Universal stabilizing mechanism for transition-metal polytetrahedrally packed phases

We describe the electronic factors leading to structural stability in the class of transition-metal alloys exhibiting polytetrahedral packing (PTP). In a cluster expansion for the total energy, the lowest-order terms which obtain the correct chemical trends correspond to angular forces. These contain an energetic penalty for four-body clusters with 90°angles. Such clusters are uniformly absent in the PTP phases. The four-body contribution is roughly structure independent within the PTP class.