Charge transfer on the metallic atom-pair bond and the bond energy
- PDF / 196,395 Bytes
- 6 Pages / 584.957 x 782.986 pts Page_size
- 63 Downloads / 245 Views
Vummethala Seshubai School of Physics, University of Hyderabad, Hyderabad 500 046, India (Received 27 February 2011; accepted 8 April 2011)
Following our recent report demonstrating the significance of the nearest-neighbor unlike atom-pair bond in metallic alloys, an equation for the energy of such a bond is presented in this study. The success of empirically derived Miedema’s equation in predicting the signs of the heats of formation of metallic alloys is explained. The negative contribution to the energy stems from the ionicity in the bond. The charge transfer on the bond, suggested by Pauling to establish electroneutrality, contributes the positive term which is quantified in this study. The value of Miedema’s empirically derived constant (Q/P)1/2 and the origin of the R/P term follow from the present model. It is shown that the energy of the atom-pair bond, calculated using the model, in the compounds of MgCu2 structure type are directly correlated to the experimental heats of formation of the compounds, and this fact enables the prediction of the heats of formation values for new compounds of the same structure type.
I. INTRODUCTION 1–4
Miedema and coworkers had proposed the following essentially empirical equation for the heat of formation DH of intermetallic phases: 1=3 2 R 2 DH 5 f ðcÞ ðD/Þ þ ðQ=PÞ DN ; ð1Þ P where / is proportional to Pauling’s electronegativity; N (in “density units”) was called “electron density at the boundary of the Wigner–Seitz cells” of the elements.1 “D” denotes the difference in two quantities for the two elements forming an alloy. 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/,DN1/3) map, referred to hereafter as the RG map. Considering 96 structure types with ;3000 compounds,7–9 it has been demonstrated that D/ } DN1/3 with pffiffiffiffiffiffiffiffiffi a slope ; þ Q=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 AuCu3 and NaCl structure types formed multiple lines. The resolution on the map between the lines belonging to different structure types is good enough to anticipate many observations that can be made from binary metallic phase diagrams. An example is predicting the total absence of A15 type, NiAs type, and c phase a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.135 J. Mater. Res., Vol. 26, No. 12, Jun 28, 2011
http://journals.cambridge.org
Downloaded: 16 Mar 2015
compounds from ;250 binary systems in which the Laves phase compounds exist.5,7,9 Another example is the prediction of 22 structure types including CaCu5 (hP6), FeB (oP8), Er2Co7 (hR18), Ce2Ni7 (hP36), NbBe3 (hR12), CFe3 (oP16), and Si3Mn5 (hP16) as possible concomitants to MgCu2 type compounds in binary systems.8 Since the choice of one crystal structure or another by intermetallic compounds is expected to be decided by small energy diffe
Data Loading...
