D.O. Pederson

A NEW 1212-TYPE PHASE: Cr-SUBSTITUTED TlSr2CaCu2O7 WITH Tc UP TO ABOUT 110 K

Superconductivity up to about 110 K in the CrTlSrCaCuO system was observed by resistance and ac susceptibility measurements. Powder X-ray diffraction analyses showed that a 1212-type phase, Cr-substituted TlSr2CaCu2O7, is responsible for the observed superconductivity. This is the first time that single-element substitution for TlSr2CaCu2O7 increases Tc of the 1212 phase to above 100 K. The Cr-substituted TlSr2CaCu2O7 is easily prepared and has very good quality suitable for practical applications.

A new superconducting 1201-type phase (Tl,Cr)Sr2CuO5

Abstract A new 1201-type phase (Tl,Cr)Sr2CuO5 has been synthesized and identified. This phase has a tetragonal unit cell (space group P4/mmm) with a = 3.795 A and c = 8.880 A , and is superconducting at 40–50 K. Cr in (Tl,Cr)Sr2CuO5 plays a similar role to Pb in (Tl,Pb)Sr2CuO5, which is instructive in the search for new Cr-based superconductors.

Self-pumped phase conjugation in the red in photorefractive Ba 0.5 Sr 1.5 K 0.25 Na 0.75 Nb 5 O 15 and Sr 0.6 Ba 0.4 Nb 2 O 6 with cerium in 9-fold coordinated sites

Self-pumped phase conjugation has been observed at selected laser wavelengths over the range 647–780 nm. Phase-conjugate reflectivities as great as 20% were measured for power levels ranging from 2 to 100 mW. In addition, phase-conjugate and beam-fanning response times were also measured. Our observations were carried out in crystals belonging to the tungsten-bronze family; these crystals were doped with cerium at the 9-fold coordinated lattice sites to give a red photorefractive response.

Semiconducting TlSr2RCu2O7 (R=rare earth) and its superconducting derivatives

Semiconducting TlSr2RCu2O7 (R=Pr or Er) with a 1212-type structure has been synthesized in the single-phase form. Partial substitution of Sr2+ for R3+ converts this semiconductor to a 90 K superconductor TlSr2(R1−ySry)Cu2O7. A combination substitution, Sr2+ for R3+ and Pb4+ for Tl3+, leads to the Ca-free 100 K superconductor (Tl, Pb)Sr2(R, Sr)Cu2O7. The results are explained in the framework of the mixed Cu2+/Cu3+ valence.

Optimum preparation and elemental addition for Tl-based 2223 phase Tl2Ba2Ca2Cu3O10−δ

Abstract TlBaCaCuO samples with 95%-pure 2223 phase Tl 2 Ba 2 Ca 2 Cu 3 O 10−gd and 120 K T c can be prepared in a non-closed system from component oxides with a starting composition of 2223 by heating at 895°C for about 48 h in flowing oxygen. With this procedure, elemental addition at 0.2 molar level maintains 2223 phase purity of 75–95% and a T c of 112–120 K for most of the investigated elements (Na, K, Rb, Ti, Zr, Hf, V, Nb, Ta, Mo, W, Zn, Cd, Hg, Se, and Te).

Derivatives of TlSr2CaCu2O7 from elemental substitutions of Pb for Tl, La for Sr, and Y for Ca

Abstract Tl(Sr,La) 2 CaCu 2 O 7 , Tl(Sr,La) 2 (Ca,Y)Cu 2 O 7 , and (Tl,Pb)(Sr,La) 2 (Ca,Y)Cu 2 O 7 were synthesized and characterized. Thus, all derivatives of the 1212-type phase TlSr 2 CaCu 2 O 7 from elemental substitutions, Pb for Tl, La for Sr, or/and Y for Ca, have been synthesized. La and Y play a similar role in the substitutions. Pb is necessary to increase the Tc from 90 K to above 100 K in the double substitutions while it plays a role similar to La and Y in the single substitution. The triple substitution of Pb, La, and Y does not increase Tc to even higher. The optimal average Cu valence is 2.20 ± 0.05, the same for all derivatives.

New thallium-lead based layered cuprates and their superconductivity

A new series of 1222-type (Tl,Pb)-based layered cuprate compounds (Tl,Pb)Sr2(Nd,Ce)2Cu2Oz have been successfully synthesized in the pure form and identified by powder X-ray diffraction analyses. The structure of the compounds is directly related to that of Nb-1222 NbSr2(Nd,Ce)2Cu2Oz with tetragonal body-center lattice. The lattice parameters are a = 3.855 Ac = 29.47 A for (Tl0.7Pb0.3)Sr2(Nd0.5Ce0.5)2Cu2Oz, and a = 3.853 A, c = 29.61 A for (Tl0.7Pb0.3)Sr2(Nd0.75Ce0.25)2Cu2Oz. Resistance measurements showed that (Tl1−xPbx)Sr2(Nd0.75Ce0.25)2Cu2Oz (x ≤ 0.6) prepared by the typical procedure exhibit superconductivity with Tc(onset) in the range 30–40 K.

Formation and superconductivity of 1212-type phase TlSr2(Sr0.5R0.5)Cu2O7-δ with R=Sc, Y and lanthanides

Abstract Nominal samples TlSr 2 (Sr 0.5 R 0.5 )Cu 2 O x with R = Sc , Y and lanthanides, have been prepared, and investigated by X-ray diffraction analysis and resistance measurement. The sample with R = Y , Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu consists mainly of a tetragonal 1212-type phase TlSr 2 (Sr 0.5 R 0.5 )Cu 2 O 7- γ , and exhibits a superconducting transition at about 90 K. The sample with R = Ce forms a very pure 1212 phase, but is a semiconductor. The sample with R = Sc forms essentially a 1201 phase, and is a semiconductor. The sample of R = La consists of 1201 phase as its major phase and a 1212 phase as its minor phase, and shows two-step superconducting transitions at 40 K and 90 K. The results are discussed from the point of view of the average valence of Cu and the ionic size of the rare earth.

Phase transformation during low temperature Tl-diffusion in the fabrication of Tl-Ba-Ca-Cu-O thin films

Abstract In the two-step fabrication of Tl-Ba-Ca-Cu-O thin films, Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl-2223) and Tl 1 Ba 2 Ca 2 Cu 3 O 9 (Tl-1223) films with good superconducting properties can be produced at a relatively low Tl-diffusion temperature. Our study shows that although a low annealing temperature generally favors the formation of Tl 2 Ba 2 Ca 1 Cu 2 O 7 (Tl-2212) superconducting film, Tl-2223 and Tl-1223 phases can still form by prolonging the annealing time. In fact, a prolonged annealing time leads to the transformation from Tl-2212 phase to Tl-2223 phase, and then to Tl-1223 phase. Therefore, the annealing time must be carefully controlled to get the needed superconducting phase.

New high‐Tc cuprate (Tl1−xCrx) (Sr2−yBay)Ca2Cu3O9

A new series of 1223‐type (Tl,Cr)‐based cuprates, (Tl,Cr)(Sr,Ba)2Ca2Cu3Oz, have been successfully synthesized and identified by powder x‐ray diffraction and electron diffraction analyses. Ba partial substitution for Sr promotes the formation of (Tl,Cr)‐based 1223‐type compounds. Nominal samples, (Tl1−xCrx)‐(Sr2−yBay) Ca2Cu3Oz, with 0.15≤x≤0.50 and 0.50≤y≤1.50 are pure or nearly pure 1223 phase and exhibit Tc(ρ=0) in the range of 104–114 K. This new (Tl,Cr)‐based high‐Tc material may be of importance in practical applications.