For synthesized TlBa2Ca2Cu3O8+δ samples, we used the two-step method. In the first stage, a Tl-free precursor has prepared before proceeding to the second stage, where Tl2O3 has added to the precursor before final sintering. We note that for both methods, starting materials have utilized powders materials BaCO3 (99.0% Oxford Chem Serve), CaCO3 (99.98% Oxford Chem Serve), CuO (99.999% Sigma-Aldrich) and Tl2O3 (99.99% Sigma-Aldrich).
Preparation of Ba:Ca:Cu = 2:2:3 multiphase ceramic precursors:
Sol-gel method (SG). The initial reactants were dissolving separately, BaCO3 and CaCO3 in acetic acid and CuO in nitric acid. When the complete dissolution has achieved, all the solutions were mixed, and the poly(vinyl alcohol)/poly(vinyl acetate) ([-CH2CHOH-]n/[CH2CH(O2CCH3)]n, (Sigma-Aldrich) has added as the complexing agent. The solution was stirring slowly, with continuous up to 80oC, then obtained green gel was dried slowly, the temperature rate 1oC per minute up to 300oC. The result was a black powder that was then grounded in an agate mortar. Then, the powdered was calcined at 900oC in the air with a heating rate of 2oC /min, for 12 h, with two intermediate grindings.
The resulting powders were ground, separated into six parts, and pressed in the form of a disc. For the eliminates the CO2 from precursors, each pellet was individually annealed tube-type furnace at different temperatures 700oC, 800oC, 900oC, 915oC, 930oC, and 945oC in flowing oxygen partial pressure of 0.5 bar for 12 h. For all synthesis temperature heating rate was 1oC/min. First, the samples heated until the temperature of the planned synthesis and then turned on the oxygen and keep on this temperature for 12 h. After the synthesized completed, we turned off the oxygen, and then samples cooled to room temperature inside the furnace.
Solid-state reaction method (SSR). The materials BaCO3, CaCO3, and CuO were mixed in the stoichiometric ratio Ba:Ca:Cu = 2:2:3, and then they were ground carefully in an agate mortar. The resulting powder mixture was calcined in an alumina crucible in the air in a muffled furnace, with four time’s intermediate grindings at 900 °C for 60 h. Then, as made above, the resulting powders also were separated into six parts, pressed and annealed under flowing O2 in various temperatures.
In the second step both Ba2Ca2Cu3Ox precursors prepared by SG and SSR methods separately was mixed with Tl2O3 according to the composition TlBa2Ca2Cu3O8+δ and After final grinding the powder was pressed into a disc-shaped pellet 6 mm in diameter, and 3 mm thick, by using a hydraulic press under a pressure of 400 MPa. The samples have wrapped in a platinum foil then individually put into quartz tubes and from quartz tubes were evacuated up to 10− 3 Torr and sealed. Thereafter, a quartz tube has inserted into a programmed muffle furnace. The temperature of the furnace was raised at a rate of 25oC/min up to 900oC and held at this temperature for 8 h. After the synthesis completed the furnace was quickly cooled to room temperature.
X-ray powder diffraction (XRD) patterns were obtained on a Dron-3 + PC diffractometer with CuKα radiation. The Fourier transformed IR of the samples was taken in the region 400–4000 cm− 1 on a Cary 600 series FTIR Spectrometer using the KBr disc technique. Scanning resolution 0.5 sm− 1. The samples were pulverized into a fine powder and then mixed with potassium bromide powder using a weight ratio of 1:100. The IR absorption spectra were measured immediately after preparing the discs.