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• 0   Introduction

  Zintl phase compounds are special polar intermetallics comprising metals with very different electronegativities, and the electropositive elements can usually be assumed to completely transfer their valence electrons to the electronegative ones; thus, each constituent atom obeys the 8-N octet rule and a closed-shell configuration is achieved. Such an electron-precise nature has been observed frequently in Zintl phases, and plays a critical role in governing the formation of the structures of these compounds^[1-4]. Because of their complex crystal structures, Zintl phase compounds exhibit specific physical and chemical properties. For example, Yb₁₄MnSb₁₁^[5-6], which has a high molecular mass of 3 783.088 and very low thermal conductivity, has a figure of merit (zT) of 1.0 at 1 200 K. BaFe₂As₂, another Zintl compound, with a tetragonal ThCr₂Si₂-type structure and space group I4/mmm, has been reported to show great superconductivity. It exhibits a structural and magnetic phase transition at 140 K, at which a spin-density-wave anomaly has been observed, and the superconducting transition temperature can be increased to 38 K by doping with K((Ba_1-xK_x)Fe₂As₂, x=0.4)^[7-9]. Many other Zintl phase compounds exhibit excellent thermoelectric perfor-mances, magnetic properties, and superconductivities.

  The formula RM₂X₂ (R=rare or alkaline earth metal, M=metal, and X=group ⅢA~ⅥA element) represents a very large family of compounds with more than five crystal types (I4/mmm, P3m1, Pnma, and Cmcm)^[10]. RM₂X₂ compounds crystallize mainly into the structure of ThCr₂Si₂^[7], with the tetragonal space group I4/mmm, and contain practically identical layers of edge-sharing MX₄ tetrahedra separated by R atom layers. The second-most common crystal structure of RM₂X₂ compounds is the CaAl₂Si₂-type structure^[11], with the space group P3m1. The trigonal structure for these RM₂X₂ compounds can be viewed as polyanionic M₂X₂ layers stacked along the _c-axis and separated by layers of R cations^[12]. The least common crystal struc-ture among RM₂X₂ compounds is the α-BaCu₂S₂-type structure^[13-14], with an orthogonal phase featuring the Pnma space group, in which MX4 tetrahedra form a three-dimensional (3D) network frame with large cavities where R cations are located.

  The different structure types are closely connected, and phase transitions may occur between them. For example, BaAl₂Ge₂^[15] and BaCu₂S₂ undergo a reversible phase change process, where the α (low-temperature phase; space group Pnma) phase changes to the β (high-temperature phase; space group I4/mmm) phase when the temperature rises, and the reverse occurs when the temperature is lowered. For BaZn₂P₂, the high-temperature phase (group I4/mmm) was reported in 1978^[16], but the low-temperature phase has not been reported yet. In this study, α-BaZn₂P₂ was obtained via the high-temperature Sn-flux reaction. The synthesis, crystal structure, and electronic band structure of α-BaZn₂P₂ are presented and discussed in this paper, and the results of differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD) show that thermal decomposition occurs during the heating process.

  1   Experimental

  1.1   Synthesis

  All manipulations were performed either in an argon-filled glovebox or under vacuum. The starting materials P (99.999%), Zn (99.99%), Ba (99+%), and Sn (99.9%) were obtained from Alfa and used as received. The elements, Ba, Zn, P, and Sn, were arranged in an alumina crucible in a molar ratio of 2: 1: 3: 20. The reactions were sealed in quartz ampoules under vacuum, and the ampoules were placed in a high-temperature programmable furnace. The reactions were heated to 300 ℃ over 5 h, and held at that temperature for an additional 5 h. Afterwards, the sealed mixtures were heated to 950 ℃ and held at that temperature for 20 h, and then slowly cooled to 400 ℃ at a rate of -5 ℃·h^-1. Then, they were centrifuged at 2 500 r·min^-1 for 5 min to separate the products from the Sn flux. The main products were silver block crystals of Ba₃Sn₂P₄ and needle crystals of α-BaZn₂P₂. The α-BaZn₂P₂ crystals were stable in air, water, and alcohol, and reacted slowly with dilute acid.

  1.2   X-ray powder diffraction

  Powder XRD patterns were obtained at room temperature using an Ultima Ⅳ X-ray powder diffractometer using Cu Kα radiation (λ=0.154 18 nm). The operating voltage and current were 40 kV and 40 mA, respectively. The data were recorded in 2θ mode with a step size of 0.02° and a counting time of 10 s, and the scanning range was from 20° to 80°. The results were used for the phase purity analysis only.

