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This work reports on the effect of grain size on the electrical, thermal, and magnetic properties of La0.7Sr0.3CoO3 samples obtained by solid-state reaction combined with ball milling. Electrical characterization made for samples with grain sizes ranging from 26 nm to 0.57 μm shows a size-induced metal-to-insulator transition for the sample with smallest grain size. As a consequence of the grain size reduction, there is a change of the thermopower sign and the thermal conductivity displays extremely low values. Interestingly, the thermoelectric figure of merit displays an enhancement as a consequence of the grain size reduction. The Curie temperature was found to be constant whereas the saturation magnetization decreases as grain size decreases. Thereby electrical and magnetic properties of nanocrystalline samples are interpreted in terms of a core-shell structure in which grain boundaries display an insulating behavior while the core region of each grain exhibits the bulk properties. [ABSTRACT FROM AUTHOR]

Thermal expansion was determined for two series of ternary compounds, Ba8MxGe46-x and Ba8MxSi46-x, with M=Cu, Zn, Pd, Ag, Cd, Pt, and Au and for several quaternary compounds for which we investigated the influence of substitution by Zn/Ni in Ba8ZnxGe46-x as well as the dependence of thermal expansion on the Si/Ge ratio in Ba8Cu5SixGe41-x. In the temperature range from 4.2 to 300 K the thermal expansion of all ternary compounds was measured with a capacitance dilatometer, whereas from 300 to 700 K for several selected samples a dynamic mechanical analyzer was employed. The low temperature data compare well with the lattice parameters of single crystals, gained from measurements at three different temperatures (100, 200, and 300 K). For a quantitative description of thermal expansion the semiclassical model of Mukherjee et al. [Phys. Rev. Lett. 76, 1876 (1996)] was used, which also provided reliable accurate values of the Debye and Einstein temperatures. Results in this respect show good agreement with the corresponding data derived from temperature dependent x-ray diffraction and specific heat measurements. Furthermore the present paper is a comprehensive collection and discussion of all thermal expansion data of intermetallic type-I-clathrate materials so far available in the literature including our results of thermal expansion measurements of the Ge- and Si-based type-I-clathrates listed above. [ABSTRACT FROM AUTHOR]

The current paper gives an overview of the newly obtained thermal expansion coefficients of skutterudites as well as those so far available in literature. Thermal expansion was determined for CoSb3, Pt4Sn4.4Sb7.6, for As- and Ge-based skutterudites as well as for various high-ZT skutterudites (micro- and nanostructured) with didymium (DD) and mischmetal (Mm) as filler atoms in frameworks of (Fe1-xCox)4Sb12 and (Fe1-xNix)4Sb12, and for double and triple-filled skutterudites such as Ca0.07Ba0.23Co3.95Ni0.05Sb12 and Sr0.025Ba0.075Yb0.1Co4Sb12. For low temperatures, a capacitance dilatometer was used (4–300 K), whereas for temperatures 300

A series of alloyed ferecrystals ([Sn1−xBixSe]1.15)1(VSe2)1 with 0 ≤x≤ 0.66 was synthesized via the modulated elemental reactants technique. X-ray diffraction of the compounds reveals systematic changes of the lattice parameter and the intensities of the Bragg peaks confirming the successful alloying of the compounds corroborated by Rietveld refinements. Interestingly, both constituents of the intergrowth compounds exhibit systematic structural changes as a function of the Bi-content indicating interlayer interaction. The a-axis lattice parameter of the VSe2 layer expands with increasing Bi-content, which signifies changes in the electronic structure of this constituent. Electrical resistivities, Hall and Seebeck coefficients of compounds with a varying Bi-content present a complex scenario. At low Bi-contents an enhancement of the charge density wave transition is observed, whereas further substitution results in a suppression of the effect. At Bi-contents exceeding x = 0.55 minority carriers from the Sn1−xBixSe layer contribute to the transport properties. [ABSTRACT FROM AUTHOR]

