Syntheses, Characterizations, and Properties of Suboxides and Layered Titanium-based Pnictide Oxide Superconductors



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We investigated the effects of oxygen interstitial in the phase Ce3Si2Ox. The binary Ce3Si2 was found to exhibit antiferromagnetic behavior, with the Neel temperature of TN = 8 K. The oxygen interstitial phase, Ce3Si2O0.82 was found to be paramagnetic. This indicates that the introduction of oxygen as interstitial leads to the loss of long range magnetic ordering in Ce3Si2 structure at low temperatures. Herein we will report the fine tuning of the interstitial oxygen content aimed at further understanding the transition from antiferromagnetic to paramagnetic behavior in the range of Ce3Si2Ox (x = 0.00-0.82). This was done by investigating the range of the interstitial content that can be incorporated into the host, and use it to “tune” the electron count of the pseudo-binary electronic structure. We also report the discovery of a new intergrowth structure, Ce8Si5O0.52, which features alternate stacking of Ce3O2O0.52 and Ce5Si3 layers.
Motivated by the search for new classes of layered pnictide oxide superconductors, the compound BaTi2Sb2O was synthesized by high-temperature solid-state reaction within inert container. Its crystal structure was determined using X-ray powder diffraction and Rietveld refinement. BaTi2Sb2O crystallizes in a layered variant of the CeCr2Si2C structure type (space group P4/mmm (No.123)), and features [Ti2Sb2O]2- layers separated by layers of Ba atoms. The [Ti2Sb2O]2- layers can be described as being formed from O-centered square nets of Ti atoms, Ti2O, inverse to the CuO2 layers in the superconducting cuprates. The Ti2O sheets are then capped by Sb atoms above and below the sheet to form a nominal network of Ti4Sb2 octahedral units bridged by oxygen. Magnetic susceptibility measurements shows BaTi2Sb2O to exhibit a magnetic transition (at Tc = 54 K) that can be attributed to spin density wave (SDW) or charge density wave (CDW) transitions. This is reminiscent of the un-doped phases of the FeAs-based high-Tc superconductors. Chemical substitution experiments were performed for the purpose of suppressing the magnetic transition and possibly inducing a superconducting state. In this regard a series of p-doped phases, Ba(1-x)NaxTi2Sb2O with x = 0.05-0.33 were prepared. Structural parameters show a systematic change, associated with an elongation of the c-axis and contraction of the a-axis, with the increasing of Na content. A systematic lowering of the magnetic transition temperature is observed with the increasing Na content. More importantly, superconducting transitions are also observed with Na content, x = 0.05-0.33. The superconducting transition temperature Tc increases from 2.8 K (Na = 5%) to 5.5 K (Na = 15%). Bulk superconductivity is observed through dc magnetization, resistivity and heat capacity measurements. Elemental chemical analysis, using inductively coupled plasma / mass spectrometer (ICP-MS), confirm the stoichiometric of the doped phases with regarding to Ba and Na content.



Suboxides, Antiferromagnetic, Layered transition based oxide superconductors, Titanium-based pnictide oxides, Spin density wave, Charge density wave, Solid state synthesis