Synthesis and structural characterization of the new Zintl phases Ba3Cd2P4 and Ba2Cd2P3. Rare example of small gap semiconducting behavior with negative thermopower within the range 300 ​K–700 ​K

Novel ternary Zintl phosphide halides Ba3P5X (X = Cl, Br) with 1D helical phosphorus chains: synthesis, crystal and electronic structure

Black single crystals of two novel ternary phosphide halides, Ba3P5Cl and Ba3P5Br, were grown using molten metal Pb-flux high-temperature reactions. These compounds were structurally characterized with the aid of the single-crystal X-ray diffraction (SCXRD) method at 100(2) K. The SCXRD shows that both compounds are isostructural and adopt a new structure type (space group R3̄c, No. 167, Z = 6) with unit cell parameters a = 14.9481(16) Å, c = 7.3954(11) Å and a = 15.045(4) Å, c = 7.537(3) Å for Ba3P5Cl and Ba3P5Br, respectively. Cl- and Br- anions are octahedrally coordinated by Ba2+ cations, thus composing a face-sharing 1D infinite chain 1∞[XBa3]5+ running along the [001] direction. Moreover, the crystal structures feature peculiar one-dimensional disordered infinite helical chains of 1∞P-, composed of partially occupied phosphorous atoms, each being a superposition of three symmetrical copies of the ordered phosphorus chain, with continuity along the c-axis. Ba3P5X (X = Cl, Br) compounds are charge-balanced heteroanionic Zintl phases according to the charge-partitioning scheme (Ba2+)3[P-]5X-. The presumed semiconducting behavior of both compounds corroborates well with the results of the electronic structure calculations performed with the aid of the TB-LMTO-ASA code.

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

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.

Enhanced Thermoelectric Performance of LiZnSb-Alloyed CaZn0.4Ag0.2Sb by Band Engineering

LiZnSb is a Zintl phase that has been predicted to be a good material in thermoelectric applications for a long time. However, experimental work indicated that the synthesized LiZnSb materials were p type, and their maximum zT value is only 0.08 at 525 K. CaZn_0.4Ag_0.2Sb, which belongs to the LiGaGe structure type and is also closely associated with the LiZnSb structure, did show high zT plateaus in a wide range of temperature, with the mixed transition metal Zn/Ag sites regulated. By comparing their crystallographic and electronic band structures, it is evident that the interlayered distances in both compounds have a great effect on the regulation of the corresponding electrical transport properties. When alloying CaZn_0.4Ag_0.2Sb with LiZnSb, solid solutions form within a specific range, which led to a marked enhancement in the Seebeck coefficient through the orbital alignment and carrier concentration optimization. In addition, a low thermal conductivity was obtained owing to the reduced electronic component. With the above optimization, a maximum zT value of ∼1.3 can be realized for (CaZn_0.4Ag_0.2Sb)_0.87(LiZnSb)_0.13 at 873 K, more than twice that of the pristine CaZn_0.4Ag_0.2Sb and about 10-fold compared to that of LiZnSb. This work may shed new light on the optimization of thermoelectric properties based on Zintl phases, for which the crystal structures are usually very complicated and a direct correlation between the structures and properties is difficult to make.

Synthesis, structural characterization, and electronic structure of the novel Zintl phase Ba2ZnP2

The novel Zintl phase dibarium zinc diphosphide (Ba₂ZnP₂) was synthesized for the first time. This was accomplished using the Pb flux technique, which allowed for the growth of crystals of adequate size for structural determination via single-crystal X-ray diffraction methods. The Ba₂ZnP₂ compound was determined to crystallize in a body-centered orthorhombic space group, Ibam (No. 72). Formally, this crystallographic arrangement belongs to the K₂SiP₂ structure type. Therefore, the structure can be best described as infinite [ZnP₂]^4- polyanionic chains with divalent Ba²⁺ cations located between the chains. All valence electrons are partitioned, which conforms to the Zintl-Klemm concept and suggests that Ba₂ZnP₂ is a valence-precise composition. The electronic band structure of this new compound, computed with the aid of the TB-LMTO-ASA code, shows that Ba₂ZnP₂ is an intrinsic semiconductor with a band gap of ca 0.6 eV.