Inverse Perovskites (Eu3O)E withE = Sn, In – Preparation, Crystal Structures and Physical Properties

Inverse-Perovskite Ba3 BO (B = Si and Ge) as a High Performance Environmentally Benign Thermoelectric Material with Low Lattice Thermal Conductivity

High energy-conversion efficiency (ZT) of thermoelectric materials has been achieved in heavy metal chalcogenides, but the use of toxic Pb or Te is an obstacle for wide applications of thermoelectricity. Here, high ZT is demonstrated in toxic-element free Ba3 BO (B = Si and Ge) with inverse-perovskite structure. The negatively charged B ion contributes to hole transport with long carrier life time, and their highly dispersive bands with multiple valley degeneracy realize both high p-type electronic conductivity and high Seebeck coefficient, resulting in high power factor (PF). In addition, extremely low lattice thermal conductivities (κlat ) 1.0-0.4 W m-1  K-1 at T = 300-600 K are observed in Ba3 BO. Highly distorted O-Ba6 octahedral framework with weak ionic bonds between Ba with large mass and O provides low phonon velocities and strong phonon scattering in Ba3 BO. As a consequence of high PF and low κlat , Ba3 SiO (Ba3 GeO) exhibits rather high ZT = 0.16-0.84 (0.35-0.65) at T = 300-623 K (300-523 K). Finally, based on first-principles carrier and phonon transport calculations, maximum ZT is predicted to be 2.14 for Ba3 SiO and 1.21 for Ba3 GeO at T = 600 K by optimizing hole concentration. Present results propose that inverse-perovskites would be a new platform of environmentally-benign high-ZT thermoelectric materials.

Mechanically Stable Magnetic Metallic Materials for Biomedical Applications

The structural, electrical, and magneto-elastic properties of lanthanide base nitride (Ln = Dy-Lu) anti-perovskites were investigated using density functional theory (DFT). The reported structural outcomes are consistent with the experiment and decrease from Dy to Lu due to the decrease ofatomic radii of Ln atoms. According to the electronic band profile, the metallic characteristics of these compounds are due to the crossing over of Ln-f states at the Fermi level and are also supported by electrical resistivity. The resistivity of these compounds at room temperature demonstrates that they are good conductors. Their mechanical stability, anisotropic, load-bearing, and malleable nature are demonstrated by their elastic properties. Due to their metallic and load-bearing nature, in addition to their ductility, these materials are suitable as active biomaterials, especially when significant acting loads are anticipated, such as those experienced by such heavily loaded implants as hip and knee endo-prostheses, plates, screws, nails, dental implants, etc. In thesecases, appropriate bending fatigue strength is required in structural materials for skeletal reconstruction. Magnetic properties show that all compounds are G-type anti-ferromagnetic, with the Neel temperatures ranging from 24 to 48 K, except Lu₃Nin, which is non-magnetic. Due to their anti-ferromagnetic structure, magnetic probes cannot read data contained in anti-ferromagnetic moments, therefore, data will be unchanged by disrupted magnetic field. As a result, these compounds can be the best candidates for magnetic cloaking devices.

Tilting structures ininverseperovskites,M3TtO (M= Ca, Sr, Ba, Eu;Tt= Si, Ge, Sn, Pb)

Single-crystal X-ray diffraction experiments were performed for a series ofinverseperovskites,M3TtO (M= Ca, Sr, Ba, Eu;Tt= tetrel element: Si, Ge, Sn, Pb) in the temperature range 500–50 K. ForTt= Sn, Pb, they crystallize as an `ideal' perovskite-type structure (Pm\bar 3m,cP5); however, all of them show distinct anisotropies of the displacement ellipsoids of theMatoms at room temperature. This behavior vanishes on cooling forM= Ca, Sr, Eu, and the structures can be regarded as `ideal' cubic perovskites at 50 K. The anisotropies of the displacement ellipsoids are much more enhanced in the case of the Ba compounds. Finally, their structures undergo a phase transition at ∼ 150 K. They change from cubic to orthorhombic (Ibmm,oI20) upon cooling, with slightly tilted OBa6octahedra, and bonding angles O—Ba—O ≃ 174° (100 K). For the larger Ba2+cations, the structural changes are in agreement with smaller tolerance factors (t) as defined by Goldschmidt. Similar structural behavior is observed for Ca3TtO. SmallerTt4−anions (Si, Ge) introduce reduced tolerance factors. Both compounds Ca3SiO and Ca3GeO with cubic structures at 500 K, change into orthorhombic (Ibmm) at room temperature. Whereby, Ca3SiO is the only representative within theM3TtO family where three polymorphs can be found within the temperature range 500–50 K: Pm\bar 3m–Ibmm–Pbnm. They show tiny differences in the tilting of the OCa6octahedra, expressed by O—Ca—O bond angles of 180° (500 K), ∼ 174° (295 K) and 170° (100 K). For largerM(Sr, Eu, Ba), together with smallerTt(Si, Ge) atoms, pronounced tilting of the OM6octahedra, and bonding angles of O—M—O ≃ 160° (295 K) are observed. They crystallize in theanti-GdFeO3type of structure (Pbnm,oP20), and no phase transitions occur between 500 and 50 K. The observed phase transitions are all accompanied by multiple twinning, in terms of pseudo-merohedry or reticular pseudo-merohedry.

Electronic Properties of Antiperovskite Materials from State-of-the-Art Density Functional Theory

We present a review on the research developments on the theoretical electronic properties of the antiperovskite materials. The antiperovskite materials have perovskite type structure with the positions of cations and anions interchanged. The electronic structures are used to explain different physical properties of materials; therefore it is crucial to understand band structures and densities of states of materials for their effective use in technology. The theoretical results of the electronic structure of antiperovskites were discussed and compared with the available experimental results to measure the accuracy of the research done so far on these materials. The important physical properties of these compounds like magnetic properties and superconductivity are also highlighted. Nevertheless the thermoelectric properties of these materials are still unexplored except for a few reports which suggest that antiperovskite materials may be potential candidates for thermoelectric generators.