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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PrCo 2 Ga 8 is an orthorhombic quasi-skutterudite type compound which crystallizes in the CaCo 2 Al 8 structure type, with space group P bam (No. 55). The Pr 3+ ion has a site symmetry of C s which predicts a crystal electric field (CEF) level splitting into 9 singlets for J = 4. However, a phase transition at T m = 1.28 K is observed in electrical resistivity and specific heat results and is reported in this paper. The electrical resistivity shows an upturn below T m due to the superzone-gap formation. This transition is tuneable in fields and is suppressed to lower temperatures with applied magnetic fields. The electronic specific heat C p (T )/T increases below T m and reaches a value of 7.37 J/(mol K 2 ) at 0.4 K. The Sommerfeld coefficient, γ extracted from the low temperature analysis of C 4f (T )/T is 637 mJ/(mol K 2 ) indicating a possible mass enhancement of the quasiparticles. The calculated entropy value of 3.05 J/(mol K) is recovered around T m exhibiting almost 53% of Rln2, where R is the universal gas constant. Magnetic susceptibility results obeys the Curie-Weiss law for data above 100 K with an estimated effective magnetic moment, µ eff = 3.37 µ B /Pr and Weiss temperature, θ p = −124 K.

The intermetallic compound PrFe2Al8 that possesses a three-dimensional network structure of Al polyhedra centered at the transition metal element Fe and the rare earth Pr is investigated through neutron powder diffraction and inelastic neutron scattering in order to elucidate the magnetic ground state of Pr and Fe and the crystal field effects of Pr. Our neutron diffraction study confirms long-range magnetic order of Pr below TN = 4.5 K in this compound. Subsequent magnetic structure estimation reveals a magnetic propagation vector k = ( 1 2 0 1 2 ) with a magnetic moment value of 2.5 µB/Pr along the orthorhombic c-axis and evidence the lack of ordering in the Fe sublattice. The inelastic neutron scattering study reveals one crystalline electric field excitation near 19 meV at 5 K in PrFe2Al8. The energy-integrated intensity of the 19 meV excitation as a function of |Q|(Å −1 ) follows the square of the magnetic form factor of Pr 3+ thereby confirming that the inelastic excitation belongs to the Pr sublattice. The second sum rule applied to the dynamic structure factor indicates only 1.6(2) µB evolving at the 19 meV peak compared to the 3.58 µB for free Pr 3+ , indicating that the crystal field ground state is magnetic and the missing moment is associated with the resolution limited quasi-elastic line. The magnetic order occurring in Pr in PrFe2Al8 is counter-intuitive to the symmetry-allowed crystal field level scheme, hence, is suggestive of exchange-mediated mechanisms of ordering stemming from the magnetic ground state of the crystal field levels.

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 Ca2–x Lu x CdSb2 (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 Ca2–x Lu x CdSb2, respectively. The convoluted atomic bonding with an attendant large unit cell volume of ∼4365 Å3 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 Ca2–x Lu x CdSb2 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.

Intrinsic 2D ferromagnetic semiconductors are an important class of materials for spin-charge conversion applications. Cr 2 Ge 2 Te 6 retains long-range magnetic order in the bilayer at cryogenic temperatures and shows complex magnetic interactions with considerable magnetic anisotropy. Here, a series of structural, magnetic, X-ray scattering, electronic, thermal transport and first-principles calculation studies are performed, which reveal that localized electronic charge carriers in Cr 2 Ge 2 Te 6 are dressed by the surrounding lattice and are involved in polaronic transport via hopping that is observed via magnetocrystalline anisotropy. This opens the possibility for manipulation of charge transport in Cr 2 Ge 2 Te 6 -based devices by electron-phonon-and spinorbit coupling-based tailoring of polaron properties.