Thermodynamic Assessment of the Pt-Sb System

The Pt-Sb system was critically assessed by means of CALPHAD technique. Based on the experimental data in the literature, the excess Gibbs energies of the solution phases (liquid, rhombohedral, fcc) were modeled with the Redlich-Kister equation. The five intermetallic compounds, Pt7Sb, Pt3Sb, Pt3Sb2, PtSb, and PtSb2 were treated as stochiometric compounds and expressed as the formula (Pt)m(Sb)n. The intermetallic compound, Pt5Sb with a homogenerity ranges 0.155 0.189 Sb, were treated as the formula (Pt,Sb)m(Pt,Sb)n. A set of self-consistent thermodynamic parameters of the Pt-Sb system was obtained.


Introduction
The precious metals, Pt, Sb, Au, Ag and their alloys, have been widely applied as catalysts for their high thermal stability and high activity, such as decompounding hydrazine, purifying automobile gas, oxygenating selectively and so on [1][2][3][4][5].Understanding the phase equilibria as well as the thermochemical behaviour of these systems is helpful for the development of the related materials.This paper intends to assess the Pt-Sb system thermodynamically and provides a set of selfconsistent parameters for calculation of the phase equilibria and thermochemical properties of the system.

Literature Review
The Pt-Sb phase diagram consists of the liquid, the facecentered cubic (fcc), terminal solid solution (Pt); the terminal solid solution, rhombohedral (Sb) with a negligible solubility of Pt.The gas phase was estimated by Itkin and Alcock [6], which is not considerated in the present work.
In the Pt-Sb system, there are six intermediate phases, Pt 7 Sb, Pt 5 Sb, Pt 3 Sb, Pt 3 Sb 2 , PtSb, and PtSb 2 .The Pt 7 Sb compound was discovered by [11], using DTA, microprobe analysis, and X-ray powder (XRD).Pt 7 Sb, a cubic structure, is formed by a peritectoid reaction.Different designations for the phase Pt 5 Sb are "Pt 4 Sb" [8], "Pt 34 Sb 7 " [10], "Pt 4+ Sb" [12], and "Pt 82 Sb 18 " [13].A compound, "Pt 5 Sb 2 " was reported in Ref. [7] and not confirmed in other publications [8,10,12,13].The compound as Pt 5 Sb has a cubic structure wth a homogeneity range of 0.155 to 0.189 Sb [11].The boundaries of the phase were determined using a temperature dependence of the lattice parameter.This phase is formed at 748˚C ± 4˚C by peritectic reaction, which was consistent with Refs.[7][8][9].It decomposed at 560˚C by eutectoid reaction.The compound Pt 3 Sb was formed by peritectic reaction [9,11].The compound was investigated using thermal arrest by [8] and explained by the phase transformation in "Pt 4 Sb".The compound Pt 3 Sb was examined and observed as a single phase with a composition of 0.275 Sb and as a second phase at 0.25 and 0.30 Sb by Srivastava et al. [14].However, Pt 3 Sb was confirmed as a stoichiometric compound in subsequent research [10].
The compound PtSb 2 was first found by chemical analysis of crystals precipitating from Pt-Sb melts containing an excess of Sb [18].The temperature of congruent melting of PtSb 2 was measured as 1226˚C [7], 1210˚C [8], and 1225˚C [11].The enthalpy increments of PtSb 2 measured by drop calorimetry in the temperature interval 196˚C to 620˚C [19].

Unary Phases
The Gibbs energy function i G T (298.15K) for the element i (i = Pt, Sb) in the phase  ( = liquid, face-centered cubic(fcc) and rhombohedral) is described by an equation of the following form: where (298.15K) is the molar enthalpy of the element i at 298.15 K in its standard element reference (SER) state, fcc for Pt and rhombohedral for Sb.The Gibbs energy of the element i, , in its SER state, is denoted by GHSER i , i.e., In the present work, the Gibbs energy functions are taken from the SGTE (Scientific Group Thermodata Europe) pure elements database compiled by Dinsdale [20].

