Abstract

Polycrystalline samples of BaLaMnO₄, oxygen deficient and oxygen excess BaLaMnO_{4 ± δ} and the solid solution Ba_xSr_{1 − x}LaMnO₄ 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 BaLaMnO_3.89 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 BaLaMnO_4.1 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, BaLaMnO_3.89, but is enhanced in the oxidized form, BaLaMnO_4.1, with the ferromagnetic onset moving to 350 K. The solid solution Ba_xSr_{1 − x}LaMnO₄ (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 T_N = 128 K and a Mn³⁺ moment of 3.3 μ_B, 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.