Bin Okai

Pressure Dependence of the Structural Phase Transition Temperature in SrTiO 3 and KMnF 3

The pressure dependence of the structural phase transition temperature, T a , in SrTiO 3 has been measured up to 30 kbar. The initial slope of 1.8 deg/kbar by Sorge et al. (1970) has been reaffirmed contrary to the prediction by Fossheim and Berre (1972): d T a /d p =0.5 deg/kbar. Above 8.5 kbar the transition temperature shifts nonlinearly with increasing pressure. As for the cubic-tetragonal transition temperature of KMnF 3 the measurements to 10 kbar show a linear shift: d T a /d p =3.9 deg/kbar. Above 10 kbar a deviation from linearity seems to set in.

Preparation and Physical Properties of PrBa2Cu3Oy

A new compound, PrBa2Cu3Oy, which is isomorphic with YBa2Cu3Oy, was prepared as single phase. With specimens quenched from 920°C, the oxygen content y was 6.8 ± 0.1 and the ratio of Pr3+ to Pr4+ was 0.4:0.6. With furnace-cooled specimens, the value of y was 7.1 ± 0.1 and the ratio became 0.1:0.9. The average valence of Cu ions was 2.0 ± 0.1 for the former and 2.1 ± 0.1 for the latter. Specimens handled both ways were nonmetallic and nonsuperconducting down to 4.2 K.

Preferred Orientation of CsCl Type RbI in the Change NaCl→CsCl Type

Under a uniform hydrostatic pressure of about 5 kbar, RbI single crystal undergoes a phase transition from NaCl type to CsCl type polycrystal having preferred orientations. The orientation relation is approximately as follows: [111](CsCl)//[100](NaCl) and [100](CsCl)//[111](NaCl). This orientation relation is different from other types as observed for temperature-induced transition of CsCl or ammonium halides.

Crystal Structure and Tc of YBa2Cu3Oy (y>7)

At a high oxygen pressure of 6 GPa, YBa2Cu3Oy was heat-treated. With an increasing supply of oxygen, the crystal structure of the processed specimens gradually changed from orthorhombic (y=7) to tetragonal (y=7.7), whereas the onset of superconducting transition temperature consistently remained at 92 K for all of the specimens. The distribution of charges among the copper-oxygen chains and the CuO2 sheets in YBa2Cu3Oy (y>7) are discussed.

Crystallographic Study in the Nd2CuO4-Sr2CuO3 System

A crystallographic study was done in the Nd2CuO4-Sr2CuO3 system. Five phases were found in this system; A: Nd2CuO4, B: Nd2SrCu2O6, C: (Nd0.45Sr0.55)3Cu2O6-y, D: (Nd1-xSrx)2CuO4-y, and E: Sr2CuO3. Phases B and C possess Sr3Ti2O7-type structures, and phase D has the same structure as La2CuO4. No superconductivity was observed in any of the above-mentioned phases down to 4.2 K.

Preferred Orientation of B2-Type Rb and K Halides in the Change B1 →B2-Type

Rubidium and potassium halides (RbX and KX; X=F, Cl, Br and I except for RbF) retain preferred orientation of (100)(B2)//(111)(B1) and (111)/(B2)//(100)(B1) at the phase transition from B1 to B2-type structure under pressure. Geometry is shown for the movements of lattice points which are compatible with the orientation relation.

High-oxygen-pressure synthesis of superconducting tetragonal YBa2Cu3O7.7

A tetragonal form of YBa2Cu3Oy whose lattice parameters were a=3.8715 A and c=11.6506 A was prepared at a high oxygen pressure of 6 GPa and 1050°C. Using the value of oxygen content determined by oxygen analysis measurement, the tetragonal YBCO was expressed as YBa2Cu3O7.7. If the oxygen is assumed to be divalent as usual, the average valence of Cu becomes 2.8. The onset of the superconducting transition temperature was 92 K, nearly the same as usually obtained with YBCO; however, the volume fraction of superconductivity was a third of the usual YBCO.

Preparation and crystal structures of some complex sulphides at high pressures

The NH4CdCl3 structure is a high-pressure form for some complex sulphides. Sr(Ti1−xZrx)S3, a mixture of hexagonal SrTiS3 (BaNiO3 structure) and orthorhombic SrZrS3 (GdFeO3 structure) at the ambient pressure, transforms to the orthorhombic NH4CdCl3 structure by heating at 1000°C under a pressure of 60 kbar. The NH4CdCl3 structure is also a high-pressure form of BaSnS3 and SrSnS3. In Ba(Ti1−xZrx)S3 having the same structural form as Sr(Ti1−xZrx)S3 at 0 kbar, only Ba(Ti0.25Zr0.75)S3 transforms to a tetragonal perovskite structure at 900°C and 60 kbar. The volume change in Sr(Ti1−xZrx)S3 from the state at 0 kbar to the NH4CdCl3 structure is much larger than that of Ba(Ti0.25Zr0.75)S3 from 0 to 60 kbar form, suggesting that Ba(Ti1−xZrx)S3 will also posess the NH4CdCl3 structure at higher pressures.

Enhancement of Jc in Y1-xCa1.1xBa2-0.1xCu3Oy

With specimens of Y1-xCa1.1xBa2-0.1xCu3Oy prepared under the same conditions, Y0.95Ca0.055Ba1.995Cu3Oy possessed a larger superconducting critical current density Jc than the nondoped YBa2Cu3Oy. The Jc of the former composition was 275 A/cm2 at 77 K, whereas it was 145 A/cm2 with the latter stoichiometric composition.

Effect of Pressure on Superconducting Transition Temperatures in the Bi–Sr–Ca–Cu–O System

Superconducting transition temperatures were measured under pressure for the Bi-Sr-Ca-Cu-O system. The transition temperature near 80 K (Tc1) slightly increases with pressure: dTc1/dp=0.06 K/GPa (onset), while Tc near 110 K (Tc2) decreases with pressure: dTc2/dp=-0.2 K/GPa (onset) and -0.3 K/GPa (complete). The negative pressure-dependence was discussed with regard to the resonating-valence-bond theory.