Przemysław J. Malinowski

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).

Redetermination of crystal structure of Ag(II)SO4 and its high-pressure behavior up to 30 GPa

Here we redetermine the crystal structure of Ag(II)SO4, an unusual d9 system, at 1 atm from powder X-ray data and we report hydrostatic pressure X-ray diffraction experiments on Ag(II)SO4 inside the diamond anvil cell. AgSO4 crystallizes in the monoclinic C2/c cell, with a = 12.8476(2) A, b = 13.6690(4) A, c = 9.36678(19) A, β = 47.5653(13)°, and V = 1214.04(5) A3 (Z = 16). AgSO4 exhibits bulk modulus, B0, of 36.9 GPa, and undergoes sluggish decomposition at ∼23 GPa yielding a high-pressure phase of Ag2S2O7 (K2S2O7-type), with the substrate and product coexisting at 30 GPa. Theoretical calculations within Density Functional Theory for the C2/c cell nicely reproduce the observed trend for lattice constants as well as the B0 values of AgSO4, and suggest that the rigidity of the infinite [Ag(SO4)] chains as well as the Jahn–Teller effect for the Ag(II) cation persist even at 30 GPa.

Silver(II) triflate with one-dimensional [Ag(II)(SO3CF3)4/2]∞ chains hosting antiferromagnetism

Silver(II) triflate, previously reported by Leung et al. in 1979 (Can. J. Chem., 1979, 57, 326–329), crystallizes in a triclinic P cell with a = 4.9117(11) A, b = 5.1136(10) A, c = 11.033(3) A, α = 79.955(14)°, β = 75.771(16)°, γ = 61.571(17)° and V = 235.68(10) A3 and is isomorphous to Cu(SO3CF3)2. The compound has a layered structure with an interlayer separation of 10.67 A; the van der Waals gaps open between the CF3 groups from neighbouring sheets. Ag(II) is coordinated by six O atoms in the form of an elongated octahedron; the adjacent Ag2+ cations are linked via –OSO– bridges; direct –O– bridges are absent. The [Ag(II)(SO3CF3)2]∞ layers consist of one-dimensional chains which interact weakly with each other via longer Ag⋯OSO–Ag contacts. This results in a 1D rather than 2D antiferromagnetic ordering, which can be described via the Bonner–Fisher model with a superexchange constant, Jintra-chain, of 104 K (9.0 meV) per pair of interacting Ag2+ cations. The magnetic ordering persists even at room temperature leading to a broad ESR signal with g = 2.199. DFT calculations confirm the 1D character of electronic structure and antiferromagnetism residing within the [Ag(II)(SO3CF3)4/2]∞ chains with a |Jinter-chain|/|Jintra-chain| ratio of ∼10−3 to 10−2. The calculated indirect bandgap at the Fermi level of ∼1 eV opens between rather flat valence and conduction bands, which are predominated by contributions from Ag and O atoms. Ag(SO3CF3)2 is extremely sensitive to moisture and decomposes instantly when exposed to atmosphere. When dry, it rapidly decomposes thermally above 120 °C, but its slow exothermic decay to AgSO3CF3 takes place even at room temperature. Silver(II) triflate is also photosensitive and irradiating it with a 632.8 nm laser radiation at a power greater than 0.17 mW leads to its decomposition to Ag(I) triflate and Ag(I)2S2O7.

Unusual Thermal Decomposition of AgIISO4 Yielding AgI2S2O7: Bending Hammond’s Rule

“It could be said that among the coinage metals (Cu, Ag, Au), only silver is normal . (...) gold is anomalous due to large relativistic effects. Copper is anomalous as it has a nodeless and therefore very compact d shell, with strong electron–electron repulsion (...)”. The “normality” of elemental silver, is reflected in the prevalence of its monovalent oxidation state, +1 (as expected for a Group 11 element), which also has severe consequences for its second oxidation state—it renders Ag an extremely powerful oxidizer. The standard redox potential for the Ag/Ag redox pair equals +1.98 V versus NHE and it is surpassed only by those of FC/F , F2/2F , ClC/Cl and OHC/OH . Thus Ag is stabilized in connections with fluoride ligands, whereas its oxo and aza compounds are rare and are quite unstable thermodynamically and thermally. Here we investigate in detail the thermal decomposition of recently synthesized Ag sulfate, AgSO4. [4] The activation energy for decomposition turns out to be substantial ( 127 kJmol ) rendering this compound metastable at ambient (p, T) conditions. We show, based on literature about thermal decomposition for over 50 different sulfates and oxo-sulfates (see the Supporting Information), that solely AgSO4 s low-temperature thermal decomposition is associated with the reduction of a metal cation (“reductive decomposition”). We consider the reaction pathway for decomposition and we point out structural links between AgSO4 and crystalline product of its decomposition, AgSO3.5 Ag2S2O7. Ag sulfate was reported to decompose thermally in a single step while yielding Ag disulfate and releasing O2 [Eq. (1)]: