Polar intermetallic compounds of the silicon and arsenic family elements and their ternary hydrides and fluorides

Abstract

<p>An investigation has been made on the effects of hydrogen and fluoride in the solid state chemistry of alkaline-earth and divalent rare-earth metal pnictide (Pn) and tetrelide (Tt) phases A5(Pn,Tt,)3Zx, where A = Ca, Sr, Ba, Sm, Eu, Yb; Pn = As, Sb, Bi; Tt = Si, Ge, Sn, Pb and Z = H, F. Several trivalent rare-earth-metal pnictides, RE5Pn3 (RE = Y, La, Gd, Tb, Dy, Ho, Er, Tm) and alkaline-earth-metal trielides, A5Tr3Zx (Tr = Ga, In, Tl) have been included in an effort to complete observed structural trends. Two main experimental techniques were followed throughout this work, (a) reactions in absence of hydrogen or under continuous high vacuum, and (b) reactions with binary metal hydrides, AHx, in closed containers. Ternary fluorides followed to a large extent the chemistry of the corresponding ternary hydrides.;The results demonstrate that all the phases reported with the [beta]- Yb5Sb3-type structure in the A5Pn3 systems are hydrogen-stabilized compounds. Reactions in absence of hydrogen lead to compounds with the Mn5Si3-type structure. The structure type [beta]- Yb5Sb3 (=Ca5Sb3F) was found to be characteristic of ternary systems and inaccurately associated with phases that form in the Y5Bi3-type, a structure that is distinctive for pnictide compounds of trivalent metals. A new series of isomorphous Zintl compounds with the Ca16Sb11-type structure were prepared and studied as well.;All the alkaline-earth-metal tetrelides, A5Tt3, that crystallize in the Cr5B3-type structure can be interstitially derivatized by hydrogen or fluoride. Stannides and plumbides that form in this structure type are, with the exception of Sr5Sn3, hydrogen-stabilized compounds. Reactions in absence of hydrogen gave compounds in the Cr5B3, Ba5Si3, Pu31Pt20, Ca5Pb3 or W5Si3 structure types. Several new compounds with these structures were prepared. Additionally, an unprecedented Yb36Sn23 phase was discovered; its structure features isolated atoms, dimers and hexamers of tin.;Binary and ternary compounds were characterized by Guinier powder patterns, single crystal X-ray (30 structural refinements) and powder neutron (Ca5Bi3D and Ca5Sn3D samples) diffraction techniques. In an effort to establish property-structure relationships, electrical resistivity and magnetic measurements were performed on selected systems, and the results were explained in terms of the Zintl concepts, aided by extended Huckel band calculations. The structures of hydrogen-stabilized and electron deficient A5Tt3Hx compounds were rationalized in terms of multiple Tt-Tt bond interactions in these systems. ftn1This work was performed at the Ames Laboratory under contract no. W-7405-eng-82 with the U.S. Department of Energy. DOE Report no. IS-T 1730.</p>