Dmitri O. Charkin

A reinvestigation of quaternary layered bismuth oxyhalides of the Sillén X1 type

Abstract A family of layered bismuth oxyhalides, L I 0.5 Bi 1.5 O 2 X and L II BiO 2 X has been reinvestigated. Formation of X1-type Sillen compounds has been established for L I =Li, Na, L II =Ca, Sr, Ba, and X =Cl, Br, I, but the details of their crystal structures are different. While all L I 0.5 Bi 1.5 O 2 X , CaBiO 2 Br, and CaBiO 2 I adopt the disordered tetragonal Nd 2 O 2 Te structure, all compounds of L II =Sr and Ba are orthorhombic and isostructural to PbSbO 2 Cl, due to L /Bi cation ordering. Crystal structures have been determined for CaBiO 2 I, SrBiO 2 Br, SrBiO 2 I, and BaBiO 2 I. We discuss the factors which determine the occurrence and type of cation ordering in the quaternary bismuth and antimony X1-type oxyhalides. We also predict that more isostructural compounds can be prepared with antimony.

Modular approach as applied to the description, prediction, and targeted synthesis of bismuth oxohalides with layered structures

A version of the modular approach to the description, crystal-chemical analysis, and prediction of layered compounds is proposed. Criteria for the existence of new compounds with set structures are formulated and considered in detail for layered bismuth compounds. The potential of this approach is verified by the successful synthesis of more than 100 new bismuth oxohalides, which refer to more than ten predicted structure types.

A crystallographic re-investigation of Cu2Sb-related binary, ternary, and quaternary structures: how many structure types can exist upon the same topology of a unit cell?

Crystal chemical analysis of several large sets of topologically related structures confirms earlier suggestions that within the same cell content, space group, and Wyckoff site occupancies, there may exist several structure types of sometimes quite different coordination chemistry and bonding arrangement. The original Flahauts suggestion of demarcating anti-Cu2Sb and PbFCl structure types has been extended to an analysis of structures of over 350 compounds in terms of various structural characteristics as free parameters. For the Cu2Sb family, structural field maps indicate formation of at least three distinct structure types: Cu2Sb, PbFCl, CeFeSi (anti-type of PbFCl) and UAs2 (which is not an anti-type of Cu2Sb). The formerly suggested ternary ZrSiS structure type is in fact an ordered version of UAs2. Extension of this approach to “filled” versions of these structure types permits clear demarcation of two other topologically related structure types, LaOAgS and HfCuSi2, the latter being a derivative of UAs2 but not Cu2Sb. Very similar relationships have been observed for several smaller structural families. Perspectives of using different structural parameters as free coordinates are discussed.

Synthesis, Crystal Structure, Transport, and Magnetic Properties of Novel Ternary Copper Phosphides, A2Cu6P5 (A = Sr, Eu) and EuCu4P3

Three new ternary copper phosphides, Sr(2)Cu(6)P(5), Eu(2)Cu(6)P(5), and EuCu(4)P(3), have been synthesized from the elements in evacuated silica capsules. Eu(2)Cu(6)P(5) and Sr(2)Cu(6)P(5) adopt the Ca(2)Cu(6)P(5)-type structure, while EuCu(4)P(3) is isostructural to BaMg(4)Si(3) and still remains the only representative of this structure type among the ternary Cu pnictides. All three materials show metallic conductivity in the temperature range 2 K ≤ T ≤ 290 K, with no indication for superconductivity. For Eu(2)Cu(6)P(5) and EuCu(4)P(3), long-range magnetic order was observed, governed by 4f local moments on the Eu atoms with predominant ferromagnetic interactions. While Eu(2)Cu(6)P(5) shows a single ferromagnetic transition at T(C) = 34 K, the magnetic behavior of EuCu(4)P(3) is more complex, giving rise to three consecutive magnetic phase transitions at 70, 43, and 18 K.

Novel lanthanoid–cadmium oxide pnictides with the tetragonal LaOAgS structure

Abstract Five new equiatomic lanthanoid–cadmium oxide pnictides LnOCdPn (Ln=rare-earth, Pn=pnictogen) have been prepared. All of them crystallize in the tetragonal LaOAgS structure type. The phosphide series is represented by La compound only; arsenides were found for Ln=La–Nd. The structure of CeOCdAs ( P 4/ nmm , a =4.1852(1)A, c =9.1595(3)A, R p =0.054, R wp =0.071) has been solved from X-ray powder data. Similarities between structures of CeOCdAs, isoelectronic CeOZnSb, and related LaOCd 0.5 Se are discussed.

