**1. Introduction**

Phase diagrams are employed extensively in material science and associated technological applications in industry. For decades, phase diagrams at ambient pressure have been explored by measuring thermophysical quantities and by theoretical modeling. Phase boundaries can be determined directly by experimental techniques such as thermal analysis, including using differential thermal analysis (DTA) or differential scanning calorimetry (DSC), volume changes (e.g., dilatometry), electrical resistivity measurements, or X-ray diffraction (XRD) to identify structural changes [1].

Experimental methods are costly when mapping a phase diagram of binary and ternary systems over a wide range of compositions and temperatures, due to the large number of experiments required. Hence, direct measurements of phase transitions are often complemented by thermodynamic modeling of phase diagrams, typically determined by the Gibbs free energy, which describes the lowest energy, most stable phases, as a function of the thermodynamic parameters, temperature, composition and occasionally pressure. Calculation of phase diagrams (CALPHAD) methodology is commonly used to model phase diagrams. It is based on thermodynamic considerations and empirical databases, extrapolating the thermodynamic parameters to calculate these complex systems [2,3]. Several software packages have been developed (e.g., ChemSage, WinPhad, PANDAT, and ThermoCalc) that employ the CALPHAD methodology to calculate the phase diagrams of complex systems at ambient pressure [4]. A major challenge of this approach is the need for reliable thermodynamic property databases to calculate the phase diagram. Thus, extensive thermodynamic measurements of enthalpy, heat capacity. and activity (through electromotive force (EMF) measurements and vapor pressure studies) have been conducted and analyzed [5] to support this venture.

Pressure can a ffect physical properties and, in many cases, stabilize new phases. Physical properties of alloys under high pressure are at the focus of planetary research, as well as having advanced technological applications [6–8]. The emergence of high pressure technologies enables the scientific community to explore regions of phase diagrams at elevated pressures and high temperatures, which were inaccessible in the past [9,10]. However, numerous experimental challenges remain. The versatile and popular high-pressure apparatus of the diamond anvil cell (DAC) is widely used to measure structural changes under high pressure. Currently, measurements can be performed at a pressure range of 0–500 GPa and at temperatures ranging from cryogenic to thousands of degrees. At a lower pressure range one can find other techniques such as piston cylinder, multi-anvil press, and the Paris–Edinburgh large volume cell. To date, phase diagrams of elements under pressure are relatively well-known and established [11] but the database for binary systems is limited. For ternary systems, the database is practically nonexistent.

To establish phase diagrams in the liquid state, additional experimental challenges are presented, for example, chemical reactivity of the liquid at high temperatures with structural materials and effects associated with high vapor pressure. Hence, information on the properties of liquids at high temperatures, and even more so at high pressures, is limited. The scarce data that exists has relatively large experimental error, and as a result, there are discrepancies between reported thermophysical values. Even for elemental systems there are open questions, for example, the shape of the melting curve [12] and the possible existence of liquid–liquid transitions [13]. Direct measurements of the changes in the liquid structure and their interpretation can be challenging [14]. Indirect studies of phase transformations in the liquid using classical thermal analysis approaches are rare.

The study of liquid binary systems requires even greater experimental e ffort, due to the need to carefully measure a range of compositions and include phase separation phenomena. Therefore, this area of research remains relatively unexplored. Recently, we have developed a methodology to determine the pressure and temperature dependence of the excess interaction parameters in liquid solution, from ambient pressure measurements of density, heat capacity, and thermal expansion. This approach enables the construction of binary alloy phase diagrams at low pressures of up to several GPa [15,16].

The purpose of this paper is to provide an overview of thermophysical measurements and associated models that support the construction of phase diagrams in liquid binary alloys of columns III to V and their constituent elemental systems. These systems exhibit complex phase diagrams under pressure and their thermophysical properties are often anomalous. Their low melting points make them easily accessible for experimental study, and thus they are ideal candidates for developing new approaches to phase diagram studies. In particular, we have developed techniques to measure the density and sound velocity to high accuracy in liquid metals, which is required to establish reliable diagrams. The results of such measurements have been incorporated into a model, describing the free energy of the liquid alloys and predicting the pressure dependence of binary phase diagrams. Electrical measurements provide a complementary technique to explore phase transitions and the limits of liquid phase stability. Such measurements have been applied at both ambient and high pressures to identify phase stability limits. This set of techniques and accurate measurements provides new pathways to study the liquid phase diagram under pressure.
