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Paramagnetism of layered organic conductors (BEDO-TTF)_2ReO_4*H_2O and (BEDT-TTF)_xDy(NO_3)_z: analysis of phase transition at 200 K

Yu. N. Shvachko·D. V. Starichenko·A. V. Korolyov·N. D. Kushch·E. B. Yagubskii
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Abstract

Layered organic conductors (BEDO-TTF)_2ReO_4*H_2O (metal) and (BEDT-TTF)_xDy(NO_3)_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 T_AO=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)_2ReO_4*H_2O consists of a fraction of delocalized pi-type holes (I_epr=n(e_F)), I_epr(300K)=1,62*10^(-4) emu/mol and antiferromagnetically correlated local moments S=1/2, chi_p(300K)=1,86*10^(-3) emu/mol. The phase transition Me-Me at T_AO=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(NO_3)_z does not have solvent bridges (H_2O) between neighboring anions and the disorder in anion layer converts this compound into paramagnetic insulator. Moreover, the anion metal-complexes Dy(NO_3)_z^n(-) contain magnetic ions Dy^3(+). 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^(+)), I_epr(300K)=6,3*10^(-4) emu/mol and strongly localized 4f^9-electrons, J=15/2 (Dy^3(+)), chi_p(300K)=4*10^(-2) emu/mol.

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Yu. N. Shvachko, D. V. Starichenko, A. V. Korolyov, N. D. Kushch, E. B. Yagubskii. 2005-06-24. Paramagnetism of layered organic conductors (BEDO-TTF)_2ReO_4*H_2O and (BEDT-TTF)_xDy(NO_3)_z: analysis of phase transition at 200 K. https://arxiv.org/abs/cond-mat/0506634

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