K. Sinzger et al., THE ORGANIC METAL (ME2-DCNQI)2CU - DRAMATIC CHANGES IN SOLID-STATE PROPERTIES AND CRYSTAL-STRUCTURE DUE TO SECONDARY DEUTERIUM EFFECTS, Journal of the American Chemical Society, 115(17), 1993, pp. 7696-7705
Although (R1,R3-DCNQI)2Cu salts (1) are isomorphic (space group I4(1)/
a), they behave differently upon cooling: The group M salts remain met
allic down to the lowest temperatures, whereas the group M-I salts sho
w phase transitions to semiconductors at temperatures T(M-I) = 160-230
K. With regard to the steric requirements of the methyl group, 1a (CH
3/CH3/H2) is expected to belong to group M-I, but it remains metallic
even at 0.4 K (sigma = 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 with
in a few kelvins, transforming 1c-e from three-dimensional into strict
ly one-dimensional systems. This unprecedented strong secondary deuter
ium isotope effect is also reflected in significant changes in ESR sig
nals and magnetic susceptibilities. Structure determinations by X-ray
analysis of 1a and 1d at various temperatures (20 K, 156 K, and room t
emperature) reveal characteristic differences of the crystal structure
s above and below the phase-transition temperature T(M-I) for both com
pounds. The very special structural features of the crystals are relat
ed 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 tempera
ture-dependent structural and conductivity data, which shows that the
conductive properties of the salts depend dramatically on the N-Cu-N a
ngle alpha(co). A minimal threshold value (alpha(co) almost-equal-to 1
26.4-degrees) is evaluated which determines the phase behavior of 1 up
on cooling or applying pressure. In order to understand their unique p
roperties as compared to those of other DCNQI metal salts, a new appro
ach to DCNQI copper salts is presented in terms of a ''two conductors
in one'' concept, which is a unique combination of a common one-dimens
ional (anisotropic) conduction path along segregated stacks and a thre
e-dimensional (isotropic) conduction path within a Robin-Day class III
network of delocalized mixed-valent metal ions bridged by organic spa
cers allowing electron transfer.