Dominik Kurzydłowski

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.

KAgF3, K2AgF4 and K3Ag2F7: important steps towards a layered antiferromagnetic fluoroargentate(II),

Crystal structure and magnetic properties of K2AgF4, related to recently studied Cs2AgF4, have been scrutinized. It crystallizes orthorhombic (Cmca No.64) with a = 6.182(3) A, b = 12.632(5) A, c = 6.436(3) A (Z = 4, V = 502.6(7) A3). K2AgF4 exhibits slightly puckered [AgF2] sheets and a compressed octahedral coordination of Ag(II) and it is not isostructural to related Cs2AgF4. Violet–coloured K2AgF4 orders ferromagnetically below 26 K. The DFT calculations reproduce semiconducting properties and ferromagnetism of K2AgF4 at the LSDA + U level but only if substantial values of Mott–Hubbard on-site electron–electron repulsion energies for Ag and F are used in calculations. We have also succeeded to solve the crystal structure of a brown KAgF3 (1D antiferromagnet below 64 K; GdFeO3–type, PnmaNo. 62, a = 6.2689(2) A, b = 8.3015(2) A, c = 6.1844(2) A, Z = 4, V = 321.84(2) A3) and to prepare K3Ag2F7, a novel KAgF3/K2AgF4 intergrowth phase and a member of the Ruddlsden–Popper KnAgFn+2 series (n = 1.5). Dark brown K3Ag2F7 crystallizes orthorhombic (K3Cu2Cl7-type, CccaNo. 68, setting 2) with a = 20.8119(14) A, b = 6.3402(4) A, c = 6.2134(4) A (Z = 4, V = 819.87(9) A3).

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.

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.

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.

Large exchange anisotropy in quasi-one-dimensional spin- 12 fluoride antiferromagnets with a d(z2)1 ground state

Hybrid density functional calculations are performed for a variety of systems containing d9 ions (Cu2+, Ag2+), and exhibiting quasi-one-dimensional magnetic properties. In particular we study fluorides containing these ions in a rarely encountered compressed octahedral coordination which forces the unpaired electron in the d(z2) orbital. We predict that such systems should exhibit magnetic anisotropies surpassing that of Sr2CuO3 - one of the best realizations of an one-dimensional system known to date. In particular we predict that the inter-chain coupling in the Ag2+ containing [AgF][BF4] should be nearly five orders of magnitude smaller than the intra-chain interaction. Our results indicate that quasi-one-dimensional spin-1/2 systems containing chains with spin sites in a d(z2)1 local ground state could constitute a versatile model for testing modern theories of quantum many-body physics in the solid state.

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.

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).