Structure‐Bonding Relationships in the Laves Phases

Abstract

The Laves phase structures (MgCu2 MgZn2 and MgNi2 types) are adopted by a large number of binary intermetallic compounds of composition AB2. These phases belong to the class of tetrahedrally closepacked alloys. Two of the structural features of the Laves phases which have aroused interest are, firstly the dependence of the structure adopted (especially the choice between the cubic MgCu2 and hexagonal MgZn2 types) on the number of valence electrons of the constituent metal atoms and, secondly the structural distortions observed in a number of the hexagonal MgZn2 type phases. By performing extended Hückel band calculations on model AB2 compounds with both the cubic and hexagonal structures and on the B sub‐nets alone, structure electron‐count and deformation‐electron count correlations are derived. These correlations depend on the highly peaked nature of the density of states of the Laves phases, primarily due to the B network.

The Influence of the Valence Electron Concentration on the Structural Variation of the Laves Phases MgNi2–xGex

“…Thus, an ordered superstructure of the cubic C15 Laves phase is formed. The hexagonal Laves phase hP36-MgNi 1.3 Ge 0.7 crystallizes in its own structure type with space group P6 3 /mcm (Pearson code hP36) and unit cell parameters of a = 8.6946 (2) and c = 7.8127(3) Å. The crystal structure can be viewed as a superstructure of the hexagonal Laves phase MgZn 2 and is presented in Figure 3.…”

“…[14] Mg 2 Ni 3 Si crystallizes in the Y 2 Rh 3 Ge-type structure (superstructure of C15), with the Ni atoms in the Kagomé and the Si atoms in the hexagonal 3 6 net, [15] and the isoelectronic compound Ca 2 Pd 3 Ge also adopts the Y 2 Rh 3 Ge structure. [16] A partial Cu/As substitution within the graphite-like 3 6 Cu-net of the cubic CdCu 2 Laves phase (C15) results in a new ordered, orthorhombically distorted Cd 4 Cu 7 As structure (superstructure of MgCu 2 ). [17] The system Mg/Ni/Ge shows a large variety of structure types, but only a few of them are structurally related to Laves phases.…”

X-ray crystallography of simple molecular solids up to megabar pressures: application to solid oxygen and carbon dioxide

“…For hard sphere mixtures, stoichiometric compounds of formula AB, AB 2 , AB 13 are stable for atomic diameter ratios in the range 0.2-0.42, 0.42-0.59, and 0.48-0.62, respectively [24]. Laves phases compounds occur for a diameter ratio of 0.816 [25]. Outside these concentration ranges stand miscibility gaps where the two components are predicted to unmix.…”

“…Knowledge of the phase diagram of the binary mixture is thus requested beforehand, to determine the solubility of He in the com- [104] (fluid H 2 ), [13] (solid H 2 ), [105] (solid CO 2 ) and [30] (solid O 2 ). The dash-doted line indicates the ratio for Laves phase [25]. …”

Universal mechanism of tetrahedral metal cluster formation in structures with breathing pyrochlore sublattices

“…The small distortion of the B-tetrahedra is due to the difference in size between A and A atoms [48]. For a more detailed discussion on the crystal chemistry and chemical bonding in such Laves phases we refer to review articles [49][50][51] and references which present there in. (the structural type of MgCu 4 Sn).…”