CsSnI(3) is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature. Experimental and theoretical studies of CsSnI(3) have been limited by the lack of detailed crystal structure characterization and chemical instability. Here we describe the synthesis of pure polymorphic crystals, the preparation of large crack-/bubble-free ingots, the refined single-crystal structures, and temperature-dependent charge transport and optical properties of CsSnI(3), coupled with ab initio first-principles density functional theory (DFT) calculations. In situ temperature-dependent single-crystal and synchrotron powder X-ray diffraction studies reveal the origin of polymorphous phase transitions of CsSnI(3). The black orthorhombic form of CsSnI(3) demonstrates one of the largest volumetric thermal expansion coefficients for inorganic solids. Electrical conductivity, Hall effect, and thermopower measurements on it show p-type metallic behavior with low carrier density, despite the optical band gap of 1.3 eV. Hall effect measurements of the black orthorhombic perovskite phase of CsSnI(3) indicate that it is a p-type direct band gap semiconductor with carrier concentration at room temperature of ∼ 10(17) cm(-3) and a hole mobility of ∼585 cm(2) V(-1) s(-1). The hole mobility is one of the highest observed among p-type semiconductors with comparable band gaps. Its powders exhibit a strong room-temperature near-IR emission spectrum at 950 nm. Remarkably, the values of the electrical conductivity and photoluminescence intensity increase with heat treatment. The DFT calculations show that the screened-exchange local density approximation-derived band gap agrees well with the experimentally measured band gap. Calculations of the formation energy of defects strongly suggest that the electrical and light emission properties possibly result from Sn defects in the crystal structure, which arise intrinsically. Thus, although stoichiometric CsSnI(3) is a semiconductor, the material is prone to intrinsic defects associated with Sn vacancies. This creates highly mobile holes which cause the materials to appear metallic.

The solid-state transformation phenomena of spinodal decomposition and nucleation and growth are presented as tools to create nanostructured thermoelectric materials with very low thermal conductivity and greatly enhanced figure of merit. The systems (PbTe)(1-x)(PbS)(x) and (Pb(0.95)Sn(0.05)Te)(1-x)(PbS)(x) are not solid solutions but phase separate into PbTe-rich and PbS-rich regions to produce coherent nanoscale heterogeneities that severely depress the lattice thermal conductivity. For x > approximately 0.03 the materials are ordered on three submicrometer length scales. Transmission electron microscopy reveals both spinodal decomposition and nucleation and growth phenomena the relative magnitude of which varies with x. We show that the (Pb(0.95)Sn(0.05)Te)(1-x)(PbS)(x) system, despite its nanostructured nature, maintains a high electron mobility (>100 cm(2)/V x s at 700 K). At x approximately 0.08 the material achieves a very low room-temperature lattice thermal conductivity of approximately 0.4 W/m x K. This value is only 28% of the PbTe lattice thermal conductivity at room temperature. The inhibition of heat flow in this system is caused by nanostructure-induced acoustic impedance mismatch between the PbTe-rich and PbS-rich regions. As a result the thermoelectric properties of (Pb(0.95)Sn(0.05)Te)(1-x)(PbS)(x) at x = 0.04, 0.08, and 0.16 were found to be superior to those of PbTe by almost a factor of 2. The relative importance of the two observed modes of nanostructuring, spinodal decomposition and nucleation and growth, in suppressing the thermal conductivity was assessed in this work, and we can conclude that the latter mode seems more effective in doing so. The promise of such a system for high efficiency is highlighted by a ZT approximately 1.50 at 642 K for x approximately 0.08.

Lead sulfide, a compound consisting of elements with high natural abundance, can be converted into an excellent thermoelectric material. We report extensive doping studies, which show that the power factor maximum for pure n-type PbS can be raised substantially to ~12 μW cm(-1) K(-2) at >723 K using 1.0 mol % PbCl(2) as the electron donor dopant. We also report that the lattice thermal conductivity of PbS can be greatly reduced by adding selected metal sulfide phases. The thermal conductivity at 723 K can be reduced by ~50%, 52%, 30%, and 42% through introduction of up to 5.0 mol % Bi(2)S(3), Sb(2)S(3), SrS, and CaS, respectively. These phases form as nanoscale precipitates in the PbS matrix, as confirmed by transmission electron microscopy (TEM), and the experimental results show that they cause huge phonon scattering. As a consequence of this nanostructuring, ZT values as high as 0.8 and 0.78 at 723 K can be obtained for nominal bulk PbS material. When processed with spark plasma sintering, PbS samples with 1.0 mol % Bi(2)S(3) dispersion phase and doped with 1.0 mol % PbCl(2) show even lower levels of lattice thermal conductivity and further enhanced ZT values of 1.1 at 923 K. The promising thermoelectric properties promote PbS as a robust alternative to PbTe and other thermoelectric materials.

In situ nanostructuring in bulk thermoelectric materials through thermo-dynamic phase segregation has established itself as an effective paradigm for optimizing the performance of thermoelectric materials. In bulk PbTe small compositional variations create coherent and semicoherent nanometer sized precipitates embedded in a PbTe matrix, where they can impede phonon propagation at little or no expense to the electronic properties. In this paper the nanostructuring paradigm is for the first time extended to a bulk PbS based system, which despite obvious advantages of price and abundancy, so far has been largely disregarded in thermoelectric research due to inferior room temperature thermoelectric properties relative to the pristine fellow chalcogenides, PbSe and PbTe. Herein we report on the synthesis, microstructural morphology and thermoelectric properties of two phase (PbS)(1-x)(PbTe)(x)x = 0-0.16 samples. We have found that the addition of only a few percent PbTe to PbS results in a highly nanostructured material, where PbTe precipitates are coherently and semicoherently embedded in a PbS matrix. The present (PbS)(1-x)(PbTe)(x) nanostructured samples show substantial decreases in lattice thermal conductivity relative to pristine PbS, while the electronic properties are left largely unaltered. This in turn leads to a marked increase in the thermoelectric figure of merit. This study underlines the efficiency of the nanostructuring approach and strongly supports its generality and applicability to other material systems. We demonstrate that these PbS-based materials, which are made primarily from abundant Pb and S, outperform optimally n-type doped pristine PbTe above 770 K.