  Temperature-dependent XRD was conducted using an Empyrean with a temperature platform in order to analyze the changes in α-BaZn₂P₂ during the heating process from 30 to 920 ℃. A powdered sample was placed on the platform and heated at 10 ℃·min^-1 from room temperature. Measurements were taken at 30, 200, 400, 600, 800, 850, 870, 900 and 920 ℃ and the results were compared with DSC.

  1.3   Single-crystal X-ray diffraction

  Single crystal of the title compound were selected and cut in Patatone-N oil to suitable size (0.3 mm×0.06 mm×0.06 mm) and then mounted on glass fiber for data collection, which was performed on a Bruker SMART APEX-Ⅱ CCD area detector with graphite-monochromated Mo Kα radiation (λ=0.071 073 nm) at 296 K using ω scan. Data reduction and integration, along with global unit cell refinements, were performed by the INTEGRATE program incorporated in the APEX2 software. Semi-empirical absorption corrections were applied using the SCALE program for the area detector. The structures were solved using direct methods and refined using full matrix least-squares methods on F² using SHELX-2014^[17]. All structures were refined to converge with anisotropic displacement parameters.

  CCDC: 433850.

  1.4   Elemental analysis

  Several single crystals of the title compound were selected and energy dispersive X-ray spectroscopy (EDS) measurements were performed using a Zeiss Auriga scanning electron microscope with an energy dispersive spectrometer to determine the elemental composition. The spectra obtained from visibly clean surfaces provided results identical to those of the crystallographic data. The compositions were homo-geneous within one crystal, and, within standard unce-rtainty, identical to those of crystals selected at random from different reactions (Supporting Information).

  1.5   Thermogravimetry (TG)/DSC

  A Netzsch Thermal Analysis STA 449 F3 Jupiter® was used to evaluate the thermal properties of α-BaZn₂P₂ between 50 and 950 ℃. After a baseline was established, 166.598 6 mg of prepared powdered sample was placed in an alumina crucible and heated under argon at 10 K·min^-1.

  1.6   Calculation of electronic and energy band structures

  The calculations were based on density functional theory (DFT) and used first-principles approaches. Similar calculation results for BaZn₂As₂ were reported by Shein et al in 2014^[18]. The Vienna ab initio simul-ation package using the projector augmented wave method was used to calculate the equilibrium structural parameters and elastic properties of the polymorphs^[19-20]. The exchange correlation potentials were calculated using the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA)^[21]. A kinetic energy cutoff of 500 eV and k-mesh of 5×10×5 for an orthorhombic structure were used. The geometry optimization was performed with a force cutoff of 10 meV·nm^-1.

  2   Results and discussion

  2.1   Structure description

  The α-BaZn₂P₂ single crystals were prepared via a Sn-flux reaction. The EDS results were consistent with the stoichiometry as written. Single-crystal X-ray diffraction confirm that BaZn₂P₂ crystallizes with the α-BaCu₂S₂-type structure (space group Pnma). Crys-tallographic data and structural refinements are summarized in Table 1. The cell parameters are a=0.976 78(5) nm, b=0.413 34(2) nm, c=1.060 55(5) nm. Selected atomic positional and displacement parameters are shown in Table S1, and the most important interatomic distances and angles are presented in Table S2.

  Table 1.  Crystal data and structure refinement for α-BaZn₂P₂

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  Formula	BaZn2P2
  Formula weight	330.2
  Crystal ststem	Orthorhombic
  Space group	Pnma
  a/nm	0.976 78(5)
  b/nm	0.413 34(2)
  c/nm	1.060 55(5)
  V/nm3	0.428 19(4)
  Z	4
  Dc/(g·cm-3)	5.ll9
  GOF	l.04
  Final R indices[I>2σ(I)]a	R1=0.025 8, wR2=0.063 9
  Final R indices(all data)a	R1=0.025 8, wR2=0.064 0
  aR1=[Σ||Fo|-|Fc||]/Σ|Fo|, wR2=[Σw(|Fo2|-|Fc2|)2]/Σw(|Fo|2)2]1/2

