Influence of a Second Cation (M= Ca2+, Mg2+) on the Phase Evolution of (BaxM1-x)F2Starting from Amorphous Deposits

Unexpected Coexisting Solid Solutions in the Quasi-Binary Ag(II) F2 /Cu(II) F2 Phase Diagram

High-temperature solid-state reaction between orthorhombic AgF2 and monoclinic CuF2 (y=0.15, 0.3, 0.4, 0.5) in a fluorine atmosphere resulted in coexisting solid solutions of Cu-poor orthorhombic and Cu-rich monoclinic phases with stoichiometry Ag1-x Cux F2 . Based on X-ray powder diffraction analyses, the mutual solubility in the orthorhombic phase (AgF2  : Cu) appears to be at an upper limit of Cu concentration of 30 mol % (Ag0.7 Cu0.3 F2 ), while the monoclinic phase (CuF2  : Ag) can form a nearly stoichiometric Cu : Ag=1 : 1 solid solution (Cu0.56 Ag0.44 F2 ), preserving the CuF2 crystal structure. Experimental data and DFT calculations showed that AgF2  : Cu and CuF2  : Ag solid solutions deviate from the classical Vegard's law. Magnetic measurements of Ag1-x Cux F2 showed that the Néel temperature (TN ) decreases with increasing Cu content in both phases. Likewise, theoretical DFT+U calculations for Ag1-x Cux F2 showed that the progressive substitution of Ag by Cu decreases the magnetic interaction strength |J2D | in both structures. Electrical conductivity measurements of Ag0.85 Cu0.15 F2 showed a modest increase in specific ionic conductivity (3.71 ⋅ 10-13 ±2.6 ⋅ 10-15  S/cm) as compared to pure AgF2 (1.85 ⋅ 10-13± 1.2 ⋅ 10-15  S/cm), indicating the formation of a vacancy- or F adatom-free metal difluoride sample.

Towards quantitative treatment of electron pair distribution function

The pair distribution function (PDF) is a versatile tool to describe the structure of disordered and amorphous materials. Electron PDF (ePDF) uses the advantage of strong scattering of electrons, thus allowing small volumes to be probed and providing unique information on structure variations at the nano-scale. The spectrum of ePDF applications is rather broad: from ceramic to metallic glasses and mineralogical to organic samples. The quantitative interpretation of ePDF relies on knowledge of how structural and instrumental effects contribute to the experimental data. Here, a broad overview is given on the development of ePDF as a structure analysis method and its applications to diverse materials. Then the physical meaning of the PDF is explained and its use is demonstrated with several examples. Special features of electron scattering regarding the PDF calculations are discussed. A quantitative approach to ePDF data treatment is demonstrated using different refinement software programs for a nanocrystalline anatase sample. Finally, a list of available software packages for ePDF calculation is provided.

Ionic conductivity and structure of M1-xPbxF2 (M = Ca, Sr, Ba) solid solutions prepared by ball milling

Nanocrystalline M1-xPbxF2 (M = Ca, Sr, Ba) solid solutions were prepared by ball milling mixtures of binary parent materials. The structure of the obtained materials was investigated by 19F MAS NMR spectroscopy and XRPD, and their ionic conductivity by impedance spectroscopy. It was found that fluorite-structured solid solutions over the whole range of compositions can be prepared by ball milling for all three systems, closing the broad miscibility gap of the CaF2-PbF2 system. The fluoride ion conductivity increased with increasing Pb content of the solid solutions, with Ba0.10Pb0.90F2 showing the highest conductivity of all samples prepared, being 1.5 orders of magnitude smaller than the one of PbSnF4. Ca1-xPbxF2 showed a fluoride ion conductivity increase which can be assigned to geometric frustration induced disorder. Ball milling of pure PbF2 revealed an increase of the fluoride ion conductivity by 2.5 orders of magnitude in the case of PbF2 containing a large amount of β-PbF2 compared to microcrystalline β-PbF2. Its fluoride ion conductivity is also 2.5 orders of magnitude larger than the fluoride ion conductivity of ball milled PbF2 consisting of a small amount of β-PbF2 but a large amount of α-PbF2 pointing to differently conducting and structured grain boundaries of α-PbF2 and β-PbF2.