High-pressure stabilization of open-shell bromine fluorides.

Halogen fluorides are textbook examples of how fundamental chemical concepts, such as molecular orbital theory or the valence-shell electron-repulsion (VSEPR) model, can be used to understand the geometry and properties of compounds. However, it is still an open question whether these notions are applicable to matter subject to high pressure (>1 GPa). In an attempt to gain insight into this phenomenon, we present a computational study on the phase transitions and reactivity of bromine fluorides at pressures of up to 100 GPa (≈106 atm). We predict that at a moderately high pressure of 15 GPa, the bonding preference in the Br/F system should change considerably with BrF3 becoming thermodynamically unstable and two novel compounds emerging as stable species: BrF2 and BrF6. Calculations indicate that both these compounds contain radical molecules while being non-metallic. We propose a synthetic route for obtaining BrF2 which does not require the use of highly reactive elemental fluorine. Finally, we show how molecular orbital diagrams and the VSEPR model can be used to explain the properties of compressed bromine fluorides.

Silver route to cuprate analogs.

The parent compound of high-[Formula: see text] superconducting cuprates is a unique Mott insulator consisting of layers of spin-[Formula: see text] ions forming a square lattice and with a record high in-plane antiferromagnetic coupling. Compounds with similar characteristics have long been searched for without success. Here, we use a combination of experimental and theoretical tools to show that commercial [Formula: see text] is an excellent cuprate analog with remarkably similar electronic parameters to [Formula: see text] but larger buckling of planes. Two-magnon Raman scattering and inelastic neutron scattering reveal a superexchange constant reaching 70% of that of a typical cuprate. We argue that structures that reduce or eliminate the buckling of the [Formula: see text] planes could have an antiferromagnetic coupling that matches or surpasses the cuprates.

Fluorides of Silver Under Large Compression*.

The silver-fluorine phase diagram has been scrutinized as a function of external pressure using theoretical methods. Our results indicate that two novel stoichiometries containing Ag+ and Ag2+ cations (Ag3 F4 and Ag2 F3 ) are thermodynamically stable at ambient and low pressure. Both are computed to be magnetic semiconductors under ambient pressure conditions. For Ag2 F5 , containing both Ag2+ and Ag3+ , we find that strong 1D antiferromagnetic coupling is retained throughout the pressure-induced phase transition sequence up to 65 GPa. Our calculations show that throughout the entire pressure range of their stability the mixed-valence fluorides preserve a finite band gap at the Fermi level. We also confirm the possibility of synthesizing AgF4 as a paramagnetic compound at high pressure. Our results indicate that this compound is metallic in its thermodynamic stability region. Finally, we present general considerations on the thermodynamic stability of mixed-valence compounds of silver at high pressure.

Hydrogen-Bonded Cyclic Dimers at Large Compression: The Case of 1H-pyrrolo[3,2-h]quinoline and 2-(2'-pyridyl)pyrrole.

1H-pyrrolo[3,2-h]qinoline (PQ) and 2-(2'-pyridyl)pyrrole (PP) are important systems in the study of proton-transfer reactions. These molecules possess hydrogen bond donor (pyrrole) and acceptor (pyridine) groups, which leads to the formation of cyclic dimers in their crystals. Herein, we present a joint experimental (Raman scattering) and computational (DFT modelling) study on the high-pressure behaviour of PQ and PP molecular crystals. Our results indicate that compression up to 10 GPa (100 kbar) leads to considerable strengthening of the intermolecular hydrogen bond within the cyclic dimers. However, the intramolecular N-H∙∙∙N interaction is either weakly affected by pressure, as witnessed in PQ, or weakened due to compression-induced distortions of the molecule, as was found for PP. Therefore, we propose that the compression of these systems should facilitate double proton transfer within the cyclic dimers of PQ and PP, while intramolecular transfer should either remain unaffected (for PQ) or weakened (for PP).

