Control of magnetic ordering by Jahn-Teller distortions in Nd2GaMnO6 and La2GaMnO6

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+ cations are disordered over the six coordinate sites. La2GaMnO6 is a ferromagnetic 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 displacement parameter data suggest Jahn-Teller distortions which are both coherent and incoherent with the Pnma space group symmetry of the perovskite structure (a = 5.51122(4) Angstrom, b = 7.80515(6) Angstrom, c = 5.52947(4) Angstrom) at room temperature. The coherent distortion is strongly suppressed in comparison with the parent LaMnO3 phase, but the displacement ellipsoids suggest that incoherent distortions are significant and arise from local Jahn-Teller distortions. The preparation of the new phase Nd2GaMnO6 has been found to depend on sample cooling rates, with detailed characterization necessary to ensure phase separation has bt en avoided. This compound also adopts the GdFeO3-type orthorhombically distorted perovskite structure (space group 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 totally different effect on the Jahn-Teller distortion at the Mn3+ centers. This phase exhibits a Q(2) mode Jahn-Teller distortion similar to that observed in LaMnO3, although reduced in magnitude as a result of the introduction of Ga3+ onto the B site. There is no evidence of a dynamic Jahn-Teller distortion. 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 ordered in the xy plane making an angle of 29(2)degrees with the y-axis. The Mn3+ moments couple ferromagnetically in the xz plane. However, along the y-axis the moments couple ferromagnetically while the x components are coupled antiferromagnetically. This results in a canted antiferromagnetic arrangement in which the dominant exchange is ferromagnetic. Nd2GaMnO6 is paramagnetic 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) per Nd2GaMnO6 formula unit was measured by magnetometry at 5 K in an applied magnetic field of 5 T.