Ternary Bismuthide SrPtBi2: Computation and Experiment in

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Ternary Bismuthide SrPtBi2: Computation and Experiment in Synergism to Explore Solid-State Materials Xin Gui,† Xin Zhao,‡,* Zuzanna Sobczak,§ Cai-Zhuang Wang,‡ Tomasz Klimczuk,§ Kai-Ming Ho,‡ and Weiwei Xie†,* †

Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States Department of Physics and Astronomy, Iowa State University and Ames Laboratory, United States Department of Energy, Ames, Iowa 50011, United States § Faculty of Applied Physics and Mathematics, Gdansk University of Technology, Narutowicza 11/12, Gdansk 80-233, Poland ‡

S Supporting Information *

ABSTRACT: A combination of theoretical calculation and the experimental synthesis to explore the new ternary compound is demonstrated in the Sr−Pt−Bi system. Because Pt−Bi is considered as a new critical charge-transfer pair for superconductivity, it inspired us to investigate the Sr−Pt−Bi system. With a thorough calculation of all the known stable/metastable compounds in the Sr−Pt−Bi system and crystal structure predictions, the thermodynamic stability of hypothetical stoichiometry, SrPtBi2, is determined. Following the high-temperature synthesis and crystallographic analysis, the first ternary bismuthide in Sr− Pt−Bi, SrPtBi2 was prepared, and the stoichiometry was confirmed experimentally. SrPtBi2 crystallizes in the space group Pnma (S.G. 62, Pearson Symbol oP48), which matches well with theoretical prediction using an adaptive genetic algorithm. Using first-principles calculations, we demonstrate that the orthorhombic structure has lower formation energies than other 112 structure types, such as tetragonal BaMnBi2 (CuSmP2) and LaAuBi2 (CuHfSi2) structure types. The bonding analysis indicates that the Pt−Bi interactions play a critical role in structural stability. The physical property measurements show the metallic properties at the low temperature, which agrees with the electronic structure assessment.

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topological Dirac semimetal Na3Bi.10,11 Such materials can be used as transistors and other electronic devices.12 Usually, simple Bicontaining semiconductors or semimetals can be predicted using chemical valence arguments or Zintl−Klemm concepts.13−15 Half-Heusler RPtBi (R = rare earth elements) compounds following 18e− rules show intriguing properties such as superconductivity in LuPtBi.16−18 The Zintl−Klemm concepts can be employed for interpreting the charge balance in BaMnBi2, which is an antiferromagnetic Dirac material coexisting with superconductivity at high pressure.19 BaMnBi2 can be formulated as Ba2+Mn2+Bi−Bi3− with Bi−Bi square planar in the structure.20 However, if a system turns more complex with a small difference in electronegativity among elements, chemical valence rules may fail in predicting crystal structures and properties. It is even harder to rationalize some subtle structural distortions or atomic distributions because of the complexity. For example, ternary rare-earth gallium bismuthide, LaGaBi2, off the chemical balance, crystallizes in

INTRODUCTION Exploring novel functional solid-state materials, which are key to technology innovation, is a long-standing goal in materials chemistry and physics. Because of various possibilities of combining atoms, discovering novel materials has long been found to be a time-consuming process. Traditionally, prediction of new compounds heavily relies on empirical or semiempirical rules, such as electron counts and packing rules.1−3 Current strategies involving informatics, machine learning, and using of materials databases, such as Materials Project and AFLOWLIB, opened up an efficient approach to expedite the search for functional solid-state materials.4−7 Briefly, truly representative atomic combinations will be selected based on the highly performed computation. These atomic combinations will be evaluated by experimental synthesis to see which one is more stable and reliable. The process with a combination of experiments and theory has effectively reduced the scope for finding new materials.8 As nontoxic heavy metals, bismuth shows the strong spin− orbital coupling effects, which provide bismuthides many unique and desirable properties for optoelectronic, thermoelectric, and electronic device applications.9 For example, threedimensional Dirac fermions have been detected in the © XXXX American Chemical Society

