Ca_(14)Au_(46)Sn_5: A 'colored' Gd _(14)Ag_(51)-type structure containing columns of well-differentiated hexagonal gold stars

摘要

A novel hexagonal phase discovered near the Ca_(15)Au _(60)Sn_(25) quasicrystal and its cubic approximants (ACs) was synthesized by means of high-temperature solid-state reactions. Single-crystal structural analyses show that this is a Gd_(14)Ag_(51) isotype with composition within the range Ca_(14)Au _(45.56(4)-46.67)(4)Sn_(5.14(3)-4.14(3)), space group P6/m (No. 175), and lattice parameters a = 12.763(3)-12.879(3) ? and c = 9.326(3)-9.3815(4) ?. In this phase, Sn mixes with Au in two of seven anionic sites to give a strong coloring that generates a narrow honeycomb-like Au/Sn template, in which sizable columns of hexagonal Au stars are confined. This phase transforms into the cubic 2/1 AC phase through a peritectic reaction at ～678 °C. The valence electron count per atom (e/a) of the present phase is in the range 1.41-1.45. However, it does not appear to follow a Hume-Rothery mechanism.