  The crystal structures of α-and β-BaZn₂P₂ are shown in Fig. 1; the Ba atoms are orange, Zn atoms are yellow, and P atoms are green. As shown in Fig. 1(a), α-BaZn₂P₂ features a 3D network of Zn and P atoms that form large cavities in which Ba atoms reside. Ba is in seven-fold coordination with respect to Zn, with Ba-Zn distances ranging from 0.326 17(7) to 0.364 30(6) nm (distances here and later in the article were obtained from X-ray data at 296 K). Seven P atoms surround each Ba, and the Ba-P distances range from 0.319 66(14) to 0.350 26(12) nm. The Zn1 and Zn2 atoms both have tetrahedral coordination with P atoms, with Zn-P distances ranging from 0.243 26(8) to 0.255 04(15) nm. The polyhedral view with ZnP₄ tetrahedra is shown in Fig.S1. Fig.S1(a) shows that the ZnP₄ tetrahedra form the anion frame by sharing sides or vertices and that Ba²⁺ cations are located in the large cavities. Fig.S1(b, c) show the chains in the b-axis formed by sharing sides of Zn1-and Zn2-centered ZnP₄ tetrahedra. Fig.S1(d) shows the cavity where Ba atoms are located; the cavities were formed through the sharing of the vertices (P atoms) of the ZnP₄ tetrahedra. The bond angles of ZnP₄ tetrahedra are presented in Table S2, and are 94°~117° for Zn1, 106°~115° for Zn2, 84°~143° for P1, and 73°~138° for P2.

  图1 (a) Polyhedral and ball-and-stick models of the crystal structure of α-BaZn₂P₂; (b) Ball-and-stick model of β-BaZn₂P₂ Figure1. (a) Polyhedral and ball-and-stick models of the crystal structure of α-BaZn₂P₂; (b) Ball-and-stick model of β-BaZn₂P₂

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  For comparison, Fig. 1(b) shows the ball-and-stick model of β-BaZn₂P₂, which crystallizes in a tetragonal phase, with the ThCr₂Si₂-type structure and the space group I4/mmm. The high-temperature β phase of BaZn₂P₂ contains layers of edge-sharing ZnP₄ tetra-hedra separated by Ba atom layers (Fig.S1(e, f)).

  2.2   XRD

  XRD was conducted for α-BaZn₂P₂, and the results are shown in Fig. 2(a). For comparison, the calculated XRD patterns of α-BaZn₂P₂ (Pnma) and β-BaZn₂P₂ (I4/mmm) are shown in Fig. 2(b) and 2(c), respectively. Fig. 2 shows that the collected samples are α-BaZn₂P₂ (Pnma); the small peaks indicated with an asterisk (^*) in the collected data curve are Sn peaks. The α-BaZn₂P₂ peak (2θ=32.23°) in the collected data spectrum is very intense because it partially overlaps with a Sn peak (2θ=32.04°).

  图2 XRD pattern of α-BaZn₂P₂ single crystals ground into a powder (a), calculated XRD patterns of α-BaZn₂P₂ with space group Pnma (b) and β-BaZn₂P₂ with space group I4/mmm (c) Figure2. XRD pattern of α-BaZn₂P₂ single crystals ground into a powder (a), calculated XRD patterns of α-BaZn₂P₂ with space group Pnma (b) and β-BaZn₂P₂ with space group I4/mmm (c)

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  Fig. 3 shows the temperature-dependent XRD results. As the temperature increased to 920 ℃, α-BaZn₂P₂ was basically retained, while the peak positions were shifted to the left and the small peaks of α-BaZn₂P₂ shrank and gradually disappeared. During the experiment, the general structure of α-BaZn₂P₂ was retained even at 920 ℃. However, with the increase in temperature during the heating process, the atomic vibration of α-BaZn₂P₂ increased, the interplanar spacing became larger, and the degree of disorder increased. At the same time, the main peak (2θ≈29°) of α-BaZn₂P₂ indicate cleavage with increasing temperature, which was the result of the exacerbation of atomic vibration.

  图3 Temperature-dependent XRD patterns of α-BaZn₂P₂ from 30 to 920 ℃ Figure3. Temperature-dependent XRD patterns of α-BaZn₂P₂ from 30 to 920 ℃

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  In addition, the peaks changed during the heating process (Fig. 3). At 30 ℃, the main species is α-BaZn₂P₂ with a bit of Sn (small peaks indicated with a triangle △), and the Sn peaks disappeared when the temperature reached 400 ℃. When the temperature reached 800 ℃, some new peaks of Ba₄P₅ appeared, which were indicated with an asterisk (^*). When the temperature reached 900 ℃, new peaks of ZnP₄ appe-ared, which were indicated with a white star (☆), and there may be some other new binary phases. There-fore, α-BaZn₂P₂ can decompose during the heating process. This is consistent with the results of DSC.

