ABSTRACT
We report an extraordinary pressure dependence of the magnetic interactions in the metal-organic system [CuF_{2}(H_{2}O)_{2}]_{2}pyrazine. At zero pressure, this material realizes a quasi-two-dimensional spin-1/2 square-lattice Heisenberg antiferromagnet. By high-pressure, high-field susceptibility measurements we show that the dominant exchange parameter is reduced continuously by a factor of 2 on compression. Above 18 kbar, a phase transition occurs, inducing an orbital re-ordering that switches the dimensionality, transforming the quasi-two-dimensional lattice into weakly coupled chains. We explain the microscopic mechanisms for both phenomena by combining detailed x-ray and neutron diffraction studies with quantitative modeling using spin-polarized density functional theory.
ABSTRACT
CaMg(2)AlN(3) was synthesized in a closed system by solid state reaction from binary nitrides. Structure refinements based on powder X-ray diffraction data suggested ambiguity about the occupancy of magnesium and aluminum tetrahedral sites. Solid-state (27)Al and (25)Mg NMR studies were used to adjudicate amongst possible space groups. With reference to projector augmented wave calculations of the quadrupolar coupling constants, the measured values of CQ and the numbers of crystallographically inequivalent Al and Mg sites indicate that CaMg(2)AlN(3) crystallizes in the space group P6(3)/mmc with partial occupancy of the distorted tetrahedral Al site and possibly also mixing of Mg(2+) and Al(3+) ions on opposite sites. The compound obtained by synthesis with a flux shows orange defect-related luminescence at room temperature.
