Structure and magnetism in BaLaMnO 4 +/− δ ( δ = 0.00, 0.10) and Ba x Sr 1 − x LaMnO 4 . Disappearance of magnetic order for x > 0.30.

Polycrystalline samples of BaLaMnO4, oxygen deficient and oxygen excess BaLaMnO4 ± δ and the solid solution BaxSr1 − xLaMnO4 have been prepared and investigated. Magnetic susceptibility, powder X-ray diffraction and variable temperature powder neutron diffraction data are presented for these compounds. BaLaMnO4 and BaLaMnO3.89 crystallize in space group I4/mmm with unit cell constants a = 3.90252(6) Å, c = 13.2660(3) Å and a = 3.9089(1) Å, c = 13.3243(5) Å. The unit cell constants for the tetragonal BaLaMnO4.1 are a = 3.9314(2) Å, c = 13.051(1) Å. For the δ = 0.00 compound a small ferromagnetic response appears in the susceptibility below 300 K and the Curie–Weiss θ = +5 K but no evidence for long range magnetic order of any type is seen above 6 K in neutron diffraction data. The ferromagnetic response is supressed in the reduced form, BaLaMnO3.89, but is enhanced in the oxidized form, BaLaMnO4.1, with the ferromagnetic onset moving to 350 K. The solid solution BaxSr1 − xLaMnO4 (0.00 ≤ x ≤ 0.34) also crystallizes in space group I4/mmm. We find antiferromagnetic long range ordering for SrLaMnO4 (x = 0.0), ordering vector k = [1/2 1/2 0], with TN = 128 K and a Mn3+ moment of 3.3 μB, contrary to a previous report. The Nèel temperatures and ordered moment magnitudes decrease as the Ba concentration is increased from x = 0.00 to x = 0.29. For x > 0.29 all signs of antiferromagnetic long range order vanish. An explanation for the destruction of three-dimensional (3D) magnetic long range ordering is proposed based on the distance dependence of the 2D in-plane correlation length, the interplanar exchange coupling and competing interactions.