The Organic Metal (Me2-DCNQI)2Cu: Dramatic Changes in Solid-State Properties and Crystal Structure Due to Secondary Deuterium Effects

Klaus Sinzger, Siegfried Hünig, Martina Jopp, Dagmar Bauer, Werner Bietsch, Jost Ulrich von Schütz, Hans Christoph Wolf, Reinhard Karl Kremer, Tobias Metzenthin, Robert Bau, Saeed I. Khan, Andreas Lindbaum, Christian L. Lengauer, Ekkehart Tillmanns

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149 Citations (Scopus)

Abstract

Although (R1,R3-DCNQI)2Cu salts (1) are isomorphic (space group I41/a), they behave differently upon cooling: The group M salts remain metallic down to the lowest temperatures, whereas the group M-I salts show phase transitions to semiconductors at temperatures TM-I = 160–230 K. With regard to the steric requirements of the methyl group, 1a (CH3/CH3/H2) is expected to belong to group M-I, but it remains metallic even at 0.4 K (σ = 500 000 S cm−1)- Deuterated 1a, however, namely 1c (CH3/CD3/H2), 1d (CD3/CD3/H2), and 1e (CD3/CD3/D2), undergo sharp phase transitions at 58, 73, and 82 K, respectively, the lowest phase-transition temperatures observed so far for copper salts 1 at ambient pressure. Thereby, conductivities drop by 6–8 orders of magnitude within a few kelvins, transforming 1c-e from three-dimensional into strictly one-dimensional systems. This unprecedented strong secondary deuterium isotope effect is also reflected in significant changes in ESR signals and magnetic susceptibilities. Structure determinations by X-ray analysis of 1a and 1d at various temperatures (20 K, 156 K, and room temperature) reveal characteristic differences of the crystal structures above and below the phase-transition temperature TM-I for both compounds. The very special structural features of the crystals are related to their 7-fold diamondoid superstructure, which includes infinite superhelices in the stacks of ligands and copper ions. A phase diagram of (R1,R3-DCNQI)2Cu salts (1) is derived from a comparison of temperature-dependent structural and conductivity data, which shows that the conductive properties of the salts depend dramatically on the N-Cu-N angle αco.. A minimal threshold value (αco ≈ 126.4°) is evaluated which determines the phase behavior of 1 upon cooling or applying pressure. In order to understand their unique properties as compared to those of other DCNQI metal salts, a new approach to DCNQI copper salts is presented in terms of a “two conductors in one” concept, which is a unique combination of a common one-dimensional (anisotropic) conduction path along segregated stacks and a three-dimensional (isotropic) conduction path within a Robin-Day class III network of delocalized mixed-valent metal ions bridged by organic spacers allowing electron transfer.

Original languageEnglish
Pages (from-to)7696-7705
Number of pages10
JournalJournal of the American Chemical Society
Volume115
Issue number17
DOIs
Publication statusPublished - 1 Aug 1993
Externally publishedYes

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