Thermodynamic properties of { xCa + (1 − x)P}

Abstract

A Knudsen-effusion technique in combination with a mass-spectral analysis of the vapour phase has been applied to study thermodynamic properties of {xCa + (1 − x)P} alloys (0.5 ⩽ x ⩽ 1) over the temperature range 650 K to 1537 K. The compound CaP was found to be unstable and to decompose at low temperatures into Ca3P2 and gaseous phosphorus. Evaporation of the phosphide Ca3P2 was followed by congruent decomposition. The standard molar Gibbs energy of formation of Ca3P2 from red phosphorus and solid calcium has been expressed by the equation: Δ f G m o ( Ca 3 P 2 ) / ( J ⋅ mol − 1 ) = − ( 527000 ± 3300 ) + ( 35.93 ± 2.36 ) ⋅ ( T /K ) . Thermodynamic functions of {xCa + (1 − x)P}(1) have been estimated with help of ΔfG°m(Ca3P2) values and the coordinates of a point on the line of the two-phase equilibrium (Ca3P2 + 1). It has been shown that the liquid solution can be described by the ideal-associated-solution model if complexes of Ca3P2 type are assumed to exist.