High-Pressure Phase Transitions of Zinc Difluoride up to 55 GPa.

Studying the effect of high pressure (exceeding 10 kbar) on the structure of solids allows us to gain deeper insight into the mechanism governing crystal structure stability. Here, we report a study on the high-pressure behavior of zinc difluoride (ZnF2)-an archetypical ionic compound which at ambient pressure adopts the rutile (TiO2) structure. Previous investigations, limited to a pressure of 15 GPa, revealed that this compound undergoes two pressure-induced phase transitions, i.e., TiO2 → CaCl2 at 4.5 GPa and CaCl2 → HP-PdF2 at 10 GPa. Within this joint experimental-theoretical study, we extend the room-temperature phase diagram of ZnF2 up to 55 GPa. By means of Raman spectroscopy measurements we identify two new phase transitions, HP-PdF2 → HP1-AgF2 at 30 GPa and HP1-AgF2 → PbCl2 at 44 GPa. These results are confirmed by density functional theory calculations which indicate that in the HP1-AgF2 polymorph the coordination sphere of Zn2+ undergoes drastic changes upon compression. Our results point to important differences in the high-pressure behavior of ZnF2 and MgF2, despite the fact that both compounds contain cations of similar size. We also argue that the HP1-AgF2 structure, previously observed only for AgF2, might be observed at large compression in other AB2 compounds.

The contamination of inland waters by microplastic fibres under different anthropogenic pressure: Preliminary study in Central Europe (Poland).

The contamination of freshwater ecosystems with microfibres has not yet been studied in Poland. We analysed samples from a river and three lakes located in central and northeastern Poland. A significantly higher number of fibres were reported in the river, which runs through large cities, compared with the lake situated within the Landscape Park. Fibres smaller than 1.0 mm dominated, especially in the river where they constituted 39% of all fibres detected. We found more microplastics (⩽ 4930 fibres·m-3) by using a mesh size of 20 µm compared with other studies of inland waters. The use of Raman spectroscopy allowed us to identify conventional plastic polymers: polyethylene terephthalate, polyester and polyurethane. We estimated that up to 25 g of microplastic in the form of fibres might be in the lake water under the surface. We found microplastic fibres in Majcz Lake situated within the Masurian Landscape Park. This suggests that microfibres are carried by the wind and rain and enter freshwater isolated from sewage outlets. By using the control sample and an air-test of microfibres in the laboratory, we observed that there is a high probability of contamination with microplastic in the field samples (up to 30% of all fibres detected). The contamination risk noted from the samples cannot be ignored; this could be particularly important for analysis of microplastic in remote freshwater ecosystems.

High-Pressure Behavior of Silver Fluorides up to 40 GPa.

A combined experimental-theoretical study of silver(I) and silver(II) fluorides under high pressure is reported. For AgI, the CsCl-type structure is stable to at least 39 GPa; the overtone of the IR-active mode is seen in the Raman spectrum. Its AgIIF2 sibling is a unique compound in many ways: it is more covalent than other known difluorides, crystallizes in a layered structure, and is enormously reactive. Using X-ray diffraction and guided by theoretical calculations (density functional theory), we have been able to elucidate crystal structures of high-pressure polymorphs of AgF2. The transition from ambient pressure to an unprecedented nanotubular structure takes place via an intermediate orthorhombic layered structure, which lacks an inversion center. The observed phase transitions are discussed within the broader framework of the fluorite → cotunnite → Ni2In series, which has been seen for other metal difluorides.

Prediction of Extremely Strong Antiferromagnetic Superexchange in Silver(II) Fluorides: Challenging the Oxocuprates(II).