Received: December 29, 2017 Revised: February 13, 2018 Published: February 14, 2018 A

DOI: 10.1021/acs.jpcc.7b12801 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

of a specimen. The samples were examined at 15 kV. Spectra were collected for 100 s. Heat Capacity Measurements. The specific heat measurements were performed using a physical properties measurement system on pieces of SrPtBi2 polycrystalline samples shown in Figure S1. The measurements were operated over a temperature range of 1.8−300 K without the applied magnetic field.

the hexagonal structure with 24 atoms per unit cell and La6 trigonal prisms centered by Bi atoms.21 Recently, we discovered the superconductivity in new monoclinic BaPt2Bi2.22 The research indicates that the Pt−Bi antibonding is significant for inducing superconductivity. Thus, we extended our study to the Sr−Pt−Bi system, which was unexplored before. To accelerate the targeted synthesis, we employed the crystal structure prediction and total energy calculation first to determine the thermodynamic stable stoichiometry. Using the predicted stoichiometry, we successfully synthesized the new SrPtBi2. Unlike other tetragonal 112CuSmP2 (BaMnBi2)19 or CuHfSi2 (LaAuBi2),23 SrPtBi2 adopts the orthorhombic structure with space group Pnma, which is analogous to YNiSn2.24 Electronic structure calculations based on the experimental crystal information have confirmed the structural preference and chemical stability of orthorhombic SrPtBi2. The heat capacity measurements show the metallic properties of SrPtBi2 with weak electron−phonon interactions at the low temperature.

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COMPUTATIONAL METHODS Adaptive Genetic Algorithm. The crystal structure search was performed using the adaptive genetic algorithm (AGA) method.30,31 AGA integrates auxiliary interatomic potentials and first-principles calculations together in an adaptive manner to ensure high efficiency and accuracy. Interatomic potentials based on the embedded-atom method (EAM) were selected as the auxiliary classical potentials in the study of the Sr−Pt−Bi system.32 Structure searches of SrPtBi2 were performed for unit cell sizes of 2, 3, 4, 6, and 12 formula units. During the searches, no symmetry constraint was applied, and the total structure population was set to be 128. Convergence was considered to be reached when the lowest energy in the structure pool remained unchanged for 500 consecutive generations. At the end of each classical GA search, the lowest-energy structures were selected to perform first-principles calculations according to the AGA procedure, whose energies, forces, and stress were used to adjust the parameters of the EAM potential using the Potf it code.33,34 Finally, all of the structures used to tune the interatomic potentials during the AGA search were collected for higher-accuracy optimization by first-principles calculations. Vienna Ab Initio Simulation Package. First-principles calculations were carried out using the density functional theory (DFT) within a generalized gradient approximation (GGA) by the Vienna Ab Initio Simulation Package code.35,36 The projector-augmented wave method37 was used to describe the valence configuration: 4s24p65s2 for Sr, 5d96s1 for Pt, and 5d106s26p3 for Bi. The GGA exchange−correlation energy functional parametrized by Perdew, Burke, and Ernzerhof was used.38 The plane-wave basis was used with a kinetic energy cutoff of 520 eV. Monkhorst−Pack’s scheme was adopted for Brillouin zone sampling with a k-point grid of 2π × 0.05 Å−1 during the AGA searches.39 In the final structure refinements of the collected SrPtBi2 structures from the AGA searches as well as the known binary/ternary compounds in the Sr−Pt−Bi system, a denser grid of 2π × 0.025 Å−1 was used, and the ionic relaxations stopped when the forces on every atom became smaller than 0.01 eV/Å. To characterize the thermodynamical stability, the formation energy of any given structure SrmPtnBip was calculated using face-centered cubic (fcc) Pt, fcc Sr, and rhombohedral Bi as references, that is, EF(SrmPtnBip) = [E(SrmPtnBip) − m × E(Sr) − n × E(Pt) − p × E(Bi)]/(m + n + p). Tight-Binding, Linear Muffin-Tin Orbital-Atomic Spheres Approximation. Calculations of the electronic and bonding features were performed by tight-binding, linear muffin-tin orbital-atomic spheres approximation using the Stuttgart code.40−42 Exchange and correlation were treated by the local density approximation.43 In the atomic sphere approximation method, the space is filled with overlapping Wigner−Seitz (WS) spheres.44 During the calculation of SrPtBi2, the empty spheres are used to make the overlap of WS spheres limited to no larger than 16%. The WS radii are as follows: 2.14−2.17 Å for Sr; 1.50−1.53 Å for Pt; and 1.63−1.79

