Structural instability under pressure in the HTSL (Nd, Gd, Ce)2CuO4

The partial substitution of Nd by Gd and Ce on the structural properties of Nd2CuO4 n-type superconductor was studied by using X-ray diffraction measurements. The maximum value of critical transition temperature T-c was obtained in Gd doped (Nd2-x-yCex Gd-y)CuO4 for x=0.15 Ce. The structural distortion of the T'-phase was investigated by X-ray diffraction measurements function of temperature for samples with high Gd content. Under normal-pressure, for small anomalies of the lattice parameter c, especially the deviation of the c/a-relationship, a more continuously structural transition was evidenced as a function of Gd concentration and temperature. The influence of pressure and Gd concentration on the lattice parameters and unit cell volume was also studied at room temperature in the optimal doped sample (x=0.15 Ce). In this case a higher pressure stabilizes the smallest undistorted phase.