Polyamorphism in Yb-based metallic glass induced by pressure

The Yb62.5Zn15Mg17.5Cu5 metallic glass is investigated using synchrotron x-ray total scattering method up to 38.4 GPa. The polyamorphic transformation from low density to high density with a transition region between 14.1 and 25.2 GPa is observed, accompanying with a volume collapse reflected by a discontinuousness of isothermal bulk modulus. This collapse is caused by that distortional icosahedron short range order precedes to perfect icosahedron, which might link to Yb 4f electron delocalization upon compression, and match the result of in situ electrical resistance measurement under high pressure conditions. This discovery in Yb-based metallic glass, combined with the previous reports on other metallic glass systems, demonstrates that pressure induced polyamorphism is the general behavior for typical lanthanide based metallic glasses.

Scientific RepoRts | 7:46762 | DOI: 10.1038/srep46762 distance r 1 in real space shifts to shorter distance with increasing pressure. The shifts of Q 1 of structure factor S(Q) and the nearest-neighbor distance r 1 reflect changes in volume or density induced by pressure, which is illustrated by a power law 3,[11][12][13] . Using normalized first peak position Q 1 with a power of 2.5 to estimate the relative volume change as a function of pressure, a fast decrease of the average atomic volume under pressure range from 14.1 to 25.2 GPa, a transition region, is observed. Two amorphous states, low density amorphous states (LDAS) and high density amorphous states (HDAS), were separated by the transition region. Fitting the pressure-volume data by the second-order Birch-Murnaghan equation of state (EoS) 14 , the isothermal bulk modulus are determined as B 0 = 24(1) GPa and 45 (7) GPa at different state region respectively, presenting a discontinuousness of isothermal bulk modulus, as shown in Fig. 2. The volume difference between the two states is as large as 15.2% at ambient pressure. Furthermore, the volume collapse taking place continuously and undergoing a broad pressure range rather than abruptly at certain pressure point shows an anomalous compressed region, which is in agreement with polyamorphic transition in Ce 55 Al 45 metallic glass with a smooth and continuous transition region between 2 and 13.5 GPa 5 . This anomalous compressed curve is also observed in phase transition of ytterbium monotelluride under pressure 15 .
As shown in Fig. 3(a,b), both Q 1 and r 1 move with a relatively fast pace at transition region and the changes of both Q 1 and r 1 as a function of pressure exhibit a trend similar to that of volume, which suggest that the polyamorphic transformation is embodied as well in medium range and short range length scale besides long range length scale. Intuitive local structure change of Yb 62.5 Zn 15 Mg 17.5 Cu 5 within short range length scale induced by pressure was reflected by gradually disappeared the low-r shoulder of the first peak in G(r), as shown in Fig. 1(b).
As is displayed in Fig. 1, the second peak in S (Q) and PDFs are of splitting at each pressure point which are characteristic for conventional amorphous alloy systems 16,17 . In real space, the changes splitting of second peak under pressure reflect structural changes of Yb 62.5 Zn 15 Mg 17.5 Cu 5 in medium range length scale. Such change is consistent with phase transition characterized by shifts of Q 1 which embody the medium range order. In reciprocal space the relationship between the first peak and splitting second peak as a function of pressure is indicative of short range order change 18,19 . Define position of the second peak and its shoulder as Q 2 and Q 2shoulder . Using two Gaussian functions to fit the second peak, the ratios of peak position show Q 2shoulder /Q 1 = 1.93 and Q 2 /Q 1 varying from 1.66 to 1.72 with increased pressure, as displayed in Fig. 4. In contrast to a perfect icosahedron short range order, Q 2shoulder /Q 1 = 2.04 and Q 2 /Q 1 = 1.71 20 , short range order changes in the Yb 62.5 Zn 15 Mg 17.5 Cu 5 metallic glass  under pressure go through from a more distortional icosahedra to a more perfect one with a different pace corresponding to different phase states. The polyamorphic transformation in the Yb 62.5 Zn 15 Mg 17.5 Cu 5 metallic glass under pressure is thus mainly related to the reduction of icosahedra distortion with a different pace leading to volume collapse. This result is consistent with the change of PDFs whose each peak is sharper increasingly with increasing pressure, indicating the structure gradually becomes more order compared to initial structure, as presented in Fig. 1(b). At stage of LDAS, the change of distortion with pressure is slight and it contributes to a weak volume reduction. The diminution in degree of icosahedra distortion proceeds with faster pace within a transition region, then, distortional icosahedron is close to a more perfect one reaching the range of HDAS, which leads to a large volume change. Whereas, such changes of distortional icosahedra result from the decrease of average atomic distance in first neighbor shell with a different step as shown in Fig. 3(b), which should be radically associated with electronic transformation.