Study of cationic substitution in Bi2WO6 and derived structures in the framework of the modular approach

The possibilities of substitution of lead, alkaline and rare earth, antimony, and tellurium cations for bismuth ions in the structure of the Bi2WO6 ferroelectric and compounds with more complicated derived structures have been studied. The trends in the formation of solid solutions are well described in the framework of the modular approach in which layers are treated as building units of crystal structures. The underlying existence criteria (electroneutrality, geometric and chemical compatibility of layers) formulated for simple (two-layer) structures can be easily extended to more complicated (multilayer) structures. On the basis of the results obtained, the existence of new series of layered bismuth oxohalides was predicted.

A New Approach to Synthesis of Layered Fluorites Containing Molecular Anions : Synthesis of Ln2O2CO3, K(LnO)CO3, and Ln2O2CrO4 via Metathesis Reactions

A new synthetic approach is suggested for preparation of layered rare-earth oxide compounds containing [Ln2O2] slices and molecular anion sheets (CO32-, SO42-, and CrO42-). It is based on exchange reactions of rare-earth oxychlorides, [Ln2O2]Cl2, and alkali carbonates, sulfates, or chromates. Five new rare-earth oxychromates [Ln2O2]CrO4 (Ln=Pr-Gd) have been prepared which adopt a new, probably layered, structure type. In addition, significantly easier and more efficient synthetic pathways were found to the known compounds [Ln2O2]K2(CO3)2 and [La2O2]CrO4. The structure of the latter compound has been determined from neutron powder diffraction data. Factors affecting reaction pathways and products are discussed, as well as prospects for applying the approach to more complex layered compounds.

A novel family of layered bismuth compounds II: the crystal structures of Pb0.6Bi1.4Rb0.6O2Z2, Z=Cl, Br, and I

Abstract Using various synthetic approaches, we have prepared over 50 new multinary bismuth oxyhalides which crystallize in four layered structure types. Most of the compounds belong to the three previously reported structure types involving fluorite- and CsCl-like metal–oxygen vs. metal–halogen layers as well as single or double halide ion sheets. The majority of Bi2−xAxQ0.6O2Z2 (A=Li, Na, K, Ca, Sr, Ba, Pb; Q=Rb, Cs; Z=Cl, Br, I) compounds crystallize in the tetragonal structure of Pb0.6Bi1.4Cs0.6O2Cl2 (Y2) while both Bi1.4Ba0.6Q0.6O2I2 (Q=Rb, Cs) oxyiodides adopt its orthorhombically distorted, partially ordered version. Due to the lower degree of substitution, the fluorite-like layers in the Y2 structure accommodate more A cations than previously known for related Bi compounds. However, very large Tl+ or Rb+ give compounds with another, as yet unknown, structure. We discuss the influence of size and charge of A cations and stoichiometry of [Bi2−xAxO2] fluorite layers on structure and stability of layered oxyhalides of bismuth. Also, we predict formation of isostructural compounds with smaller Q cations like Tl+ and K+.

Two new arsenides, Eu7Cu44As23 and Sr7Cu44As23, with a new filled variety of the BaHg11 structure.

Two new ternary arsenides, namely, Eu7Cu44As23 and Sr7Cu44As23, were synthesized from elements at 800 °C. Their crystal structure represents a new filled version of the BaHg11 motif with cubic voids alternately occupied by Eu(Sr) and As atoms, resulting in a 2 × 2 × 2 superstructure of the aristotype: space group Fm3̅m, a = 16.6707(2) Å and 16.7467(2) Å, respectively. The Eu derivative exhibits ferromagnetic ordering below 17.5 K. In agreement with band structure calculations both compounds are metals, exhibiting relatively low thermopower, but high electrical and low thermal conductivity.

Synthesis and crystal structure of layered oxyhalides BaPbBi3Nb2O11X (X = Cl, Br, I)

Three new bismuth oxyhalides BaPbBi3Nb2O11X (X = Cl, Br, I), including the first perovskite bismuth oxyiodide, were prepared by ceramic route. Their crystal structure is formed by intergrowth of Sillén (PbBiO2X) and Aurivillius (BaBi2Nb2O9) phases. The results of Rietveld refinements show that the peculiarities of the building blocks (in particular, the distribution of Ba2+ and Bi3+) remain intact upon formation of the intergrowth structure. The Ba2+ cations prefer pure-oxygen to mixed oxygen-halogen environment which can be explained on the basis of bond valence method.