Signatures of structural differences in Pt–P- and Pd–P-based bulk glass-forming liquids

Abstract

The structural differences between the compositionally related Pt-P-and Pd-P-based bulk glass-forming liquids are investigated in synchrotron X-ray scattering experiments. Although Pt and Pd are considered to be topologically equivalent in structural models, we show that drastic changes in the total structure factor and in the reduced pair distribution function are observed upon gradual substitution. These variations indicate the existence of significant structural differences on the short-(SRO) and medium-range order (MRO) length scale. The structural data suggest that the distribution of the dominant polyhedra and the distribution of their connection schemes gradually change from Pt-P-to Pd-P-based alloys, which is likely connected to the different sensitivities to annealing or cooling rate induced embrittlement. The evolution of the total structure factor and the reduced pair distribution function with increasing temperature indicate the (partial) dissolution of both, the MRO and the SRO, which reflects the thermodynamic properties of the liquids.

Wave-Vector Dependence of the Dynamics in Supercooled Metallic Liquids

“…Its origin appears associated to the presence of independent locally fluctuating elastic moduli on the probed length scale [34,35] and suggests the presence of a crossover to a diffusive-like dynamics at larger length scales [34,36]. The Q 0 mode of relaxation in the Pd42.5Cu27Ni9.5P21 also implies the absence of any important contribution to the dynamics from the diffusion of small components in the highly viscous state, and is compatible with the stiff structure of this system which is suggested made by 3-atom cluster connections of icosahedra [18].…”

“…The decrease of βðQÞ for Q values smaller than that of the main peak in SðQÞ implies the occurrence of larger dynamical heterogeneities consistent with the notion of medium-range-order domain fluctuations. The smaller measured Q values correspond generally to length scales comparable to the typical size of icosahedra clusters whose occurrence in the Pd-based alloy has been confirmed by structural studies [25,41]. The dynamics of such clusters was recently identified to strongly dominate the evolution of the ISFs in supercooled metallic liquids [12].…”

“…The structural relaxation times and shape parameters measured at all Qs are reported in Fig. 3 for both Au49Cu26.9Si16.3Ag5.5Pd2.3 (left column) and Pd42.5Cu27Ni9.5P21 alloys (right column) and compared with the S(Q) measured with x-ray diffraction [17,18]. Both τ(Q) and β(Q) mimic the S(Q), showing a significant slowdown of the dynamics at Qm accompanied by a simultaneous increase in β(Q) (panels a-b and d-e).…”

“…XPCS provides information on the microscopic dynamics by monitoring the temporal evolution of the intensity fluctuations g 2 ðQ; tÞ, which are related to the ISF through the Siegert relation g 2 ðQ; tÞ ¼ 1 þ γ • jfðQ; tÞj 2 , with γ the experimental contrast [19,23]. Despite the multicomponent nature of the probed alloys, in both cases, XPCS mainly measures the average dynamics coming from the noble-noble atoms correlation which dominates the scattered signal [22,24,25]. .…”

Different Thermal Responses of Local Structures in Pd43Cu27Ni10P20 Alloy from Glass to Liquid

“…The first peak ( q 1 ) is relatively symmetric, while a shoulder peak ( q 2 s ) appears on the higher q side of the second main peak ( q 2 ), likely due to the icosahedral-like short-range order (SRO) . For the ideal icosahedral SRO, the ratios of peak positions between the first two main peaks q 2 / q 1 and between the shoulder peak and the first main peak q 2 s / q 1 are about 1.71 and 2.04, respectively. , By applying one Gaussian function to fit the first peak and a two-Gaussian function for the second peak, as done in previous studies, , the ratios of q 2 / q 1 and q 2 s / q 1 can be estimated ranging from 1.70 to 1.72 and from 1.95 to 1.96 upon heating, in good agreement with the expected values. The slight divergence indicates a little distortion of the icosahedral-like SROs in the Pd 43 Cu 27 Ni 10 P 20 alloy.…”