Solution Phases
In the Pt-Sb system, there are three solution phases: liquid, fcc and rhombohedral.The gas phase was not considerated in the Pt-Sb system, similar to the results re-ported by Durussel and Feschotte [11].Their Gibbs energies are described by the following expression: where R is the gas constant, x Pt and x Sb are the mole fraction of Pt and Sb, respectively, and E m G  is the excess Gibbs energy, expressed by the Redlich-Kister polynomial [21].
where j L  is the interaction parameter between element Pt and Sb, which is to be evaluated in the present work.Its general form is In most cases, only the first one or two terms are used according to the temperature dependence on the experimental data.

Intermetallic Compounds
There were six intermediate phases, Pt 7 Sb, Pt 5 Sb, Pt 3 Sb, Pt 3 Sb 2 , PtSb, and PtSb 2 in the Pt-Sb system.The phases crytal structure data was shown in Table 1.According to the composition and crystal structure data , the Sb in the five compounds, Pt 7 Sb, Pt 3 Sb, Pt 3 Sb 2 , PtSb and PtSb 2 of the system with no or little solid solubility will be treated as the stoichiometric compounds.While the compound Pt 5 Sb with more homogenertiy ranges 0.155 -0.189 Sb will treated as solid solution compound.
So the Pt 5 Sb phase had a homogenertiy ranges, 0.155 -0.189 Sb.The two-sublattice model, (Pt,Sb) 0.833 (Pt,Sb) 0.167 is used to describe this phase in the present work.
The parameters i y and i are the site fractions of Pt or Sb on the first and second sublattices, respectively; the parameter represents the Gibbs energies of the compound Pt 5 Sb when the first and second subletices are occupied by only one element Pt or Sb, respectively, which are relative to the enthalpies of pure fcc for Pt and rhombohedral for Sb in their SER state; * and * represent the jth interaction parameters (j = 0) between the element Pt and Sb on the first and second sublattice, respectively.

Optimization
Most of the above experimental info namic model parameformation temperatures based on the phase diagram of the Pt-Sb system [11].
The optimization was carried out by means of the Thermo-Calc software [25], which can handle various kinds of experimental data.The program works by minimizing an error sum where each of the selected data values is given a certain weight.The weight is chosen by personal judgment and changed by trial and error during the work until most of the selected experimental information is reproduced within the expected uncertainty limits.
The ways to get the phase diagram summarized as several steps using the Thermo-Calc software.Firstly, it is the Unary phases to get the GES file, which is the basis of the assessment.Secondly, it is the thermodynamic mization.When the phase diagram is successfully optimized, all the phases in the system will get a set of consistent thermodynamic parameters to express Gibbs energies.Using the set of consistent thermodynamic param

Results and Discussions
A thermodynamic description of the Pt-Sb system obtained in the present work is shown in Table 2.The Pt-Sb phase diagram calculated ermodynamic parameters is presented in Figure 1, and nearly identical to the one determined by Bhan et al. [9], Durussel and Feschotte [11].Because of the symmetry of liquid at both sides of the compound PtSb 2 , there is great different about the liquid at 0.20 -0.40 Sb between the calculated results and the experimental data [7][8][9].
The invariant equilibria of the Pt-Sb system are listed in Table 3.In the Table 3, some calculating data is nearly to experiment data but some is very different to experiment data.There is some ers.Other is that the liquid of the s the symmetry during the assessed procedure.So it is very difficult to treat and optimize to get the same as the experimental data.In order not to change the types of reactions, the temperatures of reactions were revised in this work.As shown in the table, most satisfactory agreement is obtained between the calculations and experiments [7][8][9]11], where there is uncertainty in the invariant reaction temperature at 0.20 -0.40 Sb.

Conclusion
The phase relations and the thermodynamic descri of the Pt-Sb system were critically evaluated from the experimental information available in the literature. of consistent thermodynamic parameters were deri With the thermodynamic description available, one can now make various calculations of practical interest or in the optimization of high-order systems.
the phase diagram and build the database such as the TDB file.

able 3 .
Invariant reactions of the Pt-Sb system.

Table 2 . Thermodynamic parameters of the Pt-Sb system a .
a In J•mole 1 of the formula units.