Correction: Eu2+-Eu3+ valence transition in double, Eu-, and Na-doped PbSe from transport, magnetic, and electronic structure studies.

Correction for 'Eu2+-Eu3+ valence transition in double, Eu-, and Na-doped PbSe from transport, magnetic, and electronic structure studies' by Bartlomiej Wiendlocha et al., Phys. Chem. Chem. Phys., 2017, 19, 9606-9616.

Eu2+-Eu3+ valence transition in double, Eu-, and Na-doped PbSe from transport, magnetic, and electronic structure studies.

The Eu atoms in Pb1-xEuxSe have long been assumed to be divalent. We show that p-type doping of this magnetic semiconductor alloy with Na can modify the effective Eu valence: a mixed, Eu2+-Eu3+ state appears in Pb1-x-yEuxNaySe at particular values of y. Magnetization, carrier concentration, resistivity, and thermopower of Pb1-x-yEuxNaySe are reported for a number of samples with different x and y. A pronounced increase in thermopower at a given carrier concentration was identified and attributed to the presence of enhanced ionized impurity scattering. A strong decrease in the hole concentration is observed in Pb1-yNaySe when Eu is added to the system, which we attribute to a Eu2+-Eu3+ self-ionization process. This is evidenced by magnetization measurements, which reveal a significant reduction of the magnetic moment of Pb1-xEuxSe upon alloying with Na. Further, a deviation of magnetization from a purely paramagnetic state, described by a Brillouin function, identifies antiferromagnetic interactions between the nearest-neighbor Eu atoms: a value of Jex/kB = -0.35 K was found for the exchange coupling parameter. The conclusion of a Eu2+-Eu3+ self-ionization process being in effect is supported further by the electronic structure calculations, which show that an instability of the 4f7 configuration of the Eu2+ ion appears with Na doping. Schematically, it was found that the Eu 4f levels form states near enough to the Fermi energy that hole doping can lower the Fermi energy and trigger a reconfiguration of a 4f electronic shell.

Solvothermal Synthesis of Tetrahedrite: Speeding Up the Process of Thermoelectric Material Generation.

Derivatives of synthetic tetrahedrite, Cu12Sb4S13, are receiving increasing attention in the thermoelectric community due to their exploitation of plentiful, relatively nontoxic elements, combined with a thermoelectric performance that rivals that of PbTe-based compounds. However, traditional synthetic methods require weeks of annealing at high temperatures (450-600 °C) and periodic regrinding of the samples. Here we report a solvothermal method to produce tetrahedrite that requires only 1 day of heating at a relatively low temperature (155 °C). This allows preparation of multiple samples at once and is potentially scalable. The solvothermal material described herein demonstrates a dimensionless figure of merit (ZT) vs temperature curve comparable to that of solid-state tetrahedrite, achieving the same ZT of 0.63 at ∼720 K. As with the materials from solid-state synthesis, products from this rapid solvothermal synthesis can be improved by mixing in a 1:1 molar ratio with the Zn-containing natural mineral, tennantite, to achieve 0.9 mol equiv of Zn. This leads to a 36% increase in ZT at ∼720 K for solvothermal tetrahedrite, to 0.85.

Highly efficient (In2Te3)x(GeTe)(3-3x) thermoelectric materials: a substitute for TAGS.

GeTe is a versatile base compound to produce highly efficient p-type thermoelectric materials such as the TAGS materials (AgSbTe2)1-x(GeTe)x and GeTe-PbTe nanocomposites. The pure GeTe composition shows a very high power factor, ~42 µW cm(-1) K(-2), between 673 K and 823 K, which is among the highest power factors that have ever been reported in this temperature range. However, its relatively high thermal conductivity limits the dimensionless figure of merit ZT to values of only unity. In this paper, we present an efficient approach to reduce the thermal conductivity by preparing (In2Te3)x(GeTe)(3-3x) solid solutions. In spite of a slight degradation of the electronic properties, the drastic reduction of the thermal conductivity due to a synergistic combination of reduced electronic thermal conductivity, strong alloy scattering, and vacancy phonon scattering leads to ZT values as high as 1.35 at 823 K for the x = 0.05 sample. Our results show that (In2Te3)x(GeTe)(3-3x) is a prospective substitute for TAGS as a p-leg element for high-temperature power generation.

Natural mineral tetrahedrite as a direct source of thermoelectric materials.

We show that a simple powder processing procedure using natural mineral tetrahedrite, the most widespread sulfosalt on earth, provides a low cost, high throughput means of producing thermoelectric materials with high conversion efficiency. These earth-abundant thermoelectrics can open the door to many new and inexpensive power generation opportunities.

Role of lone-pair electrons in producing minimum thermal conductivity in nitrogen-group chalcogenide compounds.

Fully dense crystalline solids with extremely low lattice thermal conductivity (κ(L)) are of practical importance for applications including thermoelectric energy conversion and thermal barrier coatings. Here we show that lone-pair electrons can give rise to minimum κ(L) in chalcogenide compounds that contain a nominally trivalent group VA element. Electrostatic repulsion between the lone-pair electrons and neighboring chalcogen ions creates anharmonicity in the lattice, the strength of which is determined by the morphology of the lone-pair orbital and the coordination number of the group VA atom.

Resistance, magnetoresistance, and thermopower of zinc nanowire composites.

We report a very large enhancement of the thermopower of 4 nm diameter metallic Zn nanowires, with a temperature dependence that is consistent with that of their electrical resistivity and the Mott formula. The temperature dependence of the resistance, magnetoresistance, and thermopower of composites consisting of 15, 9, and 4 nm diameter Zn nanowires imbedded in porous host materials is reported. The 15 nm wires are metallic. The smaller wires show 1D weak localization, but the electrical resistivity mostly follows a T(-1/2) law, and the thermopower of the 4 nm wires saturates at -130 microV/K.

Thermoelectric power of bismuth nanocomposites.

Because of the increase in the electronic density of states in low-dimensional systems, semiconductor quantum wires constitute a most promising thermoelectric material. We report here the first experimental observation of a very large enhancement of the thermoelectric power of composites containing bismuth nanowires with diameters of 9 and 15 nm, embedded in porous alumina and porous silica. The temperature dependence of the electrical resistance shows that the samples are semiconductors with energy gaps between 0.17 and 0.4 eV, consistent with the theoretical predictions.