Unique octahedral rotation pattern in the oxygen-deficient Ruddlesden–Popper compound Gd3Ba2Fe4O12

A novel Ruddlesden–Popper-related compound, Gd3Ba2Fe4O12, was discovered and its crystal structure was determined via single-crystal X-ray diffraction. The structure has an ordered structure of octahedra and pyramids along the c axis. Gd3Ba2Fe4O12 belongs to the tetragonal system P42/ncm, with a = 5.59040 (10) Å and c = 35.1899 (10) Å. The A-site ions in the Ruddlesden–Popper structure, i.e. Gd3+ and Ba2+, exhibit an ordering along the c axis. The perfect oxygen deficiency is accommodated at the GdO layers in the proper Ruddlesden–Popper structure. Using the bond-valence-sum method, the Fe ions in the FeO6 octahedra and FeO5 pyramids represent valence states of +3 and +2.5, respectively, demonstrating a two-dimensional charge disproportionation. The corner-sharing FeO6 octahedra and FeO5 pyramids are tilted in opposite directions, with the neighbours around one axis of the simple perovskite configuration, which, using Glazer's notation, can be represented as a − b 0 c 0/b 0 a − c 0. In the perovskite blocks, the facing FeO5 pyramids across the Gd layer rotate in the same sense, which is a unique rotation feature related to oxygen deficiency.

Evidence for Local Structural Distortion and Mixed States of Fe in β-NaFeO2: A Bond Valence Sum Analysis

We have analyzed the crystal structure, interatomic distances and bond valence sums in β-phase of sodium ferrite based on neutron diffraction measurement at room temperature. The Rietveld analysis performed using Pna21 space group obtains smaller lattice constants compared to the previous reports. This discrepancy is attributed to the sodium deficiency. We notice that the Na-O bonds are shortened, while Fe-O bonds are elongated. The calculated bond valence sum around the sodium and the iron ions are 1.08 and 2.64, respectively. This indicates the presence of Fe2+-Fe3+ mixed valence state.

Empirical Electronic Polarizabilities for Use in Refractive Index Measurements III. Structures with Short [5]Ti–O and Vanadyl Bonds

ABSTRACT The electronic polarizabilities of most cations, such as Na+, Ca2+, Fe2+, Fe3+, and Zr4+, show a monotonic decrease as the cation coordination increases. However, polarizabilities of the ions [5]Ti4+, [5]V5+, and [6]V5+ show strong deviations from a regular decrease. In this paper we characterize the [5]Ti and vanadyl compounds by infrared frequencies, by the short [5]Ti4+– O, [5]V4+–O, [6]V4+–O, [5]V5+–O, and [6]V5+–O bonds and the polarizabilities of [5]Ti4+, [5]V4+, [6]V4+, [5]V5+, and [6]V5+ determined from refractive index measurements. Analysis of the structures of 18 compounds containing short [5]Ti–O bonds supports the concept of the short Ti–O bond being associated with the bond valence sum (omitting Ti) around the oxygen atom O*. The short Ti–O* bond occurs to satisfy the bond valence requirement of (O2–) of ∼2.0 vu. Plotting the [5]Ti–O* distances of 18 minerals versus the bond valence sum (BVS) around O* shows an approximately linear relationship. Extrapolation to BVS = 0 yields a minimum distance of 1.65 Å. The mean value is 1.693 Å. The mean short distances in V4+ vanadyl minerals are 1.597 Å (CN = 5) and 1.590 Å (CN = 6), whereas the mean short distance in five V5+ minerals is 1.647 Å (CN = 5) and in 14 V5+ minerals is 1.644 Å (CN = 6). We compare the polarizabilities of [5]Ti and [5,6]V4+ and [5,6]V5+ ions with the polarizabilities of [4]-coordinated Ti4+ ([4]Ti4+ ) and [6]-coordinated Ti4+ ([6]Ti4+ ) and of [4]-, [5]-, and [6]-coordinated V4+ and V5+ ([n]V4+ and [n]V5+) and hypothesize that the reduced polarizability of [5]Ti4+, [5]V5+, and [6]V5+ ions is caused by the short Ti–O* and V=O bonds.

