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B. T. iVI. WILLIS 689 APPENDIXThe treatment in § 2 can be extended to allow ~or the change of layer spacing accompanying segregation.Let A~, Aa(l+s/2~), Aa(l+s/~) be the spacings between layers with average scattering factors of f~ and f~, f~ and f~, f~ and f~. It is easily shown that formula (15) for the intensity of the 3M±l reflexions is then modified, assuming s ~ 1, to For a given alloy, with finite values of a, fl and s, the profiles of the 3M+l and 3M-1 reflexions are no longer equivalent; moreover, these profiles are different for the successive peaks occurring with increasing h a . Evaluation of (17) California, Los Alamos Scientific Laboratory, Los Alamos, ~Vew Mexico, U.S.A. (Received 9 December 1958) The structure of 1)u_Ni 3 and the structure and composition of CeNi 3 have been determined by single crystal X-ray methods. Pul~i 3 has three formula units in a rhombohedral unit cell with a = 6.22 A and a = 33 ° 44', probable space group R3m. CeNi 3 has six formula units in a hexagonal cell with a=4.98 and c=16.54 A, probable space group P63/mmc. These structures are both derived from stacking single layers of the MNi 5 structure (CaCus-type) and double layers of the MNi~ structure (Cu~Mg-type).

Neutron diffraction experiments on metallic europium have been conducted over a temperature range from room temperature to 4°K using a sample in the form of europium filings. The data show that the metal undergoes a transition to an antiferromagnetic state at a Neel temperature TN of 91 °K and that this ordering process continues until saturation develops at about 20°K. A model which conforms to the data consists of a helical spin structure with the magnetic moments lying parallel to a cube face and with the rotation axis directed perpendicular to the moments or along the [100] direction. The pitch of the helix was observed to change very slightly with temperature; the period was 3.5a at T/TN=1 and increased to 3.6a at T/TN=0.05 where a is the lattice spacing. On the basis of the above model and the intensities of the magnetic reflections, the calculated ordered moment at each atom site is 5.9±0.4 /J,B per atom. This measured moment is somewhat less than the maximum theoretical value of 7 [XB expected from divalent europium characterized by a spin only ground state of 8 57/2. Values of magnetic moment times magnetic form factor, /*/, as calculated from Eu coherent magnetic reflections are in agreement with similar values of /if derived from the compound EuO. The intensities of the magnetic diffraction peaks deviate from a Brillouin behavior and instead follow a temperature dependence proportional to (TN-T) 112 over a considerable region below the Neel temperature. The Debye temperature, as derived from the nuclear peak intensities, exhibited a variation from 70 to 120°K over a corresponding sample temperature range of 100 to 293°K.

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The angular distribution of the 501-keV y radiation from low-temperature polarized I80mjj£ ^g^g found to exhibit a very large parity-nonconserving forward-backward asymmetry. At a polarization of 71 %, the measured asymmetry is -(1.49± 0.25) %. This asymmetry corresponds to an£2/M2 mixing ratio of magnitude 0.038±0.007 in fairly good agreement with circular-polarization experiments. We also deduce the £'3/M2 mixing ratio to be +6.0±0.5, agreeing with angular-correlation measurements,

The magnetic properties of two dialuminides, DyAl2 and NdAl2, have been investigated by neutron diffraction and susceptibility measurements. Both compounds show Curie-Weiss behavior in the paramagnetic region and are basically ferromagnetic below Curie temperatures of 62°K and 65°K, respectively. At low temperatures, the compound NdAl2 is a pure ferromagnet, whereas DyAl2 is almost completely ferromagnetic, but has the additional property of a weak antiferromagnetic ordering. The susceptibility measurements show an effective moment per molecule of 9.7±0.1 and 3.1±0.05 μB for DyAl2 and NdAl2, respectively. The neutron diffraction data show a rare-earth-ordered saturation moment of 9.1±0.4 and 2.5±0.1 μB on the Dy and Nd atom sites, respectively. The Al atom sites exhibit no measureable moment within the limits of experimental detection (0.5 μB). Both compounds show a temperature-dependent magnetic ordering process which obeys a Brillouin function. The experimental magnetic form factor for both the Dy3+ and Nd3+ ions diminishes more rapidly with increasing (sinθ)/λ than present theory postulates.

The magnetic properties of PrAb and ErAb have been investigated by neutron-diffraction and susceptibility measurements on powder samples of the above compounds. The susceptibility data show that both compounds obey the Curie-Weiss law; the effective moment per molecule in the paramagnetic region is 3.5±0.05 J!.B for PrAh and 9.2±0.1 J!.B for ErAh. The neutron-diffraction measurements confirm that these compounds are ferromagnetically ordered below a Curie temperature of 34°K for PrAh and 14°K for ErAh; the respective saturated ordered moments at the rare earth atom sites are 2.94±0.05 J!.B and 8.3±0.3 J!.B. The magnetic form factor data from the ErAl2 coherent magnetic reflections are in good agreement with similar data obtained by other investigators on an Er single crystal. The magnetic scattering intensity of the (111) reflection from PrAl2 was measured just below the Curie temperature To in order to determine the long-range magnetic-order temperature dependence. The results show that the spontaneous magnetization M follows a power law in the vicinity of To of the form M ex: (To-T)fl, where ,8=0.6±0.1.