Face-Centered Cubic Refractory Alloys Prepared from Single-Source Precursors

Three binary fcc-structured alloys (fcc–Ir0.50Pt0.50, fcc–Rh0.66Pt0.33 and fcc–Rh0.50Pd0.50) were prepared from [Ir(NH3)5Cl][PtCl6], [Ir(NH3)5Cl][PtBr6], [Rh(NH3)5Cl]2[PtCl6]Cl2 and [Rh(NH3)5Cl][PdCl4]·H2O, respectively, as single-source precursors. All alloys were prepared by thermal decomposition in gaseous hydrogen flow below 800 °C. Fcc–Ir0.50Pt0.50 and fcc–Rh0.50Pd0.50 correspond to miscibility gaps on binary metallic phase diagrams and can be considered as metastable alloys. Detailed comparison of [Ir(NH3)5Cl][PtCl6] and [Ir(NH3)5Cl][PtBr6] crystal structures suggests that two isoformular salts are not isostructural. In [Ir(NH3)5Cl][PtBr6], specific Br…Br interactions are responsible for a crystal structure arrangement. Room temperature compressibility of fcc–Ir0.50Pt0.50, fcc–Rh0.66Pt0.33 and fcc–Rh0.50Pd0.50 has been investigated up to 50 GPa in diamond anvil cells. All investigated fcc-structured binary alloys are stable under compression. Atomic volumes and bulk moduli show good agreement with ideal solutions model. For fcc–Ir0.50Pt0.50, V0/Z = 14.597(6) Å3·atom−1, B0 = 321(6) GPa and B0’ = 6(1); for fcc–Rh0.66Pt0.33, V0/Z = 14.211(3) Å3·atom−1, B0 =259(1) GPa and B0’ = 6.66(9) and for fcc–Rh0.50Pd0.50, V0/Z = 14.18(2) Å3·atom−1, B0 =223(4) GPa and B0’ = 5.0(3).


Introduction
Traditionally, high-entropy alloys were prepared using conventional melting of pure metals. Nevertheless, catalytic applications as well as preparation of high-entropy alloys based on metals with ultra-high melting points need the development of new techniques for a preparation of high-entropy alloys as fine nanostructured powders. Recently, single-source precursors strategy has been applied to access high-entropy alloys based on platinum group metals. The strategy requires a synthesis of coordination compounds from water solutions and their further thermal decomposition in a hydrogen flow based on the following principle general scheme [1]: in water solution: a[Ir(NH 3 ) 5 Cl]Cl 2 + b[Rh(NH 3 ) 5 Cl]Cl 2 + (1-a-b)[Ru(NH 3 ) 5

Materials and Methods
[Ir(NH 3 ) 5 Cl]Cl 2 and [Rh(NH 3 ) 5 Cl]Cl 2 were prepared from IrCl 4 ·xH 2 O and RhCl 3 ·xH 2 O according to published protocols [9,10]. ( 2 O respectively. Details about the synthesis of solid-state precursors and alloys can be found in our earlier publications [8,10]. Briefly, the precursors, [Ir(NH 3 ) 5 Cl][PtCl 6 ], was first crystallized at room temperature from a mixture of water solutions of [Ir(NH 3 ) 5 5 Cl]Cl 2 . The mixture was kept in darkness at room temperature for a week. Orange crystals were filtered and dried on air. [Rh(NH 3 ) 5 Cl] 2 [PtCl 6 ]Cl 2 crystals were decomposed in hydrogen flow at 700 • C. Similarly, all compounds were also decomposed in He flow. All preparatory conditions for all alloys are summarized in Table 1. Phase composition and cell parameters of metallic alloys were obtained by in house powder Xray diffraction (PXRD) using an ARL X'TRA diffractometer (CuKα-radiation, Ni-filter, position sensitive detectors, Bragg-Brentano reflection geometry, 2Θ = 5-100°, Δ2Θ = 0.03°, 10 s/step, room temperature, Thermo Electron Corporation, Waltham, MA, USA). Polycrystalline samples were slightly ground with hexane using an agate mortar, and the resulting suspensions were deposited on the polished side of a quartz sample holder, a smooth thin layer formed after drying. Silicon powder was taken as an external standard (a = 5.4309 Å, full width at half maximum 2Θ = 0.1°) for the calibration of the zero-shift of the goniometer and instrumental line broadening. Only single-and two-phase fcc-structured alloys were found as products of thermal decomposition of single-source precursors mentioned above (Table 1).