Triple molybdates K3–x Na1+x M 4(MoO4)6 (M = Ni, Mg, Co) and K3+x Li1–x Mg4(MoO4)6 isotypic with II-Na3Fe2(AsO4)3 and yurmarinite: synthesis, potassium disorder, crystal chemistry and ionic conductivity

The triple molybdates K3–x Na1+x M 4(MoO4)6 (M = Ni, Mg, Co) and K3+x Li1–x Mg4(MoO4)6 were found upon studying the corresponding ternary molybdate systems, and their structures, thermal stability and electrical conductiviplusmnty were investigated. The compounds crystallize in the space group R 3 c and are isostructural with the sodium-ion conductor II-Na3Fe2(AsO4)3 and yurmarinite, Na7(Fe3+, Mg, Cu)4(AsO4)6; their basic structural units are flat polyhedral clusters of the central M1O6 octahedron sharing edges with three surrounding M2O6 octahedra, which combine with single NaO6 octahedra and bridging MoO4 tetrahedra to form open three-dimensional (3D) frameworks where the cavities are partially occupied by disordered potassium (sodium) ions. The split alkali-ion positions in K3–x Na1+x M 4(MoO4)6 (M = Ni, Mg, Co) give their structural formulae as [(K,Na)0.5□0.5)]6(Na)[M1][M2]3(MoO4)6, whereas the lithium-containing compound (K0.5□0.5)6(Mg0.89K0.11)(Li0.89Mg0.11)Mg3(MoO4)6 shows an unexpected (Mg, K) isomorphism, which is similar to (Mn, K) and (Co, K) substitutions in isostructural K3+x Li1–x M 4(MoO4)6 (M = Mn, Co). The crystal chemistry of the title compounds and related arsenates, phosphates and molybdates was considered, and the connections of the cationic distributions with potential 3D ionic conductivity were shown by means of calculating the bond valence sum (BVS) maps for the Na+, Li+ and K+ ions. Electrical conductivity measurements gave relatively low values for the triple molybdates [σ = 4.8 × 10−6 S cm−1 at 390°C for K3NaCo4(MoO4)6 and 5 × 10−7 S cm−1 at 400°C for K3LiMg4(MoO4)6] compared with II-Na3Fe2(AsO4)3 (σ = 8.3 × 10−4 S cm−1 at 300°C). This may be explained by a low concentration of sodium or lithium ions and the blocking of their transport by large potassium ions.

Synthesis, crystal structure determination of a novel phosphate Ag1.64Zn1.64Fe1.36(PO4)3 with an alluaudite-like structure

Single crystals of Ag1.64Zn1.64Fe1.36(PO4)3 [silver zinc iron phosphate (1.64/1.64/1.36/3)] have been synthesized by a conventional solid-state reaction and structurally characterized by single-crystal X-ray diffraction. The title compound crystallizes with an alluaudite-like structure. All atoms of the structure are in general positions except for four, which reside on special positions of the space group, C2/c. The Ag+ cations reside at full occupancy on inversion centre sites and at partial occupancy (64%) on a twofold rotation axis. In this structure, the unique Fe3+ ion with one of the two Zn2+ cations are substitutionally disordered on the same general position (Wyckoff site 8f), with a respective ratio of 0.68/0.32 (occupancies were fixed so as to ensure electrical neutrality for the whole structure). The remaining O and P atoms are located in general positions. The three-dimensional framework of this structure consists of kinked chains of edge-sharing octahedra stacked parallel to [10\overline{1}]. These chains are built up by a succession of [MO6] (M = Zn/Fe or Zn) units. Adjacent chains are connected by the PO4 anions, forming sheets oriented perpendicular to [010]. These interconnected sheets generate two types of channels parallel to the c axis, in which the Ag+ cations are located. The validity and adequacy of the proposed structural model of Ag1.64Zn1.64Fe1.36(PO4)3 was established by means of bond-valence-sum (BVS) and charge-distribution (CHARDI) analysis tools.

Bond Valence Sum: A Powerful Tool for Determination of Oxidation States of Metal Ions in Coordination Compounds

Transition metal ions in coordination compounds can adopt different oxidation states. The Bond Valence Sum (BVS) model, based solely on structural information, relates the bond lengths around a metal center to its oxidation state. This model can provide details on the oxidation states of the metal ions and serves as an additional support for the accuracy of crystal structure determination. Herein, we introduce the fundamental concept of the BVS method and summarize the empirical BVS parameters for selective metal ions that have more than one oxidation state. Applications of the method to mononuclear and polynuclear complexes will be discussed.