Hydrogen Impurity Effects. A_5Tt_3 Intermetallic Compounds between A = Ca, Sr, Ba, Eu and Tt = Si, Ge, Sn with Cr_5B_3-like Structures that are Stable Both as Binary and as Ternary Hydride and Fluoride Phases

摘要

All of the binary systems Ca, Sr, Ba, or Eu (A) with Tt (tetrel) = Si or Ge as well as Sr-Sn form both binary Cr_5B_3-type A_5Tt_3 phases and the corresponding ternary hydrides with stuffed Cr_5B_(3-) (Ca_5Sn_3F-) type structures. All of those tested, Ca-Si, Ba-Si, Ca-Ge, also yield the isotypic A_5Tt_3F_x phases. The tetragonal structures of Ca_5Si_3, Ca_5Si_3F_(o.42), Sr_5Si_3, Eu_5Si_3H_x Ca_5Ge_3, Ca_5Ge_3H_x Ca_5Ge_3F_(o.66) (I4/mcm, No.140) and of Ba_5Si_3F_(o.16) (P4cc, Ba_5Si_3-type) were refined from single-crystal X-ray diffraction data. The interstitial H, F atoms are bound in a constricted tetrahedral (A~(2+))_4 cavity in the Cr_5B_3-type heavy atom structure, which can be described ideally as (A~(2+))_5(Tt_2)~(6-)(Tt)~(4-). Many of 14 previous reports of the phases reported here were apparently hydrides according to lattice constant differences or, for Sr_5Si_3, the fractional coordinates of Sr2 about the tetrahedral site. An articulated model is developed that allows description of the relationship between the dimensions of the tetrabedral interstitial site and the cation cavity about Tt_2 and for some matrix effects in this structure type. The model suggests limitations on the stability of these binary A_5Tt_3 compounds for tbe heavier tetrels, as observed. The resistivities of Ca_5Ge_3 and Ca_5Ge_3H_x are both cbaracteristic of poor metals, and Pauli-like magnetic susceptibilities are exhibited by CasGeJ' Ca_5Ge_3H_x, Ca_5Ge_3F_(0.66), Sr_5Ge_3, and Sr_5Sn_3. Tbe characteristic ideal Tt_2~(6-) dimers are evidently not realistic descriptions for these pbases; rather at least some of the #pi#~(*4) electrons in the dimers are delocalized in a conduction band. This effect appears to be greater in two europium salts. Bond lengths of dimers in the Ca-Si and Ca-Ge families appear to shorten slightly in three instances of their oxidation to form the hydride or the fluoride, as might be expected.