High-temperature Behavior of PtBi₂ and Possibility of Using the Mineral Insizwaite as a Geothermometer

Abstract

The article presents investigation of polymorphic transformations of the PtBi₂ phases that occur during heating in inert atmosphere and in vacuum. A synthetic medium-temperature β-PtBi₂ modification corresponding to the mineral insizwaite was studied by high-temperature X-ray diffraction and differential thermal analysis in the temperature range of 20–640°C. Two phase transitions are established by in situ X‑ray powder diffraction: from β into γ-PtBi₂, and then into δ-PtBi₂. Two endothermic peaks are recorded on the DTA curve, which correspond to the phase transitions. High-temperature X-ray powder diffraction data determined γ- and δ-PtBi₂ formation at high temperature in inert atmosphere and in vacuum. Reverse polymorphic transformation were not observed by X-ray powder diffraction during cooling. It is suggested that insizwaite is a mineral geothermometer and its presence in the geological system implies certain temperature ranges of mineral formation environments.

INTRODUCTION

The phase relations in the binary diagram Pt–Bi were reviewed by H. Okamoto (1991). The Pt–Bi system contains six binary compounds: PtBi, α-PtBi₂, β‑PtBi₂, γ-PtBi₂, δ-PtBi₂ and Pt₂Bi₃. The crystallographic data for the phases are shown in Table 1. The phase with the composition PtBi₂ forms different structural modifications depending on the crystallization temperature (Okamoto, 1991; Zhuravlev and Stepanov, 1962).

Table 1. Crystallographic data for Pt–Bi system

The low-temperature modification α-PtBi₂ was synthesized by T. Biswas and K. Schubert in 1969. Its structure was solved later (Bhatt and Schubert, 1980b). It is orthorhombic, sp. gr. Pbca; the structure type is AuSn₂.

The β-PtBi₂ modification has a cubic crystal structure, sp. gr. Pa$\bar {3}$, unit cell parameters: a = 6.625(A), V = 290.775 (A3), Z = 4 with a pyrite structure type (Zhuravlev et al., 1962, Furuseth et al., 1965, Brese and Schnering, 1994). This synthetic phase corresponds to the mineral insizwaite, which was found and described by L. Cabri and D. Harris (1972).

The crystal structure and properties of γ-PtBi₂ modification have been studied several times. The first structural model for the γ-PtBi₂ in trigonal sp. gr. P$\bar {3}$ was obtained by Schubert et al. (1968). The same model was used in structure refinement by Xu et al. (2016). Synthesis of γ-PtBi₂ was performed using a melt-growth method in both cases. M. Kaizer et al. (2014) discovered that γ-PtBi₂ can also be obtained at 70°C by reductive decomposition of layered Bi₁₃Pt₃I₇. Using this unconventional growth method, the γ‑PtBi₂ single crystals were synthesized, and structure was solved in another sp. gr. P31m.

Recently in 2020, one more structural study of γ‑PtBi₂ published (Shipunov et.al., 2020). Both structural models were considered: in sp. gr. P$\bar {3}$ and P31m. The structure refinement in the group P31m gave the lowest R-factor: 5.2% against 11.8%. Selected area electron diffraction (SAED) also confirmed the group P31m.

The only information about the highest modification contains in the paper (Bhatt and Shubert, 1980b). Authors found that δ-PtBi₂ forms solid solution with a compound Pt₄Bi₇Pb.

Zhuravlev et al. (1962a, 1962b) provides first high-temperature X-ray diffraction study of the PtBi₂ phase. Three modifications were obtained, and thermal expansion coefficients were calculated. The crystallographic data were obtained for cubic PtBi₂. Polymorphic transitions under cooling were not studied. It needs to be mentioned that names of the PtBi₂ polymorphs in the Zhuravlev’s works do not correspond to modern nomenclature.

We report the results of PtBi₂ polymorphic transitions temperature dependence study using several methods: thermal analysis coupled with thermogravimetric analysis (DTA+TG); high-temperature X-ray powder diffraction and electron microprobe analysis. Experiments were performed under inert atmosphere and in vacuum. Reversibility of transitions was addressed. The authors suggest that mineral insizwaite, β-PtBi₂, can be used as a geothermometer for estimating temperature of crystallization in a geological system.

EXPERIMENTAL

Synthesis

The β-PtBi₂ polymorph was obtained via solid phase synthesis in vacuum-sealed silica-glass tubes. Mixture of pure metals Pt and Bi in molar proportion Pt : Bi = 1 : 2 was placed in a silica tube, evacuated and sealed. To obtain the precursor compound the tube with the charge was placed in a furnace and heated at 850°C for a week, then quenched by dropping in cold water. The precursor material was grounded and divided in to several portions. Each portion was placed in the tube, evacuated, and sealed again. One tube was heated at the temperature 350°C during a week leading to the β-PtBi₂ polymorph formation. Another tube heated at the temperature 250°C during four months with a purpose to obtain the α-PtBi₂ polymorph.

