Influence of Ruthenium Doping on the Crystal Structure and Magnetic Properties of Pr_0.67Ba_0.33Mn_1–xRu_xO₃ Manganites

Abstract

This study reports the structural, morphological, and magnetic properties of ruthenium doping at the manganese site in Pr_0.67Ba_0.33MnO₃ manganites. Rietveld refinement X-ray powder diffraction (XRD) data show that Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ crystallize in an orthorhombic perovskite structure with the Pnma space group. Doping with Ru yields an increment in the lattice parameter and unit cell volume. In addition, small changes in the Mn–O–Mn bond angle and bond distance are observed. Field emission scanning electron microscopy (FESEM) is used to examine the surface morphology of the samples. Fourier transform infrared spectroscopy (FTIR) reveals that the Mn–O and metal–oxygen bonds appear at the 600 and 900 cm⁻¹ bands, respectively. The AC magnetic susceptibility measurement studies confirm that a paramagnetic (PM) to ferromagnetic (FM) transition exists at 130 and 153 K for the Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ samples, respectively.

1. Introduction

Doped perovskite manganite compounds with the general chemical formula A_1−xM_xMnO₃ (A = La, Pr, Nd, and M = Ca, Sr, Ba) are gaining increasing attention because the compounds in this family exhibit a colossal magnetoresistance (CMR) effect, metal-to-insulator transition, charge ordering, and phase separation, as well as a remarkable variety of structural, physical, and magnetic properties [1,2,3]. The compounds exhibit a range of structural, electronic, and magnetic phase transitions by substituting different ions at the A or Mn sites. Doped Pr-based manganite is one of the existing CMRs which have interesting structural and physical properties that are correlated with the coupling within the spin phonon interaction, lattice, and orbital degrees of freedom between Mn³⁺ and Mn⁴⁺. A strong spin phonon interaction arises from deformation of the MnO₆ octahedron, which plays an important role in the lattice, magnetic, and electrical behavior of these materials. Several studies on manganites have stated that the doping of other elements at the Mn site is vital to the resulting new exchange interactions between Mn and doped transition metal ions [4,5]. Partial substitution at the Mn site alters the local magnetic coupling between the magnetic moments of the substituents and the Mn ions. Most of the doping on the Mn site decreases with the transition temperature due to weakening of the double exchange interaction [6,7].

The structural and magnetic properties of the compound also depend on the type of ions and doping concentration. The Ru-doped manganite modifies the distance between Mn ions, as well as the Mn³⁺/Mn⁴⁺ ratio. Ru has an atomic radius of 0.62 Å, which is greater than that of the Mn ion (0.53 Å). In addition, Ru has the same valence as the host Mn⁴⁺ ion [8]. Vabitha et al. reported that Ru is a new potential candidate that induces a metal-to-insulator transition in Nd_0.5Ca_0.5MnO₃ compounds [9]. Another report suggested that half-doped manganites with Ru exhibit metal to insulator (M-I) transition within the ferromagnetic (FM) regions [10]. The substitution of Ru at the Mn site has demonstrated that double exchange (DE) was induced because the electronic configurations of Ru (IV) and Ru (V) are similar to those of Mn (III) and Mn (IV) [11]. Numerous works have reported the structure of these compounds. Manganite perovskites exhibit a variety of structural formations, such as orthorhombic and rhombohedral formations, depending on the substitution ions and preparation of the compounds [12]. However, unsystematic variations in the magnetic and structural properties of manganites were observed for doped Ru at Mn site compounds. Therefore, the effect of Ru substitution at the Mn site on modifications of the structural and magnetic behaviors of Pr_0.67Ba_0.33MnO₃ should be investigated. This work presents the results of investigations into synthesized crystalline samples of Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ by using the solid-state method and an analysis of the influence of doping Ru substitution at the Mn site on the crystal structural and magnetic properties. To the best of the author’s knowledge, Ru substitution at the Mn site for Pr_0.67Ba_0.33Mn_1-xRu_xO₃ has not been reported.

2. Materials and Methods

Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ were synthesized through a solid-state reaction method. Stoichiometric amounts of high-purity (>99.9%) praseodymium oxide (Pr₆O₁₁), barium carbonate (BaCO₃), ruthenium oxide (RuO₂), and manganese oxide (Mn₂O₃) were mixed thoroughly using an agate mortar and pestle, ground for 2 h, and calcined in air at 850 °C for 24 h. The mixed oxide powders were reground and calcined at 900 °C for another 24 h. The final compounds were pressed into circular pellets with a thickness of about 2–3 mm under a pressure of 5 tons per cm². Subsequently, the pellets were sintered in air at 950 °C for 24 h. The phase identification and crystallinity of the Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ samples were determined through powder X-ray diffraction (XRD) at room temperature.

The structure of the samples was characterized using an X-ray diffractometer (PANalytical model X’pert PRO MPD with Cu-K_α radiation (λ = 1.5406 Å) at room temperature. Data were collected at a scattering angle range of 10° ≤ 2θ ≤ 90°. The data were collected with a step size of 0.017^o and a counting time of 18 s per step. Rietveld refinement and structural analysis were executed using the general structure analysis system (GSAS) program and graphical user interface (EXPGUI) [13,14], and were visualized using the visualization for electronic and structure analysis (VESTA) program [15]. The peak shape was modeled using the pseudo-Voight function, which was refined with the cell parameter, scale factor, zero factor, and background function.

