The standard molar Gibbs energy of formation of YbPt 3 and LuPt 3 intermetallics

Abstract

The standard molar Gibbs energies of formation of YbPt 3 and LuPt 3 intermetallic compounds have been measured in the temperature range 880 K to 1100 K using the solid-state cells: Ta|Yb+YbF 2|CaF 2|YbPt 3+Pt+YbF 2|Ta, and Ta|Lu+LuF 3|CaF 2|LuPt 3+Pt+LuF 3|Ta, The trifluoride of Yb is not stable in equilibrium with Yb or YbPt 3. The results can be expressed by the equations: Δ f G m ° ( YbPt 3) (J· mol −1) =−322100+0.39( T K )±400, Δ f G m ° ( LuPt 3) (J· mol −1) =−366800+3.82( T K )±400. The standard molar Gibbs energy of formation of LuPt 3 is −41.1 kJ · mol −1 more negative than that for YbPt 3 at 1000 K. Ytterbium is divalent in the pure metal and trivalent in the intermetallic YbPt 3. The energy required for the promotion of divalent Yb to the trivalent state is responsible for the less negative Δ f G m o of YbPt 3. The enthalpies of formation of the two intermetallics are in reasonable agreement with Miedema's model. Because of the extraordinary stability of these compounds it is possible to reduce oxides of Yb and Lu with hydrogen in the presence of platinum at T K > 1473 . The equilibrium chemical potential of oxygen corresponding to the reduction of Yb 2O 3 and Lu 2O 3 by hydrogen in the presence of platinum is presented in the form of an Ellingham diagram.