Preparation, crystal structure and enthalpy of formation of dicaesium hexabromoneptunate

Cs2NpBr6 has been prepared by precipitation from a saturated aqueous hydrobromic acid solution. X-ray powder diffraction patterns showed that the compound is face-centred cubic and isomorphous with the corresponding uranium compound. The lattice parameter is 11.082 +- 0.002 A at 296 +- 2 K and the crystal contains four molecules per unit cell. The enthalpy of solution in 1M HCl at 298.15 +- 0.05 K has been measured as - 107.3 +- 0.4 kJ mol-1. Combination of this value with other thermodynamic data yield - 1682.5 +- 1.9 kJ mol-1 for the standard enthalpy of formation of the compound at 298 K. (author)