Browsing by Author "Chou, Ta-Lei"

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Competition between ferromagnetism and antiferromagnetism in the rutile Cr1-xVxO2 system
(2016) Mustonen, Otto; Vasala, Sami; Chou, Ta-Lei; Chen, Jin-Ming; Karppinen, Maarit
School of Chemical Technology | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We present a comprehensive computational and experimental examination of the Cr1−xVxO2 (0 ≤ x ≤ 0.5) system. The entire series crystallizes in the rutile structure, but the compounds exhibit significantly different magnetic properties depending on x. Lattice parameter a increases linearly with x, but the c parameter is slightly reduced due to vanadium-vanadium bonding. The V-for-Cr substitution creates Cr3+-V5+ pairs; this leads to competition between ferromagnetic (Cr4+-Cr4+) and antiferromagnetic (Cr3+-Cr3+) interactions such that the materials change from ferromagnetic to antiferromagnetic with increasing x. Weak ferromagnetic interactions arising from Cr4+ are observed even in the seemingly antiferromagnetic phases with the exception of x = 0.5, which contains only Cr3+. Density functional theory calculations are performed, but they incorrectly predict the x = 0.5 phase to be a half-metal. This is caused by an incorrect prediction of the oxidation states of chromium and vanadium.
Competition between ferromagnetism and antiferromagnetism in the rutile Cr1−xVxO2 system
(2016-01-06) Mustonen, Otto; Vasala, Sami; Chou, Ta-Lei; Chen, Jin-Ming; Karppinen, Maarit
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We present a comprehensive computational and experimental examination of the Cr1−xVxO2 (0≤x≤0.5) system. The entire series crystallizes in the rutile structure, but the compounds exhibit significantly different magnetic properties depending on x. Lattice parameter a increases linearly with x, but the c parameter is slightly reduced due to vanadium-vanadium bonding. The V-for-Cr substitution creates Cr3+−V5+ pairs; this leads to competition between ferromagnetic (Cr4+−Cr4+) and antiferromagnetic (Cr3+−Cr3+) interactions such that the materials change from ferromagnetic to antiferromagnetic with increasing x. Weak ferromagnetic interactions arising from Cr4+ are observed even in the seemingly antiferromagnetic phases with the exception of x=0.5, which contains only Cr3+. Density functional theory calculations are performed, but they incorrectly predict the x=0.5 phase to be a half-metal. This is caused by an incorrect prediction of the oxidation states of chromium and vanadium.
Layered thermoelectric materials : misfit cobalt oxides and oxyselenides
(2015) Chou, Ta-Lei
School of Chemical Technology | Doctoral dissertation (article-based)
Because of the growing awareness of environmental and energy-related issues, harvesting waste heat by thermoelectric materials is getting increasing attention. A decent thermoelectric material is typically a heavily doped semiconductor with high thermopower and low thermal conductivity. To simultaneously achieve these characteristics, a complex structure is an essence. Inspired by the artificial superlattices various multi-layered compounds with a repetitive stacking of conducting and blocking layers which enables to decouple electrical and thermal transport are currently investigated as candidates to approach the ideal "phonon glass, electron crystal" scenario. Metal oxides feature advantages like availability and thermal stability in air at elevated temperatures yet poor carrier mobility without appropriate doping. Replacing oxygen with less electronegative anions, for instance chalcogens, would turn the framework more covalent and lead to readily improved electrical conduction. This dissertation presents several layered misfit cobalt oxides and oxyselenides as thermoelectrics. To begin with, double-interlayered phases [Sr2O2]0.52CoO2 and [Ca1.7(OH)2]0.58CoO2 were successfully made under compression and investigated by x-ray absorption spectroscopy (XAS) and thermogravimetry (TG) which unveil subtle local geometry, electronic structure and defects embedded in the lattice. Given that a mixed-anion oxide-chalcogenide framework might effectively benefit the electrical transport, some layered oxyselenides, such as BiOCuSe, Bi2MO4Cu2Se2 (M = Y or early rare earth element) and AE2CoO2Cu2Se2 (AE = alkaline earth element) which in common possess an anti-fluorite-type [Cu2Se2] sub-layer, were then investigated. Temperature-dependent extended x-ray absorption fine structure (EXAFS) is found to be a powerful probe to resolve the lattice characteristics of BiOCuSe upon finite doping. Meanwhile, Cu vacancies and alkaline-earth dopants are found to affect distinctively in the BiOCuSe matrix. Bi2MO4Cu2Se2 compounds, unfortunately, are metals with poor thermopower. The origin of metallicity is readily evidenced in the XAS analysis. Lastly the Sr2CoO2Cu2Se2 p-type semiconductor was visited. Computational work suggests there is no way to overlook the contribution from the oxide sub-layer at the valence band maximum (VBM). Replacing Sr with Ba, in contrast to the Ca-for-Sr substitution case, would more efficiently raise the thermoelectric power factor. It is quite convincing once an appropriate doping scheme is employed, either in the oxide or the selenide layers, the Sr2CoO2Cu2Se2 oxyselenides could be a promising option for intermediate-temperature thermoelectrics.