A magnesium deuteride preparation of composition Mg(D0.gH0.1) 2 is found to be tetragonal with a=4.5025, c=3.0123/~. The atomic positions are:2 Mg in (000) ~922/ , 1~1-~ 4(D +H) in _+ (xxO) (½ +x, t-x, 1) with x =0.306 _+0"003.The bond lengths are Mg-6D = 1.95 + 0.02 A. The structure is of the rutile type, Some years ago it was shown by X-ray diffraction (Ellinger et al., 1955) that magnesium hydride is tetragonal with two molecules in a unit cell of dimensions a--4.5618_+ 0.0005, c=3.0205_+ 0.0005 A.The two magnesium atoms were found to be in positions (0, 0, 0)( I, ½, I). It was suggested that the hydrogen atoms were located at the sites +_(x,x, 0)G+x, ½-x, I) * Work done under the auspices of the U.S. Atomic Energy Commission. with x ~ 0.306, i.e. that MgHe has the rutile type of structure. However, the hydrogen atoms had no measurable effect on the diffraction intensities, and accordingly there was no experimental proof for the proposed hydrogen positions. A direct determination of the hydrogen sites by means of neutron diffraction was therefore undertaken, using the deuteride in preference to the hydride.Magnesium deuteride was prepared by reaction of deuterium gas with magnesium metal (obtained by decomposition of MgH2) for four days at 510 °C.
low, but becoming less than 2 as the temperature increases. The "M" represents metal which is saturated with hydrogen in solid solution. Since the lines in Fig. 8 are straight, the AH's do not change with composition or temperature within the accuracy of the measurements, and thus the numbers in column 3 of Table I may be regarded as the Table I Heats of Formation of Dihydrides" Calorimetric AH Differential AH (kcal./mole Ha) (kcal./mole Ha) La -40.09 (LaH2.76) -49.7 ± 0.1 Ce -42.26 (CeH2.69) -33.9 ± 0.9 Pr -39.52 (PrH2.86) -47.8 ±0.2 Nd -44.8 ± 0.8 " Errors listed are the probable errors as computed from the deviations of the experimental points from the leastsquare lines.heats of formation of one mole of the "ideal" MH2 phase formed from the metal and gaseous hydrogen. No strict comparison with Sieverts and Gotta's calorimetric data is possible, because Sieverts and Gotta's data were obtained for compositions other than MH2 and do not represent differential heats of occlusion. The calorimetric data should, however, be close to ours, and general agreement is evident from the table.Equations for the variation of decomposition pressure with temperature of the MH2 phases may be obtained by fitting expressions of the form, log p = A -B/T, to the available points of Fig. 8 by least-square methods. The equations so obtained are for La, logl0 p = (10.758 ± 0.001) -(10858 ± 23)/T for Nd, logio p = (9.370 ± 0.179) -(9796 ± 171)/ for Pr, logic P = (10.229 ± 0.048) -(10446 ± 46)/T for Ce, log10 p = (7.708 ± 0.226) -(7417 ± 190)/P Temperatures are in degrees Kelvin, pressures in mm. of mercury. The limits of error indicated are the probable errors as calculated from the deviations of the data from the fitted curves. measurements, O.
Pressure-temperature-composition data are presented for the lanthanum-, cerium-, praseodymium-and neodymiumhydrogen systems in the ranges T = 150 to 800°, p = 0.01 to 400 mm. It is concluded that La, Ce, Pr and Nd all behave similarly with hydrogen, exhibiting two solid phases, metal and a hydride phase, for compositions between M and MIL, and a single solid hydride phase for compositions between MIL and approximately ». (M = La, Ce, Pr or Nd.) In the latter range, composition is dependent on temperature and pressure. The heats of formation of the MEL hydride phases, as obtained from the pressure-temperature measurements are:
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