Thermodynamic properties of the alkali metal hexachloroniobate and hexachlorotantalate compounds by vapour pressure measurements

The vapour pressures for the decomposition of the solid and liquid compounds AMCl6 where A = Na, K, Rb, or Cs, and M = Nb or Ta, were measured by a fused quartz Bourdon spiral pressure gauge. The hexachlorotantalates were found to be more stable thermally than the hexachloroniobates. Enthalpies of decomposition were calculated. The dissociation temperatures of each sequence of compounds increase with increasing size of the radius of the alkali-metal cation. Partial pressures of NbCl5 or TaCl5 vapour in equilibrium with solutions representing the binary Systems KCl–KNbCl6 and KCl–KTaCl6, respectively, are also reported.