Control of magnetic ordering by Jahn-Teller distortions in Nd2CaMnO6 and La2CaMnO6

The substitution of Ga3+ into the Jahn-Teller distorted, antiferromagnetic perovskites LaMnO3 and NdMnO3 strongly affects both the crystal structures and resulting magnetic ordering. In both compounds the Ga3+ and Mn3+ cation s are disordered over the six coordinate sites. La2GaMnO6 is a ferromagneti c insulator (T-c = 70 K); a moment per Mn cation of 2.08(5) mu (B) has been determined by neutron powder diffraction at 5 K. Bond length and displacem ent parameter data suggest Jahn-Teller distortions which are both coherent and incoherent with the Pnma space group symmetry of the perovskite structu re (a = 5.51122(4) Angstrom, b = 7.80515(6) Angstrom, c = 5.52947(4) Angstr om) at room temperature. The coherent distortion is strongly suppressed in comparison with the parent LaMnO3 phase, but the displacement ellipsoids su ggest that incoherent distortions are significant and arise from local Jahn -Teller distortions. The preparation of the new phase Nd2GaMnO6 has been fo und to depend on sample cooling rates, with detailed characterization neces sary to ensure phase separation has bt en avoided. This compound also adopt s the GdFeO3-type orthorhombically distorted perovskite structure (space gr oup Pnma, alpha = 5.64876(1) Angstrom, b = 7.65212(2) Angstrom, c = 5.41943 (1) Angstrom at room temperature). However, the B site substitution has a t otally different effect on the Jahn-Teller distortion at the Mn3+ centers. This phase exhibits a Q(2) mode Jahn-Teller distortion similar to that obse rved in LaMnO3, although reduced in magnitude as a result of the introducti on of Ga3+ onto the B site. There is no evidence of a dynamic Jahn-Teller d istortion. At 5 K a ferromagnetically ordered Nd3+ moment of 1.06(6) mu (B) is aligned along the y-axis and a moment of 2.8(1) mu (B) per Mn3+ is orde red in the xy plane making an angle of 29(2)degrees with the y-axis. The Mn 3+ moments couple ferromagnetically in the xz plane. However, along the y-a xis the moments couple ferromagnetically while the x components are coupled antiferromagnetically. This results in a canted antiferromagnetic arrangem ent in which the dominant exchange is ferromagnetic. Nd2GaMnO6 is paramagne tic above 40(5) K, with a paramagnetic moment and Weiss constant of 6.70(2) mu (B) and 45.9(4) K, respectively. An ordered moment of 6.08(3) mu (B) pe r Nd2GaMnO6 formula unit was measured by magnetometry at 5 K in an applied magnetic field of 5 T.