X-ray Powder Diffraction

The synthesis products were ground in an agate mortar and analyzed by X-ray diffraction method on the Rigaku DMAX 2200/PC diffractometer at the Institute of Geology of Ore Deposits RAS (IGEM RAS, Moscow, Russia). X-ray diffraction data were acquired at room temperature using Cu-λK_α radiation, time-counting scanning mode, step size 0.05°θ, exposure 1.5 sec, in the 2θ range from 3° to 80°. Phase analysis was performed with the Jade 6.5 program (Materials Data Incorporation LTD).

Electron Microprobe Analysis

The chemical composition and homogeneity of the grains were determined using a Microanalyzer Cameca SX100 with five wavelength spectrometers and a Bruker Xflash6 energy dispersive spectrometer at the Common User Center Geoanalyst of the Zavaritzkiy Institute of Geology and Geochemistry UB RAS (IGG UB RAS, Ekaterinburg, Russia).

Differential Thermal Analysis

Thermal effects were studied via differential thermal analysis (DTA+TG) using an STA 449F5 Jupiter analyzer, NETZSCH (Germany) at Zavaritskiy Institute of Geology and Geochemistry UB RAS (Ekateriburg). DTA experiments were conducted in α‑Al₂O₃ crucible in argon flow 50 mL/min with ramp rate 10°/min in the temperature range 30–800°C. Prior to the heating two corundum crucibles with the same mass were annealing up to 1100°C on air and were subsequently used for crucib the background line (“blank line”) measurement. Inert α-Al₂O₃ used as standard. The work chamber was evacuated then filled with argon.

High-temperature Powder X-ray Diffraction

High-temperature powder X-ray diffraction (HTXRD) experiments of PtBi₂ phase were done using an Empyrean Panalytical BV diffractometer with Anton Paar HTK-1200N high-temperature attachment at the Frumkin Institute of Physical chemistry and Electrochemistry RAS (IPCE RAS, Moscow, Russia). β-PtBi₂ powder were deposited on a Al₂O₃ sample holder. HTXRD experiments were performed in vacuum in a temperature cycle 50–640–50°C (heating and cooling) using Cu-λK_α radiation in 2θ range from 10° to 80°. Temperature steps were 10/20/50°C, average heating rate was about 5°/min, collecting time at each temperature step was about 30 min. The total time of the experiments, including heating, cooling, and diffraction data collection, was about 17.5 h.

RESULTS AND DISCUSSION

X-ray powder diffraction data of the samples synthesized at temperature 250 and 350°C, correspond to benchmark diffraction patterns of the β-PtBi₂ modification (PDF card no. 26-0221) and insizwaite (Cabri and Harris, 1972) (Table 2). The low temperature α‑modification of PtBi₂ was not obtained despite the long time of the synthesis experiment.

Table 2. X-ray diffraction data for β-PtBi₂

Homogeneity of the samples was confirmed by electron microprobe analysis. The composition was obtained at 16 points on three grains (Table 3). The average element contents (in wt %) are Pt 32.82, Bi 67.59. The empirical formula for synthetic insizwaite is Pt_1.03Bi_1.97, calculated based on atoms per formula unit.

Table 3. Microprobe analysis of β-PtBi₂

The DTA curve of the PtBi₂ sample contains two endothermic peaks at the temperatures 421.0 and 639.4°C, respectively (Fig. 1, Table 4). These data are in agreement with the temperature of polymorphic transformations (Okamoto, 1991). The thermal effect at the temperature of 653.2°C corresponds to incongruent melting of the PtBi₂ and to solidus temperature (t_s), above which the two-component field exists. The melting ends at the temperature 717.3°C—the liquidus (t_l), at which the small endothermic peak on the DTA curve appears. The sample weight change was 0.05% in the temperature range 30–775°C, this value is within the measurement error of. The endothermic peak at 639.4°C temperature just before melting indicates the formation of the δ-PtBi₂ phase, which exists in a narrow temperature range.

Fig. 1.

Temperature dependence of DTA signal for PtBi₂ in argon atmosphere with ramp rate of 10°C/min.