Thermoelectric (TE) power generation has come to be appreciated as an attractive means of low-cost conversion of waste heat to useful electrical energy with a small environmental impact. For a compound to qualify as an efficient thermoelectric material it should exhibit the highest TE figure of merit, ZT, possible at the temperature of operation, T. ZT is defined asand it involves the simultaneous manipulation of the TE power (absolute Seebeck coefficient) S, the electrical conductivity r, and the thermal conductivity j. The search for efficient TE materials mainly focuses on degenerate semiconductors since the underlying physics of these systems allow the coexistence of high thermopower values with high electrical conductivity to achieve high power factors: PF = S 2 r. The Seebeck coefficient is inversely related to the electrical conductivity according to the Boltzmann transport equation, and, as a result, maximization of one cannot be achieved without minimization of the other. An interesting alternative that has been recently suggested to achieve high power factors is the quantum-confinement effect; however, definite experimental verification of this is still lacking.[1]Another route to achieving high-performance TEs is through the minimization of the thermal conductivity. To this end, many suggestions have been made to increase ZT. These include the phonon-glass electron-crystal approach [2] (where loosely bound atoms rattle in cage structures [3] ) as in clathrates, [4] and the thin-film multilayer approach where the introduction of interfaces significantly reduces phonon propagation.[ [10] where compositional fluctuations at the nanoscopic level, resulting in a distinct type of nanostructuring, seem to play a key role in the previously reported very low thermal conductivity. [11] In contrast to the thin-film multilayers, bulk nanocomposite systems offer the advantages of large-scale industrial production and the sustenance of large thermal gradients for extended time. The challenge, therefore, lies in identifying equally efficient p-type materials so that they can be employed in the fabrication of TE modules.Here we report on the Ag(Pb 1 -y Sn y ) m SbTe 2 + m series and show that certain compositions exhibit high performance p-type TE properties (e.g., ZT ∼ 1.45 at 630 K) as a result of their very low thermal conductivity. We show as well that the Ag(Pb 1 -y Sn y ) m SbTe 2 + m systems are in fact bulk nanocomposites. We demonstrate that varying the m and y values, as well as the Ag and Sb concentrations, allows for control over a wide range of properties such as carrier concentration, TE power, and thermal conductivity. These exceptional properties, derived from specific compositions, outperform the standard state-of-the-art p-type systems like TAGS ((AgSbTe 2 ) 0.15 (GeTe) 0.85 , ZT ∼ 1.2 at 720 K [12] ), PbTe (ZT ∼ 0.7 at 740 K [13] ), and Zn 4 Sb 3 (ZT ∼ 1.3 at 670 K [14] ).The electronic-transport properties of the Ag(Pb 1 -y Sn y ) mSbTe 2 + m system can be tuned primarily through carefully control...

Semiconductor materials for efficient hard radiation detection are identified by combining a powerful chemical concept called dimensional reduction and precise theoretical electronic structure calculations. After more than 50 years of research and development in the field, this constitutes a significant step forward in the search for new detector materials.

We report production of nanostructured magnetic carbon foam by a high-repetition-rate, high-power laser ablation of glassy carbon in Ar atmosphere. A combination of characterization techniques revealed that the system contains both sp 2 and sp 3 bonded carbon atoms. The material is a form of carbon containing graphitelike sheets with hyperbolic curvature, as proposed for "schwarzite." The foam exhibits ferromagnetic-like behavior up to 90 K, with a narrow hysteresis curve and a high saturation magnetization. Such magnetic properties are very unusual for a carbon allotrope. Detailed analysis excludes impurities as the origin of the magnetic signal. We postulate that localized unpaired spins occur because of topological and bonding defects associated with the sheet curvature, and that these spins are stabilized due to the steric protection offered by the convoluted sheets.

We explored the effect of Cd substitution on the thermoelectric properties of PbTe in an effort to test a theoretical hypothesis that Cd atoms on Pb sites of the rock salt lattice can increase the Seebeck coefficient via the formation of a resonance level in the density of states near the Fermi energy. We find that the solubility of Cd is less than previously reported, and CdTe precipitation occurs to create nanostructuring, which strongly suppresses the lattice thermal conductivity. We present detailed characterization including structural and spectroscopic data, transmission electron microscopy, and thermoelectric transport properties of samples of PbTe-x% CdTe-0.055% PbI(2) (x = 1, 3, 5, 7, 10), PbTe-1% CdTe-y% PbI(2) (y = 0.03, 0.045, 0.055, 0.08, 0.1, 0.2), PbTe-5% CdTe-y% PbI(2) (y = 0.01, 0.03, 0.055, 0.08), and PbTe-1% CdTe-z% Sb (z = 0.3, 0.5, 1, 1.5, 2, 3, 4, 5, 6). All samples follow the Pisarenko relationship, and no enhancement of the Seebeck coefficient was observed that could be attributed to a resonance level or a distortion in the density of states. A maximum ZT of approximately 1.2 at approximately 720 K was achieved for the PbTe-1% CdTe-0.055% PbI(2) sample arising from a high power factor of approximately 17 microW/(cm K(2)) and a very low lattice thermal conductivity of approximately 0.5 W/(m K) at approximately 720 K.