Strong magnetic coupling between the spins of unpaired electrons is an essential ingredient of many fascinating physical phenomena. Here we report calculations using the hybrid HSE06 functional of magnetic superexchange constants, J, for a series of low-dimensional CuII and AgII binary and ternary systems with fluoride and oxide ligands. The calculations correctly reproduce the sign and size of the magnetic superexchange constants for prototypical antiferromagnetic (AFM) 1D (J1D ) and 2D (J2D ) systems, while overestimating the absolute values of J by about 11 %. We find that [AgF][BF4 ], a quasi-1D system with linear infinite [AgII F+ ] chains, is predicted to exhibit an unprecedented strong AFM superexchange via one atom (F), with J1D about -300 meV. Compression of [AgF][BF4 ] to 10 GPa should lead to a further increase in AFM interactions with J1D reaching -360 meV at 10 GPa.

Local and Cooperative Jahn-Teller Effect and Resultant Magnetic Properties of M2AgF4 (M = Na-Cs) Phases.

The crystal structure, magnetic properties, heat capacity, and Raman spectra of double-perovskite M2AgF4 (M = K, K3/4Rb1/4, K1/2Rb1/2, K1/4Rb3/4, and Rb) phases have been examined, adding to the body of previous results for the M = Na, Cs derivatives. The results suggest that double-perovskite K2AgF4 adopts a disordered orthorhombic Bmab structure with an antiferrodistortive arrangement of the elongated and tilted [AgF6] octahedra rather than the structure with the ferrodistortive arrangement of compressed octahedra, as suggested previously (Mazej, Z.; Goreshnik, E.; Jaglicic, Z.; Gawel, B.; Lasocha, W.; Grzybowska, D.; Jaron, T.; Kurzydlowski, D.; Malinowski, P. J.; Kozminski, W.; Szydlowska, J.; Leszczynski, P. J.; Grochala, W. KAgF3, K2AgF4 and K3Ag2F7: important steps towards a layered antiferromagnetic fluoroargentate(II). CrystEngComm 2009, 11, 1702-1710). A re-examination of the previously collected single-crystal X-ray diffraction data confirms the current structure assignment, and it is also in agreement with recent theoretical calculations. High-field electron paramagnetic resonance spectra reaffirm the presence of elongated [AgF6] octahedra in the crystal structure of all M2AgF4 phases studied. The local structure of the M = K derivative is most complex, with regions of the sample that are quite orthorhombically distorted, whereas other regions more closely resemble the tetragonal phase. The mixed-cation K/Rb phases are also inhomogeneous, containing regions of the pure K compound and regions of another high-symmetry phase (likely tetragonal) of a mixed (Rb-richer) compound with unknown composition. The temperature-resolved phase diagram of all K/Rb phases has been established and positioned within the entire M = Na, K, Rb, Cs series.

High-pressure stabilization of argon fluorides.

On account of the rapid development of noble gas chemistry in the past half-century both xenon and krypton compounds can now be isolated in macroscopic quantities. The same does not hold true for the next lighter group 18 element, argon, which forms only isolated molecules stable solely in low temperature matrices or supersonic jet streams. Here we present theoretical investigations into a new high-pressure reaction pathway, which enables synthesis of argon fluorides in bulk and at room temperature. Our hybrid DFT calculations (employing the HSE06 functional) indicate that above 60 GPa ArF2-containing molecular crystals can be obtained by a reaction between argon and molecular fluorine.

Potential energy barrier for proton transfer in compressed benzoic acid.

Benzoic acid (BA) is a model system for studying proton transfer (PT) reactions. The properties of solid BA subject to high pressure (exceeding 1 kbar = 0.1 GPa) are of particular interest due to the possibility of compression-tuning of the PT barrier. Here we present simulations aimed at evaluating the value of this barrier in solid BA in the 1 atm - 15 GPa pressure range. We find that pressure-induced shortening of O⋯O contacts within the BA dimers leads to a decrease in the PT barrier, and subsequent symmetrization of the hydrogen bond. However, this effect is obtained only after taking into account zero-point energy (ZPE) differences between BA tautomers and the transition state. The obtained results shed light on previous experiments on compressed benzoic acid, and indicate that a common scaling behavior with respect to the O⋯O distance might be applicable for hydrogen-bond symmetrization in both organic and inorganic systems.

