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.

Defect Trapping and Phase Separation in Chemically Doped Bulk AgF2.

We report a computational survey of chemical doping of silver(II) fluoride, which has recently attracted attention as an analogue of La2CuO4-a known precursor of high-temperature superconductors. By introducing fluorine defects (vacancies or interstitial adatoms) into the crystal structure, we obtain nonstoichiometric, electron- and hole-doped polymorphs of AgF2±x. We find that the ground-state solutions show a strong tendency for localization of defects and of the associated electronic states, and the resulting doped phases exhibit insulating or semiconducting properties. Furthermore, the distribution of Ag(I)/Ag(III) sites which appear in the crystal structure points to the propensity of the AgF2 system for phase separation upon chemical doping, which is in line with observations from previous experimental attempts. Overall, our results indicate that chemical modification may not be a feasible way to achieve doping in bulk silver(II) fluoride, which is considered essential for the emergence of high-Tc superconductivity.

Gigantic work function in layered AgF2.

AgF2 is a layered material and a correlated charge transfer insulator with an electronic structure very similar to the parent compounds of cuprate high-TC superconductors. It is also interesting as it is a powerful oxidizer. Here we present a first principles computation of its electronic properties in a slab geometry including its work function for the (010) surface (7.76 eV) which appears to be the highest among known materials with non-dipolar surfaces, and surpassing even that of fluorinated diamond (7.24 eV). We demonstrate that AgF2 will show a "broken-gap" type alignment becoming electron doped and promoting injection of holes in many wide band gap insulators if chemical reaction can be avoided. Novel junction devices involving p type and n type superconductors have been proposed. The issue of chemical reaction is discussed in connection with the possibility to create flat AgF2 monolayers achieving high-TC superconductivity. As a first step in this direction, we studied the stability and properties of an isolated AgF2 monolayer.

[Ag(OH2 )2 ][Ag(SO4 )2 ]: A Hydrate of a Silver(II) Salt.

When exposed to air at ambient conditions, AgSO4 slowly reacts with moisture, yielding AgSO4 ⋅H2 O. The crystal structure determination (powder data) shows that it may be described as [Ag(OH2 )2 ][Ag(SO4 )2 ], with some sulfate groups being shared between different Ag2+ cations, resembling in that way its Cu2+ analogue. [Ag(OH2 )2 ][Ag(SO4 )2 ], the first hydrate of a compound of Ag2+ , was extensively characterized using many physicochemical methods.

Persistence of Mixed and Non-intermediate Valence in the High-Pressure Structure of Silver(I,III) Oxide, AgO: A Combined Raman, X-ray Diffraction (XRD), and Density Functional Theory (DFT) Study.

The X-ray diffraction data collected up to ca. 56 GPa and the Raman spectra measured up to 74.8 GPa for AgO, or AgIAgIIIO2, which is a prototypical mixed valence (disproportionated) oxide, indicate that two consecutive phase transitions occur: the first-order phase transition occurs between 16.1 GPa and 19.7 GPa, and a second-order phase transition occurs at ca. 40 GPa. All polymorphic forms host the square planar [AgIIIO4] units typical of low-spin AgIII. The disproportionated Imma form persists at least up to 74.8 GPa, as indicated by Raman spectra. Theoretical hybrid density functional theory (DFT) calculations show that the first-order transition is phonon-driven. AgO stubbornly remains disproportionated up to at least 100 GPa-in striking contrast to its copper analogue-and the fundamental band gap of AgO is ∼0.3 eV at this pressure and is weakly pressure-dependent. Metallization of AgO is yet to be achieved.

Canted Antiferromagnetism in Two-Dimensional Silver(II) Bis[pentafluoridooxidotungstate(VI)].

By slow reaction between colorless AgIW2O2F9 and elemental F2 in liquid anhydrous HF, violet platelike single crystals of Ag(WOF5)2 were grown. The crystal structure of Ag(WOF5)2 consists of layers built from Ag2+ cations bridged by [WOF5]- anions and not, as previously assumed, from infinite [AgII-F]+∞ chains and [W2O2F9]- anions. A majority (97%) of the disordered AgII cations are found with square-planar coordination of F/O ligands within the same layer, and they form additional long contacts with O/F atoms originating from the neighboring layers. The remaining 3% the of Ag(II) ions are coordinated only by F atoms in a square-planar fashion. The magnetic moments of Ag2+ from the same layer are almost perfectly antiferromagnetically aligned. Weak ferromagnetic interlayer interactions cause a small tilt (∼1.5°) of the magnetic moments, resulting in canted antiferromagnetism. Because of the lowering of the symmetry of [WOF5]- in the solid state, the vibrational spectra show more bands than expected for regular C4v symmetry. The electronic spectrum of Ag(WOF5)2 is reported and analyzed.

