Hydrogen impurity effects. A

Journal Article · Fri Jun 01 00:00:00 EDT 2001 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1006/jssc.2001.9144· OSTI ID:782911

Leon-Escamilla, E Alejandro; Corbett, John D

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{sub 5}B{sub 3}-type A{sub 5}Tt{sub 3} phases and the corresponding ternary hydrides with stuffed Cr{sub 5}B{sub 3}- (Ca{sub 5}Sn{sub 3}F-) type structures. All of those tested, Ca-Si, Ba-Si, Ca-Ge, also yield the isotypic A{sub 5}Tt{sub 3}F{sub x} phases. The tetragonal structures of Ca{sub 5}Si{sub 3}, Ca{sub 5}Si{sub 3}F{sub 0.42}, Sr{sub 5}Si{sub 3}, Eu{sub 5}Si{sub 3}H{sub x}, Ca{sub 5}Ge{sub 3}, Ca{sub 5}Ge{sub 3}H{sub x}, Ca{sub 5}Ge{sub 3}F{sub 0.66} (I4/mcm, No. 140) and of Ba{sub 5}Si{sub 3}F{sub 0.16} (P4/ncc, Ba{sub 5}Si{sub 3}-type) were refined from single-crystal X-ray diffraction data. The interstitial H, F atoms are bound in a constricted tetrahedral (A{sup 2+}){sub 4} cavity in the Cr{sub 5}B{sub 3}-type heavy atom structure, which can be described ideally as (A{sup 2+}){sub 5}(Tt{sub 2}){sup 6{minus}}(Tt){sup 4{minus}}. Many of 14 previous reports of the phases reported here were apparently hydrides according to lattice constant differences or, for Sr{sub 5}Si{sub 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{sub 2} and for some matrix effects in this structure type. The model suggests limitations on the stability of these binary A{sub 5}Tt{sub 3} compounds for the heavier tetrels, as observed. The resistivities of Ca{sub 5}Ge{sub 3} and Ca{sub 5}Ge{sub 3}H{sub x} are both characteristic of poor metals, and Pauli-like magnetic susceptibilities are exhibited by Ca{sub 5}Ge{sub 3}, Ca{sub 5}Ge{sub 3}H{sub x}, Ca{sub 5}Ge{sub 3}F{sub 0.66}, Sr{sub 5}Ge{sub 3}, and Sr{sub 5}Sn{sub 3}. The characteristic ideal Tt{sub 2}{sup 6{minus}} dimers are evidently not realistic descriptions for these phases; rather, at least some of the {pi}*{sup 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.

• Research Organization: Department of Chemistry, Iowa State University, Ames, IA (US); Ames Lab., Ames, IA (US)
• Sponsoring Organization: USDOE Office of Science (US)
• DOE Contract Number: W-7405-ENG-82
• OSTI ID: 782911
• Journal Information: Journal of Solid State Chemistry, Vol. 159, Issue 1; Other Information: PBD: 1 Jun 2001; ISSN 0022-4596
• Country of Publication: United States
• Language: English

Related Subjects

08 HYDROGEN
36 MATERIALS SCIENCE
37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ATOMS
BOND LENGTHS
CATIONS
DIMENSIONS
DIMERS
ELECTRONS
EUROPIUM
HYDRIDES
HYDROGEN
INTERSTITIALS
OXIDATION
STABILITY
X-RAY DIFFRACTION
CALCUIM COMPOUNDS
STRONTIUM COMPOUNDS
BARIUM COMPOUNDS
EUROPIUM COMPOUNDS
SILICON COMPOUNDS
GERMANIUM COMPOUNDS
FLUORIDES
INTERMETALLIC COMPOUNDS
ELECTRIC CONDUCTIVITY
MAGNETIC SUSCEPTIBILITY
CRYSTAL STRUCTURE
LATTICE PARAMETERS