Atomic Volume Contraction in Rare Earth Nickel Intermetallics as a Function of Partial Coordination Number Coefficient

Abstract

Molecular volume contraction upon formation of rare earth inter-metallic compounds from the elements is a very well-known phenomenon. While a method for calculating intermetallic atomic volumes was introduced by the author in 1971 as a partial argument for predicting the unstability of SmCo5 (1) and again in 1974 in the discussion of the Pu2C3 structure (2), systematic studies have only recently included rare earth semimetals and intermetallics (3, 4). As might be expected from their large size it is the rare earth atoms that are primarily responsible for the volume contraction. The rare earth atom volume is found herein to decrease approximately linearly with respect to its coefficient of partial coordination number with nickel. This latter concept is a measure of the relative importance and number of rare earth-nickel bonds and arises from the generalization of the concept of coordination number (3, 5). The amount of the rare earth volume contraction is surprisingly large, i.e., greater than 30%. Nickel on the other hand shows a smaller volume expansion upon alloying. These volume changes are thought to be due primarily to size difference effects upon bonding density distribution.