Neutron-diffraction measurementswere performed on polycrystalline samples of 13 different compounds of the type RM2X& (R, a heavy lanthanide from Gd to Tm; M, the 3d element Fe, Co, Ni, or Cu', X, Si or Ge}, at room temperature and at low temperatures. The crystallographic structure of these compounds belongs to the tetragonal space group I4/mmm (D4q ). It was found that these compounds, paramagnetic at room temperature, undergo a transition to antiferromagnetism at low temperatures.The transition temperatures and most of magnetic structures were determined. Below the transition temperature, magnetic ordering was found only on the R sublattice, and no magnetic ordering was detected on the M sublattice. The magnetic lattices were found to be characterized by propagation vectors of k=(0, 0, 1} in the cobalt compounds investigated (TbCo2Si2, HoCo2Si2, DyCo2Ge2, and ErCo&Ge2), k = (~,0, 2 ) in the copper compounds investigated (TbCu2Si2, HoCu2Si» TbCu2Ge2, and HoCu2Ge2), and incommensurate k =(0,0,0.76) in HoNi2Geq (unsolved incommensurate in other nickel compounds and in iron compounds). Together with previously published results on other RM2X2 compounds, the magnetic lattice is found to vary with M and be almost invariant to R.

Single crystals of the ternary semiordered alloys UFe x Al 12Ϫx with xϭ3. 8, 4.0, 4.2, 4.5, 4.7, 5.0, 5.5, and 5.8, were prepared, and found to crystallize in the tetragonal ThMn 12 -type structure ͑space group I4/mmm͒. Studies of monocrystalline and polycrystalline samples of these materials, by ac susceptibility, magnetization, and 57 Fe Mössbauer spectroscopy, show two magnetic transitions in the range of 4.0ϽxՇ4.7, where only a single transition has been previously reported for polycrystalline materials. These measurements yield the magnetic phase diagram of this system, extending the magnetic structures of single crystal UFe 4 Al 8 , determined by ͑unpolarized and polarized͒ neutron diffraction, of polycrystalline UFe 5 Al 7 , determined elsewhere by neutron diffraction, and of single crystal UFe 5.8 Al 6.2 , determined by magnetization. The four magnetic regions, identified in the magnetic phase diagram, and their proposed magnetic structures, are discussed.

Neutron-diffraction measurements on TbCo3B2, with the isotope B11, show ferromagnetic structure below 35(2) K. Ordered magnetic moments of Tb and Co were found to be in the basal planes directed parallel to each other. The Tb ordered moment is significantly smaller than its free-ion value. The Co ordered moment is nearly zero. Ac-susceptibility and SQUID magnetometer measurements agree with the neutron results.

The magnetic properties of NaCl-type uranium compounds with group VA and group VIA elements are discussed. The assumption that / is a good quantum number is shown to be a good approximation for the ground multiplet of the uranium ion. The uranium ions are dealt with by the usual methods of crystalfield theory. They are assumed to be in the electrostatic field of their neighboring cations and conduction electrons and also to experience the exchange field of neighboring uranium ions. This model enables us to calculate the magnetic saturation moment in the ordered state and the susceptibility in the paramagnetic state. The results agree with experiment, except that the calculated (paramagnetic) magneton numbers of the Uvi compounds are higher than those measured. The computed (total) splittings of the ground state in those compounds are between 500 and 1200 cm -1 . The exchange interaction is derived from the RudermanKittel-Kasuya-Yosida type of interaction and is used to explain the occurrence of the observed ferromagnetic and antiferromagnetic (type-I) ordering. The problems involved in such a model are discussed at length. A schematic band structure is proposed, the effective conduction electron mass being computed to be 3-9m, while the s-/coupling constant is 12-22 eV A 3 , appreciably larger than in lanthanide compounds.

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