Heusler type structure

The famous Heusler type structure can be considered as a ternary ordered variant of the BiF3 type. On the other hand, the YPd2Pb structure can also be derived from a cubic closest packing of the lead atoms, where the octahedral voids are filled with yttrium (rocksalt substructure) and the tetrahedral voids are filled by the palladium atoms. 

Magnetic properties of rare earth metal-transition metal-indides with CsCl superstructures or Heusler type structure... 

A quantum-mechanical interpretation of Miedema s parameters has already been proposed by Chelikowsky and Phillips (1978). Extensions of the model to complex alloy systems have been considered. As an interesting application we may mention the discussion on the stabilities of ternary compounds presented by de Boer et al. (1988). In the case of the Heusler-type alloys XY2Z, for instance, the stability conditions with respect to mechanical mixtures of the same nominal composition (XY2+Z, X+Y2Z, XY+YZ, etc.) have been systematically examined and presented by means of diagrams. The Miedema s parameters, A t>, A ws1/3, moreover, have been used as variables for the construction of structural maps of intermetallic phases (Zunger 1981, Rajasekharan and Girgis 1983). 

An example where, due to ordering, we observe perhaps in a more immediate way, the increase of the unit cell size (formation of a multiple cell) is the MnCu2Al-type structure (representative of the so-called Heusler alloys) which can be considered a derivative structure (superstructure) of the cP2-CsCl type, which in turn is a superstructure of the W-type structure, corresponding to a non-primitive cubic cI2... 

Figure 3.31. cF16-MnCu2Al type structure (representative of the Heusler type alloys). The unit cell is shown in (a), an eighth of the cell is shown in (b). It degenerates into a cP2-CsCl type cell if the atoms at the cube comers (Mn and Al) are equivalent. If all the atoms were equivalent there would he a further degeneration into the cI2-W type. 

Finally, it is noted that besides the Laves phases the Heusler-type phases NijAlNb and NijAlTa with an L2j structure may... 

The classic Heusler-type alloys are Cu2AlMn and Cu2SnMn (Dwight, 1967 Kouvel, 1967) which will be addressed in Sec. 6. Their unique feature is their ferromagnetism, since the constituent elements are not ferromagnetic. These phases crystallize with the L2j structure which is derived from the B2 structure (see Fig. 1), and which is also adopted by a series of other ternary phases (Dwight, 1967). 

The structural similarity of MgAgAs-type compounds with Heusler alloys and with the transition-metal based half-metallic ferromagnets (de Groot et al. 1983) has provoked band-structure calculations of UNiSn (Mueller et al. 1987, Albers et al. 1987). Self-consistent-field scalar relativistic spin-polarized calculations neglecting the spin-orbit coupling revealed the following features of the valence band ... 

Closely related to the structure of the Heusler alloys described above is the Clb structure. This structure type is most easily described in terms of fig. 30. It differs from the normal Heusler structure only in the occupation of the C sites. In the normal Heusler alloys X2YZ, both the A and C sites are fully occupied by X atoms. 

This Structure type is a superstructure of the Cap2 type. The As atoms occupy the positions of the Ca atoms the Ag and Mg atoms regularly occupy the positions of the F atoms. The structure can also be obtained by removing one of the Cu atoms from the MnCu2Al unit cell and placing the vacancies and the copper atoms in a regular manner. The compounds which crystallize in the structure types MnCu2Al are called Heusler phases. 

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