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The HfFe 6 Ge 6 -type ScMn 6 Sn 6− x In x and YMn 6 Sn 6− x In x compounds (0.1≤ x ≤1.0) have been synthesized and studied by powder X-ray diffraction, microprobe analysis and magnetization measurements. The low In content compounds ScMn 6 Sn 5.88 In 0.12 and YMn 6 Sn 5.89 In 0.11 order antiferromagnetically at T N =381 and 334 K, respectively. The In-rich compounds ( x ≥0.61) are ferromagnetic in their whole ordered range (297≤ T c ≤359 K). Compounds of intermediate compositions order ferromagnetically above room temperature and undergo a ferro- to antiferromagnetic transition on cooling. The results are discussed and compared with those observed for the other HfFe 6 Ge 6 -type LuMn 6 Sn 6− x In x and RMn 6 Ge 6− x Ga x (R=Sc, Y, Lu) solid solutions.

The structures of the Y3Ge4, YGe1.82and YGe2.69compounds are refined from X-ray single-crystal data. Y3Ge4is isotypic with Er3Ge4. YGe1.82(isotypic with ErGe1.83) and YGe2.69(isotypic with DyGe3) compounds are both characterized by the occurrence of germanium vacancies which are localized, at random within AlB2slabs or, in a double layer of Ge atoms, respectively. In the latter case, very weak unindexed additional diffracted intensities in the Weissenberg patterns suggest a long-range ordering of these vacancies. Single crystal studies of several AlB2- and ThSi2-types YGe≤2derivatives clearly evidence large supercells which should also be related to various orderings of the Ge vacancies.

The NdCo2−xFexSi2(0

The AlB2-type RGe2−xcompounds (0.33

Investigations made by powder X-ray diffraction and susceptibility measurements on ternary germanides RTiGe (R≡La–Nd, Sm, Lu) are reported. All these new compounds crystallize in the well-known tetragonal structure of the CeFeSi-type (space group P4/nmm). LaTiGe and LuTiGe are Pauli paramagnets. CeTiGe is characterized by Curie–Weiss paramagnetism down to 5 K, whereas PrTiGe, NdTiGe and SmTiGe behave antiferromagnetically below 70, 150 and 265 K, respectively. Low temperature (T<77 K) investigations of the corresponding heavy rare earth compounds show that ErTiGe and TmTiGe are antiferromagnetic below 41 and 15 K, respectively. Metamagnetic-like behaviour occurs in ErTiGe, TmTiGe (Hc≅2 kOe) and HoTiGe (Hc≅5 kOe). These results are discussed and compared with those of isotypic ternary RTX (T≡Mn, Fe, Co, Ru; X≡Si, Ge) compounds.

The La1−xYxMn2Ge2(0⩽x⩽0.9) solid solution has been studied by neutron diffraction. This allows the construction of thex − Tmagnetic phase diagram of this system. Seven different magnetic structures, in which some are characterized by mixed (001) Mn planes where ferro- and antiferromagnetic components coexist, are necessary to describe this diagram. An antiferromagnetic Mn moment in-plane component occurs in all the studied compounds. This component gives rise to an incommensurate interplane arrangement, at least at low temperature, for the La1−xYxMn2Ge2(0⩽x⩽0.7) compounds and to a commensurate arrangement forx= 0.2 and 0.1. The corresponding Néel temperatures decrease fromTN= 421K forx= 0 toTN= 377K forx= 0.8. The antiferromagnetic in-plane component appears only at relatively low temperatures in La1−xYxMn2Ge2. The ferromagnetic in-plane component gives rise to a ferromagnetic interplane arrangement for0⩽x⩽0.7 (TN≈ 320K) and to an antiferromagnetic interplane arrangement for0.8⩽x⩽1 (TN= 362–427K). The total Mn moment value is large in the yttrium-poor compounds (μMn≈ 3.0μN) and weakr in the yttrium-rich compounds (μMn≈ 2.3 μN). A full description of the Mn sublattice magnetic behaviour within the whole RMn2Ge2and (R,R′)Mn2Ge2series is given.

We report on single crystal refinement of 22 ThCr2Si2-type structure germanides of composition RT2Ge2(R = Ca, Y, La, Nd, U; T = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd). The interatomic distances and their variations with R size, R valency and the position of the transition metal in the Periodic Table are analysed from 35 refined structures. The T-Ge bond is always strong except in the Mn compounds. The Ge-Ge bond is stronger in the right-hand side transition metal compounds. This effect is discussed in term of a previous theoretical work. Among the other contacts T-T, R-T and R-R, the T-T contacts seem to play an important role in the Ru and Rh compounds, whereas it is believed that the R-T contacts control the cell dimensions of Fe and Co compounds. This paper examines the relationships between the interatomic distances and the magnetic behaviour of manganese ThCr2Si2-type structure compounds.

The magnetic properties of the ThCr2Si2-type structure LaMn2−xFexGe2solid solution have been studied in the composition range 0 ⩽ x⩽ 1. For x⩽ 0.4, all samples exhibit an easy-axis ferromagnetic behaviour while an easy plane occurs when 0.4 ⩽ x⩽ 0.7. The Curie temperatures and the saturation magnetization values (4.2 K, Happl.= 1.5 T) decrease continuously with the Mn substitution rate from 325 K and 1.45 μBper Mn atom for x= 0, to 65 K and 0.62 μBfor x= 0.7. The Fe rich compounds (0.8 ⩽ x⩽ 1) do not exhibit ferromagnetism. At 2 K, LaMn1.5Fe0.5Ge2(Tc= 162 K) exhibits a canted ferromagnetic structure characterized by an antiferromagnetic component (μMn(AF) ∼ 2.7 μB) aligned along the c-axis and a ferromagnetic component (μMn(F) ∼ 1.1 μB) lying in the (001) plane. Moreover, at 300 K, the neutron diffraction study clearly shows that LaMn1.5Fe0.5Ge2is a purely easy-plane antiferromagnet (not detected by bulk magnetic measurements) characterized by a stacking of antiferromagnetic (001) Mn planes. The thermal dependence of the magnetic moment yields an ordering temperature TN∼ 385 K. These results are discussed and compared with those obtained for the RMn2Ge2(R = Ca, Ba, LaNd) series.

The crystallographic data of CeFeSi-type RTX compounds have been refined from single crystal X-ray data. The results show that the atomic coordinates vary with the size of the rare earth element in order to keep optimum values for the T–X and R–X distances on one hand, and with the nature of the transition metal as a function of the relative T–R affinity on the other hand. These behaviours are compared with those observed in the closely related ThCr2Si2-type RT2X2compounds. Attempts to correlate these results with the magnetic behaviours observed in the RTX and RT2X2series are made.

Neutron diffraction and 57FeMössbauer spectroscopy were used to establish the crystal structure of HoFe0.5Si2. Our data strongly suggest that this compound crystallizes in the CeNiSi2-type structure. This study removes the uncertainties concerning the localization of the transition metal in the heavy rare earth RFeXSi2(R=Tb, Dy, Ho, Er, Lu) compounds, previously described in the TbFeSi2-type structure.