The phase transition of the Yb 62.5 Zn 15 Mg 17.5 Cu 5 metallic glass can be ascribed to the electronic transition. In the Yb 62.5 Zn 15 Mg 17.5 Cu 5 metallic glass, the 4f shell of Yb is completely full and considered as divalent with an electronic configuration close to 4f 14 (5d6s) 2 at ambient conditions, whereas, under high pressure conditions, it transforms to a trivalent state 4f 13 (5d6s) 3 when one of the localized 4f electrons is delocalized, which is deduced from mechanism lied in polymorphism in ytterbium compounds 9,21 and polyamorphism in Ce-based metallic glass under pressure 5,6 . An Yb in the trivalent state has a smaller atomic volume compared to divalent state, and this results in a striking decrease in average bond length, which coincides with the change of icosahedral distortion and volume collapse.
The pressure dependence of electrical resistivity. Electronic transition in the Yb 62.5 Zn 15 Mg 17.5 Cu 5 metallic glass should be reflected in changes of electronic transport property. Therefore, an in situ resistance measurement under compression conditions was carried out. Figure 5 displays the resistance of Yb 62.5 Zn 15 Mg 17.5 Cu 5 metallic glass as a function of pressure. According to weak-scattering theory which is well suited for low-resistivity metallic glasses, the resistance of a metallic glass is considered as arising from disordered atom scattering described by structure factor 22,23 . The pressure dependence of electrical resistivity is effected by several factors, such as the Fermi energy, the electron-ion interaction and structure factor of the system. As a result of a rather delicate interplay of these factors, the resistivity of metallic glasses is either to increase or to decrease under compressed conditions depending on different systems 24,25 . In the Yb 62.5 Zn 15 Mg 17.5 Cu 5 metallic glass system, a positive pressure coefficient of the resistivity (PCR) is observed. Additionally, both the structure and electronic transition are arising from Yb 4f electrons delocalization as well as relation between pressure and electrical resistivity is volume dependence 23 , therefore, resistance of the Yb 62.5 Zn 15 Mg 17.5 Cu 5 metallic glass exhibits a similar transition region, which is from 19.3 to 30.2 GPa, to that of low density to high density transition displayed in synchrotron x-ray scattering experiment. The shift of transition region might be due to quasi-hydrostatic pressure conditions in resistance measurement experiment. The pressure effect on this metallic glass is basically reversible, but with sluggish effect when pressure released. Thus this polyamorphism could only be studied via in situ high pressure methods.
The positive PCR obviously reduced after transition in the Yb 62.5 Zn 15 Mg 17.5 Cu 5 metallic glass is observed in Fig. 5. Cheung and Ashcroft 26 found a positive PCR for the polyvalent metals Mg, Al, Sn and a negative PCR for the monovalent metal K in liquid metals under pressure. Particularly, the positive PCR is maximal for the divalent metals 22,25 . This conclusion may suit for Yb contribution to PCR of the Yb 62.5 Zn 15 Mg 17.5 Cu 5 metallic glass as well. The conversion from divalent to trivalent Yb leads to that contribution of Yb to positive PCR in the Yb 62.5 Zn 15 Mg 17.5 Cu 5 metallic glass decreases in case of 4f electron delocalization, which gives rise to decline of positive PCR after transition.
A polyamorphic transformation from low density to high density with a transition region between 14.1 and 25.2 GPa occurs accompanying with change of electrical transport property. The transitions could be explained by 4f electron delocalization in Yb. The 4f-5d electronic collapse is reflected in both pressure-volume and pressure-resistivity relationship. This findings, combined with the previous reports on pressure induced polyamorphism in Ce based and La based metallic glasses 5,6,17 , is valuable to understand the polyamorphic transformation, which could be a general behavior in lanthanide based metallic glasses.

Methods
To achieve Yb-based metallic glasses with good glass forming ability, constituent elements Zn, Mg and Cu were carefully selected and adjusted the composition. Metallic glass Yb 62.5 Zn 15 Mg 17.5 Cu 5 ribbons were prepared using the single-roller melt-spinning technique. Raw image data were processed using software Fit-2D 27 with masking strategy 28 to remove the diamond peaks to obtain one-dimensional scattering data. Subtracting the contributions from the sample environment and background, the reduced pair distribution function G(r) and structure factor S(Q) were extracted using the program PDFgetX2 29 , which carries out a numerical Fourier transform of S(Q) according to 0 High-pressure resistivity measurements. In situ electrical resistance was measured by a four-probe resistance system in a DAC. An aluminum oxide (Al 2 O 3 )/epoxy mixture powder layer was inserted between the T301 stainless steel gasket and diamond anvil to provide electrical insulation for the electrodes and gasket. Four electrodes made of platinum foil with a thickness of 10 μ m were arranged to contact the sample in the chamber. The pressure was determined by a ruby and up to 40.9 GPa. The resistance was calculated from the Van de Pauw method 30 .