Hexafluoro-, heptafluoro-, and octafluoro-salts, and [MₙF₅ₙ₊₁]⁻ (n=2, 3, 4) polyfluorometallates of singly charged metal cations, Li⁺–Cs⁺, Cu⁺, Ag⁺, In⁺ and Tl⁺

The AMF₆, A₂MF₇, A₃MF₈, AM₂F₁₁, AM₃F₁₆ and AM₄F₂₁ compounds (A = Li, Na, K, Rb, Cs, Cu, Ag, In, Tl; M = P, As, V, Rh, Ru, Au, Pt, Ir, Os, Re, Sb, Mo, W, Nb, Ta, Bi) are reviewed. Some of the structural data of the AM₆ compounds are based just on powder diffraction work from the middle of the last century. The crystal structure types of AMF₆ compounds have been re-classified in this review, based mainly on single crystal data. The crystal structure types of AMF₆ compounds can be classified into six main groups: LiSbF₆ type, NaSbF₆ type, structures of cubic APF₆ and AAsF₆ with orientational disorder of the anions, tetragonal KSbF₆ (T) types and similar structures, AgSbF₆ type and similar structures, and KOsF₆ type. Reported crystal structures of A₂MF₇, A₃MF₈, AM₂F₁₁, AM₃F₁₆ and AM₄F₂₁ compounds are limited. K₂WF₇ in the orthorhombic crystal system. Among the A₃MF₈ compounds the complete crystal structure has been determined only for Na₃TaF₈, which is monoclinic. The only known examples of crystal structures of AM₂F₁₁ compounds are ASb₂F₁₁1 (A = Ag, K, Cs). Crystals of KSb₂F₁₁ are orthorhombic and isostructural to AgSb₂F₁₁, while CsSb₂F₁₁ is monoclinic. CsSb₃F₁₆ is the only example of a structurally characterized AM₃F₁₆ compound. Its crystals are orthorhombic. For the rest of the known A₂MF₇, A₃MF₈, AM₂F₁₁, AM₃F₁₆ and AM₄F₂₁ compounds, only lattice parameters are known

A unique two-dimensional silver(II) antiferromagnet Cu[Ag(SO4)2] and perspectives for its further modifications

Copper(II) silver(II) sulfate crystallizes in a monoclinic CuSO4-related structure with P21/n symmetry. This quasi-ternary compound features [Ag(SO4)2]2- layers, while the remaining cationic sites may be occupied either completely or partially by Cu2+ cations, corresponding to the formula of (CuxAg1-x)[Ag(SO4)2], x = 0.6-1.0. CuAg(SO4)2 is antiferromagnetic with large negative Curie-Weiss temperature of -84.1 K and shows two characteristic ordering phenomena at 19 K and 40 K. Density functional theory calculations reveal that the strongest superexchange interaction is a two-dimensional antiferromagnetic coupling within [Ag(SO4)2]2- layers, with the superexchange constant J2D of -11.1 meV. This renders CuAg(SO4)2 the rare representative of layered Ag2+-based antiferromagnets. Magnetic coupling is facilitated by the strong mixing of Ag d(x2-y2) and O 2p states. Calculations show that M2+ sites in MAg(SO4)2 can be occupied with other similar cations such as Zn2+, Cd2+, Ni2+, Co2+, and Mg2+.Comment: 9 pages, 4 Tables, 9 Figures, and electronic supplement of 21 page

Anomalously Large Formula Unit Volume and Its Effect on the Thermal Behavior of LiBF₄