Zn doped ternary compounds Cu2ZnxSn1-xSe3 (x = 0, 0.025, 0.05, 0.075) were prepared by solid state synthesis. The undoped compound showed a monoclinic crystal structure as a major phase, while the doped compounds showed a cubic crystal structure confirmed by powder XRD (X-Ray Diffraction). The surface morphology and elemental composition analysis for all the samples were studied by SEM (Scanning Electron Microscopy) and EPMA (Electron Probe Micro Analyzer), respectively. SEM micrographs of the hot pressed samples showed the presence of continuous and homogeneous grains confirming sufficient densification. Elemental composition of all the samples revealed an off-stoichiometry, which was determined by EPMA. Transport properties were measured between 324 K and 773 K. The electrical resistivity decreased up to the samples with Zn content x = 0.05 in Cu2ZnxSn1-xSe3, and slightly increased in the sample Cu2Zn0.075Sn0.925Se3. This behavior is consistent with the changes in the carrier concentration confirmed by room temperature Hall coefficient data. Temperature dependent electrical resistivity of all samples showed heavily doped semiconductor behavior. All the samples exhibit positive Seebeck coefficient (S) and Hall coefficient indicating that the majority of the carriers are holes. A linear increase in Seebeck coefficient with increase in temperature indicates the degenerate semiconductor behavior. The total thermal conductivity of the doped samples increased with a higher amount of doping, due to the increase in the carrier contribution. The total and lattice thermal conductivity of all samples showed 1/T dependence, which points toward the dominance of phonon scattering at high temperatures. The maximum 1/TZT = 0.48 at 773 K was obtained for the sample Cu2SnSe3 due to a low thermal conductivity compared to the doped samples. [ABSTRACT FROM AUTHOR]

Abstract: Two novel compounds, EuRh3Ge7 and EuIr3Ge7, have been synthesized by arc melting of the elements and subsequent annealing at 800 °C. X-ray structure analyses confirmed for both compounds isotypism with the crystal structure of the ScRh3Si7-type : SC-data for EuRh3Ge7 (a = 0.78918(2), c = 2.07179(5) nm, R F = 0.0130) and Rietveld powder data for EuIr3Ge7 (a = 0.788399(5), c = 2.07583 (2) nm, R F = 0.0501). Magnetization, heat capacity, thermal expansion and electrical resistivity at various magnetic fields were measured revealing second order phase transitions at 20 K for EuRh3Ge7, and 87 K for EuIr3Ge7, respectively. While this transition is clearly antiferromagnetic in the case of EuRh3Ge7, the nature of this transition remains unclear for EuIr3Ge7. The evaluation of magnetic susceptibility as well as heat capacity data reveals an intermediate valence state of Eu-ions in both compounds. [Copyright &y& Elsevier]

Highlights: [•] Quinary clathrate-I compounds with n-type electrical conductivity. [•] Tuning of the band gap via Ni-substitution. [•] High thermoelectric performance (ZTmax =0.9 at 830K). [Copyright &y& Elsevier]

Intermetallic clathrates are promising materials for thermoelectric applications at elevated temperatures. In the search for cost-competitive representatives, we investigate the thermoelectric properties of quaternary type I clathrates with nominal compositions BaCuSiGe (4.5 ≤ x ≤ 7). The specimens were prepared by melting and annealing with subsequent ball milling and hot pressing. X-ray diffraction and energy-dispersive x-ray spectroscopy were used for an accurate structure determination and for an analysis of the phase purity and microstructure, respectively. The transport properties show a systematic variation with the Cu content in the clathrate phase. The highest dimensionless thermoelectric figure of merit ZT of 0.31 is achieved for BaCuSiGe at about 250°C (520 K). [ABSTRACT FROM AUTHOR]

Cu2Ge1-xInxSe3 (x = 0, 0.05, 0.1, 0.15) compounds were prepared by a solid state synthesis. The powder X-ray diffraction pattern of the undoped sample revealed an orthorhombic phase. The increase in doping content led to the appearance of additional peaks related to cubic and tetragonal phases along with the orthorhombic phase. This may be due to the substitutional disorder created by Indium doping. Scanning Electron Microscopy micrographs showed a continuous large grain growth with low porosity, which confirms the compaction of the samples after hot pressing. Elemental composition was measured by Electron Probe Micro Analyzer and confirmed that all the samples are in the stoichiometric ratio. The electrical resistivity (ρ) systematically decreased with an increase in doping content, but increased with the temperature indicating a heavily doped semiconductor behavior. A positive Seebeck coefficient (S) of all samples in the entire temperature range reveal holes as predominant charge carriers. Positive Hall coefficient data for the compounds Cu2InxGe1-xSe3 (x = 0, 0.1) at room temperature (RT) confirm the sign of Seebeck coefficient. The trend of ρ as a function of doping content for the samples Cu2InxGe1-xSe3 with x = 0 and 0.1 agrees with the measured charge carrier density calculated from Hall data. The total thermal conductivity increased with rising doping content, attributed to an increase in carrier thermal conductivity. The thermal conductivity revealed 1/T dependence, which indicates the dominance of Umklapp phonon scattering at elevated temperatures. The maximum thermoelectric figure of merit (ZT) = 0.23 at 723 K was obtained for Cu2In0.1Ge0.9Se3. [ABSTRACT FROM AUTHOR]