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EXPERIMENTAL SECTION Synthesis. Polycrystalline samples of SrPtBi2 were obtained via a high-temperature solid-state method. Elemental strontium (>99%, rod, Alfa Aesar), platinum powder (99.98%, ∼60 meshes, Alfa Aesar), and ground bismuth powder (99.999%, lump, Alfa Aesar) were pelletized in the glovebox with a ratio of 1:1:2 and placed into an alumina crucible which was subsequently sealed into an evacuated (10−5 Torr) quartz tube. The sample was heated to 900 °C at a rate of 1 °C/min and annealed at 900 °C for 2 days. After that, the tube was quenched in the air. It yielded a mixture of SrPtBi2 and minor PtBi2 impurity. All products are sensitive in the moisture. Phase Analyses and Structure Determination. All SrPtBi2 samples were ground finely and examined by a Rigaku MiniFlex 600 powder X-ray diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å, Ge monochromator). A Bragg angle ranging from 5° to 90° in a step of 0.010° at the rate of 0.08°/min was taken to get a precise pattern. The lattice parameters and phase composition refinements were accomplished using Jana2006 with the LeBail model.25,26 Multiple crystals (∼20 pieces) roughly 0.02 mm in diameter were picked up to carry out single-crystal X-ray diffraction in case of heterogeneity. A BRUKER APEX II diffractometer equipped with Mo radiation (λKα = 0.71073 Å) was utilized to determine the structure at room temperature. The sample protected with glycerol was mounted on a Kapton loop. To guarantee the accuracy, seven different angles were chosen to take the measurement at room temperature with an exposure time of 15 s per frame while the width of scanning was 0.5°. The direct methods and full-matrix least-squares on F2 models along with SHELXTL package were employed to solve the crystal structure.27 Data acquisition was made via Bruker SMART software which made corrections for Lorentz and polarization effects as well.28 On the basis of face-index modeling, numerical absorption corrections were approached by XPREP.29 Scanning Electron Microscopy. Characterization was performed using a high-vacuum scanning electron microscope (JSM-6610 LV) and energy-dispersive spectroscopy. Samples were quickly moved from the glovebox and mounted on the carbon tape because of the air sensitivity before loading into the scanning electron microscopy (SEM) chamber. Multiple points and areas were examined in each phase within multiple grains B


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Figure 2a, together with the hypothetical structures (BaMnBi2 and LaAuBi2) and the energy on the convex hull. The

Å for Bi. The basis set for the calculations included Sr 5s, 4d, Pt 6s, 6p, 5d, and Bi 6s, 6p wavefunctions. The convergence criterion was set to 0.1 meV. A mesh of 60 k points in the irreducible wedge of the first Brillouin zone was used to obtain all integrated values, including the density of states (DOS), band structure, and crystal orbital Hamiltonian population (COHP) curves.45

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RESULTS AND DISCUSSION Construction of the Convex Hull. The convex hull of the Sr−Pt−Bi system was constructed to investigate the stability of different stoichiometries, as plotted in Figure 1. Because there is

Figure 2. (a) Formation energies of the SrPtBi2 crystal structures obtained from the AGA searches. The structures are classified by their Pt−Bi coordination, where “mixed” represents structures with more than one type of Pt−Bi polyhedra. Energies of the hypothetical structures, that is, the BaMnBi2 and LaAuBi2 types (cyan and blue dash lines), and the energy on the convex hull at the composition of SrPtBi2 are plotted as references (red dash line). (b) Phonon DOS of the lowest-energy structure.

