Eu Doping in the GdCd7.88 Quasicrystal and Its Approximant Crystal GdCd6

The effect of Eu doping in the Tsai quasicrystal (QC) GdCd7.88 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 Gd1–xEuxCd7.6±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 Gd1–xEuxCd6 (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 Gd1–xEuxCd6+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.


I ) Synthesis and phases a) Synthesis comments, EDX and ICP results
Quasicrystal and tetragonal crystals were formed using the self-flux method.They were observed to crystallize simultaneously if the final annealing temperature was set below a temperature-dependent threshold, which raises with the Eu concentration x to meet with the liquidus line above x = 0.55.As the Eu concentration raises, the temperature window for which only QCs are obtained after centrifugation becomes narrows, making it nearly impossible to obtain macroscopic grains from centrifugation above x ~ 0.5.In the pure Gd-Cd phase diagram, quasicrystals can only be obtained in the Gd-poor region, up to ~1% Gd starting material.Above that threshold, crystals will form as 1/1 ACs.Adding Europium does not appear to shift this limit towards a higher rare earth (RE) percentage, as evidenced by the batch with starting material (Gd0.4Eu0.6)1.2Cd98.8,which showed only s-type 1/1 ACs at 1.2% RE.The Cd per rareearth ratio was estimated from SCXRD refinements, and the Gd/Eu ratio were established with EDX data for the 1/1 ACs and ICP for the QCs.The reason why ICP was chosen is the lower concentration of some of the samples, along with the lower at% concentration of QCs compared to 1/1 ACs, making the EDX method unreliable.We investigated the three phases, 1/1 ACs, tetragonal and QC with DSC.In the 1/1 ACs, the formation temperature of the QC increases as Eu concentration increases from 390°C to 430°C.This observation is consistent with previous investigations and the QC single crystal DSC result.The QC with the highest Eu concentration showed a similar result, which indicates the upper limit of the formation temperature for Eu-doped QC is around 430°C.In the DSC result of the (Gd, Eu)Cd11 sample, we observed two formation temperatures, 440 and 501°C.
According to the previous report, we expect the peak at 501 °C to be a consequence of the (Gd, Eu)Cd11 tetragonal phase formation, whereas the peak at 440 °C to should be linked to the QC phase.The summarized DSC results provided gives a possible answer as to why no QC in the binary Eu-Cd system appears achievable.For concentrations of Eu above x=0.25 in the (Gd1-xEux)Cd compounds, the QC and tetragonal phases coexist, and it is difficult to isolate the pure (Gd1-xEux)Cd QC phase with solution grown method to target the shrunk window.The series of the Eu-doped 1/1 ACs present the electron density isosurfaces at the 15 -e/Å 3 level in the location of the M6 site, i.e., cube interstitial position.In Figure S4, (a) to (d), the electron density map has been generated from Fobs data of the individual single crystal diffraction dataset with the selected area.The Eu concentration increases in order, as x=0.25, 0.45, 0.69, 0.73 in the (a) through (d), as well as increasing the occupancy of the M6 site, respectively.From (a)-(d), the M6 site occupancy increase from left to right is 1.3%, 8%, 9.8%, and 25.6%, respectively.Although the additional Cd position is not visible at the electron density isosurface at the 15 e/Å 3 level in the x = 0.25 sample, it is visible at the electron density isosurface at the 8 e/Å 3 level.Therefore, we still take its ~1.3% occupancy into account, which is higher than the uncertainty value of 0.6%, and use it as a comparison with other samples.The zero-field cooled (ZFC) and field-cooled (FC) magnetization measurements were performed under a field applied of 10 Oe for both the EuCd11 tetragonal phase sample and Eu4Cd25 1/1 AC.The magnetic susceptibility data was acquired up to larger temperatures under a field of 5000 Oe for both samples.

II ) Additional magnetic data
The relatively large Curie-Weiss temperature θCW = -16 K found for the Eu4Cd25 compared to the observed Néel transition at TN = 2.9 K indicates a frustrated antiferromagnet behavior.Regarding the quasicrystalline Eu-doped samples.Memory experiments were performed on the samples at doping concentration x = 0.06 and x = 0.19.The samples were cooled down under no field applied and set to wait at temperature Tw = 3.5 K below their spin freezing temperature.As expected, depending on the waiting time, a difference of magnetization appears in the ZFC data when compared to the reference acquired without stopping step.Higher Eu content relates to a larger low-temperature nuclear contribution, with the largest found in the pure Eu4Cd25 1/1 AC.

Figure S2 :
Figure S2: Proportion of Eu atom in the structure of (a) c-type and s-type 1/1 ACs and (b) quasicrystals, compared to the nominal value.

Figure
Figure S3: (From left to right) DSC measurements of the 1/1 ACs, tetragonal phase sample and a Eu-doped QC sample.

Figure S5 :
Figure S5: Unit cell of the Eu4Cd25 1/1 AC.The black tetrahedra are ordered while the light grey ones are random within the structure.All M6 sites equivalent within the structure are occupied except for the location of the 16 empty magenta cubes.In total, there are 8 × 8 = 64 M6 equivalent sites in the Eu4Cd25 structure, of which 16 are unoccupied.They are shown in the previous figure, by drawing the empty cubes given by 2 dodecahedron shell Cd positions along the (1 1 1) direction and equivalent, as well as 6 positions from the icosidodecahedra.The vacancies line up the edges of a large tetrahedron.In

Figure S6 :
Figure S6: Reciprocal space of the c-AC with x = 0.45 at (a-c) 293 K and (d-f) 100 K. Reciprocal space slice of (a,d) h k 0, (b,e) h k 0.5 and (c,f) h k .

Figure S7 :
Figure S7: Magnetization as a function of field recorded at T = 2 K for the Eu-doped (a) QCs and (b)1/1 ACs.The magnetic susceptibility plots recorded under a magnetic field of 5000 Oe are presented in (c) for the QCs and (d) for the 1/1 ACs.

Figure S8 :
Figure S8: Examples of Curie-Weiss analyses.Inverse magnetic susceptibility plots of (a) the Eu-doped QC with concentration x = 0.19, (b) the s-type 1/1 AC with x = 0.16 and (c) c-type 1/1 AC with x = 0.25.The blue are linear fits to the data.

Figure S9 :
Figure S9: Zero field cooled and field cooled magnetization of (a) the tetragonal EuCd11 phase and (b) The Eu4Cd25 1/1 (c) Curie-Weiss fits of the inverse susceptibility of EuCd11 and (d) The Eu4Cd25 1/1 AC.

Figure S10 :
Figure S10: Magnetic memory measurement for the QCs at x = 0.06 and x = 0.19.an increase in waiting time at Tw = 3.5 K during cooling increases the magnetization difference in the ZFC plot around Tw.

Figure S11 :
Figure S11: Specific heat, plotted as C/T of (a) the Eu-doped QC sample with concentration x = 0.06, (b) the c-type 1/1 AC with concentration x = 0.45 (c) Eu4Cd25 with the low temperature

Table S1 :
Summary of the syntheses conditions and crystalline phases obtained. *

Table S3 :
ICP data of the QCs.
b) DSC results and phase diagram

Table S4 :
Summary of magnetic properties.