Igor V. Plokhikh

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.

Effect of Transition Metal Substitution on the Structure and Properties of a Clathrate-Like Compound Eu7Cu44As23

A series of substitutional solid solutions—Eu7Cu44−xTxAs23 (T = Fe, Co, Ni)—based on a recently discovered clathrate-like compound (Eu7Cu44As23) were synthesized from the elements at 800 °C. Almost up to 50% of Cu can be substituted by Ni, resulting in a linear decrease of the cubic unit cell parameter from a = 16.6707(1) Å for the ternary compound to a = 16.3719(1) Å for the sample with the nominal composition Eu7Cu24Ni20As23. In contrast, Co and Fe can only substitute less than 20% of Cu. Crystal structures of six samples of different composition were refined from powder diffraction data. Despite very small differences in scattering powers of Cu, Ni, Co, and Fe, we were able to propose a reasonable model of dopant distribution over copper sites based on the trends in interatomic distances as well as on Mössbauer spectra for the iron-substituted compound Eu7Cu36Fe8As23. Ni doping increases the Curie temperature to 25 K with respect to the parent compound, which is ferromagnetically ordered below 17.5 K, whereas Fe doping suppresses the ferromagnetic ordering in the Eu sublattice.

Structural, thermal, and IR studies of β-[Nd2O2](CrO4), an unexpected analog of a slag phase [Ba2F2](S6+O3S2−)

Abstract A family of Ln2CrO6 (Ln=Pr, Nd, Sm–Tb) compounds has been re-investigated using powder X-ray diffraction and IR spectroscopy. The structure of β-Nd2CrO6≡β-[Nd2O2](CrO4) is similar to that of the slag compound [Ba2F2](S6+O3S2−) in that it exhibits a disordered arrangement of (CrO4)2− anions between [Nd2O2]2+ litharge-type blocks. Its structural architecture is also related to other layered α- and γ-[Ln2O2](AO4) species (A=S, Cr, Mo), showing various orientations of the tetrahedral anions within the interlayer space. Size relationships between the incorporated tetrahedral anions and formation of different structure types (denoted as M1-, M2- and T-type) are reviewed. The possible existence of new compounds which are isostructural with, or structurally related to, β-[Nd2O2](CrO4) and bearing other transition metal-centred tetrahedral anions are discussed.

Investigation of the substitution at the anionic positions BaT2As2 (T = Fe, Ni) superconductors

The possibilities of electron/hole doping of two ternary arsenides, BaFe2As2 and BaNi2As2, via partial substitution at the arsenic position by 16 and 14 group elements, have been studied. While no substitution has been observed for chalcogens, BaFe2As2 incorporates Sb, Si, and Ge at the As site; BaNi2As2 incorporates Sb, Ge, Sn, and Pb. The observed results can be tentatively explained suggesting that 14 group elements are incorporated into the BaT2As2 structures as X6−2 dumbbells.

On the stability boundaries of the LaOAgS structure type

Among layered inorganic structure types, that of LaOAgS, numbering over 200 representatives, has been studied extensively due to their promising physical properties (magnetic, thermoelectric, ion conducting, semiconducting, or superconducting). The relative simplicity of this structure type formed by anti-isostructural litharge and mackinawite-derived slabs suggests even larger number of representatives. However, out of over 500 possible candidates suggested from structure modeling, less than 50% could be realized synthetically. In order to gain deeper insight into the “crystal chemical stability boundaries”, we employed a complex approach based on synthesis, geometry analysis, and computational estimates of relative structural stability. Two groups of representatives were studied: i) LaOAgCh (Ch = S, Se, Te) chalcogenides, where stability was estimated (by calculating DFT ground-state energies) against La2O2Ch and Ag2Ch, and ii) AFTPn (A = Ca, Sr, Ba, Eu; T = Zn, Mn, Cd; Pn = P, As, Sb, Bi) pnictides and AeFAlTt (Ae = Sr, Ba; Tt = Si, Ge, Sn) tetrelides where stability was studied against binary alkaline-earth fluorides and ternary AT2X2 compounds (X = Group 14, 15, or 16 element). For the group i), we found that these three compounds were energetically more preferable. Indeed, our attempts to prepare both known S and Se representatives and yet unknown telluride were successful, although the latter has not yet been obtained phase-pure. For the case ii), the calculated pattern agrees well with the experimental results and predictions from geometrical considerations. No Ca compounds were found to exist, and neither do the proposed SrFCdPn. The most interesting case is the AFZnP group (A = Eu, Sr), where, apparently, just a subtle difference in the radii of Sr2+ and Eu2+ allows only the Sr compound to exist. The same pattern is also observed for AFAgS with A = Sr and Eu. Quite interesting findings are the first fluoride tetrelides BaFAlTt (Tt = Si, Ge) which are, in addidion, first examples that are free from transition metals; the latter is also true for an elusive BaFMgAs pnictide. Despite the complexity of the considered systems, our approach permitted us to successfully predict and prepare over 20 new LaOAgS-type compounds. Moreover, we can likely extend it to other related groups, e.g. iron pnictides, which can probably serve as parent compounds for iron-based superconductors. These results together will be presented in the report. This work was supported by Russian Foundation for Basic Researches under Grant No. 16-03-00661. [1] Plokhikh, I. V. (2016) Inorg. Chem. 55, 12409-12418. [2] Charkin, D. O. (2014) J. Alloys Compd. 585, 644-649. [3] Charkin, D. O. (2015) J. Alloys Compd. 627, 451-454.