Synthesis and structural, magnetic, thermal, and transport properties of several transition metal oxides and arsenides

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Das, Supriyo
Major Professor
David C. Johnston
Committee Member
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Physics and Astronomy
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Several transition metal oxides and arsenides have been synthesized and their magnetic, thermal, structural, and transport properties have been studied in this thesis. Magnetically pure spinel compound LiV2O4 is a rare d-electron heavy fermion. The presence of small concentrations of magnetic defects, which are produced by the crystal defects in the spinel structure, strongly affect the physical properties of LiV2O4. The phase relations in the Li2O-V2O3-V2O5 ternary system at 700 °C for compositions in equilibrium with LiV2O4 are reported. This study clarified the synthesis conditions under which low and high magnetic defect concentrations can be obtained within the spinel structure of LiV2O4. We confirmed that the LiV2O4 phase can be obtained containing low (0.006 mol%) to high ( 0.83 mol%) magnetic defect concentrations ndefect and with consistently high magnetic defect spin S values between 3 and 6.5. The high ndefect values were obtained in the LiV2O4 phase in equilibrium with V2O3, Li3VO4, or LiVO2 and the low values in the LiV2O4 phase in equilibrium with V3O5. A model is suggested to explain this correlation. We grew single crystals of LiV2O4 using Li3VO4 as a self-flux. The magnetic susceptibility of some as-grown crystals show a Curie-like upturn at low temperatures, showing the presence of magnetic defects within the spinel structure. The magnetic defects could be removed in some of the crystals by annealing them at 700 °C. A very high specific heat coefficient γ = 450 mJ/(mol K2) was obtained at a temperature of 1.8 K for a crystal containing a magnetic defect concentration ndefect = 0.5 mol%. A crystal with ndefect = 0.01 mol% showed a residual resistivity ratio of 50. To search for superstructure peaks or other evidence of spatial correlations in the arrangement of the crystal defects with in the crystal structure which give rise to magnetic defects, we carried out high-energy x-ray diffraction studies on LiV2O4 single crystals. Entire reciprocal lattice planes were mapped out with the help of synchrotron radiation. No noticeable differences in the x-ray diffraction data between a crystal with high magnetic defect concentration and a crystal with low magnetic defect concentration were found. This indicates the absence of any long-range periodicity or short-range correlations in the arrangements of the crystal/magnetic defects.

In addition to LiV2O4, we synthesized and studied the properties of LV4O8 (L = Yb, Y, Lu) compounds which crystallize in a structure similar to that of the orthorhombic CaFe2O4 structure-type, and contain four inequivalent V sites arranged in zigzag chains. We confirm the structure and report the magnetic, thermal, and transport properties of polycrystalline YV4O8 and LuV4O8. A first-order like phase transition is observed at 50 K in both YV4O8 and LuV4O8. The symmetry remains the same with the lattice parameters changing discontinuously. The structural transition in YV4O8 leads to partial dimerization of the V atoms resulting in a sudden sharp drop in the magnetic susceptibility. The V spins that do not form dimers order in a canted antiferromagnetic state. The magnetic susceptibility of LuV4O8 shows a sharp peak at $sim 50 K. The magnetic entropies calculated from heat capacity versus temperature

measurements indicate bulk magnetic transitions below 90 K for both YV4O8 and LuV4O8.

We also grew single crystals of EuPd2Sb2 from PdSb self-flux. Single crystal x-ray diffraction studies confirmed that EuPd2Sb2 crystallizes in the CaBe2Ge2-type structure which is closely related to the structure of the recently discovered iron-arsenide based superconductors. Antiferromagnetic ordering in the crystallographic ab-plane at ∼ 6 K with a spin-reorientation transition at 4.5 K is suggested from the magnetic susceptibility versus temperature χ (T) and heat capacity versus temperature Cp(T) data. The electrical resistivity versus temperature ρ (T) data show metallic behavior down to 1.8 K along with an anomaly at 5.5 K in zero field. The anomaly is suppressed to 2.7 K in an 8 T field. The Hall-coefficient R,sub>H measurements indicated that the dominant charge carriers are electrons. The magnetization versus magnetic

field M(H) isotherms show three field-induced transitions at 2.75 T, 3.90 T, and 4.2 T magnetic fields parallel to the ab plane at 1.8 K. No transitions are observed in M(H) for fields parallel to the c axis.

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Fri Jan 01 00:00:00 UTC 2010