Thermodynamic properties of silicides IV. Determination of the standard molar enthalpies of formation at the temperature 298.15 K of pentatungsten trisilicides of composition W5Si3and W5Si3.115by fluorine-combustion calorimetry

Thermodynamic properties of silicides. VI. Pentamolybdenum trisilicide (Mo5Si3). Fluorine combustion calorimetric determination of the standard molar enthalpy of formation at the temperature 298.15 K

“…Previous experience with fluorine bomb calorimetric studies of molybdenum silicides (3,7) and tungsten silicides (8,9) strongly suggested that Mo 5 Si 3 could be made to react with F 2 (g):…”

“…(8) The sample arrangement within the combustion vessel resembled closely that described (9) for experiments with W 5 Si 3 . Powdered Mo 5 Si 3 and a few mg of rhombohedral sulfur fuse, supported on a tungsten foil saucer, sat atop a prefluorinated nickel crucible of mass 126 g that had small holes drilled along its base to promote circulation of F 2 (g) during reaction.…”

“…Such evaluations incorporated estimated auxiliary thermodynamic functions. In a concerted effort to improve that unsatisfactory situation, we, within the past few years, have determined D f H°ms of MoSi 2 , (3) Mo 3 Si, (7) WSi 2 , (8) and W 5 Si 3 (9) at the temperature 298.15 K, and Callanan et al (4,5) measured the molar heat capacities C°p ,m at subambient temperatures of some of the same samples, and derived the corresponding thermodynamic functions.…”

Thermodynamic properties of silicides V. Fluorine-combustion calorimetric determination of the standard molar enthalpy of formation at the temperature 298.15 K of trimolybdenum monosilicide Mo3Si, and a critical assessment of its thermodynamic properties

“…Accordingly, we and others continue to address these deficiencies, and several recent thermodynamic investigations have dealt with most of the known silicides of molybdenum and tungsten: MoSi 2 , (4,5) Mo 5 Si 3 , (6) WSi 2 , (7,8) and W 5 Si 3 . (9,10) Nevertheless, values of many of the fundamental thermodynamic properties of Mo 3 Si and Mo 5 Si 3 are still not known with certainty. Work in our laboratory on the determination of D f H°m(Mo 5 Si 3 ) is nearly at an end, and we hope to report the results soon.…”

“…(6) The study of Mo 3 Si to be described here continues our recent fluorine-bomb combustion determinations of the standard massic (formerly called specific) energies of combustion and, thence, the D f H°ms of MoSi 2 , (4) WSi 2 , (8) and W 5 Si 3 . (10) We are aware from experience (4,8,10) that silicides of Mo and W can be made to react quantitatively with F 2 (g) in the calorimetric bomb when tungsten is used as an auxiliary substance to drive the fluorination to completion. We anticipated that, with a similar arrangement, the combustion reaction would proceed according to:…”