  2.3   Thermal stability analysis

  Differential thermal analysis (DTA) and TG experiments were performed over the 50~950 ℃ temperature range with the sample under the protec-tion of an Ar atmosphere, and the results are pres-ented in Fig. 4. For α-BaZn₂P₂, no significant mass loss was observed in the TG curve over the entire measured temperature range, and the endothermic peak appearing at around 230 ℃ in the DSC curve should be attributed to the melting of remaining Sn from the flux. The endothermic peaks appearing at around 860 ℃ in the DSC curve indicate the decomposition of α-BaZn₂P₂, which results in binary phases such as Ba₄P₅ and ZnP₄, as confirmed by XRD patterns.

  图4 TG (black) and DSC (blue) curves for α-BaZn₂P₂ Figure4. TG (black) and DSC (blue) curves for α-BaZn₂P₂

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  2.4   Electronic and energy band structure calculations

  To understand better the properties of the new Zintl phase, electronic structure calculations were performed for α-BaZn₂P₂. The results are presented in Fig. 5. α-BaZn₂P₂ has a small bandgap of approximately 0.4 eV, typical for a narrow-bandgap semiconductor. In Fig. 5(a), the electrons in the 4d orbitals of Ba atoms are still in the conduction band, while those in the s and p orbitals are all transferred to Zn or P atoms. In the ZnP₄ tetrahedron structures in α-BaZn₂P₂, Zn²⁺ provides four unoccupied orbitals with the lowest energy and forms coordination bonds (Zn-P bonds) with P. So Fig. 5(a) shows the orbital hybridization, which should be sp³ hybridization. The energy band structures are presented in Fig. 5(b), and direct transi-tions were found in this semiconducting material.

  图5 (a) Calculated total density of states (TDOS) and partial density of states (PDOS) for various constituent atoms in α-BaZn₂P₂; (b) Energy band structures of α-BaZn₂P₂ Figure5. (a) Calculated total density of states (TDOS) and partial density of states (PDOS) for various constituent atoms in α-BaZn₂P₂; (b) Energy band structures of α-BaZn₂P₂

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  3   Conclusions

  A new Zintl phase compound, α-BaZn₂P₂, with the space group Pnma was synthesized through metal-flux reactions. The structure was accurately determined using single-crystal XRD techniques. α-BaZn₂P₂ has a three-dimensional network structure, where ZnP₄ tetra-hedra form an anion frame by sharing sides or vertices, with Ba²⁺ cations residing within. There was no phase transition, but decomposition occurred during the heating process. The electronic and energy band structures were calculated in order to understand clearly the properties of α-BaZn₂P₂. The energy gap was approximately 0.4 eV, which indicates that α-BaZn₂P₂ is a narrow-bandgap semiconductor.

  Supporting information is available at http://www.wjhxxb.cn

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• 

  Figure 1  (a) Polyhedral and ball-and-stick models of the crystal structure of α-BaZn₂P₂; (b) Ball-and-stick model of β-BaZn₂P₂

  Ba, Zn, and P atoms are shown in orange, yellow, and green, respectively

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  Figure 2  XRD pattern of α-BaZn₂P₂ single crystals ground into a powder (a), calculated XRD patterns of α-BaZn₂P₂ with space group Pnma (b) and β-BaZn₂P₂ with space group I4/mmm (c)

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  Figure 3  Temperature-dependent XRD patterns of α-BaZn₂P₂ from 30 to 920 ℃

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  Figure 4  TG (black) and DSC (blue) curves for α-BaZn₂P₂

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  Figure 5  (a) Calculated total density of states (TDOS) and partial density of states (PDOS) for various constituent atoms in α-BaZn₂P₂; (b) Energy band structures of α-BaZn₂P₂

  Fermi energy was chosen for energy reference in (a)

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  Table 1.  Crystal data and structure refinement for α-BaZn₂P₂

  Formula	BaZn2P2
  Formula weight	330.2
  Crystal ststem	Orthorhombic
  Space group	Pnma
  a/nm	0.976 78(5)
  b/nm	0.413 34(2)
  c/nm	1.060 55(5)
  V/nm3	0.428 19(4)
  Z	4
  Dc/(g·cm-3)	5.ll9
  GOF	l.04
  Final R indices[I>2σ(I)]a	R1=0.025 8, wR2=0.063 9
  Final R indices(all data)a	R1=0.025 8, wR2=0.064 0
  aR1=[Σ||Fo|-|Fc||]/Σ|Fo|, wR2=[Σw(|Fo2|-|Fc2|)2]/Σw(|Fo|2)2]1/2

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