structures from the search are classified in terms of the Pt−Bi coordination number. It can be seen that structures below the convex hull are found for SrPtBi2. On the other hand, the hypothetical models with BaMnBi2 and LaAuBi2-type structures have energies of ∼35 meV/atom higher than the convex hull. The lowest-energy SrPtBi2 structure is found to be in the space group of Pnma with Pt@Bi5 square pyramidal Bi5 coordination. It has ∼50 meV/atom lower formation energy than the one with Pt@Bi4 tetrahedral Bi4 coordination in LaAuBi2-type and BaMnBi2-type structures. The phonon spectrum of the orthorhombic SrPtBi2 was calculated to investigate its dynamical stability. Calculations were performed using a supercell approach provided by the phonopy code,46 where supercells with sizes of 144 atoms (1 × 3 × 1) and a k-mesh of 2 × 2 × 2 were used. The phonon DOS results are plotted in Figure 2b. It can be seen that there are no imaginary phonon frequencies in this structure, indicating that it is dynamically stable. Thus far, we demonstrate that SrPtBi2 is a promising stoichiometry to discover new thermodynamically and dynamically stable compound. Experimental Confirmation of SrPtBi2. The first ternary compound in the Sr−Pt−Bi system, SrPtBi2, was produced by high-temperature synthesis. To obtain the detailed chemical stoichiometry and atomic distributions, single-crystal X-ray diffraction was used to determine the crystal structure. SrPtBi2 crystallizes in the orthorhombic LuNiSn2-type structure with the space group Pnma,23 same as our crystal structure prediction. The resulting structural analysis including the site occupancies, isotropic thermal displacements with atomic positions, and anisotropic thermal displacement data are shown in Tables 1, 2, and S1. The flexible and mixed site occupancy models were utilized to test and confirm the atomic order and stoichiometry in SrPtBi2. Because of the heavy Pt and Bi elements, the site splitting was considered for the refinement, but no site splitting was observed. The three crystallographic independent Pt atoms form Pt@Bi5 square

Figure 1. Convex hull of the formation energies in the Sr−Pt−Bi system. Black balls (and black triangles in the ternary phase diagram) represent stable compounds, while blue points indicate the metastable phases. The purple lines in the ternary phase diagram are projections of the convex hull construction into the compositional space, which forms Gibbs triangles.

currently no ternary phase existing in the Sr−Pt−Bi system, its convex hull is determined solely by elemental Sr, Pt, Bi, and the binary compounds. Thus, we performed a thorough calculation on all the known binary phases among Sr, Pt, and Bi. During our calculation, we also discovered several metastable binary phases in the Sr−Bi system, such as Sr3Bi2, Sr2Bi, and Sr3Bi marked by the blue circle. The thermodynamic stability of SrPtBi2 is determined by the red Gibbs triangle in Figure 1, that is SrPtBi 2 → x × PtBi 2 + y × Sr2Bi3 + z × SrPt 2

When the energy of the left-hand side in the abovementioned reaction (energy of SrPtBi2) is lower than that of the right-hand side (sum of the energies of x × PtBi2, y × Sr2Bi3, z × SrPt2), SrPtBi2 is considered to be thermodynamically stable. Our calculation shows that the formation energy of the right-hand side, that is, the energy on the convex hull at the composition of SrPtBi2 is −574.74 meV/atom. Exploration of the SrPtBi2 Structural Space. Our crystal structure searches of SrPtBi2 were performed using unit cell sizes of 2, 3, 4, 6, and 12 formula units. The calculated formation energies of the obtained structures are plotted in C


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The Journal of Physical Chemistry C Table 1. Single-Crystal Crystallographic Data for SrPtBi2 at 296 (2) K refined formula F.W. (g/mol) space group; Z a (Å) b (Å) c (Å) V (Å3) extinction coefficient θ range (deg) no. reflections; Rint no. independent reflections no. parameters R1: ωR2 (I > 2σ(I)) R1: ωR2 (all I) goodness of fit diffraction peak and hole (e−/Å3)

