Charge-Transfer-Induced Insulator−Superconductor Transition in TlBa2-xSrxCa0.4Pr0.6Cu2O7-δ: A Joint Neutron/X-ray Rietveld Refinement, Infrared Absorption, and Raman Scattering Studies

Abstract

Insulator-superconductor transition in TlBa2-xSrxCa0.4Pr0.6Cu2 O7-δ has been investigated by a joint Rietveld refinement of neutron and X-ray diffraction data, Raman scattering, and infrared absorption studies. For x = 0, the system is an insulator. Substitution of the Ba2+ ions by smaller Sr2+ ions leads to a transition from semiconductor to metal then superconductor for x = 1.0. Further, the superconducting transition temperature (Tc) increases with x. Results of the joint neutron/X-ray Rietveld refinement revealed that the Tl−O(1) apical bond length does not change appreciably while the Cu−O(1) bond length decreases largely when x increases, consistent with an electron transfer from Cu to Tl. This idea is supported by the Raman scattering measurements showing a large frequency shift of the phonon mode involving the motion of O(1) toward Tl, when Ba is replaced by Sr.