New Bi-based high-T-c superconducting phases obtained by low-temperature fluorination

A new superconducting phase, Bi2Sr2Ca2Cu3O8F4, was obtained by fluorination of standard Bi(2223) at moderate temperatures (250-300 degrees C), using as fluorine source ammonium hydrogen difluoride, NH4HF2. The presence of fluorine in this new phase was confirmed by energy-dispersive X-ray analysis. X-ray and neutron diffraction experiments furthermore showed that fluorine atoms replace oxygen atoms in the BiO layers in the ratio 2:1. The additional anions form a square-mesh layer between neighboring BiF layers. The incorporation of fluorine increases the crystallographic c-axis parameter by similar to 1.8 Angstrom (a = 5.409 Angstrom