SrPtBi2 700.67 Pnma; 12 17.072(4) 4.893(1) 15.806(4) 1320.4(6) 0.00008(1) 1.756−33.169 24 272; 0.1035 2770 74 0.0511; 0.0876 0.0687; 0.1071 0.961 5.552; −3.756

Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters of the SrPtBi2 Systema atom

wyck.

occ.

x

y

z

Ueq

Bi1 Bi2 Bi3 Bi4 Bi5 Bi6 Sr1 Sr2 Sr3 Pt1 Pt2 Pt3

4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c 4c

1 1 1 1 1 1 1 1 1 1 1 1

0.0335(1) 0.0429(1) 0.2882(1) 0.3136(1) 0.3332(1) 0.4773(2) 0.1200(1) 0.1524(1) 0.3559(1) 0.2021(1) 0.2036(1) 0.4533(2)

1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4

0.2610(1) 0.5442(1) 0.3907(1) 0.6726(1) 0.1234(1) 0.5776(2) 0.7661(1) 0.0273(1) 0.8896(2) 0.2425(1) 0.5426(1) 0.3990(2)

0.0182(2) 0.0259(3) 0.0122(2) 0.0118(2) 0.0131(2) 0.0114(2) 0.0171(6) 0.0114(5) 0.0164(6) 0.0157(2) 0.0154(2) 0.0133(2)

Figure 3. (a) Crystal structure of SrPtBi2 emphasizing the Pt@Bi5 square pyramidal Bi coordinations. (b) Schematic figure showing the total energies of SrPtBi2 with different structures screened by AGA.

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor (Å2).

pyramids, which share the edges and vertex with neighboring polyhedra. The Sr atoms were in the voids of Pt−Bi frames according to Figure 3a. As shown in Figure 3b and by the formation energies in Figure 2a, among the potential structures with 112 stoichiometry adopted by SrPtBi2, Pt@Bi5 polyhedra in space group Pnma is determined to hold ∼7 and ∼48 meV/ atom lower energy than Pt@Bi4 and Pt@Bi6 polyhedra, respectively. The refined powder X-ray diffraction pattern is shown in Figure 4. The LeBail fitting was used to refine all lattice parameters. PtBi2 was found as impurity (∼10%) existing in the sample. Without refinement of the atomic sites or displacement parameters of all atoms, the obtained profile residuals (Rp) is 5.98%, with weighted profile residuals (Rwp) being 9.52%. Refined lattice parameters for SrPtBi2 (orthorhombic symmetry, a = 17.093(2) Å; b = 4.8739(4) Å; and c = 15.725(1) Å) were found to be consistent with single-crystal X-ray diffraction data. In comparison, our theoretical calculation using DFTGGA slightly overestimates the lattice parameters and gives a = 17.334 Å; b = 4.949 Å; and c = 16.012 Å at equilibrium conditions. The chemical composition was further confirmed as Sr1.1(1)Pt1.0(1)Bi1.8(2) by using SEM, as shown in Figure S2. Electronic Structure and Bonding Features of SrPtBi2. To understand the structural stability from the bonding aspect, electronic structures were calculated and are shown in Figure 5.

Figure 4. Powder XRD pattern for SrPtBi2. The red dots, blue line, and green line represent calculated powder XRD pattern, observed powder XRD pattern, and the intensity difference between calculated and observed patterns, respectively. PtBi2 peaks were found as minor impurity (∼10%).

