| Summary: | New organic (super)conductors of the β″-(BEDT-TTF)<sub>4</sub>A<sup>+</sup>[M<sup>3+</sup>(C<sub>2</sub>O<sub>4</sub>)<sub>3</sub>]G family, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene; M is Fe; A is the monovalent cation NH<sub>4</sub><sup>+</sup>; G is 2-fluoropyridine (2-FPy) (1); 2,3-difluoropyridine (2,3-DFPy) (2); 2-chloro-3-fluoropyridine (2-Cl-3-FPy) (3); 2,6-dichloropyridine (2,6-DClPy) (4); 2,6-difluoropyridine (2,6-DFPy) (5), have been prepared and their crystal structure and transport properties were studied. All crystals have a layered structure in which the conducting layers of BEDT-TTF radical cations alternate with paramagnetic supramolecular anionic layers {A<sup>+</sup>[Fe<sup>3+</sup>(C<sub>2</sub>O<sub>4</sub>)<sub>3</sub>]<sup>3−</sup>G<sup>0</sup>}<sup>2−</sup>. Crystals <b>1</b> undergo a structural phase transition from the monoclinic (<i>C</i>2/<i>c</i>) to the triclinic (<i>P</i><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover accent="true"><mn>1</mn><mo stretchy="true">¯</mo></mover></mrow></semantics></math></inline-formula>) symmetry in the range 100–150 K, whereas crystals <b>2</b>–<b>5</b> have a monoclinic symmetry in the entire range of the X-ray experiment (100–300 K). The alternating current (ac) conductivity of salts <b>1</b>–<b>4</b> exhibits metallic behavior down to 1.4 K, whereas the salt <b>5</b> demonstrates the onset of a superconducting transition at 3.1 K. The structures and conducting properties of <b>1</b>–<b>5</b> are compared with those of the known monoclinic phases of the family containing different monohalopyridines as “guest” solvent molecules G.
|
|---|
