Influence of dopant concentration on the structure and physical properties of Nd_1-xPb_xMnO₃ single crystals

Abstract

The structure of Nd_1-xPbxMnO₃ crystal for x=0.25 is determined at room temperature by single-crystal X-ray diffraction. The structural refinement reveals that the crystal is tetragonal with space group P4/mmm, Z=4 and R of 8.3%. The lattice parameters are a=7.7652(1) Å, c=3.884(1) Å and α =β =γ =90°. The structural analysis is then extended to x=0.38. It is noticed that substitution of Pb at the Nd site results in structural phase change from tetragonal (x=0.25) to cubic (x=0.38). These changes are attributed to the progressive removal of inter-octahedral tilting and minimization of the octahedral distortion leading to a higher symmetry as doping concentration increases. While the unit cell volume of tetragonal structure (P4/mmm) is comparable to that of parent NdMnO₃ (Pnma), the volume of cubic unit cell (Pm3^-m) is doubled. Electron diffraction patterns support these results and rule out the possibility of twinning. Changes in transport properties as a function of temperature at different doping levels are in accordance with the observed structural changes. It is observed that T_θ and T_MI increase with x. Packing diagram of Nd_0.75Pb_0.25MnO_2.76 along b-direction for tetragonal structure.