Eu Doping in the GdCd_7.88 Quasicrystal and Its Approximant Crystal GdCd₆

The effect of Eu doping in the Tsai quasicrystal (QC) GdCd_7.88 and its periodic 1/1 approximant crystal (AC) GdCd₆ are investigated. This represents the first synthesis of Eu-containing stable QC samples, where three samples with the final composition Gd_1-EuCd_7.6±α at Eu doping concentrations x = 0.06, 0.13, and 0.19 are obtained (α ∼ 0.2). They are compared to two 1/1 ACs with compositions Gd_1-EuCd₆ (x = 0.12, 0.16). In addition, a new type of 1/1 AC, differing only by the inclusion of extra Cd sites unique to the Eu₄Cd₂₅ 1/1 AC, has been discovered and synthesized for the concentrations Gd_1-EuCd_6+δ (x = 0.25, 0.33, 0.45, 0.69, 0.73, and 0 < δ ≤ 0.085). Due to the preferred cube morphology of its single grains, we refer to them as c-type 1/1 ACs and to the conventional standard ones as s-type. In both QCs and s-type ACs, the Eu content appears to saturate at a concentration of ∼20%. On the other hand, any Gd| Eu ratio is allowed in the c-type ACs, varying continuously between GdCd₆ and Eu₄Cd₂₅. We describe and contrast the changes in composition, atomic structure, specific heat, and magnetic properties induced by Eu doping in the quasicrystalline phase and the s-type and c-type 1/1 ACs. By comparing our results to the literature data, we propose that the occupancy of the extra Cd sites can be used to predict the stability of Tsai-type quasicrystalline phases.