The X-ray diffraction study of [Ir(NH3)5Cl][PtBr6] single crystal at 150 K was performed on an automated Bruker APEX-II CCD diffractometer (MoKα radiation, graphite monochromator, twodimensional CCD detector, Bruker Corporation, Karlsruhe, Germany). One hundred and fifty nine structural parameters with 48 restrains were refined and 4159 reflexes were used. The corresponding divergence factors were Rall = 9.07% and wRref = 15.41%; for 3150 reflections with I ≥ 2σ(I), Rgt = 6.28%, wRgt = 14.33% and the S factor against F2 was 1.066. X-ray crystallographic data were deposited with an Inorganic Crystal Structure Database (ICSD) under No. 1971298. Phase composition and cell parameters of metallic alloys were obtained by in house powder X-ray diffraction (PXRD) using an ARL X'TRA diffractometer (CuKα-radiation, Ni-filter, position sensitive detectors, Bragg-Brentano reflection geometry, 2Θ = 5-100 • , ∆2Θ = 0.03 • , 10 s/step, room temperature, Thermo Electron Corporation, Waltham, MA, USA). Polycrystalline samples were slightly ground with hexane using an agate mortar, and the resulting suspensions were deposited on the polished side of a quartz sample holder, a smooth thin layer formed after drying. Silicon powder was taken as an external standard (a = 5.4309 Å, full width at half maximum 2Θ = 0.1 • ) for the calibration of the zero-shift of the goniometer and instrumental line broadening. Only single-and two-phase fcc-structured alloys were found as products of thermal decomposition of single-source precursors mentioned above (Table 1).
Room temperature compressibility curves for fcc-Ir 0.50 Pt 0.50 , fcc-Rh 0.66 Pt 0.33 and fcc-Rh 0.50 Pd 0.50 binary alloys were collected at the ID15B beamline up to 40 GPa (ESRF, λ = 0.411235 Å, MAR 555 flat panel detector, beam size 10(v) × 10(h) µm 2 ). The samples were loaded in diamond anvil cells equipped with conically supported Boehler Almax anvils 250 µm culet sizes. He was used as a pressure transmitting medium. Ruby was applied as a pressure calibrant. The diffraction data were integrated using DIOPTAS [17]. The unit cell parameters, the background and the line-profile parameters were refined simultaneously using JANA2006 software [18]. The P-V data were fitted using EoS-Fit 5.2 software [19].

Preparation of fcc-Structured Binary Alloys under Ambient Pressure and their Phase Composition
All fcc-structured alloys were prepared from single-source precursors (Table 1). Among fccstructured binary metallic systems, only the Pt-Rh pair has complete miscibility in the solid state [22]. In all other binaries, there are miscibility gaps between two fcc-structured alloys: for Ir-Rh below 1335 °C; for Pd-Rh below 910 °C and for Ir-Pt below 1370 °C [23][24][25]. Nevertheless, all single-source precursors gave single-phase alloys as products of their thermal decomposition in a reductive atmosphere even below 700 °C (Table 1). Thermal decomposition in an inert atmosphere usually results in a formation of two-phase mixtures, which might be a sign for different decomposition mechanisms.
