SYNTHESIS AND STRUCTURE-PROPERTY RELATIONSHIPS OF THE AAg2M[VO4]2 TYPE OF COMPOUNDS
Amuneke, Ngozi Ebere 1986-
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This dissertation focuses on the synthesis and structural characterization of a series of new compounds belonging to the family: AAg2M[VO4]2. We have modified the A-site spacer with the divalent metals: Sr2+ and Ba2+ and monovalent metals: K+, Rb+, and Ag+ which are representatives of chemical pressure by size and charge differences. The magnetic M-site cation can also be altered with respect to size and charge differences. Here, we have investigated the spin systems Cu2+ (S = ½), Ni2+ (S = 1), Co2+ (S = 3/2), Mn2+ (S = 5/2), Fe3+ (S = 5/2), and Cr3+ (S = 3/2). These are arranged on a triangular lattice connected exclusively via vanadates as a linker. Super-super exchange pathways via the vanadates are important for the magnetic correlations, therefore the relative orientation of these non-magnetic linkers with respect the M-site cation is of utmost importance. Here we have investigated this aspect of structure-property relationships by several experimental techniques: X-ray diffraction to examine the crystal structure, spectroscopic methods aiming at the vibrational and electronic structures, thermodynamic, and neutron diffraction measurements. A detailed structural description is presented discussing the alteration on the A and M-sites and the finding of “tunable” magnetic properties related to the orientation of the vanadate. In conclusion, those with transition metal cations with dn > 5 are ferromagnets whereas those with dn ≤ 5 belong to the rare class of geometrically frustrated antiferromagnets.