Authors: Leo K. Lamontagne
Source: eScholarship (California Digital Library) (article, 2018-01-01)
Field: Materials Science — Materials Chemistry

Electronic Functionality in Complex Palladium Oxides The study of the electronic and magnetic properties of 4d transition metal oxides is crucial in developing new functional materials while also informing the origins of favorable properties in the highly studied 3d transition metal oxides. In particular, complex palladium oxides represent one subset of 4d transition metal oxides which have received comparatively little attention in regards to their electronic functionality despite reports of interesting electronic properties in the form of compositionally driven insulator-metal transitions. Several semiconducting complex palladium oxides have been shown to be driven metallic upon hole-doping with alkali metals prompting the study of their functional properties for applications such as thermoelectrics. In addition, there exists opportunities for bettering the fundamental understandings of insulator-metal transitions and, more generally, hole-doping in oxides as these complex palladium oxides are diamagnetic with comparatively small amounts of electron-electron correlation and spin orbit coupling. We present here detailed structural studies of several complex palladium oxides and their resulting electronic properties upon hole-doping. We report that some do show favorable thermoelectric performance and remark on how structural changes imparted by aliovalent hole-dopants can influence functional properties. First, we discuss the thermoelectric performance of Li-substituted PbPdO2. Upon Li substitution, a decrease in electrical resistivity by over an order of magnitude is observed without a precipitous drop in the Seebeck coefficient leading to a zT of 0.12 at 600K. The electronic properties of Li-substituted PbPdO2 are near identical to those of the high performing cobaltate thermoelectrics despite the lack of a high degree of spin degeneracy which is believed to be the origin of the favorable properties in the cobaltates. Electronic structure calculations support experimental measurements and conclude that aspects of the band structure derived from the palladium square planar coordination contribute to the performance.Following this work, we report on a structurally similar material, LiBiPd2O4. Hole-doping was achieved through Li substitution for Pd as before and through Pb substitution for Bi. Both methods of hole-doping decreased the electrical resistivity by over three orders of magnitude. Despite this decrease, the resistivity remains too high for thermoelectric applications. Owing to a differing connectivity of the Pd square planes, LiBiPd2O4 possesses a much higher band gap, leading to the higher resistivity. 7Li solid state nuclear magnetic resonance (NMR) reveals many distinct Li environments arise with Pb substitution. This implies an asymmetric distribution of hole-dopants and may be related to formations of more conductive regions of the material in an insulating matrix. Next, we study the impact of hole-doping in two isostructural compounds, SrPd3O4 and CaPd3O4. Both semiconducting materials have been shown previously to be driven metallic with substitution onto the Sr/Ca site, though reports of the doping level necessary vary widely. Under our preparation and processing condtions, Ca1-xNaxPd3O4 is driven metallic above x = 0.10, while Sr1-xNaxPd3O4 remains semiconducting up to x = 0.20. Nearly identical electronic structures imply that there are local structural differences which affect the bulk properties. We observe through sensitive probes including, synchrotron X-ray diffraction (XRD), pair distribution function analysis of total neutron scattering data, and 23Na NMR that indeed there exists larger amounts of local disorder in the Sr compounds. 23Na NMR further reveals the presence of two distinct Na environments one of which aligns with the Na environment in metallic NaPd3O4 providing evidence of the presence of a percolative insulator-metal transition mechanism.To further understand the role local disorder and dopant distributions have on the observed electrical properties, we used Li, and K as dopants in addition to Na. While Li appears to distribute evenly throughout SrPd3O4, a distinct second phase emerges in Sr0.8K0.2Pd3O4. Density functional theory calculations (DFT) support our experiments in that it predicts differing behavior when K is used as a dopant versus Li or Na. DFT suggests that K will order when substituted into SrPd3O4 though the presence of an ordered phase or of clustered KPd3O4 can not be determined through synchrotron XRD. The electrical resistivity decreases slightly with increasing size of the hole dopant, but is not drastically affected. There is no evidence of a second phase emerging in Ca0.8K0.2Pd3O4. Materials science ; Seebeck coefficient ; Palladium ; Thermoelectric effect ; Dopant ; Doping ; Transition metal ; Condensed matter physics ; Electronic structure ; Thermoelectric materials ; Metal ; Chemical physics ; Inorganic chemistry ; Chemistry ; Metallurgy ; Thermodynamics ; Catalysis ; Physics ; Optoelectronics Advanced Thermoelectric Materials and Devices ; Electrocatalysts for Energy Conversion ; Surface and Thin Film Phenomena