All It seems that thermal decomposition of described systems could be controlled by a reaction atmosphere. In the reductive flow (hydrogen), all systems Ir-Rh, Pd-Rh, Ir-Pt and Pt-Rh formed

Preparation of fcc-Structured Binary Alloys under Ambient Pressure and Their Phase Composition
All fcc-structured alloys were prepared from single-source precursors (Table 1). Among fccstructured binary metallic systems, only the Pt-Rh pair has complete miscibility in the solid state [22]. In all other binaries, there are miscibility gaps between two fcc-structured alloys: for Ir-Rh below 1335 • C; for Pd-Rh below 910 • C and for Ir-Pt below 1370 • C [23][24][25]. Nevertheless, all single-source precursors gave single-phase alloys as products of their thermal decomposition in a reductive atmosphere even below 700 • C (Table 1). Thermal decomposition in an inert atmosphere usually results in a formation of two-phase mixtures, which might be a sign for different decomposition mechanisms.
All binary systems allow us to prepare various alloys by changing compositions of single-source precursors. So, crystallization of [M I (NH 3 ) 5 4 ] for fcc-Ir 0.5 Pd 0.5 and fcc-Rh 0.5 Pd 0.5 alloys respectively. It seems that thermal decomposition of described systems could be controlled by a reaction atmosphere. In the reductive flow (hydrogen), all systems Ir-Rh, Pd-Rh, Ir-Pt and Pt-Rh formed single phase fcc-structured alloys. Single-phase alloys formed in systems with and without miscibility in the solid-state. In the inert atmosphere (argon or helium flow), Ir-Rh forms also a single-phase fcc-alloy [5]. Ir-Pt and Pt-Rh (both systems with miscibility in the solid-state) formed a two-phase mixture after heating in an inert flow, which might be due to the mechanism of their thermal decomposition.
In an inert atmosphere (He, Ar and N 2 ), hexachlorometallates(IV) decompose in a relatively narrow temperature interval [20,21] [12,26], it has been shown that their thermal decomposition in an inert atmosphere corresponds to the formation of metallic fcc-Pt and RhBr 3 as intermediates above 500 • C. Further heating results in the decomposition of RhBr 3 and formation of two-phase fcc-alloys mixture. Such a transformation is a key process responsible for the formation of two-phase metallic products in an inert gas flow. Similarly, upon heating of [Ir(NH 3 ) 5 Cl][PtBr 6 ] above 500 • C intermediate with a total composition of "Ir:Pt:Br" contains broad reflexes corresponding to IrBr 3 and two fcc-structured alloys: [Ir(NH 3 ) 5 Cl][PtBr 6 ] → IrBr 3 + "mixture of fcc-structured alloys".
Further heating results in the formation of a two-phase metallic mixture (Table 1) Single-phase alloys prepared from single-source precursors show isometric porous metallic particles (Figure 3). The shape of porous conglomerates followed the shape of crystals characteristic for single source precursors.
Materials 2020, 13, x FOR PEER REVIEW 7 of 12 single phase fcc-structured alloys. Single-phase alloys formed in systems with and without miscibility in the solid-state. In the inert atmosphere (argon or helium flow), Ir-Rh forms also a single-phase fccalloy [5]. Ir-Pt and Pt-Rh (both systems with miscibility in the solid-state) formed a two-phase mixture after heating in an inert flow, which might be due to the mechanism of their thermal decomposition.
In an inert atmosphere (He, Ar and N2), hexachlorometallates(IV) decompose in a relatively narrow temperature interval [20,21] [12,26], it has been shown that their thermal decomposition in an inert atmosphere corresponds to the formation of metallic fcc-Pt and RhBr3 as intermediates above 500 °C. Further heating results in the decomposition of RhBr3 and formation of two-phase fcc-alloys mixture. Such a transformation is a key process responsible for the formation of two-phase metallic products in an inert gas flow. Similarly, upon heating of [Ir(NH3)5Cl][PtBr6] above 500 °C intermediate with a total composition of "Ir:Pt:Br" contains broad reflexes corresponding to IrBr3 and two fcc-structured alloys: [Ir(NH3)5Cl][PtBr6] → IrBr3 + "mixture of fcc-structured alloys".
