Hexagonal MgZn2 Research Articles

Structural and magnetic characteristics of the pseudobinary system PrMn 2ErMn 2

Alloys were prepared by induction melting and subsequent annealing at 950 °K. Replacement of Er by Pr in ErMn 2 results in increasing lattice parameters in the hexagonal MgZn 2-type structure. From Pr 0.283Er 0.05Mn 0.667 to Pr 0.163Er 0.17−Mn 0.667 the alloys crystallize with a cubic face-centered structure with lattice parameters from 12.308 to 12.601 Å, isotypical with Th 6Mn 23 (G-phase). This is also the structure of (TmPr)Mn 2. PrMn 2 crystallizes as α-Mn type (χ-phase), body-centered cubic, with a = 8.902 A ̊ at 950 °K, and as G-phase with a = 12.690 A ̊ at 1000 °K. In the liquid He range, Pr and Er magnetic moments align antiferromagne-tically in the MgZn 2-type structure. High saturation moments at 4.2 °K in the face-centered cubic, phase suggest complex behavior. These alloys show two Curie temperatures around 60 ° and 420 °K and remain ferromagnetic above room temperature. Neutron diffraction at 4.2 ° and 300 °K does not reveal antiferromagnetic ordering within crystallographic sites. Some kind of magnetic order may be retained beyond 1000 °K. The compounds do not follow the Curie-Weiss law to 1000 °K. PrMn2 (χ-phase) has a Weiss constant of —80 °K and may order antiferromagnetically below 4.2 °K on Pr sites.

The Hafnium-chromium system

A phase diagram is proposed for the hafnium-chromium system based on the results of microscopic, X-ray and thermal analyses. The main features of the diagram are: a Laves phase, HfCr 2, which melts congruently at 1825 °C and undergoes a crystallographic transformation between 1320 ° and 1360 °C; eutectic reactions at 1480 °C and 29 at.% Cr and 1660 °C and 86 at.% Cr; and a eutectoid reaction at 1390 °C and 12 at.% Cr. It was confirmed that the stable form of HfCr 2 at low temperatures has the face-centered cubic, MgCu 2-type structure, while at high temperatures it has the hexagonal MgZn 2 structure.

Pseudo-binary systems involving rare earth laves phases

The phase relations in a number of pseudo-binary systems involving rare earth Laves phases have been determined. Complete series of cubic solid-solutions occur in the DyMn 2HoMn 2, HoMn 2HoFe 2, DyMn 2DyFe 2, HoMn 2HoAl 2, TbMn 2TbAl 2 and DyMn 2DyAl 2 pseudobinary systems. Deviations from linearity in the lattice constants with composition occur in all these systems. Complete series of cubic solid-solutions also exist in the GdAl 2ErAl 2, GdAl 2PrAl 2, GdAl 2NdAl 2, GdAl 2DyAl 2, TbAl 2NdAl 2 and TbAl 2DyAl 2 systems. For these systems, no deviation from linearity occurs in the lattice constants. For the DyFe 2DyAl 2 and DyCo 2DyAl 2 systems, two new ternary phases, DyFeAl and DyCoAl, form and have the MgZn 2 structure. Their structures were determined from X-ray powder data only. The electronic state giving rise to the formation of these ternary phases is discussed qualitatively. For the DyMn 2TmMn 2 system, the range of composition in which the cubic MgCu 2 and hexagonal MgZn 2 structures exist are reported. No complete series of solid solutions or intermediate phases formed in the DyNi 2DyAl 2 system.

Crystal Structure of TaCr2 and CbCr2

Abstract : It is shown in the present study that the only intermediate phase in both the tantalum-chromium and the columbium-chromium systems corresponds to the ideal stoichiometric ratio TaCr2 or CbCr2. Both structures are cubic, MgCu2 type. At high temperature, however, TaCr2 has a hexagonal MgZn2 type of structure, which can be retained at room temperature by fast cooling.