Stability of the Anomalous Large-Void CoSn Structure

Abstract

The crystal structure of CoSn represents a highly peculiar intermetallic structure type because of the presence of a large void and highly nonspherically coordinated tin atoms. Voids in crystal structures of intermetallic compounds are most unusual and always give rise to speculations whether the observed phase is the ground state or not. Here we examine this problem by means of ab initio calculations and show that the void structure of CoSn is indeed the stable phase. With pressure we predict the following sequence of structural transformations: CoSn type ! FeSi type ! CsCl type. The structural properties of CoSn are understood in terms of competition between covalent and metallic bonding in the system. [S0031-9007(97)03781-2]

High-pressure structural behavior of large-void CoSn-type intermetallics: Experiments and first-principles calculations

“…3͑a͔͒. Note that a full relaxation of structural parameters performed in the present work modifies the results of our previous study, 11 where the transitions CoSn type→ FeSi type and FeSi type→ CsCl type above 26 and 36 GPa, respectively, were predicted. The compound FeSn is an antiferromagnet with a magnetic moment of ϳ1.8 B on the Fe atoms.…”

“…For the CoSn, a pressure of up to 38 GPa was applied. This pressure exceeds the maximum pressure applied in a previous DAC study ͑26 GPa͒, 2 and is above the pressure for theoretically predicted phase transitions to the FeSi-type and CsCl-type structures at 26 and 36 GPa, 11 respectively. The FeSn and NiIn were investigated up to 28 GPa.…”

“…This is expected for dp bonded intermetallic AB compounds from space filling arguments. 11 Interestingly, the FeSi-type structure with potential interesting properties for 12 and 13 electron compounds occurs only in the sequence for CoSn.…”

“…5 Applying high-pressure synthesis conditions increases the number of germanide representatives considerably. 10,11 With Sn ͑and In͒, an appreciable size difference between transition metal A ͑smaller͒ and main group element B ͑larger͒ is introduced. The ambient pressure largevoid CoSn structure for compounds AB appears as a consequence of such a "size mismatch."…”

“…9,10 We noticed that our previous theoretical studies of the CoSn system, indeed, predicted the FeSi-type structure as a high-pressure phase above 26 GPa. 11 However, so far, no phase transformation has been observed experimentally. 2 The present combined experimental and theoretical study is devoted to the structural behavior of the large-void CoSntype intermetallics CoSn, FeSn, and NiIn at high pressure and/or temperature, and attempts to obtain other 12 and 13 electron materials.…”