RARE EARTHS. II. A MASS SPECTROMETRIC DETERMINATION OF THE HEATS OF SUBLIMATION (OR VAPORIZATION) OF NEODYMIUM, PRASEODYMIUM, GADOLINIUM, TERBIUM, DYSPROSIUM, HOLMIUM, ERBIUM AND LUTETIUM1

A disordered structure, which rests entirely on thermodynamic data and leaves the possibility that the structure is ordered and that the excess entropy arises from some other source, has been postulated from x-ray diffraction studies. Therefore, the diffraction experiment was repeated with neutrons where the more favorable ratio of scattering factors (b/sub O/ = 0.58, b/sub N/ = 0.94) allows a more sensitive test of any departures from the disordered model. Intensities were calculated for several sets of structure parameters for the disordered model (space group Pa3) and also for the ordered model. It was concluded that the completely ordered model was incompatible with the neutron diffraction data. (P.C.H.)