We emphasized the range around the Fermi level between the energy of −4.0 and 2.0 eV. In the DOS shown in Figure 5a, we can easily see the 5d orbitals from Pt atoms contribute the most below −3.0 eV, which means that the electrons on 5d orbitals are relatively localized. Above −3.0 eV, the states are dominated by the hybridization of electrons on Bi-s and p orbitals, Pt-s and d orbitals, and Sr-s orbital, particularly around the Fermi level. To specify the atomic interactions around the Fermi level, the −COHP calculations were generated in Figure 5b, in which the D


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superconducting BaPt2Bi2, but they are not observed in SrPtBi2. It may be the reason for SrPtBi2 not being a superconductor because superconductivity requires the interplay between Pt− Bi and Pt−Pt interactions. Heat Capacity Measurements. Temperature-dependent specific heat measurements were carried out as presented in Figure 6a, which plots Cp(T) in zero applied field from 1.85 to 300 K. No anomalies, such as phase transition or superconductivity, were observed between 1.85 and 300 K.

Figure 5. Electronic structure calculation of SrPtBi2 from the energy of −4.0 to 2.0 eV. (a) DOS; (b) crystal orbital Hamilton populations (−COHP) calculations (+ indicates the bonding interactions and − indicates the antibonding interactions); and (c) band structure calculations.

positive part (+) indicates the bonding interaction, whereas the negative part (−) indicates antibonding interactions. The outcome of the −COHP calculation demonstrates that the Pt− Bi interactions play a significant role in the structural stability, and the Fermi level is located on the nonbonding region in Pt− Bi interactions. Around the Fermi level, the interplay between Bi−Bi antibonding and Sr−Bi bonding interactions governs the stability of SrPtBi2. Even the peak around the Fermi level is observed in DOS, and the band structure calculation in Figure 5c shows that the bands are localized near the Fermi level with no saddle points, which hints that superconductivity can rarely been found in this compound. Similar to the integrating electronic DOS which gives the number of electrons in the system, the integrated COHP hints toward the bond strength. 47 To compare the atomic interactions within each model and understand the bonding strengths in different 112 structure models, the corresponding integrated COHP values are calculated and listed in Table 3.

Figure 6. Heat capacity measurements of SrPtBi2. (a) Temperature dependence of Cp. Blue open circles represent experimental data and blue and green lines represent Einstein and Debye contribution to Cp, respectively. A combined fit is shown by a red line as explained in the text. (b) Low temperature Cp/T vs T2 with a fit (red line). (c) Phonon heat capacity Cph./T3 vs T.

Figure 6b shows low temperature data Cp/T versus T2 with a fit to Cp/T = γ + βT2 + δT4, where γT is an electronic contribution (Cel.), and βT2 and δT4 are the phonon contribution (Cph.) parts to the specific heat, and γT is the electronic contribution to the specific heat. The Sommerfeld parameter, γ, was calculated to be 6.4(3) mJ mol−2 K−2. The Debye temperature ΘD can then be calculated with the 1/3 12π 4 nR , where 5β 48 SrPtBi2. On the basis

Table 3. Integrated COHP per Formula for SrPtBi2 in Experimental and LaAuBi2- and BaMnBi2-Type Structures SrPtBi2-type

Pt−Bi Bi−Bi Sr−Bi Sr−Pt Pt−Pt total

LaAuBi2-type

following equation: ΘD =

ICOHP (%)

ICOHP

ICOHP (%)

ICOHP

ICOHP (%)

2.2 0.3 0.6 0.2 0 3.3

68.7 7.5 18.9 4.9 0 100

3.3 1.7 2.2 1.2 0 8.1

40.7 20.4 25.7 13.3 0 100

1.4 1.5 1.1 0 0.9 4.9

28.3 30.1 22.4 0 19.2 100

)

R is the gas

constant and n = 4 for of this Debye model, the Debye temperature is calculated to be 161(2) K, which is comparable to the Debye temperature of Sr (147 K) and lower than those reported for Bi (199 K) and Pt (240 K).49 To estimate the Einstein temperature, we plotted temperature dependence of Cph./T3 (Figure 6c), where Cph. = Cp − γT. The peak at temperature about 8 K gives the Einstein temperature value ΘE = 5Tmax = 40 K. The fit to the overall temperature dependence of the specific heat Cp(T) is shown in panel (a) by a red solid line. This fit is a combined model of the electronic part and two phonon parts: Cp = Cel. + kCDebye + (1 − k)CEinstein. For this fit, estimated k = 0.87 of the weight belongs to the Debye model CDebye (green line), and 0.13 is the weight of the Einstein model CEinstein (blue line) of the phonon heat capacity

