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 oxychlorides (both powder and single crystal samples) were measured on the 50-1150 mu g scale, and their standard enthalpies of formation at 298 K calculated. The lanthanide oxychlorides (LnOCl) were prepared from the corresponding sesquioxides by treatment with gaseous HCl. Single crystals were grown from a 1:10 (Eu, Gd) or 1:1 (Lu) mixture of sesquioxide-in-trichloride flux. Prior estimations of the standard enthalpies of formation for these three oxychlorides were made, based upon the thermodynamic systematics of LnCl(3) and LnOCl as a function of Ln(III) ionic radius. Standard formation enthalpies at 298 K determined in this work are: Delta(f)H(0)(EuOCl(cr))=-903.5+/-3 kJ mol(-1); Delta(f)H(0)(GdOCl(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 experimentally, was estimated on the basis of interpolation within the lanthanide systematics to be -997+/-4 kJ mol(-1). (C) 1998 Elsevier Science S.A.