Multi-temperature X-ray diffraction studies show that twisting, rotation, and libration cause negative thermal expansion (NTE) of the nanoporous metal−organic framework MOF-5, Zn4O(1,4-benzenedicarboxylate)3. The near-linear lattice contraction is quantified in the temperature range 80−500 K using synchrotron powder X-ray diffraction. Vibrational motions causing the abnormal expansion behavior are evidenced by shortening of certain interatomic distances with increasing temperature according to single-crystal X-ray diffraction on a guest-free crystal over a broad temperature range. Detailed analysis of the atomic positional and displacement parameters suggests two contributions to cause the effect: (1) local twisting and vibrational motion of the carboxylate groups and (2) concerted transverse vibration of the linear linkers. The vibrational mechanism is confirmed by calculations of the dynamics in a molecular fragment of the framework.

In recent years the phenomenon of negative thermal expansion (NTE; that is, contraction upon warming) over a broad temperature range has been detected in a select group of materials [1] and attributed to mechanisms that include electronic and magnetic transitions [2] and transverse atomic and molecular vibrations. [1,[3][4][5][6][7][8] Among the vibrational systems, materials that have received particular attention include AM 2 O 8 , AM 2 O 7 , A 2 M 3 O 12 , and a number of zeolites, [3] which contain MÀOÀM' bridges that undergo transverse vibration to cause contraction of the M-M' distance, and a diverse family of metal cyanides, [4][5][6][7][8] which contain MÀCNÀM' bridges that show an analogous effect but with increased vibrational flexibility. The presence of a highly flexible diatomic linker in the cyanide phases leads to pronounced thermal expansion behavior, examples of which include the largest isotropic [4] and anisotropic [5] NTE reported to date. A common NTE mechanism proposed for both the oxide and cyanide systems is the coupling of these transverse vibrations into concerted low-energy lattice modes that involve the rotation and/or translation of undistorted metal-coordination polyhedra, known as rigid unit modes (RUMs).[9] With thermal population, these modes counteract the higherenergy longitudinal modes that cause bond-length expansion, thereby leading to bulk NTE behavior.Recently, NTE has also been proposed in a series of isoreticular metal-organic framework (IRMOF) materials following the detected thermal contraction of gas-sorbed samples of IRMOF-1.[10] Theoretical simulations [11] of these materials have suggested an NTE mechanism closely analogous to that of the metal cyanide phases, [6,7] involving the transverse vibration of linear organic linkers. Following a more general investigation of such materials, herein we present the NTE properties of [Cu 3 (btc) 2 ] (btc = 1,3,5-benzenetricarboxylate), a metal-organic framework that consists of dicopper tetracarboxylate "paddlewheels" and aromatic ring motifs.[12] Through crystallographic characterization we elucidate a structural mechanism that involves two unique components: transverse vibration of planar, rather than linear, linkers, and local molecular vibrations within the framework.The highly symmetric structure of [Cu 3 (btc) 2 ] can be conveniently considered as consisting of octahedral supramolecular cages that link through their vertices to form a three-dimensional cubic framework (Figure 1 inset). As the material readily binds atmospheric water and gases at the coordinatively unsaturated Cu sites, [13] samples for powder and single-crystal X-ray diffraction measurement were sealed under vacuum in glass capillaries following their thorough

Comprehensive single-crystal structural investigations of n- and p-type Ba8Ga16Ge30 have been carried out using multitemperature neutron and conventional X-ray diffraction as well as resonant synchrotron X-ray diffraction. The data show that the guest atom positions and dynamics are very similar in the two structures, although the barium atoms are slightly more displaced from the cage centers in the p-type structure than in the n-type structure (Deltad = 0.025 A). For both structures Fourier difference maps calculated from very high-resolution neutron diffraction data (sin theta/lambda > 2 A-1) show that the Ba nuclear density at lowest temperatures (15 K) is distributed in a torus around the crystallographic 6d site with maxima in the 24j positions. At room temperature the maxima have shifted to the 24k position. Analysis of atomic displacement parameters give Einstein temperatures of approximately 60(1) K for both structures. Thus, the fundamental difference in the low temperature thermal conductivity observed for p- and n-type Ba8Ga16Ge30 appear not to be directly related to the guest atom behavior as is commonly assumed in thermoelectric research. The neutron data and the resonant synchrotron X-ray data facilitate refinement of Ga/Ge framework occupancies. The Ga atoms have a clear preference for the 6c site with the preference being somewhat stronger for the n-type structure.

