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

New Trick for an Old Dog: From Prediction to Properties of "Hidden Clathrates" Ba2Zn5As6 and Ba2Zn5Sb6

The zinc-antimony phase space has been heavily investigated due to the structural complexity and abundance of high-performing thermoelectric materials. Consequentially, the desire to use zinc and antimony as framework elements to encage rattling cations and achieve phonon-glass-electron-crystal-type properties has remained an enticing goal with only two alkali metal clathrates to date, Cs₈Zn₁₈Sb₂₈ and K₅₈Zn₁₂₂Sb₂₀₇. Guided by Zintl electron-counting predictions, we explored the Ba-Zn-Pn (Pn = As, Sb) phase space proximal to the expected composition of the type-I clathrate. In situ powder X-ray diffraction studies revealed two "hidden" compounds which can only be synthesized in a narrow temperature range. The ex situ synthesis and crystal growth unveiled that instead of type-I clathrates, compositionally close but structurally different new clathrate-like compounds formed, Ba₂Zn₅As₆ and Ba₂Zn₅Sb₆. These materials crystallize in a unique structure, in the orthorhombic space group Pmna with the Wyckoff sequence i²h⁶gfe. Single-phase synthesis enabled the exploration of their transport properties. Rattling of the Ba cations in oversized cages manifested low thermal conductivity, which, coupled with the high Seebeck coefficients observed, are prerequisites for a promising thermoelectric material. Potential for further optimization of the thermoelectric performance by aliovalent doping was computationally analyzed.

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

The Zintl phase Ca₂CdSb₂ was found to be dimorphic. Besides the orthorhombic Ca₂CdSb₂ (-o), here we report on the synthesis, the structural characterization, and the thermoelectric transport properties of its monoclinic form, Ca₂CdSb₂ (-m), and its Lu-doped variant Ca_2-xLu_xCdSb₂ (x ≈ 0.02). The monoclinic structure exhibits complex structural characteristics and constitutes a new structure type with the non-centrosymmetric space group Cm (Z = 30). The electrical resistivity ρ(T) measured on single crystals of both phases portrays a transition from a semiconductor to a degenerate p-type semiconductor upon doping with Lu and with an attendant change in the Hall carrier concentration n_H from 7.15 × 10¹⁸ to 2.30 × 10¹⁹ cm^-3 at 300 K. The Seebeck coefficient S(T) of both phases are comparable and indicate a hole-dominated carrier transport mechanism with magnitudes of 133 and 116 μV/K at 600 K for Ca₂CdSb₂ (-m) and Ca_2-xLu_xCdSb₂, respectively. The convoluted atomic bonding with an attendant large unit cell volume of ∼4365 ų drives a putative low thermal conductivity in these materials resulting in a power factor PF of 1.63 μW/cm K² and an estimated thermoelectric figure of merit zT of ∼0.5 for Ca_2-xLu_xCdSb₂ at 600 K. Differential scanning calorimetry results reveal the stability of these phases up to about 960 K, making them candidates for moderate temperature thermoelectric materials.

Discovering the desirable physical properties of arsenic compounds AB2As2 and their alloys: a theoretical study

The stability, elastic, electronic, and optical properties of AB2As2 (A = Ca, Sr; B = Mg, Zn, Cd) and their alloys with a trigonal CaAl2Si2-type structure are thoroughly examined for the first time based on the first-principles calculations.