Paramagnetism of layered organic conductors (BEDO-TTF)₂ReO₄*H₂O and (BEDT-TTF)ₓDy(NO₃)z: analysis of phase transition at 200 K

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Layered organic conductors (BEDO-TTF)₂ReO₄*H₂O (metal) and (BEDT-TTF)ₓDy(NO₃)z (paramagnetic insulator) are studied by CW-EPR and SQUID methods. A correlation in resonance and transport properties of both compounds is attributed to a similar mechanism of fine anion ordering (AO) at TₐO=200 K. Due to the hydrogen bonds motif the small configuration changes in anion sub-lattice bring a profound effect on electronic properties. It is shown that the spin system of (BEDO-TTF)₂ReO₄*H₂O consists of a fraction of delocalized pi-type holes (Iₑpr=n(eF)), Iₑpr(300K)=1,62*10(-4) emu/mol and antiferromagnetically correlated local moments S=1/2, chiₚ(300K)=1,86*10(-3) emu/mol. The phase transition Me-Me at TₐO=203 K is detected by paramagnetic relaxation and resisitivity but it is not observed in spin susceptibility. Fraction of delocalized holes gradually decreases by factor 2 at cooling down to 100 K, whereas contribution from local moments reaches maximum at 50 K and falls to zero at 14 K. However, at 2 K total spin susceptibility is fully recovered approaching 6*10(-2) emu/mol. In turn, (BEDT-TTF)ₓDy(NO₃)z does not have solvent bridges (H₂O) between neighboring anions and the disorder in anion layer converts this compound into paramagnetic insulator. Moreover, the anion metal-complexes Dy(NO₃)zⁿ(-) contain magnetic ions Dy³(+). Nonetheless, the phase transition of similar origin is also detected at T=197 K by ESR and resistivity measurements. Two spin sub-systems co-exist in this compound: hopping local moments S=1/2 (BEDT-TTF(+)), Iₑpr(300K)=6,3*10(-4) emu/mol and strongly localized 4f⁹-electrons, J=15/2 (Dy³(+)), chiₚ(300K)=4*10(-2) emu/mol.