Two Polymorphs of BaZn₂P₂: Crystal Structures, Phase Transition, and Transport Properties

Journal Article · 07 September 2021 · Inorganic Chemistry

DOI: https://doi.org/10.1021/acs.inorgchem.1c02209 · OSTI ID:1876203

Balvanz, Adam ^[1]; Baranets, Sviatoslav  ^[2]; Ogunbunmi, Michael O. ^[2]; Bobev, Svilen  ^[2]

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1. Univ. of Delaware, Newark, DE (United States); University of Delaware
2. Univ. of Delaware, Newark, DE (United States)

Here, the novel α-BaZn₂P₂ structural polymorph has been synthesized and structurally characterized for the first time. Its structure, elu-cidated from single crystal X-ray diffraction, indicates that the compound crystallizes in the orthorhombic α-BaCu₂S₂ structure type, with unit cell parameters a = 9.7567(14) Å, b = 4.1266(6) Å, and c = 10.6000(15) Å. With β-BaZn₂P₂ being previously iden-tified as belonging to the ThCr₂Si₂ family, and with the precedent of structural phase transitions between the α-BaCu₂S₂ type and the ThCr₂Si₂ type, the potential for the pattern to be extended to the two different structural forms of BaZn₂P₂ was explored. Thermal analysis suggest that a first order phase transition occurs at ~1123 K, whereby the low-temperature orthorhombic α-phase transforms to a high-temperature tetragonal β-BaZn₂P₂; the structure of which was also studied and confirmed by single-crystal X-ray diffraction. Preliminary transport properties and band structure calculations indicate that α-BaZn₂P₂ is a p-type, narrow-gap semiconductor with a direct bandgap (Eg) of 0.5 eV. The Seebeck coefficient, S(T), for the material increases steadily from the room temperature value of 119 µV/K to 184 µV/K at 600 K. The electrical resistivity of α-BaZn₂P₂ is relatively high, on the order of 40 mΩ·cm, and the ρ(T) dependence shows gradual decrease upon heating. Such behavior is comparable to those of the typical semimetals or degenerate semiconductors. Although the observed values of the as-synthesized samples are not optimal, the carrier concentration can apparently be tuned to fall between the values of metals and semiconductors, thus providing an open window for optimizing this phase towards achieving an enhanced thermoelectric figure of merit zT. The calculated indirect band gap for the β-BaZn₂P₂ phase is Eg = 0.03 eV which is about an order of magnitude lower than that of α-BaZn₂P₂. It is expected that the cage-like structural motif and layered structure possessed by α-BaZn₂P₂ and β-BaZn₂P₂ phases, respectively, would promote the realiza-tion of a low thermal conductivity in both compounds.

• Research Organization: Univ. of Delaware, Newark, DE (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: SC0008885
• OSTI ID: 1876203
• Journal Information: Inorganic Chemistry, Journal Name: Inorganic Chemistry Journal Issue: 18 Vol. 60; ISSN 0020-1669
• Publisher: American Chemical Society (ACS)Copyright Statement
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY