Supramolecular metathesis: co-former exchange in co-crystals of pyrazine with (R, R)-,(S, S)-,(R, S)-and (S, S/R, R)-tartaric acid

Co-crystals of dextro-(R,R), levo-(S,S), meso-(R,S) and racemic (R,R–S,S)-tartaric acid with pyrazine were obtained by manual kneading and slurry experiments; subsequent reactions in the solid state between these co-crystals and the various forms of tartaric acid in the solid state and via slurry show that co-former exchange takes place according to the sequence of stability [(R,S)-ta]2·py > (S,S/R,R)-ta·py > (R,R)-ta·py or (S,S)-ta·py

Mechanochemical preparation of adducts (co-crystals and molecular salts) of 1, 4-diazabicyclo-2.2. 2-octane with aromatic polycarboxylic acids

Solid-state adducts (co-crystals and molecular salts) of 1,4-diazabicyclo-[2.2.2]-octane (DABCO) with aromatic polycarboxylic acids (isophthalic acid, isoH2, dinicotinic acid, dinH2 and dipicolinic acid, dipH2) were prepared in the solid state by grinding and kneading techniques, and fully characterized via X-ray diffraction. The polycarboxylic acids differ for the presence/absence and position of a nitrogen atom in the aromatic ring; the extent of proton transfer, from the carboxylic groups on the acids to the nitrogen atoms on DABCO, reflects the trend of solution acidity of the three polycarboxylic acids

Direct observation of intermediates in a thermodynamically controlled solid-state dynamic covalent reaction.

We present the first polymorph interconversion study that uses solid-state dynamic covalent chemistry (DCC). This system exhibits unexpected and rich behavior, including the observation that under appropriate conditions the polymorph interconversion of a heterodimer proceeds through reversible covalent chemistry intermediates, and this route is facilitated by one of the two disulfide homodimers involved in the reaction. Furthermore, we demonstrate experimentally that in all cases a dynamic equilibrium is reached, meaning that changing the milling conditions affects the free energy difference between the two polymorphs and thus their relative stability. We suggest that this effect is due to the surface solvation energy combined with the high surface to volume ratio of the nanocrystalline powder.We are grateful to the EPSRC (AMB, JKMS and DJW) and ERC(DJW) for financial support.This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/ja500707z

Thermal Behavior of Iron Arsenides Under Non-Oxidizing Conditions.

Fe2As has been studied in situ by synchrotron powder X-ray diffraction (PXRD) over the range of temperatures 25-850 °C and under a neutral atmosphere to understand its thermal behavior, which is potentially important for gold extraction. For the first time, incongruent high-temperature reactions of Fe2As are observed as it breaks down and the existence of a previously undiscovered high-temperature FeAs phase with an NiAs-type structure has been determined experimentally. No evidence has been found for the existence of the high-temperature Fe3As2 phase. Hence, the previously published phase diagram for the Fe-As system has to be modified accordingly

Thermal behaviour of iron arsenides under non-oxidising conditions

Fe2As has been studied in situ by synchrotron powder X-ray diffraction (PXRD) over the range of temperatures 25–850 °C and under a neutral atmosphere to understand its thermal behavior, which is potentially important for gold extraction. For the first time, incongruent high-temperature reactions of Fe2As are observed as it breaks down and the existence of a previously undiscovered high-temperature FeAs phase with an NiAs-type structure has been determined experimentally. No evidence has been found for the existence of the high-temperature Fe3As2 phase. Hence, the previously published phase diagram for the Fe–As system has to be modified accordingly

Pressure-induced oversaturation and phase transition in zeolitic imidazolate frameworks with remarkable mechanical stability.

