# High-Temperature Liquid-Liquid Phase Transition in Glass-Forming Liquid Pd43 Ni20 Cu27P10
## 1 Introduction  
Liquid-liquid phase transition (LLPT) has long been postulated to be connected with liquid anomalies in structure, thermophysical properties, and dynamic crossovers, and thus provides an opportunity in elucidating the fundamental relationships between the structure and properties of liquid, which are far from being well understood. Due to the efforts of the researchers in broad research fields, a few LLPTs have been observed in various systems, ranging from unary to multi-component systems and spanning from network-forming to glass-forming liquids, such as Si [1; 2; 3; 4], P [5; 6], Ce [7], Sn [8], water [9; 10; 11], SiO2 [12], Ga50 In50 [13], and Al2O3-Y2O3 [14; 15]. In recent years, LLPT has been found in many metal systems, such as Pd-based [16], Fe-based [17], Zr-based [18; 19], and La-based [20] systems. However, at present, the understanding of the formation mechanism of LLPT is still not sufficient. On the one hand, the small heat flow related to LLPT is difficult to detect by the traditional differential scanning calorimetry (DSC), and on the other hand, the evidence of LLPT may be obscured by the nucleation in the undercooled liquid [21]. Pd-Ni-P is a system with an excellent glass-forming ability (GFA) [22], which can be fabricated into a centimeter-sized metallic glass at a cooling rate as low as 0.17 K/s. [PERSON] et al. revealed a hidden amorphous phase and LLPT in a supercooled liquid regime when the metallic glass is heated up to liquid [16]. In our previous work, an LLPT was observed in a superheated liquid regime during the rapid cooling of liquid Pd-Ni-P [23]. This structural transition is quite anomalous since it is endothermic and not beneficial for the enhancement of the short-range order of the structure. This kind of LLPT is contrary to other exothermic LLPTs, and the mechanism remains elusive. Cu is quite similar to Ni, with their radii being 1.28 and 1.24 A, respectively. It has been illustrated that the similar element substitution could exert marked influence on the structures and properties, such as the glass forming ability [24] and magnetic properties [25]. After the substitution of Ni by the similarelement of Cu, Pd-Ni-Cu-P has shown a large GFA, a critical size being over 70 mm [26], and sluggish crystallization kinetics, enabling it as a good choice for observing structural changes [27; 28]. It is quite interesting to know whether this similar element substitution could influence the LLPT, which will further deepen our understanding of endothermic LLPT and the fundamental relationships between the structures and properties of the liquid [29; 30; 31; 32; 33]. In recent years, many novel methods have been developed to investigate the LLPT, observing the evolution of microstructure in the alloy system through experimental and simulation methods [34; 35; 36]. [PERSON] et al. found a temperature-induced LLPT in a GaIn alloy through in situ high-energy X-ray diffraction (XRD) and ab initio molecular dynamics (AIMD) simulations [13]. [PERSON] et al. found an LLPT in an AuGa alloy through in situ high-energy XRD, extended X-ray absorption fine structure analysis, and AIMD simulations [37]. Recently, rapid scanning chip calorimetry technology has been well-developed in differential scanning calorimetry (DSC) measurement. Mettler-Toledo Company has developed a flash differential scanning calorimeter using chip sensor which is sensitive to small endothermic and exothermic peaks and has played important roles in the experimental verification of high-temperature LLPT [38]. In our previous work, the endothermic LLPT in Pd-Ni-P liquid was verified by flash differential scanning calorimetry (FDSC) and in situ synchronous XRD [23]. [PERSON] et al. found through FDSC that in a Yb-Zn binary alloy, a dynamic-driven LLPT exists at a temperature above liquidus temperature [38].