Drastic Differences between the Local and the Average Structures of Sr₂MSbO_5.5 (M = Ca, Sr, Ba) Oxygen-Deficient Double Perovskites

For many disordered materials, knowing their average crystal structure is insufficient for explaining and predicting their macroscopic properties. It has been found that a description of the short-range atomic arrangements is needed to understand such materials. In order to understand the conduction pathways in ionic conductors which have random distributions of vacancies it is imperative to know the local structures which are present. In this study the local structures of three oxygen-deficient double perovskites, Sr₂MSbO_5.5 (M = Ca, Sr, Ba), have been investigated by neutron pair distribution function analysis. The ions in these compounds are all found to have local coordination environments which are radically different than those given by their average structures. While there is no long-range ordering of the oxygen vacancies in these compounds, a considerable amount of short-range order does exist. The conditions which drive the short-range ordering are discussed as are the possible mechanisms for achieving it. It is proposed that the SbO₅ polyhedra form distorted trigonal bipyramids by moving oxygen atoms into interstitial positions. In the M = Sr compound 45° rotations of SbO₆ octahedra are also present, which add additional oxygen atoms into the interstitial sites. Large displacements of the Ca²⁺, Sr²⁺, and Ba²⁺ cations are also present, the directions of which are correlated with the occupancies of the interstitial oxygen sites. Reverse Monte Carlo modeling of the pair distribution function data has provided the actual bond length distributions for the cations

Average and Local Structural Origins of the Optical Properties of the Nitride Phosphor La_3–<i>x</i>Ce_<i>x</i>Si₆N₁₁ (0 < <i>x</i> ≤ 3)

Structural intricacies of the orange-red nitride phosphor system La_3–<i>x</i>Ce_<i>x</i>Si₆N₁₁ (0 < <i>x</i> ≤ 3) have been elucidated using a combination of state-of-the art tools, in order to understand the origins of the exceptional optical properties of this important solid-state lighting material. In addition, the optical properties of the end-member (<i>x</i> = 3) compound, Ce₃Si₆N₁₁, are described for the first time. A combination of synchrotron powder X-ray diffraction and neutron scattering is employed to establish site preferences and the rigid nature of the structure, which is characterized by a high Debye temperature. The high Debye temperature is also corroborated from ab initio electronic structure calculations. Solid-state ²⁹Si nuclear magnetic resonance, including paramagnetic shifts of ²⁹Si spectra, are employed in conjunction with low-temperature electron spin resonance studies to probes of the local environments of Ce ions. Detailed wavelength-, time-, and temperature-dependent luminescence properties of the solid solution are presented. Temperature-dependent quantum yield measurements demonstrate the remarkable thermal robustness of luminescence of La_2.82Ce_0.18Si₆N₁₁, which shows little sign of thermal quenching, even at temperatures as high as 500 K. This robustness is attributed to the highly rigid lattice. Luminescence decay measurements indicate very short decay times (close to 40 ns). The fast decay is suggested to prevent strong self-quenching of luminescence, allowing even the end-member compound Ce₃Si₆N₁₁ to display bright luminescence

Local Environments of Dilute Activator Ions in the Solid-State Lighting Phosphor Y_3–<i>x</i>Ce_<i>x</i>Al₅O₁₂

The oxide garnet Y₃Al₅O₁₂ (YAG), when substituted with a few percent of the activator ion Ce³⁺ to replace Y³⁺, is a luminescent material that is nearly ideal for phosphor-converted solid-state white lighting. The local environments of the small number of substituted Ce³⁺ ions are known to critically influence the optical properties of the phosphor. Using a combination of powerful experimental methods, the nature of these local environments is determined and is correlated with the macroscopic luminescent properties of Ce-substituted YAG. The rigidity of the garnet structure is established and is shown to play a key role in the high quantum yield and in the resistance toward thermal quenching of luminescence. Local structural probes reveal compression of the Ce³⁺ local environments by the rigid YAG structure, which gives rise to the unusually large crystal-field splitting, and hence yellow emission. Effective design rules for finding new phosphor materials inferred from the results establish that efficient phosphors require rigid, highly three-dimensionally connected host structures with simple compositions that manifest a low number of phonon modes, and low activator ion concentrations to avoid quenching