Zeolitic imidazolate frameworks (ZIFs) 7 and 9 are excellent candidates for CO2 adsorption and storage. Here, high-pressure X-ray diffraction is used to further understand their potential in realistic industrial applications. ZIF-7 and ZIF-9 are shown be able to withstand high hydrostatic pressures whilst retaining their porosity and structural integrity through a new ferroelastic phase transition. This stability is attributed to the presence of sterically large organic ligands. Results confirm the notable influence of guest occupancy on the response of ZIFs to pressure; oversaturation of ZIFs with solvent molecules greatly decreases their compressibility and increases their resistance to amorphisation. By comparing the behaviours of both ZIFs under high pressure, it is demonstrated that their mechanical stability is not affected by metal substitution. The evacuated ZIF-7 phase, ZIF-7-II, is shown to be able to recover to the ZIF-7 structure with excellent resistance to pressure. Examining the pressure-related structural behaviours of ZIF-7 and ZIF-9, we have assessed the great industrial potential of ZIFs.This work was supported by the Cambridge Commonwealth, European and International Trust; China Scholarship Council; Trinity Hall, University of Cambridge; UK Science & Technology Facilities Council. We thank anonymous reviewers for their valuable suggestions on the improvement of this manuscript.This is the final version of the article. It first appeared from the Royal Society of Chemistry via http://dx.doi.org/10.1039/C4DT02680

Mechanism of powellite crystallite expansion within nano-phase separated amorphous matrices under Au-irradiation

A fundamental approach was taken to understand the implications of increased nuclear waste loading inthe search for new materials for long-term radioisotope encapsulation. This study focused on theformation and radiation tolerance of glass ceramics with selectively induced CaMoO4as a form to trapthe problematic fission product molybdenum. Several samples were synthesised with up to 10 mol%MoO3within a soda lime borosilicate matrix, exhibiting phase separation on the nano scale according tothermal analysis, which detected two glass transition temperatures. It is predicted that these two phasesare a result of spinodal decomposition with Si–O–Ca–O–Si and Si–O–Ca–O–B units, with the latterphase acting as a carrier for MoO3. The solubility limit of molybdenum within this matrix was 1 mol%,after which crystallisation of CaMoO4occurred, with crystallite size (CS) increasing and cell parametersdecreasing as a function of [MoO3]. These materials were then subjected to irradiation with 7 MeV Au3+ions to replicate the nuclear interactions resulting froma-decay. A dose of 3�1014ions per cm2wasachieved, resulting in 1 dpa of damage within a depth ofB1.5mm, according to TRIM calculations.Glasses and glass ceramics were then analysed using BSE imaging, XRD refinement, and Raman spectro-scopy to monitor changes induced by accumulated damage. Irradiation was not observed to cause anysignificant changes to the residual amorphous network, nor did it cause amorphisation of CaMoO4based on the relative changes to particle size and density. Furthermore, the substitution of Ca2+toform water-soluble Na2/NaGd–MoO4assemblages did not occur, indicating that CaMoO4is resilientto chemical modification following ion interactions. Au-irradiation did however cause CaMoO4latticeparameter expansion, concurrent to growth in CS. This is predicted to be a dual parameter mechanismof alteration based on thermal expansion from electronic coupling, and the accumulation of defectsarising from atomic displacements

Magnetoelastic coupling behavior at the ferromagnetic transition in the partially disordered double perovskite La2NiMnO6

The magnetocapacitance and magnetoresistance properties near room temperature of partially disordered double perovskite L a 2 NiMn O 6 are related, at least in part, to coupled ferroelastic and magnetic instabilities that are responsible for a ferromagnetic phase transition near 280 K. A systematic analysis of this coupling from the perspectives of strain and elasticity has revealed a system with biquadratic coupling among three order parameters belonging to irreducible representations of X + 3 , Γ + 4 and m Γ + 4 of the parent space group F m ¯ 3 m . Classical octahedral tilting drives the structural transitions at high temperatures and strong acoustic attenuation through the temperature interval ∼300–500 K, observed by resonant ultrasound spectroscopy from a polycrystalline sample, is consistent with pinning of ferroelastic twin walls by point defects. Below room temperature, stiffening of the shear modulus by up to ∼40% can be understood in terms of biquadratic coupling of the ferromagnetic order parameter with strain. Acoustic attenuation with Debye-like patterns of loss in the temperature interval ∼150–280 K yielded activation energies and relaxation times which match up with AC magnetic and dielectric spectroscopy data reported previously in the literature. The dynamic loss mechanism, perhaps related to hopping of electrons between N i 2 + and M n 4 + , is potentially multiferroic, therefore. In addition to the possibilities for tailoring the intrinsic properties of L a 2 NiMn O 6 by controlling oxygen content, B -site order or by choice of substrate for imposing a strain on thin films, it should be possible also to engineer extrinsic properties which would respond to applied electric, magnetic, and stress fields