Further heating results in the formation of a two-phase metallic mixture (Table 1) Single-phase alloys prepared from single-source precursors show isometric porous metallic particles (Figure 3). The shape of porous conglomerates followed the shape of crystals characteristic

High-Pressure Compressibility of fcc-Structured Binary Refractory Alloys
Cell parameters characteristic for prepared fcc-alloys correspond to Zen's low [27,28] and nearly linearly depend on the alloy's compositions (Figure 4). Within experimental errors there is no positive or negative deviation from linearity, which might be a sign for ideality of described fcc-structured binary alloys. For fcc-structured binary alloys (and also for hcp-structured) with hcp metals such as Ir-Re and Rh-Re alloys, significant negative deviation from linearity has been mentioned [29,30].

High-Pressure Compressibility of fcc-Structured Binary Refractory Alloys
Cell parameters characteristic for prepared fcc-alloys correspond to Zen's low [27,28] and nearly linearly depend on the alloy's compositions (Figure 4). Within experimental errors there is no positive or negative deviation from linearity, which might be a sign for ideality of described fcc-structured binary alloys. For fcc-structured binary alloys (and also for hcp-structured) with hcp metals such as Ir-Re and Rh-Re alloys, significant negative deviation from linearity has been mentioned [29,30].  Table 1).
All investigated fcc-structured refractory alloys do not show any pressure-induced phase transitions below 50 GPa at room temperature. A similar pressure temperature stability was obtained for pure Rh, Ir, Pd and Pt up to much higher pressures. Experimental compressibility curves (P-V data) for all investigated alloys can be fitted using the third-order Birch-Murnaghan equation of state (BM-EoS) [31,32] (Table 3, Figure 5): where V0 is the unit cell volume at ambient pressure, B0 is the bulk modulus and B'0 is the pressure derivative of the bulk modulus. All alloys show regular compressibility with pressure as well as with composition.
It has been previously shown that bulk moduli for binary alloys can be estimated using quite a simple model reported in [5,6,8]. Concentration dependence of the bulk modulus B0(x) of a binary metallic alloy M 1 xM 2 1-x containing x atomic fraction of the refractory metal can be calculated using the following equation: where B1 and B2 (GPa) are the bulk moduli of the metals M 1 and M 2 , and V1 and V2 (Å 3 ) are atomic volumes at ambient pressure of M 1 and M 2 , correspondently. According to Table 3, structural parameters (atomic volume and bulk moduli) for all alloys can be estimated quite well using simple models typical for ideal solid solutions. Such a finding can be used for a prediction of the compressibility of new alloys to be able to construct a complete thermodynamic database for refractory fcc-alloys. Palladium shows relatively high compressibility in the comparison with the other platinum group metals. As a result, the fcc-Rh0.50Pd0.50 alloy had the highest compressibility among other alloys. At the same time its compressibility had the better accordance with the ideal solution model (Equation (2)). A similar good satisfaction between experimentally obtained data and predicted according to Equation (2) were found for the fcc-Ir0.42Rh0.58 alloy. Both investigated platinum alloys fcc-Ir0.50Pt0.50 and fcc-Pt0.33Rh0.67 show large deviations from predicted values. A larger value for fcc-Ir0.50Pt0.50 can be explained by relatively low experimental pressure. Nevertheless, compressibility of fcc-Pt0.33Rh0.67 was investigated up to 47 GPa. Its compressibility was much higher in comparison with  Table 1).