BaMnBi2-type

ICOHP

(

Accordingly, in the experimental SrPtBi2 structure, the Pt−Bi interactions contribute most (∼69%) in the structural stability, while there exist no Pt−Pt interactions. On the other hand, the Pt−Bi interactions in LaAuBi2 and BaMnBi2 models dramatically decrease. The decreasing of Pt−Bi bonding strength leads to the instability of SrPtBi2 in other two hypothetical models. Moreover, the changing trend for total energies is consistent with the Pt−Bi bonding strength of SrPtBi2 in different models. In this regard, the Pt−Bi interactions play a significant role in determining the stability of the compound. It is worth noticing that the Pt−Pt interactions also contribute critically in the

⎛ T ⎞3 C Debye(T ) = 9nR ⎜ ⎟ ⎝ ΘD ⎠

x 4 exp(x)

∫ [exp(x) − 1]2 dx

⎤−2 ⎛ ΘE ⎞ 2 ⎛ Θ E ⎞⎡ ⎛ Θ E ⎞ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ C Einstein(T ) = 3nR exp − 1⎥ ⎢exp ⎝T ⎠ ⎝ T ⎠⎣ ⎝ T ⎠ ⎦ E


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The Journal of Physical Chemistry C The Debye temperature estimated from the fit ΘD = 187(2) K is close to a more accurate value obtained from the low temperature fit (161 K). In this fit, the Einstein temperature was fixed at ΘE = 40 K. Having calculated the density of the electronic DOS N(EF) = 2.01 st. eV−1, one can calculate the theoretical Sommerfeld coefficient γtheor. = 4.74 mJ mol−1 K−2. Assuming that γtheor. is renormalized only by the electron−phonon interaction (no magnetic present), and taking from γexp. = γtheor.(1 + λel.−ph.), we can estimate the electron−phonon coupling to be λel.−ph. = 0.35.

ORCID

Xin Zhao: 0000-0002-3580-512X Cai-Zhuang Wang: 0000-0002-0269-4785 Weiwei Xie: 0000-0002-5500-8195 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.G. and W.X. deeply thank the support from Louisiana State University and the Louisiana Board of Regents Research Competitiveness Subprogram (RCS) under contract number LEQSF(2017-20)-RD-A-08 and the Shared Instrument Facility (SIF) at Louisiana State University for the SEM-EDS. Work at Ames Laboratory was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division including a grant of computer time at the National Energy Research Scientific Computing Centre in Berkeley, CA. Ames Laboratory is operated for the U.S. DOE by Iowa State University under contract # DE-AC02-07CH11358.

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CONCLUSIONS The first ternary phase in the Sr−Pt−Bi system, SrPtBi2, has been designed and obtained by coupling theoretical computation and experiments. The SrPtBi2 phase was synthesized and structurally characterized. It exhibits in the orthorhombic structure with space group Pnma. First-principles electronic and phonon structure calculations substantiate the chemical stability of the new compound and indicate that no superconductivity may be observed in the phase. The heat capacity measurements confirmed the metallic properties. No superconductivity was detected down to 1.85 K. With our newly discovered SrPtBi2 phase, the calculated phase diagram of the Sr-Pt-Bi system at 0 K can be correspondingly updated, as shown in Figure 7. It can be seen that the existence of SrPtBi2

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Figure 7. Updated convex hull of the Sr−Pt−Bi system after including the new SrPtBi2 phase discovered in the current work.

(green star) alters the equilibrium stable phases at the compositions which are not the stable nodes, which will aid the search for the new superconductors in the Sr−Pt−Bi system with the interplay between Pt−Pt and Pt−Bi interactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12801. SEM images of SrPtBi2, heat capacity measurement, and anisotropic thermal displacement parameters (PDF)

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Ternary Bismuthide SrPtBi2: Computation and Experiment in

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