Importance of the atom-pair bond in metallic alloying

Abstract

a b s t r a c tRajasekharan and Girgis reported that binary systems with intermetallic compounds of a particular crystal structure form a straight line on a map using Miedema's parameters. In this paper, the universality of that observation is examined. Observations from a study of 143 binary systems that crystallize in six different crystal structures at AB 3 composition are discussed. Prediction of concomitant and mutually exclusive structure types in binary metallic phase diagrams, and of phase transitions among different structure types, has been demonstrated. This behaviour is unexpected because Miedema's parameters are isotropic in nature and structural energies are generally assumed to be small. We argue in this paper that each point on the map stands for the energy of an unlike atom-pair (A-B) bond, with the bond energy remaining nearly the same at all compositions in the phase diagram. This argument is confirmed by comparing the nearest-neighbour (A-B) bond lengths for the compounds of the structure types CaCu 5 and CsCl, when concomitant with MgCu 2 structure type. This fact leads to an important conclusion that one can define bond energy for the metallic unlike-atom-pair bond as is usually done for a conventional chemical bond.

Prediction of concomitant structures in binary metallic systems from RG map: With MgCu2 structure type as an example

“…Hence, apart from the tremendous practical utility of the RG map, there is also the interesting question of why the map predicts structures. The answer to this question has been attempted by Rajasekharan and Seshubai elsewhere [12]. According to them, each point on the RG map stands for the energy of the nearest neighbor atom-pair bond in a binary system and this energy remains nearly the same at all compositions in the binary system.…”

“…Corresponding to a binary system A-B, there can be two symmetrically opposite points on the ( , n wsA ) as the 'Inverse RG line' of the structure type [12]. We note that the composition at which a structure type occurs does not enter into deciding its coordinates on the RG map; thus each point on the map represents a binary system, and not a particular intermetallic compound.…”

“…The concomitance of Si 3 Mn 5 type compounds with MgCu 2 type compounds has been discussed elsewhere [12].…”

“…The universality of such a linear relationship has been studied by us recently for 96 structure types with ∼3000 binary compounds at different compositions [11]. The possibility of predicting concomitant and mutually exclusive structure types in binary systems was demonstrated [7,8,12]. The required data were collected from standard compilations in the literature [13][14][15].…”

Charge transfer on the metallic atom-pair bond and the bond energy

“…Rajasekharan and Girgis (RG) 5,6 had observed that binary systems with intermetallic compounds of the same crystal structure occur along a straight line on a (D/,DN 1/3 ) map, referred to hereafter as the RG map. Considering 96 structure types with ;3000 compounds, [7][8][9] it has been demonstrated that D/ } DN 1/3 with a slope ; þ ffiffiffiffiffiffiffiffiffi Q=P p for all the structure types. Binary systems with compounds of 88 of the structure types formed single straight lines and six of them formed dual lines, whereas AuCu 3 and NaCl structure types formed multiple lines.…”