Abstract: The thermoelectric properties of ball milled didymium (DD) based skutterudites DD y (Fe1− x Ni x )4Sb12 skutterudites were studied for 0.13⩽ x ⩽0.25 and 0.46⩽ y ⩽0.68 in the temperature range from 300 to 800K and were compared with those of DD y (Fe1− x Ni x )4Sb12 with x =0 of a previous work as well as with their Co-substituted counterparts. Didymium was used as natural double filler (4.76 mass% Pr and 95.24 mass% Nd). The figures of merit ZT of p-type DD y (Fe1− x Ni x )4Sb12 skutterudites reach ZT ⩾1 for the composition range 0.13⩽ x ⩽0.2 (for x =0.15 ZT is almost 1.1), however, ZT values rapidly decrease for a higher nickel content. The thermoelectric efficiency with more than 12% is remarkable and similar to the values for DD y (Fe1− x Co x )4Sb12 skutterudites. [Copyright &y& Elsevier]

The field of ultraviolet (UV)-laser applications is currently experiencing rapid growth in the semiconductor processing, laser micromachining and biomedical markets. Key enablers for these technologies are optical coatings used to manipulate and guide laser beams in a targeted manner. As laser power, laser fluence and pulse frequencies increase, the demands on the physical properties of the coating materials become more stringent. Ion beam sputtering is a technique that allows producing optical coatings with the low losses required in these applications. In this study, we investigate the influence of ion beam sputtering (IBS) parameters on the optical properties of HfO 2 and SiO 2 single layers as well as the impact of annealing duration at 475 °C for anti-reflective (AR) and highly reflective (HR) optical coatings at 355 nm. For HfO 2 sputtered from a metal target the O 2 flow during the coating process is a key parameter to reduce absorption. SiO 2 single layers exhibit improved transmission in the UV-range as the ion beam energy for the sputtering process is reduced. Furthermore, a complex behavior for film stress, absorption, surface roughness and coating structure was unraveled as a function of annealing duration for AR- and HR-coatings at 355 nm. The reflectance of the HR-mirror after optimized annealing exceeded 99.94% at 355 nm and a high laser induced damage threshold (LIDT) of 6.9 J/cm 2 was measured after 2 hours of annealing. For the AR-coating a LIDT-value of 15.7 J/cm 2 was observed after 12 hours of annealing.

We report on the high temperature thin film growth of BaTiO3 on Ti3C2Tx MXene flakes using van der Waals epitaxy on a degradable template layer. MXene was deposited on amorphous and crystalline substrates by spray- and dip-coating techniques, while the growth of BaTiO3 at 700 °C was accomplished using pulsed laser deposition in an oxygen rich environment. We demonstrate that the MXene flakes act as a temporary seed layer, which promotes highly oriented BaTiO3 growth along the (111) direction independent of the underlying substrate. The lattice parameters of the BaTiO3 films are close to the bulk value suggesting that the BaTiO3 films remains unstrained, as expected for van der Waals epitaxy. The initial size of the MXene flakes has an impact on the orientation of the BaTiO3 films with larger flake sizes promoting a higher fraction of the polycrystalline film to grow along the (111) direction. The deposited BaTiO3 film adopts the same morphology as the original flakes and piezoresponse force microscopy shows a robust ferroelectric behavior for individual grains. Transmission electron microscopy results indicate that the Ti3C2Tx MXene fully decomposes during the BaTiO3 deposition and the surplus Ti atoms are readily incorporated into the BaTiO3 film. Electrical measurements show a similar dielectric constant as a BaTiO3 film grown without the MXene seed layer. The demonstrated process has the potential to overcome the longstanding issue of integrating highly oriented complex oxide thin films directly on any desired substrate.