Photoelectrochemical Behavior of WO3 in an Aqueous Methanesulfonic Acid Electrolyte.

N-type semiconducting WO3 is widely investigated as a photoanode operating in water and seawater splitting devices. Because of the propensity of WO3 to favor photo-oxidation of acidic electrolyte anions and, in parallel, the formation on the electrode surface of the peroxo species, the choice of the appropriate electrolyte to allow stable operation of the photoanode is of critical importance. Our results from structural and photoelectrochemical tests performed using mesoporous WO3 photoanodes exposed to 80 h long photoelectrolysis in a 1 M aq. methanesulfonic acid supporting electrolyte demonstrate the photostability of both the WO3 photomaterial and the CH3SO3H electrolyte. The reasons for the stability of aqueous solutions of CH3SO3H are discussed on the basis of earlier literature reports.

Freezing in resonance structures for better packing: XeF2 becomes (XeF+)(F-) at large compression.

Recent high-pressure experiments conducted on xenon difluoride (XeF(2)) suggested that this compound undergoes several phase transitions up to 100 GPa, becoming metallic above 70 GPa. In this theoretical study, in contrast to experiment, we find that the ambient pressure molecular structure of xenon difluoride, of I4/mmm symmetry, remains the most stable one up to 105 GPa. In our computations, the structures suggested from experiment have either much higher enthalpies than the I4/mmm structure or converge to that structure upon geometry optimization. We discuss these discrepancies between experiment and calculation and point to an alternative interpretation of the measured cell vectors of XeF(2) at high pressure. At pressures exceeding those studied experimentally, above 105 GPa, the I4/mmm structure transforms to one of Pnma symmetry. The Pnma phase contains bent FXeF molecules, with unequal Xe-F distances, and begins to bring other fluorines into the coordination sphere of the Xe. Further compression of this structure up to 200 GPa essentially results in self-dissociation of XeF(2) into an ionic solid (i.e., [XeF](+)F(-)), similar to what is observed for nitrous oxide (N(2)O) at high pressure.

High-pressure phase transition of AB3-type compounds: case of tellurium trioxide.

Tellurium trioxide, TeO3, is the only example of a trioxide adopting at ambient conditions the VF3-type structure (a distorted variant of the cubic ReO3 structure). Here we present a combined experimental (Raman scattering) and theoretical (DFT modelling) study on the influence of high pressure (exceeding 100 GPa) on the phase stability of this compound. In experiments the ambient-pressure VF3-type structure (R3̄c symmetry) is preserved up to 110 GPa. In contrast, calculations indicate that above 66 GPa the R3̄c structure should transform to a YF3-type polymorph (Pnma symmetry) with the coordination number of Te6+ increasing from 6 to 8 upon the transition. The lack of this transition in the room-temperature experiment is most probably connected with energetic barriers, in analogy to what is found for compressed WO3. The YF3-type phase is predicted to be stable up to 220 GPa when it should transform to a novel structure of R3̄ symmetry and Z = 18. We analyse the influence of pressure on the band gap of TeO3, and discuss the present findings in the context of structural transformations of trioxides and trifluorides adopting an extended structure in the solid state.

Unexpected persistence of cis-bridged chains in compressed AuF3.

Raman scattering measurements indicate that cis-bridged chains are retained in AuF3 even at a compression of 45 GPa - in contrast to meta-GGA calculations suggesting that structures with such motifs are thermodynamically unstable above 4 GPa. This metastability implies that novel gold fluorides (e.g. AuF2) might be attainable at lower pressures than previously proposed.

Elusive AuF in the solid state as accessed via high pressure comproportionation.

Density Functional Theory (DFT) calculations indicate that AuF might be synthesized at 22.6 GPa from AuF3 and Au (1 : 2), and subsequently quenched down to at least 5 GPa in the Cmcm (bent chain) structure.