All investigated fcc-structured refractory alloys do not show any pressure-induced phase transitions below 50 GPa at room temperature. A similar pressure temperature stability was obtained for pure Rh, Ir, Pd and Pt up to much higher pressures. Experimental compressibility curves (P-V data) for all investigated alloys can be fitted using the third-order Birch-Murnaghan equation of state (BM-EoS) [31,32] (Table 3, Figure 5): where V 0 is the unit cell volume at ambient pressure, B 0 is the bulk modulus and B' 0 is the pressure derivative of the bulk modulus. All alloys show regular compressibility with pressure as well as with composition. It has been previously shown that bulk moduli for binary alloys can be estimated using quite a simple model reported in [5,6,8]. Concentration dependence of the bulk modulus B 0 (x) of a binary metallic alloy M 1 x M 2 1-x containing x atomic fraction of the refractory metal can be calculated using the following equation: where B 1 and B 2 (GPa) are the bulk moduli of the metals M 1 and M 2 , and V 1 and V 2 (Å 3 ) are atomic volumes at ambient pressure of M 1 and M 2 , correspondently. According to Table 3, structural parameters (atomic volume and bulk moduli) for all alloys can be estimated quite well using simple models typical for ideal solid solutions. Such a finding can be used for a prediction of the compressibility of new alloys to be able to construct a complete thermodynamic database for refractory fcc-alloys. Palladium shows relatively high compressibility in the comparison with the other platinum group metals. As a result, the fcc-Rh 0.50 Pd 0.50 alloy had the highest compressibility among other alloys. At the same time its compressibility had the better accordance with the ideal solution model (Equation (2)). A similar good satisfaction between experimentally obtained data and predicted according to Equation (2) were found for the fcc-Ir 0.42 Rh 0.58 alloy. Both investigated platinum alloys fcc-Ir 0.50 Pt 0.50 and fcc-Pt 0.33 Rh 0.67 show large deviations from predicted values. A larger value for fcc-Ir 0.50 Pt 0.50 can be explained by relatively low experimental pressure. Nevertheless, compressibility of fcc-Pt 0.33 Rh 0.67 was investigated up to 47 GPa. Its compressibility was much higher in comparison with ideal solutions model. The mentioned large deviation from the ideal solutions model should be further studied theoretically.
Single-phase refractory high-entropy alloys, namely hcp-Ir 0.19 Os 0.22 Re 0.21 Rh 0.20 Ru 0.19 and fcc-Ir 0.26 Os 0.05 Pt 0.31 Rh 0.23 Ru 0.15 , prepared from single-source precursors show similar numbers for atomic volumes and room-temperature compressibility (Table 3). Their behavior suggests that they can be described as ideal solid solutions [1,5]. Prepared fcc-structured binary alloys can be used as reliable models for modeling thermodynamic and structural properties of high-entropy alloys in a broad range of compositions. Phase instability upon heating and compression typical for high-entropy alloys with light metals such as Al, Co, Ni and Fe is not typical for refractory alloys based on platinum group metals. Platinum group metals are known as stable substances upon heating and compression. As soon as pure platinum metals and their binary alloys show extraordinary phase stability, their multicomponent alloys as well as high-entropy alloys did not show any phase transitions upon heating and compression, which makes them unique for high-temperature applications under extreme chemical impact and mechanical stress. fcc-Rh (up to 64 GPa) 13.73 (7) -301(9) 3.1(2) - [5] fcc-Pt (up to 100 GPa) 15.094(2) -277(2) 4.95(2) - [33] fcc-Pd (up to 100 GPa) 14 Figure 5. Room temperature high-pressure compressibility curves for fcc-structured Rh, Ir, Pt and Pd binary alloys and pure metals in V/Z vs. P scale (according to Table 3).

Conclusions
Single-source precursors strategy can be successfully applied for the preparation of highentropy alloys of various compositions and structures. Double complex salts can be considered as effective single-source precursors for refractory multicomponent alloys. A systematic investigation Figure 5. Room temperature high-pressure compressibility curves for fcc-structured Rh, Ir, Pt and Pd binary alloys and pure metals in V/Z vs. P scale (according to Table 3).