Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment: Editorial

Crystallography of nanocrystalline materials has witnessed a true revolution in the past 10 years, thanks to the introduction of protocols for 3D acquisition and analysis of electron diffraction data. This method provides single-crystal data of structure solution and refinement quality, allowing the atomic structure determination of those materials that remained hitherto unknown because of their limited crystallinity. Several experimental protocols exist, which share the common idea of sampling a sequence of diffraction patterns while the crystal is tilted around a noncrystallographic axis, namely, the goniometer axis of the transmission electron microscope sample stage. This Outlook reviews most important 3D electron diffraction applications for different kinds of samples and problematics, related with both materials and life sciences. Structure refinement including dynamical scattering is also briefly discussed.

3D Electron Diffraction: The Nanocrystallography Revolution.

Detailed spectroscopic analysis of the electronic configuration of Cr3+ in Bi2Ga4O9 is reported. The material exhibits unique luminescent properties arising from the crystal field experienced by Cr3+, with simultaneous strong sharp and broadband near-infrared emissions from the 2E and 4T2 excited states, in a wide range of temperature. The system displays dual near-infrared emission characterized by a remarkable thermal sensitivity over the whole explored range of temperatures, reaching a value of 0.7%·K–1 in the physiological range. Moreover, the possibility to absorb and emit in the first biological window, allows one to consider the system as a new promising candidate for ratiometric fluorescent thermal sensing in biotechnological applications.

Ratiometric Optical Thermometer Based on Dual Near-Infrared Emission in Cr3+-Doped Bismuth-Based Gallate Host

Mullite is certainly one of the most important oxide materials for both conventional and advanced ceramics. Mullite belongs to the compositional series of orthorhombic aluminosilicates with the general composition Al2(Al2+2xSi2-2x)O10-x. Main members are sillimanite (x = 0), stoichiometric 3/2-mullite (x = 0.25), 2/1-mullite (x = 0.40), and the SiO2-free phase ι-alumina (x = 1, crystal structure not known). This study gives an overview on the present state of research regarding single crystal mullite. Following a short introduction, the second part of the review focuses on the crystal structure of mullite. In particular, the characteristic mullite-type structural backbone of parallel chains consisting of edge-sharing MO6 octahedra and their specific cross-linkage by TO4 tetrahedra is explained in detail, the role of cation disorder and structural oxygen vacancies is addressed, and the possibility of cation substitution on different sites is discussed. The third part of the study deals with physical properties being relevant for technical applications of mullite and includes mechanical properties (e.g., elasticity, compressibility, strength, toughness, creep), thermal properties (e.g., thermal expansion, heat capacity, atomic diffusion, thermal conductivity), electrical conductivity, and optical properties. Special emphasis is put on structure–property relationships which allow for interpretation of corresponding experimental data and offer in turn the possibility to tailor new mullite materials with improved properties. Finally, the reported anomalies and discontinuities in the evolution of certain physical properties with temperature are summarized and critically discussed.

Mullite: Crystal Structure and Related Properties

Infrared Spectroscopy of Minerals and Related Compounds

Temperature-dependent structural studies of mullite-type Bi2Fe4O9

Mullite-type A 2 M 4 O 9 phases (M = Al, Ga, Fe), representing promising oxygen conducting materials for solid oxide fuel cells (SOFCs), were synthesized using the glycerine- and the EDTA/citric acid synthesis method. For strontium-doped material pure phases could be obtained only by washing the samples after the heating in both synthesis methods. Temperature dependent investigations were carried out to show the influence of the metal atoms on the structural stability and thermal expansion coefficients. Whereas the Sr-free phases show a quasi linear thermal expansion behavior in all three directions up to their incongruent melting points, a discontinuity in the measured range is observed for the investigated strontium doped dibismuth-nonaoxotetrametallate(III) caused by the decomposition into Bi 2 M 4 O 9 , strontium metallates and bismuthoxide. Big single crystals were only observed for the Sr-free compound, of which the structure of Bi 2 (Ga 0.45 Fe 0.55 ) 4 O 9 will be presented here in the Barnighausen tree corresponding mullite-type setting.

Synthesis and properties of mullite-type (Bi1−xSrx)2(M11−yM2y)4O9−x (M = Al, Ga, Fe)

Boron compounds are included in the family of mullite-type structures if they contain chains of edge-sharing M O 6 octahedra ( M are the octahedrally coordinated cations in the chains) similar to those in mullite. The crystal structures of all boron compounds in the mullite family are derived from a tetragonal aristotype and should obey the criteria defining the limits for the deviation from the root structure. The compounds complying with these criteria for mullite-type crystal structures have an observed maximum deviation from orthogonality by Δγ′ = 4° (ominelite: γ ′ = 86.0°, mullite: 90°), a range of octahedral tilting angles ω between 61.5° (boromullite) and 90° (for 2:1 mullite it is 59.8°), a range of distances between neighboring chains normalized to the ionic radii of the central atoms between Q r = 8.2 % (PbCrBO 4 ) and 13.8 % (PbMnBO 4 , for 2:1 mullite it is 9.9 %), and a maximum deviation from tetragonal metric ( a = b , Q a = 1) with Q a = 0.78 (PbMnBO 4 ; for 2:1 mullite Q a = 0.986). The average value of all mean B–O distances in BO 3 groups is 1.383 A, which compares well with known trigonally coordinated B–O distances. It is recommended to designate all boron compounds with the characteristic mullite-type M O 4 chains of M O 6 octahedra as “mullite-type boron compounds” and to use the term “boron-mullite” or “B-mullite”, initially introduced by Werding & Schreyer (1984) (Geochim. Cosmochim. Acta, 48 , 1331–1344) for the subgroup of Al borates and Al borosilicates with mullite-type structures. The name “boromullite” is reserved for a natural mineral with sillimanite-like and Al 5 BO 9 -like modules.

Crystal chemistry of borates and borosilicates with mullite-type structures: a review

Boron-doped mullites were synthesized using aluminium nitrate-nonahydrate, tetraethoxysilane and boric acid in a sol–gel process with subsequent annealing at 950 and 1300 °C for five hours. Two different bulk compositions with constant Al2O3 contents (60 and 70 mol%, respectively) and varying SiO2 plus B2O3 contents were investigated. X-ray powder diffraction analyses yielded a linear decrease of the lattice parameters with increasing bulk B2O3 content, which was interpreted as to be due to boron incorporation. Related to the increasing boron content, corresponding infrared spectra revealed a slight and continuous shift for most of the absorption bands. These data show that mullite is able to incorporate large amounts of boron into its structure (up to about 20 mol% B2O3 depending on the bulk composition of the starting materials). Infrared analyses suggest that boron is incorporated into the mullite structure in form of planar three-fold coordinated BO3 groups.

Hartmut Schneider

Papers

Mullite: Crystal Structure and Related Properties

Temperature-dependent structural studies of mullite-type Bi2Fe4O9

Synthesis and properties of mullite-type (Bi1−xSrx)2(M11−yM2y)4O9−x (M = Al, Ga, Fe)

Crystal chemistry of borates and borosilicates with mullite-type structures: a review

Boron incorporation into mullite