B-Site-Ordered and Disordered Structures in A-Site-Ordered Quadruple Perovskites RMn3Ni2Mn2O12 with R = Nd, Sm, Gd, and Dy

Abstract

ABO₃ perovskite materials with small cations at the A site, especially with ordered cation arrangements, have attracted a lot of interest because they show unusual physical properties and deviations from general perovskite tendencies. In this work, A-site-ordered quadruple perovskites, RMn₃Ni₂Mn₂O₁₂ with R = Nd, Sm, Gd, and Dy, were synthesized by a high-pressure, high-temperature method at about 6 GPa. Annealing at about 1500 K produced samples with additional (partial) B-site ordering of Ni²⁺ and Mn⁴⁺ cations, crystallizing in space group Pn-3. Annealing at about 1700 K produced samples with disordering of Ni²⁺ and Mn⁴⁺ cations, crystallizing in space group Im-3. However, magnetic properties were nearly identical for the Pn-3 and Im-3 modifications in comparison with ferromagnetic double perovskites R₂NiMnO₆, where the degree of Ni²⁺ and Mn⁴⁺ ordering has significant effects on magnetic properties. In RMn₃Ni₂Mn₂O₁₂, one magnetic transition was found at 26 K (for R = Nd), 23 K (for R = Sm), and 22 K (for R = Gd), and two transitions were found at 10 K and 36 K for R = Dy. Curie-Weiss temperatures were close to zero in all compounds, suggesting that antiferromagnetic and ferromagnetic interactions are of the same magnitude.

Keywords: A-site-ordered quadruple perovskites; B-site double ordering; crystal structures; structural disorder.

Figure 1

Experimental (black crosses), calculated (red line), and difference (blue line at the bottom) room-temperature synchrotron X-ray powder diffraction patterns of NdMn₃Ni₂Mn₂O₁₂ (in the Pn–3 modification, prepared at 1500 K) in a 2θ range of 6° and 59°. The tick marks show possible Bragg reflection positions for the main phase and NiO impurity (from top to bottom). Inset shows a zoomed part in a 2θ range of 16° and 17.8° and emphasizes the presence of the (311) reflection from the B-site ordering. Inset shows a scanning electron microscopy (SEM) image, where the scale bar is 20 µm.

Figure 2

Experimental (black crosses), calculated (red line), and difference (blue line at the bottom) room-temperature synchrotron X-ray powder diffraction patterns of NdMn₃Ni₂Mn₂O₁₂ (in the Im–3 modification, prepared at 1700 K) in a 2θ range of 6° and 59°. The tick marks show possible Bragg reflection positions for the main phase and NiO impurity. Inset shows a zoomed part in a 2θ range of 16° and 17.9° and emphasizes the absence of the (311) reflection and the absence of B-site ordering.

Figure 3

(a) The room-temperature cubic lattice parameter in RMn₃Ni₂Mn₂O₁₂ (R = La [28], Nd, Sm, Gd, and Dy) as a function of the ionic radius R³⁺ (for the coordination number 8 as ionic radii for the coordination number XII are not available for small R³⁺ cations (R = Gd and Dy) [30]). NPD: from neutron powder diffraction. XRD: from X-ray powder diffraction. (b) R–O bond length (the left-hand axis) and bond-valence sum for R³⁺ (the right-hand axis) in RMn₃Ni₂Mn₂O₁₂ (R = La [28], Nd, Sm, Gd, and Dy) as a function of the ionic radius R³⁺.

Figure 4

(a) ZFC (filled symbols) and FCC (empty symbols) dc magnetic susceptibility curves (χ = M/H) of two modifications of NdMn₃Ni₂Mn₂O₁₂ (the Pn–3 modification, prepared at 1500 K, and the Im–3 modification, prepared at 1700 K) measured at H = 10 kOe. The inset shows the dχT/dT versus T curves (all). (b) ZFC and FCC curves of two modifications of NdMn₃Ni₂Mn₂O₁₂ measured at H = 100 Oe. The inset shows the FCC dχT/dT versus T curves.

Figure 5

(a) ZFC (filled symbols) and FCC (empty symbols) dc magnetic susceptibility curves (χ = M/H) of two modifications of SmMn₃Ni₂Mn₂O₁₂ (the Pn–3 modification, prepared at 1500 K, and the Im–3 modification, prepared at 1700 K) measured at H = 10 kOe. The inset shows FCC dχT/dT versus T curves. (b) ZFC and FCC curves of two modifications of SmMn₃Ni₂Mn₂O₁₂ measured at H = 100 Oe.

Figure 6

ZFC (filled symbols) and FCC (empty symbols) dc magnetic susceptibility curves (χ = M/H) of GdMn₃Ni₂Mn₂O₁₂ (the Pn–3 modification, prepared at 1500 K) measured at H = 10 kOe. The first inset shows ZFC and FCC curves of GdMn₃Ni₂Mn₂O₁₂ measured at H = 100 Oe. The second inset shows the FCC dχ/dT versus T curves.

Figure 7

ZFC (filled symbols) and FCC (empty symbols) dc magnetic susceptibility curves (χ = M/H) of DyMn₃Ni₂Mn₂O₁₂ (the Pn–3 modification, prepared at 1500 K) measured at H = 10 kOe (the left-hand axis). The right-hand axis shows the FCC χ⁻¹ versus T curve with the Curie–Weiss fit (black line). The fitting parameters are given in the figure. The inset shows dχ/dT versus T curves.

Figure 8

M versus H curves of two modifications of RMn₃Ni₂Mn₂O₁₂ (the Pn–3 modification, prepared at 1500 K, and the Im–3 modification, prepared at 1700 K) measured at T = 5 K with (a) R = Nd and (b) R = Sm. The insets show zoomed parts near the origin.

Figure 9

M versus H curves of GdMn₃Ni₂Mn₂O₁₂ and DyMn₃Ni₂Mn₂O₁₂ (the Pn–3 modification, prepared at 1500 K) measured at T = 5 K. The inset shows zoomed parts near the origin.

Figure 10

C_p/T versus T curves of RMn₃Ni₂Mn₂O₁₂ measured at H = 0 (black curves) and 90 kOe (red curves) for (a) R = Nd (the Pn–3 modification), (b) R = Sm (the Pn–3 modification and the Im–3 modification (blue and brown curves)), (c) R = Gd (the Pn–3 modification), and (d) R = Dy (the Pn–3 modification). Arrows show magnetic transition temperatures. Data below 100 K are shown; inset on panel (a) shows full data up to 270 K (at H = 0 Oe).

Figure 11

(a) Real χ′ versus T and (b) imaginary χ″ versus T curves of NdMn₃Ni₂Mn₂O₁₂ (the Pn–3 modification) at different frequencies (f). Inset shows the χ′ versus T curves at different H_ac (H_ac = 0.05, 0.5, and 5 Oe) and one frequency (f = 300 Hz).

Figure 12

Temperature dependence of (a) dielectric constant and (b) loss tangent at different frequencies (f: indicated on the figure) in NdMn₃Ni₂Mn₂O₁₂ (the Pn–3 modification) at H = 0 Oe. Inset shows frequency dependence of peak positions on loss tangent as T_max versus log(f) (black circles with line) and 1000/T_max versus log(f) (red squares with line).