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.

AuO: Evolving from Dis- to Comproportionation and Back Again.

The structural, electronic, and dynamic properties of hypothetical gold(II) oxide (AuO) are studied theoretically, at atmospheric and elevated pressures, with the use of hybrid density functional theory. At p = 1 atm, hypothetical AuO (metastable with respect to the elements) is predicted to crystallize in a new structure type, unique among the late-transition-metal monoxides, with disproportionation of the Au ions to Au(I/III) and featuring aurophilic interactions. Under pressure, familiar structure types are stabilized: a semiconducting AgO-type structure at ∼2.5 GPa and, with a further increase of the pressure up to ∼80 GPa, an AuSO4-type structure containing Au2 pairs. Finally, above 105 GPa, distorted NaCl- and CsCl-type Au(II)O structures dominate, and metallization is predicted at 329 GPa.

Comment on "Pressure-induced structural and valence transition in AgO" by C. Hou, J. Botana, X. Zhang, X. Wang and M. Miao, Phys. Chem. Chem. Phys., 2016, 18, 15322.

The recent article by Hou et al. has focused on a theoretical study of mixed valence compound AgO in order to elucidate the nature of the electronic structure of this system as a function of external pressure. The authors claim that '…the effects of pressure on the Ag valence state are not investigated yet.' This statement is incorrect in view of the theoretical study published in 2015 by Wlodarska et al., which answers most of the questions posed by these authors, including the key one: '…does the Ag cation exhibit similar behaviour to the Au cation in M2Au2X6halides, experiencing mixed-valence to single-valence transition under pressure? If so, what is the mechanism?'.

Structures of late transition metal monoxides from Jahn-Teller instabilities in the rock salt lattice.

Most late transition metal (LTM) monoxides crystallize in other than a rock salt structure, which is so common in the earlier transition metal monoxides. Here we present theoretical evidence based on density functional theory that an electron-phonon coupling involving a single soft mode in the cubic cell is responsible for the onset of the experimentally observed structures of the late transition metal monoxides.

Theoretical study of ternary silver fluorides AgMF4 (M = Cu, Ni, Co) formation at pressures up to 20 GPa.

Only several compounds bearing the Ag(ii) cation and other paramagnetic transition metal cations are known experimentally. Herein, we predict in silico stability and crystal structures of hypothetical ternary silver(ii) fluorides with copper, nickel and cobalt in 1 : 1 stoichiometry at a pressure range from 0 GPa up to 20 GPa employing the evolutionary algorithm in combination with DFT calculations. The calculations show that AgCoF4 could be synthesized already at ambient conditions but this compound would host diamagnetic Ag(i) and high-spin Co(iii). Although none of the compounds bearing Ag(ii) could be preferred over binary substrates at ambient conditions, at increased pressure ternary fluorides of Ag(ii) featuring Cu(ii) and Ni(ii) could be synthesized, in the pressure windows of 7-14 and 8-15 GPa, respectively. All title compounds would be semiconducting and demonstrate magnetic ordering. Compounds featuring Ni(ii) and particularly Co(ii) should exhibit fundamental band gaps much reduced with respect to pristine AgF2. The presence of Cu(ii) and Ni(ii) does not lead to electronic doping to AgF2 layers, while Co(ii) tends to reduce Ag(ii) entirely to Ag(i).

Ag6Cl4: the first silver chloride with rare Ag6 clusters from an ab initio study.

The first diamagnetic semiconducting silver subhalide, Ag6Cl4, featuring rare subvalent Ag6 clusters with 2e--6c bonds, 1D argentophilic d10-d10 intercluster interactions and 3D ionic connectivity ensured by Cl atoms has been predicted employing Density Functional Theory. Ag6Cl4 carries all unique features of Ag+ so far observed only in selected metal-rich oxides and as such represents an important addition to the discussion of the special bonding properties of silver with a filled d10-shell. Having appreciable formation enthalpy and dynamical stability, Ag6Cl4 should be in principle possible to synthesize as a metastable phase relative to AgCl.

The theoretical quest for sulfate of Ag(2+): genuine Ag(II)SO4, diamagnetic Ag(I)2S2O8, or rather mixed-valence Ag(I)[Ag(III)(SO4)2]?

