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

Abstract

The novel α-BaZn2P2 structural polymorph has been synthesized and structurally characterized for the first time. Its structure, elucidated 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 identified 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 suggests 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 of 0.5 eV, which is an order of magnitude lower than the calculated indirect bandgap for the β-BaZn2P2 phase. 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 (ρ) 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.

Structural Complexity and Tuned Thermoelectric Properties of a Polymorph of the Zintl Phase Ca2CdSb2 with a Non-centrosymmetric Monoclinic Structure

“…The magnitude of ρ­( T ) varies from the room temperature value of 40.2 to ca. 35.8 mΩ cm at 500 K, which is comparable to the magnitude of ρ­( T ) observed in α-BaZn 2 P 2 . Such a weak temperature dependence, relatively high magnitude of ρ­( T ), as well as the observed negative slope indicate a semiconducting behavior in support of the Zintl closed-shell electronic configuration.…”

“…It is also possible that strain is also a factor in the crystal growth, as seen before on the example of the α-CaGe 2 phase, which is stabilized either in thin film growth or using In flux, while the β-CaGe 2 phase readily crystallizes from melt . Also, it is noted that a contrasting situation is at play in BaZn 2 P 2 where both polymorphs form from Pb-flux …”

“…35.8 mΩ cm at 500 K, which is comparable to the magnitude of ρ(T) observed in α-BaZn 2 P 2 . 31 Such a weak temperature dependence, relatively high magnitude of ρ(T), as well as the observed negative slope indicate a semiconducting behavior in support of the Zintl closed-shell electronic configuration. ρ(T) of Ca 2−x Lu x CdSb 2 (-m), on the other hand, is presented on the right axis and features a clear contrast from that of Ca 2 CdSb 2 (-m).…”

“…30 Also, it is noted that a contrasting situation is at play in BaZn 2 P 2 where both polymorphs form from Pb-flux. 31 Results of thermal stability of Ca 2−x Lu x CdSb 2 (-m) investigated in the temperature range of 300 to 1073 K are presented in Figure S2. From the DSC trace, it is apparent that the observed thermal event which manifests as a valley in the temperature region of 960 to 1025 K is associated with the decomposition of the material and vaporization of elemental Cd.…”

On the Effects of Aliovalent Substitutions in Thermoelectric Zintl Pnictides. Varied Polyanionic Dimensionality and Complex Structural Transformations─The Case of Sr3ZnP3 vs Sr3AlxZn1–xP3

“…The refined bond lengths, taking the disorder into account, are in the range of 2.33−2.41 Å (Table S10), slightly shorter than those in previously reported Zintl phosphides, yet comparable to those in [ZnP 3 ] units with trigonal-planar coordination. 11,12,31,43,44 The Eu3 atom was also modeled by splitting into two positions, with the Eu3A/Eu3B ratio of ca. 0.67:0.33.…”