by the substitution of as little as 0.5 mol % of Ce3+ for Ca2+ ions, with higher extents of Ce3+ substitution leading to decreased photoluminescent quantum yields. Solid-state Ca-43 and Sc-45 magic-angle spinning (MAS) NMR spectra are sensitive to the effects of the paramagnetic Ce3+ dopant ions on nearby atoms in the host structure and yield evidence for local structural distortions. EPR measurements provide direct insights on structures of the Ce3+ ions, as a function of Ce3+ substitution. The combined scattering and spectroscopic analyses yield detailed new understanding of the local and long-range structures of Ce3+-doped CaSc2O4, which account for the sensitive composition-dependent optical properties of this important phosphor material

Correlating Local Compositions and Structures with the Macroscopic Optical Properties of Ce³⁺-Doped CaSc₂O₄, an Efficient Green-Emitting Phosphor

Calcium scandate (CaSc₂O₄) substituted with small amounts (<1%) of Ce³⁺ is a recently discovered bright-green-emitting phosphor with favorable light absorption and emission properties and robust temperature stability that make it well-suited for solid-state white-lighting applications. Combined analyses of scattering, solid-state nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), and photoluminescence measurements establish the compositional and structural origins of the macroscopic optical properties of this phosphor material. Simultaneous refinements of synchrotron X-ray and neutron diffraction data of Ce³⁺-doped CaSc₂O₄ enable the average crystal structure to be determined, which is shown to correspond to an exceedingly rigid host structure, as corroborated by density functional theory (DFT) calculations. Such structural rigidity leads to high quantum efficiency, which is optimized by the substitution of as little as 0.5 mol % of Ce³⁺ for Ca²⁺ ions, with higher extents of Ce³⁺ substitution leading to decreased photoluminescent quantum yields. Solid-state ⁴³Ca and ⁴⁵Sc magic-angle spinning (MAS) NMR spectra are sensitive to the effects of the paramagnetic Ce³⁺ dopant ions on nearby atoms in the host structure and yield evidence for local structural distortions. EPR measurements provide direct insights on structures of the Ce³⁺ ions, as a function of Ce³⁺ substitution. The combined scattering and spectroscopic analyses yield detailed new understanding of the local and long-range structures of Ce³⁺-doped CaSc₂O₄, which account for the sensitive composition-dependent optical properties of this important phosphor material