Dramatic enhancement of spin-spin coupling and quenching of magnetic dimensionality in compressed silver difluoride.

Meta-GGA calculations of the ambient and high-pressure polymorphs of silver difluoride indicate that the compression-induced structural changes lead to a 3.5-fold increase in the strength of antiferromagnetic spin-spin interactions resulting in coupling constant values higher than those found for record-holding oxocuprates(ii).

Hexacoordinated nitrogen(V) stabilized by high pressure.

In all of its known connections nitrogen retains a valence shell electron count of eight therefore satisfying the golden rule of chemistry - the octet rule. Despite the diversity of nitrogen chemistry (with oxidation states ranging from + 5 to -3), and despite numerous efforts, compounds containing nitrogen with a higher electron count (hypervalent nitrogen) remain elusive and are yet to be synthesized. One possible route leading to nitrogen's hypervalency is the formation of a chemical moiety containing pentavalent nitrogen atoms coordinated by more than four substituents. Here, we present theoretical evidence that a salt containing hexacoordinated nitrogen(V), in the form of an NF6- anion, could be synthesized at a modest pressure of 40 GPa (=400 kbar) via spontaneous oxidation of NF3 by F2. Our results indicate that the synthesis of a new class of compounds containing hypervalent nitrogen is within reach of current high-pressure experimental techniques.

Crystal, electronic, and magnetic structures of M2AgF4 (M = Na-Cs) phases as viewed from the DFT+U method.

Theoretical investigations of the magneto-structural correlations of M2AgF4 (M = Na-Cs) compounds show that they adopt two polymorphs, the layered perovskite and post-perovskite structures, which differ greatly in the connectivity of the Ag/F sub-lattice and hence in their magnetic properties. With the use of the DFT+U method, the relative stabilities of various M2AgF4 phases were established and the collective JT effect within the Ag/F sub-lattice of these systems was modelled. Calculations show that for all studied stoichiometries, the preferred scenario of the collective JT effect in the layered perovskite phase corresponds to an antiferrodistortive order of elongated octahedra, which leads to 2D ferromagnetic coupling, in agreement with the experimental findings for the M = Cs, and Rb systems. The layered perovskite phase is found to be progressively destabilized with respect to the post-perovskite structure when moving from Cs to Na, again in agreement with the experimental findings. Our results strongly indicate that the layered polymorph of K2AgF4 should not exhibit a ferrodistortive order of compressed octahedra, which contradicts the previous experimental results.

AgPO2F2 and Ag9(PO2F2)14: the first Ag(i) and Ag(i)/Ag(ii) difluorophosphates with complex crystal structures.

The reaction of AgF2 with P2O3F4 yields a mixed valence Ag(I)/Ag(II) difluorophosphate salt with AgAg(PO2F2)14 stoichiometry - the first Ag(ii)-PO2F2 system known. This highly moisture sensitive brown solid is thermally stable up to 120 °C, which points at further feasible extension of the chemistry of Ag(ii)-PO2F2 systems. The crystal structure shows a very complex bonding pattern, comprising of polymeric Ag(PO2F2)14(4-) anions and two types of Ag(I) cations. One particular Ag(II) site present in the crystal structure of Ag9(PO2F2)14 is the first known example of square pyramidal penta-coordinated Ag(ii) in an oxo-ligand environment. Ag(i)PO2F2 - the product of the thermal decomposition of Ag9(PO2F2)14 - has also been characterized by thermal analysis, IR spectroscopy and X-ray powder diffraction. It has a complicated crystal structure as well, which consists of infinite 1D [Ag(I)O4/2] chains which are linked to more complex 3D structures via OPO bridges. The PO2F2(-) anions bind to cations in both compounds as bidentate oxo-ligands. The terminal F atoms tend to point inside the van der Waals cavities in the crystal structure of both compounds. All important structural details of both title compounds were corroborated by DFT calculations.