Traditionally, the ferroelectric Curie temperature can be manipulated by chemical substitution, e.g., in Ba1-xSrxTiO3 as one of the archetypical representatives. Here, we show a novel approach to tune the ferroelectric phase transition applicable for nanostructured thin films. We demonstrate this effect in nano-grained BaTiO3 films. Based on an enhanced metastable cation solubility with Ba/Ti-ratios of 0.8 to 1.06, a significant shift of the phase transition temperature is discovered. The transition temperature increases linearly from 212 K to 350 K with increasing Ba/Ti ratio. For all Ba/Ti ratios, a completely diffused phase transition is present resulting in a negligible temperature sensitivity of the dielectric constant. Schottky defects are identified as the driving force behind the off-stoichiometry and the shift of the phase transition temperature as they locally induce lattice strain. Complementary temperature dependent Raman experiments reveal the presence of the hexagonal polymorph in addition to the perovskite phase in all cases. Interestingly, the hexagonal BaTiO3 influences the structural transformation on the Ba-rich side, while on the Ti-rich side no changes for the hexagonal polymorph at the ferroelectric transition temperature are observed. This concerted structural change of both polymorphs on the Ba-rich side causes a broad phase transition region spanning over a wide range up to 420 K including the transition temperature of 350 K obtained from dielectric measurements. These findings are promising for fine adjustment of the phase transition temperature and low temperature coefficient of permittivity.

Thin films of ≈50 nm thickness with Ba/Ti-ratios ranging from 0.8 to 1.06 were prepared by depositing alternating layers of Ba(OH)2 and TiO2. Annealing at 750 °C promoted the solid-solid transformation into polycrystalline BaTiO3 films containing a mixture of the perovskite and the hexagonal polymorphs with average crystallite sizes smaller than 14 nm and without impurity phases. This, together with an increase of the cubic lattice parameters for Ba-rich films, suggests an extended metastable solubility range for the perovskite-phase in these nanocrystalline thin films on both sides of the stoichiometric composition. Mapping of the cation distribution utilizing energy-filtered transmission electron microscopy corroborates defect accommodation within the BaTiO3 grains. While the cation off-stoichiometry in thermodynamic equilibrium is negligible for BaTiO3, the metastable extended solubility range in the thin films can be directly correlated to the low annealing temperature and nanocrystalline nature. The leakage current behavior can be explained by the formation of Schottky defects for nonstoichiometric films, and the cation ratio has a distinct impact on the dielectric properties: while excess-BaO has a marginal detrimental effect on the permittivity, the dielectric constant declines rapidly by more than 50% towards the Ti-rich side. The present findings highlight the importance of compositional control for the synthesis of nanocrystalline BaTiO3 thin films, in particular for low annealing and/or deposition temperatures. Our synthesis approach using alternating layers of Ba(OH)2 and TiO2 provides a route to precisely control the cation stoichiometry.

We report on the growth of polycrystalline BiFeO 3 thin films on SiO 2 /Si(001) and Pt(111) substrates by atomic layer deposition using the precursors ferrocene, triphenyl-bismuth, and ozone. By growing alternating layers of Fe 2 O 3 and Bi 2 O 3 , we employ a superlattice approach and demonstrate an efficient control of the cation stoichiometry. The superlattice decay and the resulting formation of polycrystalline BiFeO 3 films are studied by in situ X-ray diffraction, in situ X-ray photoelectron spectroscopy, and transmission electron microscopy. No intermediate ternary phases are formed and BiFeO 3 crystallization is initiated in the Bi 2 O 3 layers at 450 °C following the diffusion-driven intermixing of the cations. Our study of the BiFeO 3 formation provides an insight into the complex interplay between microstructural evolution, grain growth, and bismuth oxide evaporation, with implications for optimization of ferroelectric properties., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We show how finite-size scaling of a bulk photovoltaic effect-generated electric field in epitaxial ferroelectric insulating BaTiO_{3}(001) films and a photo-Hall response involving the bulk photovoltaic current reveal a large room-temperature mean free path of photogenerated nonthermalized electrons. Experimental determination of mesoscopic ballistic optically generated carrier transport opens a new paradigm for hot electron-based solar energy conversion, and for facile control of ballistic transport distinct from existing low-dimensional semiconductor interfaces, surfaces, layers, or other structures.