Crystal structure and magnetic properties of geometrically frustrated face centered cubic (f.c.c.) double perovskites, lanthanum lithium (M) oxides and barium yttrium (M) oxides (M = molybdenum, rhenium and rubidium)

This thesis reports a systematic study of geometrically frustrated f.c.c. double perovskites with both monoclinic (P21/n) La 2LiMO6 and cubic (Fm3m) Ba2YMO6 symmetries, where M=Mo (S=1/2), Re (S=1) and Ru (S=3/2). The roles of both the spin quantum number, i.e. quantum spin fluctuations, and the local site symmetry, i.e. orbital ordering, on the determination of the ground magnetic state were studied. All the compounds were prepared by solid state reaction and the structural information and magnetic properties of the compounds were collected using diffraction techniques (X-ray and neutron), dc susceptibility, heat capacity, muon spin relaxation (muSR) and solid state NMR.;For the S=1 materials, monoclinic La2LiReO6 shows collective singlet like behavior as zero magnetization was observed in the ZFC susceptibility and a static and diluted spin system was indicated by muSR. On the other hand, the cubic phase, Ba2YReO6 surprisingly shows a spin glass behavior confirmed by muSR while no Y/Re site mixing was observed by MAS NMR. It is also a surprising observation that this compound retains cubic symmetry down to 3K where it would have a structure transition subject to the Jahn-Teller theorem.;Finally, the S=1/2 compounds, La2LiMoO6 and Ba 2YMoO6, show quite different magnetic behavior. Monoclinic La2LiMoO6 shows the presence of at least short range order achieved at 18K according to the heat capacity and muSR measurements. Ba2>YMoO6 retains cubic symmetry down to 3K and no Jahn-Teller distortion was observed at the limit of the resolution of neutron diffraction. This compound surprisingly appears to remain paramagnetic down to 2K, yet evidence for a collective singlet state was observed by a paramagnetic Knight shift measurement in NMR. This is consistent with an existing theoretical prediction.;An extended study on other S=1/2 Mo analogues, Ba2LuMoO 6 and Ba2ScMoO6 is also presented. Both compounds show cubic structure confirmed by X-ray diffraction and paramagnetic behavior down to 2K in the susceptibility.;The S=3/2 materials, La2LiRuO6 and Ba2YRuO 6, while highly frustrated with frustration indices f ∼ 16 and 8, respectively, both show antiferromagnetic (AF) long range ordering at 24K and 37K, respectively. The Neel temperature of the latter compound was determined for the first time by the heat capacity and neutron diffraction. This compound shows an unusual AF transition as two broader peaks were observed in the susceptibility while La2LiRuO6 shows a typical AF behavior. There is about 1% of Y/Ru site mixing observed by 89Y MAS NMR in Ba 2YRuO6.