Synthesis and structural characterization of the ternary Zintl phases AE3Al2Pn4 and AE3Ga2Pn4 (AE=Ca, Sr, Ba, Eu; Pn=P, As)

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Abstract

Ten new ternary phosphides and arsenides with empirical formulae AE3Al2Pn4 and AE3Ga2Pn4 (AE=Ca, Sr, Ba, Eu; Pn=P, As) have been synthesized using molten Ga, Al, and Pb fluxes. They have been structurally characterized by single-crystal and powder X-ray diffraction to form with two different structures—Ca3Al2P4, Sr3Al2As4, Eu3Al2P4, Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Sr3Ga2As4, and Eu3Ga2As4 crystallize with the Ca3Al2As4 structure type (space group C2/c, Z=4); Ba3Al2P4 and Ba3Al2As4 adopt the Na3Fe2S4 structure type (space group Pnma, Z=4). The polyanions in both structures are made up of TrPn4 tetrahedra, which share common corners and edges to form [TrPn2]32 layers in the phases with the Ca3Al2As4 structure, and [TrPn2]31 chains in Ba3Al2P4 and Ba3Al2As4 with the Na3Fe2S4 structure type. The valence electron count for all of these compounds follows the Zintl–Klemm rules. Electronic band structure calculations confirm them to be semiconductors.

Graphical abstract

AE3Al2Pn4 and AE3Ga2Pn4 (AE=Ca, Sr, Ba, Eu; Pn=P, As) crystallize in two different structures—Ca3Al2P4, Sr3Al2As4, Eu3Al2P4, Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Sr3Ga2As4, and Eu3Ga2As4, are isotypic with the previously reported Ca3Al2As4 (space group C2/c (No. 15)), while Ba3Al2P4 and Ba3Al2As4 adopt a different structure known for Na3Fe2S4 (space group Pnma (No. 62). The polyanions in both structures are made up of TrPn4 tetrahedra, which by sharing common corners and edges, form [TrPn2]32layers in the former and [TrPn2]31 chains in Ba3Al2P4 and Ba3Al2As4.

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Highlights

AE3Ga2Pn4 (AE=Ca, Sr, Ba, Eu; Pn=P, As) are new ternary pnictides. ► Ba3Al2P4 and Ba3Al2As4 adopt the Na3Fe2S4 structure type. ► The Sr- and Ca-compounds crystallize with the Ca3Al2As4 structure type. ► The valence electron count for all title compounds follows the Zintl–Klemm rules.

Introduction

In recent years, there have been numerous reports on the crystal chemistry and physical properties of ternary pnictides in the systems AETrPn (AE=Ca, Sr, Ba, Eu, Yb; Tr=Al, Ga, In; and Pn=P, As, Sb). Examples include BaGa2Sb2 [1], Yb5Al2Sb6 [2], Ba2In5As5 [3], Eu3InP3 [4], Eu3In2P4 [5], EuIn2P2 [6], EuGa2As2 [7], Ca3AlSb3 [8] and Ca5Al2Sb6 [9], to name just a few. Almost exclusively, such compounds can be classified as Zintl phases [10], where the alkaline-earth metals are the “cations” and they donate their valence electrons to the post-transition elements, which, in turn, form covalent bonds within (poly)anionic sub-structure. The electron transfer is typically considered to be “complete”, and all constituent atoms achieve closed-shell configurations [10], [11]. These are desirable characteristics in thermoelectrics development, and many research groups are turning their attention to Zintl phases as candidate materials for solid-state energy conversion. Recent papers have already demonstrated the favorable balance of charge and heat-transport properties for the compounds Yb5Al2Sb6 [2], and Ca3AlSb3 [8]; EuIn2As2 [12] can be cited as an example showing colossal magnetoresistance.

Our research group has previously explored considerable sections of the ternary AE–Ga–Sb and AE–In–Sb phase diagrams (AE=Ca, Sr, Ba, Eu and Yb). Since these early studies had proven fruitful [13], [14], [15], [16], not long ago, we embarked on investigations of the corresponding arsenide and phosphide systems. For the synthesis of new compounds with novel structures, by and large, we have focused on the metal flux method [17], [18], since the triel elements Ga and In are particularly well-suited for such endeavors. Published structures from our prior systematic work include BaGa2Pn2 (Pn=P, As) [19], and CaGa2P2, CaGa2As2, and SrGa2As2 [20], which all crystallize with different structures. Attempts to extend the “1–2–2” chemistry to the AE–Al–Pn system led to the identification of two series of new compounds AE3Ga2Pn4 and AE3Al2Pn4, which are the subject of this paper. Herein, we present the ten newly synthesized compounds—Ba3Al2P4 and Ba3Al2As4 with the Na3Fe2S4 structure type [21], and Ca3Al2P4, Sr3Al2As4, Eu3Al2P4, Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Sr3Ga2As4, and Eu3Ga2As4, isostructural with the previously reported Ca3Al2As4 [22] and Sr3Al2P4 [23]. Their bonding characteristics are elaborated and the topological relationships to the structures of compounds with formulae AE3TrPn3 are discussed as well. The electronic band structures, calculated with the aid of the TB-LMTO method [24], are also discussed.

Section snippets

Synthesis

All synthetic and post-synthetic manipulations were performed inside an argon-filled glove box or under vacuum. The elements with stated purity greater than 99.9 wt% were purchased from either Alfa Aesar or Aldrich and used as received. All initial reactions were done in a manner, consistent with the synthesis of CaGa2P2, CaGa2As2, and SrGa2As2 [20], where excess Ga was utilized as a reactive flux; details of the metal flux method can be found elsewhere [17], [18]. Such reactions were aimed at

Structure of Ba3Al2P4 and Ba3Al2As4

Ba3Al2P4 and Ba3Al2As4 are isotypic, and crystallize with the orthorhombic space group Pnma (No. 62, Pearson symbol oP36) [28]. This structure formally belongs to the Na3Fe2S4 structure type, which has been discussed in an earlier publication [21]. The asymmetric unit of the structure contains two Ba, one Al, and three pnictogen atoms, located at either the general site 8d or the special position 4c (Table 3). Al and Pn atoms constitute AlPn4 tetrahedra, which by sharing common edges form [AlPn4

Conclusions

Ten new Zintl compounds from the series of AE3Tr2Pn4 (AE=Ca, Sr, Ba, Eu; Tr=Al, Ga; Pn=P, As) have been synthesized from flux reactions. While eight of them, Ca3Al2P4, Sr3Al2As4, Eu3Al2P4, Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Eu3Ga2As4, and Sr3Ga2As4 crystallize with the Ca3Al2As4 structure type [22], the heavier alkaline-earth metal analogs, Ba3Al2P4 and Ba3Al2As4, adopt the Na3Fe2S4 type structure [21]. Both structures feature corner- and edge-shared TrPn4 tetrahedra, forming [TrPn2]32 layers or [

Supplementary material

The information consists of a figure showing the crystal structure of Sr3In2P4; figures showing the experimental and simulated X-ray powder diffraction patterns of Ba3Al2As4 and Sr3Ga2P4; tables with the refined atomic coordinates of Ca3Al2P4, Eu3Al2P4, Eu3Al2As4, Ca3Ga2P4, Sr3Ga2P4, Sr3Ga2As4, and Eu3Ga2As4; as well as tables with selected interatomic distances and angles.

Acknowledgments

Svilen Bobev acknowledges financial support from the US Department of Energy through a grant DE-SC0001360. Maia Saito acknowledges NSF REU fellowship.

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