Crystal structure and antiferromagnetic spin ordering of $Ln\mathrm{F}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3(Ln=\mathrm{Nd},\mathrm{Pr},\mathrm{Ce},\mathrm{La})$ perovskites

Stoichiometric polycrystalline samples of $Ln\mathrm{F}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3(Ln=\mathrm{Nd},\mathrm{Pr},\mathrm{Ce},\mathrm{La})$ have been prepared by solid-state reaction and studied by means of x-ray and neutron powder diffraction as well as Mössbauer spectroscopy and magnetic measurements. All samples were found to be of single phase and to have Pnma symmetry with valence state +3 of Fe and Mo. It is demonstrated that the B-site cations of $Ln\mathrm{F}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3$ in accord with $Ln\mathrm{Fe}{\mathrm{O}}_3$ order in a G-type antiferromagnetic structure with the magnetic moments aligned along the $b$ axis. However, with significantly lower Néel temperatures than their $Ln\mathrm{Fe}{\mathrm{O}}_3$ parent compounds. The Fe-O-Fe bond lengths and bond angles and thus the magnitude of the antiferromagnetic superexchange interaction are found to systematically change with the ionic radius of Ln such that $T_N$ increases with increasing radius. Only the $\mathrm{CeF}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3$ compound experiences a low temperature spin reorientation from alignment along the $b$ axis to the $a$ axis.

Figure 1

Rietveld refinement of XRPD data for $Ln\mathrm{F}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3$ perovskites $(Ln=\mathrm{Nd}$ (a), Pr(b), Ce(c), La(d)). The upper traces illustrate the observed data as dots, and the calculated pattern as solid line; the lower solid trace is a plot of the difference, observed intensity minus the calculated one. The tick marks show positions calculated for Bragg reflections.

Figure 2

(left panels) Field-cooled (FC, in red (gray)) and zero-field-cooled (ZFC, in black) magnetization curves recorded under a magnetic field $\mathrm{H}=1\mathrm{kOe}$ and (right panels) magnetic field dependence of the magnetization at $\mathrm{T}=5\mathrm{K}$ for the different perovskites $Ln\mathrm{F}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3$ with $Ln=\mathrm{Nd}(\mathrm{a},\mathrm{e}),\mathrm{Pr}(\mathrm{b},\mathrm{f}),\mathrm{Ce}(\mathrm{c},\mathrm{g}),\mathrm{La}(\mathrm{d},\mathrm{h})$.

Figure 3

(a) Field-cooled and zero-field-cooled magnetization curves recorded under a magnetic field $\mathrm{H}=1\mathrm{kOe}$ for all samples. (b) Magnetic field dependence of the magnetization for $\mathrm{CeF}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3$ at $\mathrm{T}=5,100$ and $300\mathrm{K}$.

Figure 4

Mössbauer spectrum of $\mathrm{PrF}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3$ recorded at room temperature. The dots are the measured spectrum, the thin lines are the fitted three sextets and one singlet. The stronger line is the total sum of the fitted functions. See main text for more information about model considered.

Figure 5

Thermal evolution of the NPD powder patterns for $Ln\mathrm{F}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3$ perovskites: $(Ln=\mathrm{Nd}$ (a), Pr (b), Ce (c), La (d)).

Figure 6

Observed (dots), calculated (full line), and difference (bottom) NPD Rietveld profiles for $\mathrm{CeF}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3$ at different temperatures: (a) 600 K, (b) 450 K, (c) 300 K, (d) 4 K. The series of tick marks correspond to the allowed Bragg reflections for the nuclear (upper set of tick marks) and magnetic (lower set of tick marks) contributions.

Figure 7

Comparison of the long angle reflections of the NPD diffractograms of $\mathrm{CeF}{\mathrm{e}}_{2/3}\mathrm{M}{\mathrm{o}}_{1/3}{\mathrm{O}}_3$ at 300 K and 4 K; the corresponding crystal and magnetic structures are sketched beside each set of data. Fe/Mo spins locate at the center of the octahedron. The arrows show the different direction of Fe/Mo magnetic moments.

Figure 8

Variation of ${\mathrm{T}}_{\mathrm{N}}$ of pure and Mo-doped samples with (right) the Fe–O–Fe bond angles and (left) tilting angles Θ and Φ estimated at room temperature. Filled symbols represent Mo-doped samples, using crystallographic data extracted from the present NPD data measured at 295 K, while open symbols represent data extracted from XRPD data at room-temperature from the literature [18, 19] on the undoped compounds. No data was found for pure Ce compound. Linear fits of each dataset are included, as guides to the eye.