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

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

156 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

Fingerprint

Dive into the research topics of 'The Organic Metal (Me2-DCNQI)2Cu: Dramatic Changes in Solid-State Properties and Crystal Structure Due to Secondary Deuterium Effects'. Together they form a unique fingerprint.

Cite this