Volume 239, Issue 5-6

Because of the neutron’s special properties, neutron diffraction may be considered one of the most powerful techniques for structure determination of crystalline and related matter. Neutrons can be released from nuclear fission, from spallation processes, and also from low-energy nuclear reactions, and they can then be used in powder, time-of-flight, texture, single crystal, and other techniques, all of which are perfectly suited to clarify crystal and magnetic structures. With high neutron flux and sufficient brilliance, neutron diffraction also excels for diffuse scattering, for in situ and operando studies as well as for high-pressure experiments of today’s materials. For these, the wave-like neutron’s infinite advantage (isotope specific, magnetic) is crucial to answering important scientific questions, for example, on the structure and dynamics of light atoms in energy conversion and storage materials, magnetic matter, or protein structures. In this primer, we summarize the current state of neutron diffraction (and how it came to be), but also look at recent advances and new ideas, e.g., the design of new instruments, and what follows from that.

An X-ray diffraction study of ceramic samples of the BiScO 3 –PbTiO 3 –PbMg ⅓ Nb ⅔ O 3 system with compositions close to the morphotropic phase boundary was carried out at room temperature. The existence of wide areas of solid solutions has been established. The symmetry of the PbTiO 3 -based solid solutions is tetragonal (space group P 4 mm ). The symmetry of the PbMg ⅓ Nb ⅔ O 3 -based solid solutions is cubic (space group Pm 3‾ m ). Near the BiScO 3 side, the solid solutions are rhombohedral (space group R 3 m ). During the morphotropic phase transition from the cubic solid solutions to the tetragonal ones, additional phases appear. If a tetragonal phase prevails ((1 − 2 x )BiScO 3 – x PbTiO 3 – x PbMg ⅓ Nb ⅔ O 3 x  = 0.46; (1 − 2 x )BiScO 3 –1.1 x PbTiO 3 –0.9 x PbMg ⅓ Nb ⅔ O 3 x  = 0.45 and 0.42), a satisfactory model is a model with a minority cubic phase (space group Pm 3‾ m ). If a cubic phase prevails ((1 − 2 x )BiScO 3 –1.1 x PbTiO 3 –0.9 x PbMg ⅓ Nb ⅔ O 3 x  = 0.39 and 0.36; (1 − 2 x )BiScO 3 –0.8 x PbTiO 3 –1.2 x PbMg ⅓ Nb ⅔ O 3 x  = 0.4), a model with a minority monoclinic phase (space group Cm ) or with two minority phases: tetragonal (space group P 4 mm ) and monoclinic (space group Cm ) is satisfactory.

A new uranyl silicate Cs 4 [(UO 2 ) 5 (SiO 3 OH) 2 O 2 F 4 ] ( 1 ), was obtained via a hydrothermal route. The new compound is monoclinic, P 2 1 / n , a  = 8.3870(2), b  = 13.4612(2), c  = 10.9503(2) Å, β  = 91.223(2)°, V  = 1236.00(4) Å 3 ; the structure has been solved and refined down to R 1  = 0.022. Therein, the phosphouranylite units ( PU s) associate into a new type of uranyl-silicate layers, [(UO 2 ) 5 (SiO 3 OH) 2 O 2 F 4 ] 4− , which interleave with the Cs + cations. Topological analysis of PU based structures indicates that these layers in 1 provide a unique example of complexes constructed only via association of the PU and not involving other building units.

Fe-dominant sogdianite, a cyclosilicate compound with the chemical formula (Fe 3+ 0.74 Zr 0.64 Ti 0.46 Al 0.15 )(□ 1.02 Na 0.98 )K[Li 3 Si 12 O 30 ], was studied. The investigation involved a comprehensive analysis of the mineral sample, including crystal-chemical analysis, Raman and FTIR spectroscopy, optical absorption, and luminescence study. Crystallographic site populations were determined through single crystal structure refinement and electron probe microanalysis. The thermoelastic behavior of a powder was studied using in situ  high-temperature X-ray diffraction (30–750 °C). Notably, no phase transition was detected; sogdianite exhibited anisotropic thermal expansion. The first time study of vibrational spectra and spectral bands assigning were performed. The electronic transitions in d 5 -ion impurities of sogdianite were studied using optical absorption and luminescence spectroscopy. The origin of pink color and luminescence of sogdianite was clarified. The broad spectral bands in the visible UV spectral region are responsible for the pink color exhibited by sogdianite and could be attributed to d – d transitions occurring in Fe 3+ ions.

Introduction of Th into synthetic disordered fluorite-type Gd 2 Zr 2 O 7 induces a transition to an ordered pyrochlore-type phase at a Th concentration of 10 % at the Gd site (Gd 1.8 Th 0.2 Zr 2 O 7 composition). The degree of order of the fluorite-type phase reaches 50 % for a Th concentration of 25 % (Gd 1.5 Th 0.5 Zr 2 O 7 composition). Upon application of high pressure, the Gd 2 Zr 2 O 7 phase retains the fluorite-type structure until 33 GPa ( K 0  = 167(1) GPa), where it undergoes reversible amorphization. The Gd 1.7 Th 0.3 Zr 2 O 7 phase was found to be stable up to at least a pressure of 25 GPa ( K 0  = 169(3) GPa). Upon heating to T max of 1135 K, the Gd 2 Zr 2 O 7 phase retains its disordered fluorite-type structural arrangement ( α  = 3.03 × 10 −5  K −1 ). The excellent stability of the Gd 2− x Th x Zr 2 O 7 phases under extreme conditions of temperature and pressure makes Gd 2 Zr 2 O 7 a promising candidate as a host matrix for radioactive elements for safe long-term underground storage of nuclear waste.

The correlations of values of the crystal structure complexity indices were calculated using the major modern approaches (10 indices in total), in particular, those, proposed by Krivovichev, Oganov, the concept of implicit hierarchical depth, as well as, for the comparison, the measure of regularity of ( r , R )-system introduced by Blatov. To find the correlations, a series of 33 mercury-containing mineral structures with a non-repeating system of crystallographic positions occupied by atoms without any partial occupancy was used. It was shown that almost all pairs of complexity indices are statistically significantly ( p  < 0.05) positively or negatively correlated. The discriminating power of the indices for the set of structures was calculated. It is discussed that all the indices can be used to assess the complexity for any series of the crystal structures, but each of them has its particular advantages and limitations.

A new supramolecular cocrystal of bis(3-hydroxy-1-methylpyridin-1-ium iodide)-18-crown-6 (I) has been synthesized successfully from 3-hydroxypyridine, methyl iodide and 18-crown-6 (18C6). The cocrystal was characterized by FT-IR, UV-DRS and PL spectroscopy, single crystal and powder X-ray diffraction (XRD) analysis, SEM and EDS analysis, and thermogravimetric analysis. The bulk phase purity of the compound is confirmed by powder XRD analysis, whereas single-crystal XRD confirms that it packs in a monoclinic system with centrosymmetric space group ( P 2 1 / c ). Single crystal XRD shows that the asymmetric unit is C 6 H 12 O 3 , [C 6 H 8 NO] + [I]¯. The presence of C–H⋯O, C–H⋯I and O–H⋯I intermolecular contacts helps to achieve crystal cohesion. The direct band gap energy has been estimated to be 3.21 eV using diffuse reflectance spectral measurements. Thermogravimetric analysis was employed to determine the compound’s thermal behaviour concerning temperature. EDS and SEM investigations were used to examine the elemental composition and surface morphology of the compound.