In an attempt to extend the chemistry of Ag(II) compounds to novel ligand environments, we have examined various polymorphs of an as yet unknown compound of AgSO(4) stoichiometry by means of a quantum chemistry DFT approach. Since AgSO(4) has not yet been prepared, we were interested whether genuine divalent silver, Ag(II), could be stabilized in the presence of sulfate (SO(4)(2-)) anions or whether it would rather have a tendency toward disproportionation to Ag(I)/Ag(III) (as known from binary oxide, "AgO") or maybe toward the formation of peroxodisulfate (S(2)O(8)(2-)) of Ag(I). Considering all important electromeric forms, Ag(II)SO(4), Ag(I)Ag(III)(SO(4))(2), and Ag(I)(2)S(2)O(8), in a variety of crystal structures, we examined their dynamical (vibrational) stability and discussed the most stable phases from the point of view of structural features thought to be crucial for appearance of high-T(C) superconductivity. We show that a compound of a nominal stoichiometry, AgSO(4), should most likely be a genuine paramagnetic sulfate of divalent silver which forms 3D ...Ag-O-S(O(2))-O... networks, and--unlike CuSO(4)--shows no direct connections of paramagnetic Ag(II) centers via Ag-O bridges. This must lead to low electronic dimensionality and concomitant low potential for the appearance of high-T(C) superconductivity. Finally, we discuss structural features of the most stable phase of AgSO(4) predicted here in a broader context of crystal structures of group 11 metals.

Structural polymorphism of pyrazinium hydrogen sulfate: extending chemistry of the pyrazinium salts with small anions.

Two polymorphs (alpha, beta) of pyrazinium hydrogen sulfate (pyzH(+)HSO(4)(-), abbreviated as PHS) with distinctly different hydrogen-bond types and topologies but close electronic energies have been synthesized and characterized for the first time. The alpha-polymorph (P2(1)2(1)2(1)) forms distinct blocks in which the pyzH(+) and HSO(4)(-) ions are interconnected through a network of NH...O and OH...O hydrogen bonds. The beta-form (P1) consists of infinite chains of alternating pyzH(+) and HSO(4)(-) ions connected by NH...O and OH...N hydrogen bonds. Density functional theory (DFT) calculations indicate the possible existence of a hypothetical polar P1 form of the beta-polymorph with an unusually high dipole moment.

High-temperature superconductivity as viewed from the maximum hardness principle.

The Maximum Hardness Principle - and its reformulation by Chattaraj as the Minimum Polarizability Principle - is an immensely useful concept which works in support of a chemical intuition. As we show here, it may also be used to rationalize the scarcity of high-temperature superconductors, which stems - inter alia - from rarity of high-density of state metals in Nature. It is suggested that the high-temperature oxocuprate superconductors as well as their iron analogues - are energetically metastable at T ➔ 0 K and p ➔ 0 atm conditions, and their tendency for disproportionation is hindered only by the substantial rigidity of the crystal lattice, while the phase separation and/or superstructure formation is frequently observed in these systems. This hypothesis is corroborated by hybrid density functional theory theoretical calculations for Na- (thus: hole) or La- (thus: electron) doped CaCu(II)O2 precursor. Non-equilibrium synthetic methods are suggested to be necessary for fabrication of high-temperature superconductors of any sort. Graphical abstract Doped oxocuprate superconductors are shown to be unstable with respect to phase separation (disproportionation) in accordance with the Maximum Hardness Principle; their metastability is mostly due to rigidity of [CuO2] sheets and preparation using high-temperature conditions.

Structural and electronic properties of Fe monolayer on BaTiO3(0 0 1).

The structural, electronic, and phonon properties of the BaTiO3(0 0 1) surface and the Fe/BaTiO3(0 0 1) interface have been studied within the density functional theory. Attention is paid to the lattice instabilities (soft phonon modes) that induce ferroelectric distortions in the surface and the interface. A phonon-induced monoclinic (Cm) thin-film counterpart of the low-temperature rhombohedral (R3m) ferroelectric bulk BaTiO3 phase is found. The changes in crystal structure, electronic density of states, atomic charges, and magnetic moments associated with the ferroelectric distortions are discussed comparing the results of the standard GGA and the hybrid DFT calculations. The magnetoelectric coupling at the Fe/BaTiO3(0 0 1) interface is investigated by the analysis of changes in magnetic moments on Fe and Ti atoms induced by the atomic displacements perpendicular and parallel to the surface.

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).

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.