Recently, there has been a lot of interest in topological insulators (TIs), being electronic materials, which are insulating in their bulk but with the gapless exotic metallic state on their...
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.
Zintl phases with complex crystal structures have been studied as promising candidate materials for thermoelectric (TE) applications. Here, we report the syntheses of the family of rare-earth metal Zintl phases with the general formula Ca 4−x RE x Sb 3 (x ≈ 1; RE = La−Nd, Sm, Gd−Tm, Lu). The structural elucidation is based on refinements of single-crystal X-ray diffraction data for 12 unique chemical compositions. The cubic structure is confirmed as belonging to the anti-Th 3 P 4 structure type (space group I4̅ 3d, no. 220, Z = 4), where the Ca and RE atoms share the same atomic site with ca. 75 and 25% occupancies, respectively. Such crystallographic disordering of divalent Ca and trivalent RE atoms in the structure provides a pathway to intricate bonding. The latter, together with the presence of heavy elements such as Sb and the lanthanides, are expected to enhance the scattering probability of phonons, thereby leading to low thermal conductivity κ comparable to that of the ordered RE 4 Sb 3 . The drive of the hypothetical parent compound Ca 4 Sb 3 to be stabilized by alloying with rare-earth metals can be understood following the Zintl−Klemm concept, as the resultant formula may be rationalized as (Ca 2+ ) 3 RE 3+ (Sb 3− ) 3 , indicating the realization of closed-shell electronic configurations for all elements. This notion is confirmed by electronic structure calculations, which reveal narrow bandgaps E g = 0.77 and 0.53 eV for Ca 3 LaSb 3 and Ca 3 LuSb 3 , respectively. In addition, the incorporation of RE atoms into the structure drives the phase into a state of a degenerate semiconductor with dominant hole charge carriers.
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