Influence of the Bi3+ electron lone pair in the evolution of the crystal and magnetic structure of La(1-x)Bi(x)Mn2O5 oxides.

La(1-x)Bi(x)Mn2O5 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1) oxides are members of the RMn2O5 family. The entire series has been prepared in polycrystalline form by a citrate technique. The evolution of their magnetic and crystallographic structures has been investigated by neutron powder diffraction (NPD) and magnetization measurements. All the samples crystallize in an orthorhombic structure with space group Pbam containing infinite chains of Mn(4+)O6 octahedra sharing edges, linked together by Mn(3+)O5 pyramids and (La/Bi)O8 units. These units become strongly distorted as the amount of Bi increases, due to the electron lone pair of Bi(3+). All the members of the series are magnetically ordered below TN = 25-40 K and they present different magnetic structures. For the samples with low Bi content (x = 0.2 and 0.4) the magnetic structure is characterized by the propagation vector k = (0,0,1/2). The magnetic moments of the Mn(4+) ions placed at octahedral sites are ordered according to the basis vectors (Gx, Ay, 0) whereas the Mn(3+) moments, located at pyramidal sites, are ordered according to the basis vectors (0, 0, Cz). When the content of Bi increases, two different propagation vectors are needed to explain the magnetic structure: k1 = (0,0,1/2) and k2 = (1/2,0,1/2). For x = 0.6 and 0.8, k2 is predominant over k1 and for this propagation vector (k2) the magnetic arrangement is defined by the basis vectors (Gx, Ay,0) and (Fx, Cy, 0) for Mn(4+) and Mn(3+) ions, respectively.