The crystal structure of LiBF₄ has been determined by single-crystal X-ray diffraction measurements. LiBF₄ crystallizes as a merohedral twin in the trigonal space group P3₁21 with a = 4.892(5) Å, c = 11.002(12) Å, V = 228.0(4) Å3, and Z = 3 at 200 K. The twin is generated by a 2-fold rotation about the [11̄0] direction. The lithium cation is coordinated by four fluorine atoms in a distorted tetrahedral manner, wherein two Li−F distances of 1.862(5) and 1.846(5) Å are observed. The formula unit volume (FUV = V/Z) of 77.9 ų for LiBF₄ at 298 K is considerably larger than 72.7 ų for NaBF₄ and 72.5 ų for AgBF₄, despite the smaller size of Li+, indicating loose ionic packing of LiBF₄. The thermodynamic evaluation of the decomposition temperature for LiBF₄ was performed using the empirical relationship between the standard entropy and the FUV obtained. The results indicate that the large FUV of LiBF₄ contributes to its higher decomposition temperature compared to that of LiPF₆

Crystal structures of frozen room temperature ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4), hexafluoroniobate (EMImNbF6) and hexafluorotantalate (EMImTaF6), determined by low-temperature X-ray diffraction

The crystal structures of three salts, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄), hexafluoroniobate (EMImNbF₆) and hexafluorotantalate (EMImTaF₆), all of which form room-temperature ionic liquids (RTILs), have been determined by low-temperature X-ray diffraction studies of their single crystals. EMImBF₄ crystallizes in the monoclinic space group P2₁/c with a = 8.653(5)Å, b = 9.285(18)Å, c = 13.217(7), β = 121.358(15)Å, V = 906.8(19)ų, Z = 4 at 100 K. EMImBF₄ exhibits a unique structure wherein EMIm cations form one-dimensional pillars facing the imidazolium ring to the next ring linked by H(methylene)⋯π electron interaction. The BF₄ anion also forms one-dimensional pillars along the same direction with the nearest F⋯F contact distance of 3.368(3) Å. EMImNbF₆ and EMImTaF₆ are isostructural to each other and crystallize in the orthorhombic space group P2₁2₁2₁: EMImNbF₆, a = 9.204(4)Å, b = 9.770(15)Å, c = 12.499(13)Å, V = 1124(2)ų, Z = 4 at 200 K; EMImTaF₆, a = 9.216(5)Å, b = 9.763(2)Å, c = 12.502(17)Å, V = 1124.9(17)ų, Z = 4 at 200K. In EMImNbF₆ and EMImTaF₆, EMIm cations also form a one-dimensional pillar structure and the hexafluorocomplex anions are located in a zig-zag arrangement along the same direction with the nearest F⋯F distance of 3.441(12) Å. This structure (Type-B(MF₆)) is different from the Type-A(MF₆) structure previously reported for EMImPF₆, EMImAsF₆ and EMImSbF₆. Hydrogen bonds in the Type-A(MF₆) (EMImPF₆(333 K), EMImAsF₆ (326 K) and EMImSbF₆ (283 K)) crystal lattice are weaker than those in the Type-B(MF₆) (EMImNbF₆(272 K) and EMImTaF₆ (275 K)) crystal lattice. This suggests that the strength of the hydrogen bond is not always a decisive and determining factor for the melting points of RTILs. The measurement of cell parameters for EMImBF₄ between 100 K and its melting point revealed that EMImBF₄ essentially preserves the same structure in this temperature range and increases its volume by only 4% due to the melting

Low temperature magnetism of KAgF3

KAgF$_3$ is a quasi one-dimensional quantum antiferromagnet hosting a series of intriguing structural and magnetic transitions. Here we use powder neutron diffraction, $\mu$SR spectroscopy, and Density Functional Theory calculations to elucidate the low temperature magnetic phases. Below $T_{N1}=29$K we find that the material orders as an A-type antiferromagnet with an ordered moment of 0.47$\mu_{\rm B}$. Both neutrons and muons provide evidence for an intermediate phase at temperatures $T_{N1}<T<T_{N2}$ with $T_{N2}\approx 66$ K from a previous magnetometry study. However, the evidence is at the limit of detection and its nature remains an open problem.Comment: 11 pages, 8 figures. Supplementary information is included in a separate fil

