STANDARD ENTHALPIES OF FORMATION FOR EUROPIUM, GADOLINIUM, AND LUTETIUM OXYCHLORIDES, CALCULATED FROM MEASURED ENTHALPIES OF SOLUTION

The enthalpies of solution in 1.000 N HCl of Eu, Gd, and Lu oxychlorid es (both powder and single crystal samples) were measured on the 50-11 50 mu g scale, and their standard enthalpies of formation at 298 K cal culated. The lanthanide oxychlorides (LnOCl) were prepared from the co rresponding sesquioxides by treatment with gaseous HCl. Single crystal s were grown from a 1:10 (Eu, Gd) or 1:1 (Lu) mixture of sesquioxide-i n-trichloride flux. Prior estimations of the standard enthalpies of fo rmation for these three oxychlorides were made, based upon the thermod ynamic systematics of LnCl(3) and LnOCl as a function of Ln(III) ionic radius. Standard formation enthalpies at 298 K determined in this wor k are: Delta(f)H(0)(EuOCl(cr))=-903.5+/-3 kJ mol(-1); Delta(f)H(0)(GdO Cl(cr))=-981.4+/-3 kJ mol(-1); and Delta(f)H(0)(LuOCl(cr))=-987.2+/-4 kJ mol(-1). These results are in accord with the estimations made. In addition, the standard enthalpy of formation of promethium oxychloride , Delta(f)H(0)(PmOCl(cr)), which is unknown and difficult to obtain ex perimentally, was estimated on the basis of interpolation within the l anthanide systematics to be -997+/-4 kJ mol(-1). (C) 1998 Elsevier Sci ence S.A.