The formation and growth mechanisms in the hydrothermal synthesis of SnO(2) nanoparticles from aqueous solutions of SnCl(4)·5H(2)O have been elucidated by means of in situ X-ray total scattering (PDF) measurements. The analysis of the data reveals that when the tin(IV) chloride precursor is dissolved, chloride ions and water coordinate octahedrally to tin(IV), forming aquachlorotin(IV) complexes of the form [SnCl(x)(H(2)O)(6-x)]((4-x)+) as well as hexaaquatin(IV) complexes [Sn(H(2)O)(6-y)(OH)(y)]((4-y)+). Upon heating, ellipsoidal SnO(2) nanoparticles are formed uniquely from hexaaquatin(IV). The nanoparticle size and morphology (aspect ratio) are dependent on both the reaction temperature and the precursor concentration, and particles as small as ~2 nm can be synthesized. Analysis of the growth curves shows that Ostwald ripening only takes place above 200 °C, and in general the growth is limited by diffusion of precursor species to the growing particle. The c-parameter in the tetragonal lattice is observed to expand up to 0.5% for particle sizes down to 2-3 nm as compared to the bulk value. SnO(2) nanoparticles below 3-4 nm do not form in the bulk rutile structure, but as an orthorhombic structural modification, which previously has only been reported at pressures above 5 GPa. Thus, adjustment of the synthesis temperature and precursor concentration not only allows control over nanoparticle size and morphology but also the structure.

Solar-driven photocatalysis has attracted significant attention in water splitting, CO2 reduction and organic synthesis. The syntheses of valuable azo- and azoxyaromatic dyes via selective photoreduction of nitroaromatic compounds have been realised using supported plasmonic metal nanoparticles at elevated temperatures (≥90 °C); however, the high cost, low efficiency and poor selectivity of such catalyst systems at room temperature limit their application. Here we demonstrate that the inexpensive graphitic C3N4 is an efficient photocatalyst for selective syntheses of a series of azo- and azoxy-aromatic compounds from their corresponding nitroaromatics under either purple (410 nm) or blue light (450 nm) excitation. The high efficiency and high selectivity towards azo- and azoxy-aromatic compounds can be attributed to the weakly bound photogenerated surface adsorbed H-atoms and a favourable N-N coupling reaction. The results reveal financial and environmental potential of photocatalysis for mass production of valuable chemicals.

A method to count the number of electrons accumulated in carbon nitride green radicals using a methylviologen redox indicator is reported.

The extraordinary thermoelectric properties of lead chalcogenides have attracted huge interest in part due to their unexpected low thermal conductivity. Here, it is shown that anharmonicity and large cation disorder are present in both PbTe and PbS, based on elaborate charge density visualization using synchrotron powder X‐ray diffraction (SPXRD) data analyzed with the maximum entropy method (MEM). In both systems, the cation disorder increases with increasing temperature, whereas the Te/S anions appear to be centered on the expected lattice positions. Even at the lowest temperatures of 105 K, the lead ion is on average displaced by ≈0.2 Å from the rock‐salt lattice position, creating a strong phonon scattering mechanism. These findings provide a clue to understanding the excellent thermoelectric performance of crystals with atomic disorder. The SPXRD–MEM approach can be applied in general opening up for widespread characterization of subtle structural features in crystals with unusual properties.

The crystal structure and defect chemistry of hydrothermally synthesized LiFe 1-x Mn x PO 4 (x = 0, 0.25, and 0.50) particles have been characterized by simultaneous neutron and X-ray Rietveld refinement as well as X-ray and neutron pair distribution function (PDF) analysis, crystallinity determination, Mossbauer spectroscopy, ion coupled plasma (ICP) studies, and scanning electron microscopy (SEM). The very detailed structural refinements show that fast hydrothermal synthesis causes partial Feoccupancy and vacancies on the Li (M1) site, while the Fe (M2) site is always fully occupied by iron. Thus, the defect is not merely a Li/ Fe antisite defect, and excessive amounts of Fe are the origin of the disorder in the structure. Neutron and X-ray total scattering with PDF analysis show that after fast hydrothermal synthesis, the crystalline, defective Li x Fe y PO 4 coexists with amorphous Li/ Fe-PO 4 structures having just short-range order. Iron excess is only seen in the crystalline part of the particles, and as the crystallinity of the samples increases with longer synthesis time, the crystalline Fe/Li ratio approaches 1. The present data thus suggest that when crystalline particles initially form, Fe is included faster in the structure from the amorphous precursor than Li, causing the defects in the structure. Only when all Li have been incorporated into the crystal structure and 100% crystallinity is achieved, fully ordered, defect free samples can be obtained. The Fe occupancy on the M1 site is therefore directly linked to the crystallinity of the sample. In LiFe 1-x Mn x PO 4 samples, the transition metal defect on the M1 site is only Fe and not Mn. Furthermore, the presence of Mn locks in the defects, and thus the Fe disorder is not suppressed with extended synthesis time.