Table 4.   Temperature of polymorphic transformations, solidus (t_s) and liquidus temperature (t_l) for PtBi₂

Table 5 presents the diffraction data of the γ-PtBi₂, obtained by high-temperature X-ray diffraction at 500°С, in comparison with the theoretical data. Card no. 9-269 of the International Powder Diffraction Database Powder Diffraction File (PDF) is considered the γ-PtBi₂ standard. However, our experimental data and structural studies by other authors (Schubert et al., 1968; Biswas & Schubert, 1969; Xu et al., 2016) indicate that this card for γ-PtBi₂ is apparently erroneous. The X-ray diffraction pattern of the high-temperature δ-PtBi₂ modification obtained at 620°C (Table 6). It was indexed in hexagonal crystal system, possible space groups are—P6₃/mmc, P6₃/m, P6₁22. The unit cell parameters were calculated: a = 4.391 Å, c = 5.552 Å. The hypothesis (Bhatt and Schubert, 1980b) of the isostructural nature of the δ-PtBi₂ and Pt₄Bi₇Pb phases is not confirmed.

Table 5.   X-ray diffraction data for γ-PtBi₂

Table 6. X-ray Diffraction data for δ-PtBi₂ collected at 620°С

High-temperature powder X-ray diffraction data confirm the stability of the β-PtBi₂ polymorph in the temperature range 30–420°C. Additional peaks appear in the diffraction patterns starting from 420°C. The intensity of the β-PtBi₂ peaks becomes smaller at the same time (Fig. 2). Process of transformation from the cubic β-polymorph to the trigonal γ-polymorph is going on. At 500°C the peaks of the β-PtBi₂ disappear completely_: the first polymorphic transformation ends. Only intensity of diffraction peaks varies at the temperature range 500–600°C. Diffraction pattern is changing again at New peaks appear at 600°C corresponding to polymorphic transformation from γ-PtBi₂ to δ‑PtBi₂. Decomposition process is beginning at 630°C. At this point diffraction peaks of metallic platinum appear. Peaks of the δ-PtBi₂ polymorph persist up to 640°C. Heating above 640°C was not carried out to avoid melting of the sample. After that the sample was cooled to the room temperature. During the cooling process X-ray diffraction patterns did not change.

Fig. 2.

High-temperature X-ray diffraction experiments for PtBi₂ in temperature range from 30 to 640°C in vacuum.

The stability field of the β-PtBi₂ polymorph extends from 20 to 420°C, the γ-PtBi₂ polymorph forms and exists in the temperature range 450–600°C, δ-PtBi₂ polymorph crystalizes and exists from 600 to 640°C (Kaizer et. al., 2014). The high-temperature X‑ray diffraction experiment is an in situ method, and it dynamically shows the transformation process. That is why the peaks of two polymorphs coexist in some X‑ray patterns collected near the transformation temperatures. The peak intensities of the polymorphs and their ratio change gradually in such diffraction patterns. The main diffraction peaks of the β-PtBi₂ polymorph appear together with γ-polymorph peaks in the X-ray diffraction patterns taken in the temperature range 450–550°C. The diffraction pattern acquired at 600°C contains the highest-intensity peak of the γ-PtBi₂ together with the peaks of δ-PtBi₂ polymorph. Also, all collected diffraction patterns contain the peaks of Al₂O₃ phase, which is the sample holder and internal standard.

Upon successive cooling of the sample from 640 to 50°C, the diffraction pattern did not shown significant changes (Fig. 3).

Fig. 3.

X-ray diffraction experiments collected during cooling PtBi₂ in temperature range from 640 to 50°C in vacuum.

CONCLUSIONS

High-temperature transformations of the PtBi₂ phase were studied by a set of methods. It is shown that the transformation from the δ-PtBi₂ polymorph in to the γ-PtBi₂ and then in to the β-PtBi₂ one did not occur during cooling in HTXRD experiments. The results of the DTA and HTXRD analyzes agree well and correlate with the temperatures of polymorphic transformations. The β-PtBi₂ polymorph is stable under an inert atmosphere at temperatures up to 450°C, γ-PtBi₂ is stable in the temperature rangs from 450 to 620°C, and δ-PtBi₂, from 620°C to 640°C. The existence of the high-temperature polymorph is confirmed.

The low temperature polymorph α-PtBi₂ was not obtained. Instead, the β-PtBi₂ (insizwaite) crystallized during annealing at 250°C. That is why, the α‑PtBi₂ polymorph behaviour at high temperatures and its transformation in to β-form are not studied.

The experimental results indicate that insizwaite, β-PtBi₂, can be used as a mineral geothermometer. Insizwaite crystallizes at up to 450°C under inert atmosphere and in vacuum. The γ- and δ-polymorphs form at temperatures higher than 450°C. That is true only for processes under inert atmosphere and vacuum conditions. Other transformations occur during heating of insizwaite in air (Mezhueva et al., in press). The possibility of using insizwaite as geothermometer requires further study. It is necessary to estimate the influence of different factors, such as pressure, annealing time, and the presence of additional phases in the system on crystallization of high-temperature polymorphs.