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The deuterated dicyanoquinonediimine salt Cu(2-CD3,5-CD3-DCNQI)2, (d6), a highly conductive organic metal, exhibits a phase transition from the metallic to the insulating state at Tc≊60 K. In a wide temperature range we observe an anticoincidence of high conductivity and electron spin resonance (ESR). These experiments were performed simultaneously in our ESR apparatus equipped for application of He pressure up to 200 bar. Near Tc there is a coexistence of conducting (ESR-silent) and insulating (ESR-detectable) domains. This correlation of ESR and conductivity σ points to a percolation limited conductivity. Upper and lower limits of the size of the domains are given. Light shifts Tc to lower temperatures, pressure to higher ones. This could be due to shrinking of the lattice which induces the phase transition when critical structural parameters are approached. This is most probably explained by a simultaneous development of a charge density wave (CDW) and a spin pairing within the DCNQI stack together with the trimerization of the lattice. Systematic measurements by variation of temperature, pressure, and light led to phase diagrams of (d6) and to the understanding of the mechanism of the phase transitions. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]