Modifying the valence phase transition in Eu₂Al₁₅Pt₆ by the solid solutions Eu₂Al₁₅(Pt_1-xT_x)₆ (T = Pd, Ir, Au; x=1/6)

The solid solutions Eu2Al15(Pt1-xT x )(6) with T = Pd, Ir, Au and x = 1/6 were prepared by arc-melting the stoichiometric mixture of the elements, and subsequent annealing. For x = 1/6, all three solid solutions adopt the same structure type as Eu2Al15Pt6 according to powder X-ray diffraction data. Since the platinide Eu2Al15Pt6 exhibits a (3 + 1)D modulated structure (approximant in space group P12(1)/m1), only the averaged hexagonal unit cell (P6(3)/mmc, Sc0.6Fe2Si4.9 type) was refined by the Rietveld method. Scanning electron microscopy in combination with energy-dispersive X-ray spectroscopy (SEM/EDX) showed that the degree of substitution is in line with the weighed amounts. For values of x > 1/6, no phase-pure samples could be obtained. The results of the magnetic susceptibility measurements indicate that the isoelectronic substitution of Pd for Pt lowers the temperature of the first-order valence phase transition from T-trans = 45 K in Eu2Al15Pt6 to T-trans = 42 K in Eu2Al15(Pt5/6Pd1/6)(6). For Eu2Al15(Pt5/6Ir1/6)(6) and Eu2Al15(Pt5/6Au1/6)(6) a change in the electronic situation occurs since the Ir substituted compound exhibits one electron less compared to the pristine Pt compound, while Eu2Al15(Pt5/6Au1/6)(6) has one additional electron. As a consequence, Eu2Al15(Pt5/6Ir1/6)(6) shows a higher valence phase transition temperature of T-trans = 52 K while for Eu2Al15(Pt5/6Au1/6)(6) no such transition is obvious.