Polycrystalline tungsten-substituted CaMn 1Àx W x O 3Àd (0.00 x 0.05) powders were synthesized from a polymeric precursor, pressed and sintered to high density. The impact of tungsten substitution on the crystal structure, thermal stability, phase transition, electronic and thermal transport properties is assessed. Tungsten acts as an electron donator and strongly affects high-temperature oxygen stoichiometry. Oxygen vacancies form in the high figure-of-merit (ZT)-region starting from about T ¼ 1000 K and dominate the carrier concentration and electronic transport far more than the tungsten substitution. The analysis of the transport properties yields that in the investigated regime the band filling is sufficiently high to overcome barriers of polaron transport. Therefore, the Cutler-Mott approach describes the electrical transport more accurately than the Mott approach for small polaron transport. The lattice thermal conductivity near room temperature is strongly suppressed with increasing tungsten concentration due to mass-difference impurity scattering. A ZT of 0.25 was found for x ¼ 0.04 at 1225 K. V

The relation among structure, electric transport, and photovoltaic effect is investigated for a pn heterojunction with strong correlation interactions. A perovskite interface is chosen as a model system consisting of the p-doped strongly correlated manganite Pr 0.64 Ca 0.36 MnO 3 (PCMO) and the n-doped titanate SrTi 1−y Nb y O 3 (y = 0.002 and 0.01). High-resolution electron microscopy and spectroscopy reveal a nearly dislocation-free, epitaxial interface and give insight into the local atomic and electronic structure. The presence of a photovoltaic effect under visible light at room temperature suggests the existence of mobile excited polarons within the band-gapfree PCMO absorber. The temperature-dependent rectifying current-voltage characteristics prove to be mainly determined by the presence of an interfacial energy spike in the conduction band and are affected by the colossal electroresistance effect. From the comparison of photocurrents and spatiotemporal distributions of photogenerated carriers (deduced from optical absorption spectroscopy), we discuss the range of the excited polaron diffusion length.

Articles you may be interested inThe thermoelectric performance of Ta substituted (ZrHf)NiSn-based half-Heusler compounds is studied. Here, Ta is used on the Hf site for controlling the charge carrier concentration in contrast to the widely used Sb substitution on the Sn site. The influence of the Ta content on the thermoelectric and transport properties of (Zr 0.3 Hf 0.7-x Ta x )NiSn (x ¼ 0, 0.01, 0.05) is investigated by means of Seebeck coefficient, electrical resistivity, thermal conductivity, and Hall coefficient measurements. The results are analyzed in context of the single parabolic band model. Ta substitution increases the charge carrier concentration and suppresses the influence of impurity band, which is present in the pristine (Zr 0.3 Hf 0.7 )NiSn. Moreover, Ta substitution decouples and simultaneously increases the density-of-states effective mass (m*) and the charge carrier mobility (l), leading to a larger weighted mobility lÁ(m*) 3/2 . The lattice thermal conductivity is slightly suppressed due to increased point defect scattering. As a result, a Figure

We have to make substantial corrections to the part of the paper dealing with the impact of the high-pass filter on the photocurrent and consequently also to the determination of photocarrier diffusion lengths (Sec. III C). After publication, a new measurement of the transmission curve of the used 2.3 eV high-pass filter revealed that photon energies above 3.5 eV are also blocked, in contrast to the original transmission curve. Thus, the massive drop in the photocurrent [ Fig. 6(a)] cannot be attributed unambiguously to the blanked low-energy part of the light source spectrum. Quite the contrary, newer (unpublished) measurements with a low-pass filter reveal that the contribution of excitations from photon energies lower than 2.3 eV amounts to only 2% of the total photocurrent under unfiltered illumination (Xe lamp; T = 300 K). Consequently, the photocarrier diffusion length as presented in Fig. 7 has to be corrected: According to the new dataset I low SC is exclusively generated from excitations inside the PCMO space charge region without additional inflow from the PCMO bulk. Nevertheless, transient pump probe absorption spectroscopy sets a lower limit to the lifetime of the optical excitations in the 1.5-eV polaron band of several 100 ps. 1,2 We suggest that both the presence of the interfacial energy spike (Fig. 4) and the trapping centers, like Mn 4+ sites 3 or acceptor defects, 4 may be detrimental to harvesting these low-energy excitations.A precise estimate of the contribution from UV excitations in PCMO to the total photocurrent is hindered by the yet undetermined share of the STNO (including possible doped midgap states from the Mn/Ti interdiffusion). Apart from the aforementioned constraints on the carrier mobilities, the general observation of a strong UV absorption in hole-doped manganites points toward significant contributions from the PCMO, i.e., from polaronic charge-transfer excitations involving O 2p as initial states. 5

Highly dense CaMn 1-x M x O 3-δ (with M = Nb, Mo, Ta, W and 0 ≤ x ≤ 0.08) n-type thermoelectric materials with low electrical resistivities are prepared from nano-crystalline powders. Their room temperature power factors outperform the best reported results by 30% or more. In combination with the thermal conductivities promising figure-of-merits of ZT M=Ta,x=0.04 = 0.21 and ZT M=W,x=0.04 = 0.20 were achieved at 1160 K. The relative changes and temperature dependencies of the Seebeck coefficient, the electrical resistivity, and the power factor are described with a small-polaronhopping-based mechanism. In the limits of high-temperatures and low substitution levels the Seebeck coefficients are in good agreement with Heikes formula. At high substitutions the efficiency of the doping presumably decreases due to trapping states caused by the formation of bands from Jahn-Teller lowered e g orbitals of Mn 3+ . Jahn-Teller distortion of Mn 3+ also leaves its footprints in the orthorhombic distortion of the crystal structure along the b-axis.