Strength of Correlations in a Silver Based Cuprate Analogue

AgF2 has been proposed as a cuprate analogue which requires strong correlation and marked covalence. On the other hand, fluorides are usually quite ionic and 4d transition metals tend to be less correlated than their 3d counterparts, which calls for further scrutiny. We combine valence band photoemission and Auger-Meitner spectroscopy of AgF and AgF2 together with computations in small clusters to estimate values of the Ag 4d Coulomb interaction U 4d and charge-transfer energy. Based on these values, AgF2 can be classified as a charge-transfer correlated insulator according to the Zaanen-Sawatzky-Allen classification scheme. Thus, we confirm that the material is a cuprate analogue from the point of view of correlations, suggesting that it should become a high-temperature superconductor if metallization is achieved by doping. We present also a computation of the Hubbard U in density functional "+U" methods and discuss its relation to the Hubbard U in spectroscopies.Comment: 12 pages, 10 figure

Charge Transfer and dddd excitations in AgF2_{2}

Charge transfer (CT) insulators are the parent phase of a large group of today's unconventional high temperature superconductors. Here we study experimentally and theoretically the interband excitations of the CT insulator silver fluoride AgF$_2$, which has been proposed as an excellent analogue of oxocuprates. Optical conductivity and resonant inelastic x-ray scattering (RIXS) on AgF$_2$ polycrystalline sample show a close similarity with that measured on undoped La$_2$CuO$_4$. While the former shows a CT gap $\sim$3.4~eV, larger than in the cuprate, $dd$-excitations are nearly at the same energy in the two materials. DFT and exact diagonalization cluster computations of the multiplet spectra show that AgF$_2$ is more covalent than the cuprate, in spite of the larger fundamental gap. Furthermore, we show that AgF$_2$ is at the verge of a charge transfer instability. The overall resemblance of our data on AgF$_2$ to those published previously on La$_2$CuO$_4$ suggests that the underlying CT insulator physics is the same, while AgF$_2$ could also benefit from a proximity to a charge density wave phase as in BaBiO$_3$. Therefore, our work provides a compelling support to the future use of fluoroargentates for materials' engineering of novel high-temperature superconductors.Comment: 13 pages, 9 Figures (including SI

Crystal Structures of Xenon(VI) Salts: XeF₅Ni(AsF₆)₃, XeF₅AF₆ (A = Nb, Ta, Ru, Rh, Ir, Pt, Au), and XeF₅A₂F₁₁ (A = Nb, Ta)

Experiments on the preparation of the new mixed cations XeF5M(AF6)3 (M = Cu, Ni; A = Cr, Nb, Ta, Ru, Rh, Re, Os, Ir, Pt, Au, As), XeF5M(SbF6)3 (M = Sn, Pb), and XeF5M(BF4)x(SbF6)3-x (x = 1, 2, 3; M = Co, Mn, Ni, Zn) salts were successful only in the preparation of XeF5Ni(AsF6)3. In other cases, mixtures of different products, mostly XeF5AF6 and XeF5A2F11 salts, were obtained. The crystal structures of XeF5Ni(AsF6)3, XeF5TaF6, XeF5RhF6, XeF5IrF6, XeF5Nb2F11, XeF5Ta2F11, and [Ni(XeF2)2](IrF6)2 were determined for the first time on single crystals at 150 K by X-ray diffraction. The crystal structures of XeF5NbF6, XeF5PtF6, XeF5RuF6, XeF5AuF6, and (Xe2F11)2(NiF6) were redetermined by the same method at 150 K. The crystal structure of XeF5RhF6 represents a new structural type in the family of XeF5AF6 salts, which crystallize in four different structural types. The XeF5A2F11 salts (M = Nb, Ta) are not isotypic and both represent a new structure type. They consist of [XeF5]+ cations and dimeric [A2F11]− anions. The crystal structure of [Ni(XeF2)2](IrF6)2 is a first example of a coordination compound in which XeF2 is coordinated to the Ni2+ cation