OXYGEN NONSTOICHIOMETRY AND PHASE-TRANSITIONS OF THE NEODYMIUM-RICH ND1-XCU3OZ SOLID-SOLUTION(XBA2)

On the basis of chemical, thermal analysis and Cu K-edge X-ray absorpt ion measurements, oxygen content in the Nd1 + xBa2 - xCu3Oz solid solu tion was determined between 1000 degrees C in air and 400 degrees C in oxygen for x = 0.05-0.9 compositions. It has been observed that the o xygen nonstoichiometry Delta z of the Nd1 + xBa2 - xCu3O7 + x/2 -Delta z solid solution decreases 2-2.5 times for a large substitution (Delt a z = 0.3-0.33 for x = 0.9), despite of the acclaimed higher total oxy gen content. The difference in nonstoichiometry is explained by a high er average value of the copper oxidation state (ACV), which is vital f or the solid solution with large x even at elevated temperatures (ACV approximate to 2-2.05 for x > 0.3 at 1000 degrees C, pO(2) = 0.21 atm) . On the contrary, the ACV after complete oxygenation is almost consta nt (about 2.25-2.3) for the whole series. The x-dependence of the oxyg en content is not monotonous and structural phase transitions can be o bserved at x = 0.3 and x = 0.6, as confirmed by the X-ray diffraction and the Raman scattering spectroscopy. The first well-known transition is connected with the oxygen disorder due to the Nd substitution for Ba at random Ba-sites. In the present work, it is proved by the apical oxygen mode broadening in Raman spectra. Ordering of the Nd and Ba at oms with a subsequent orthorhombic distortion of the lattice may occur even at 1000 degrees C in air due to the second transformation at x = 0.6. The invariable orthorhombicity of the Nd-rich solid solution wit h x > 0.6 is not caused by the oxygen absorption as in the x = 0.05 ca se. Existence of high- and low-temperature orthorhombic modifications of this solid solution has been observed for the first time. Finally, a tentative 3D (z-x-T) diagram is suggested for the Nd1 + xBa2 - xCu3O z solid solution up to 1000 degrees C in air, including the new x = 0. 6-0.9 region. (C) 1998 Elsevier Science B.V. All rights reserved.