(DI−DCNQI)2Cuas a unique member of the DCNQI-Cu family: A theoretical study of high-pressure phases

We have studied the pressure dependence of the electronic structures of organic solids ${\mathrm{}(\mathrm{D}\mathrm{M}\mathrm{e}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu}$ and ${\mathrm{}(\mathrm{D}\mathrm{I}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu}$ by first-principles calculations based on density-functional theory. We have found that the electronic structure of ${\mathrm{}(\mathrm{D}\mathrm{I}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu}$ changes drastically by applying pressure while that of ${\mathrm{}(\mathrm{D}\mathrm{M}\mathrm{e}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu}$ is almost unchanged. The present calculation strongly suggests that the insulating phase of ${\mathrm{}(\mathrm{D}\mathrm{I}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu}$ under pressure (≃16 kbar) will not have threefold periodicity along the $c$ axis in clear contrast to other Cu salts. The metallic phase under high pressure (>35 kbar) is also studied.