Crystal structure of AgTCNQ and contrasting magnetic properties of electrochemically synthesised AgTCNQ and CuTCNQ (7,7,8,8-tetracyanoquinodimethane)
The low resistivity of copper salts of TCNQ and the associated broad e.s.r. signals suggest that the characteristic narrow-band structure of the TCNQ columns may be modified by electronic coupling to metal d bands. Crystalline AgTCNQ and CuTCNQ have been synthesised electrochemically. The crystal structure of AgTCNQ reveals that the silver and TCNQ ions are columnated parallel to the short axis. The silver ions are four-fold coordinated by nitrogen atoms in a distorted tetrahedron. While AgTCNQ is Curie type with a 0.7% mole fraction intrinsic defect, CuTCNQ is narrow-band type, in which the paramagnetism is thermally activated across a small band gap, Eg/k= 584. The e.s.r. characteristics of the AgTCNQ signal are g= 2.0035 (5) and ΔB(295 K)= 3.0 (3) G. The CuTCNQ displays a broad signal g= 2.010 (2) and ΔB(295 K)= 86 (4) G over an even broader complementary signal, g= 2.010 (2) and ΔB(77 K)= 1150 (50) G. The positive g shift and linewidth characteristics of these signals indicate a coupled-band system between the electron acceptor levels of the TCNQ ion and the d band of the copper ions.