Hydrogen Impurity Effects. A5Tt3 Intermetallic Compounds between A=Ca, Sr, Ba, Eu and Tt=Si, Ge, Sn with Cr5B3-like Structures that are Stable Both as Binary and as Ternary Hydride and Fluoride Phases

✍ Scribed by E.Alejandro Leon-Escamilla; John D. Corbett

• Publisher: Elsevier Science
• Year: 2001
• Tongue: English
• Weight: 619 KB
• Volume: 159
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.2001.9144

✦ Synopsis

I4/mcm, No. 140) and of Ba 5 Si 3 F 0.16 (P4/ncc, Ba 5 Si 3 -type) were re5ned from single-crystal X-ray di4raction data. The interstitial H, F atoms are bound in a constricted tetrahedral (A 2 ؉ ) 4 cavity in the Cr 5 B 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 di4erences or, for Sr 5 Si 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 tetrahedral interstitial site and the cation cavity about Tt 2 and for some matrix e4ects in this structure type. The model suggests limitations on the stability of these binary A 5 Tt 3 compounds for the heavier tetrels, as observed. The resistivities of Ca 5 Ge 3 and Ca 5 Ge 3 H x are both characteristic of poor metals, and Pauli-like magnetic susceptibilities are exhibited by Ca 5 Ge 3 , Ca 5 Ge 3 H x , Ca 5 Ge 3 F 0.66 , Sr 5 Ge 3 , and Sr 5 Sn 3 . The characteristic ideal Tt 6 ؊ 2 dimers are evidently not realistic descriptions for these phases; rather at least some of the * 4 electrons in the dimers are delocalized in a conduction band. This e4ect 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 6uoride, as might be expected.