Chemistry and structural chemistry of phosphides and polyphosphides. 34

The vapor pressure of the europium phosphides EuP7, α-EuP3, β-EuP3, EuP2 and Eu3P4 were measured using the Knudsen-Effusion technique. These compounds were vaporized incongruently according to the general reaction a EuP/sub m/, (s) = a EuP/sub n/, (s) + P4, (g) where a (m-n) = 4. The following enthalpies and entropies of reaction were calculated (ΔH(kJ mol-1)/ΔS(J K-1 mol-1)/T(K). [ΔH/ΔS/T]: EuP7 → β-EuP3 [191/215/698]; α-EuP3 → EuP2 [212/162/847]; β-EuP3 → EuP2 [261/212/847]; EuP2 → Eu3P4 [209/144/868]; Eu3P4 → EuP [240/131/1110]. The temperature and pressure ranges over which these phosphides exist were determined from the above values. Standard enthalpies of formation relative to EuP(s) were calculated by combining different reaction enthalpies. The results demonstrate the relatively high stability of β-EuP3, EuP7 and the instability of EuP2. (author)