The surface morphology was investigated using field emission scanning electron microscopy (FESEM) equipment, with the LEO Gemini model 982. The Fourier transform infrared spectroscopy (FTIR) results were recorded with FTIR-Raman Drift Nicolet 6700 equipment in the range of 400–2000 cm⁻¹, with a resolution of 1 cm⁻¹. The samples were thoroughly mixed with KBr before the FTIR characterization. Temperature-dependent AC magnetic susceptibility measurements were performed using a CryoBIND-T system, along with an SR830 lock-in amplifier and an oscillator at 240 Hz, within a temperature range of 30–300 K.

3. Results

3.1. Structural Analysis

Figure 1 displays the XRD patterns of Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ samples at room temperature (black and well-crystallized powder). Figure 1a presents a single phase with small impurity peaks (labeled as “*”), which was identified as Mn₂O₃, whereas Figure 1b shows small impurity peaks at 35° and 60°, which were identified as Mn₃O₄, as indicated by the (#) symbol in the XRD patterns. The appearance of a secondary phase in the Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ sample is an indication of the incomplete crystallization of the perovskite phase at a sintering temperature. The diffraction peaks correspond to (1 1 0), (2 0 0), (0 2 2), (2 2 0), (2 2 2), (3 1 2), (4 0 0), (3 1 4), and (3 3 2) hkl planes and match the previously reported data of Pr_0.67Ba_0.33MnO₃ [16]. The Rietveld analysis of the diffraction patterns (Figure 2) shows that the samples were comprised of a single phase with an orthorhombic Pnma space group. The space group was obtained from the International Crystal Structure Database (JCPDS file card no 01-072-0841) [17,18]. The atomic positions and refined lattice parameters are listed in

Table 1. Structural parameters for Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ obtained from Rietveld refinement analysis.

Atom	x	y	z	g	biso (å2)	Site
Mn	0.000000	0.000000	0.500000	0.9000	0.651	4b
Pr	0.022580	0.250000	0.995214	0.6700	0.270	4c
Ba	−0.011257	0.250000	1.006197	0.3300	0.214	4c
O1	0.490000	0.250000	0.060800	1.0000	0.531	4c
O2	0.275900	0.034400	0.723900	1.0000	0.540	8d
Ru	0.000000	0.000000	0.500000	0.1000	0.260	4b

and

Table 2. Lattice parameters, unit cell volume, and goodness of fit for Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ obtained from Rietveld refinement.

Parameters	Pr0.67Ba0.33MnO3	Pr0.67Ba0.33Mn0.9Ru0.1O3
Symmetry	Orthorhombic	Orthorhombic
Space Group	Pnma	Pnma
a (Å)	5.5216	5.5040
b (Å)	7.7720	7.7812
c (Å)	5.4971	5.5288
A = β = γ	90	90
Unit cell volume (V)	235.91	236.78
χ2	1.017	1.030
RF (%)	8.58	11.96
RP (%)	10.45	11.74
RWP (%)	14.22	15.07

, respectively. In the orthorhombic setting Pnma, Pr and Ba were fixed at the 4c site (x, 0.25, z), Mn and Ru were fixed at the 4b site (0, 0, 0.5), O₁ was fixed at the 4c site (x, 0.25, z), and O₂ was fixed at the 8d site (x, y, z). The unit cell volume slightly increased from 235.91 to 236.78 ų after the Ru doping. The increase in cell volume can be ascribed to the larger ionic radii of Ru⁴⁺ (0.62 Å) compared to that of Mn⁴⁺ (0.53 Å) [19,20]. The peak slightly shifted towards the low angle, signifying a slight increase in the lattice parameter and unit cell volume. The crystallite size D was calculated using the Scherrer equation for the full width at half maximum and integral breadth of reflection (110). The Scherer equation was used as presented below:

$$D=\frac{K\lambda }{\beta \left(\theta \right)\cos \theta },$$

(1)

where D is the crystallite size (nm); K is a constant with 0.94; λ is the wavelength of XRD, which is 0.1541 nm for CuK_α radiation Å; β is the full width at half maximum (FWHM), after subtracting the observed data and reference, in radians; and θ is the angle of the peak of XRD. It is clear that the average crystallite size values were found in the range of 63–73 nm.

Figure 3a displays the crystal structure of Pr_0.67Ba_0.33MnO₃, which shows an octahedral MnO₆ constructed with the VESTA software using several pieces of information, such as refined cell parameters, the space group, and the positional parameters of atoms. The A site in the ABO₃-type perovskite structure occupied by Pr/Ba cations was surrounded by 12 oxygen ions, as shown in Figure 3b. The octahedral MnO₆ formed by the position of Mn ions at the B site was surrounded by six oxygen ions. For the Pr_0.67Ba_0.33MnO₃ sample, the average bond distance between Mn and O was 1.9751 Å and between Ba/Pr and O was 2.7634 Å. These distances are slightly shorter than those obtained for the Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃, where the distance between Mn and O was 1.9777 Å and Ba/Pr and O was 2.7671 Å, respectively. The bond lengths and bond angles for the Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ are listed in

Table 3. Bond angles and bond lengths for Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ obtained from Rietveld refinement.

Distances	Pr0.67Ba0.33MnO3	Pr0.67Ba0.33Mn0.9Ru0.1O3
2 × Mn-O1 (Å)	1.9723	1.9749
2 × Mn-O2 (Å)	1.9767	1.9774
2 × Mn-O2 (Å)	1.9764	1.9807
<Mn-O>(Å)	1.9751	1.9777
Mn-O1-Mn (˚)	158.6	158.5
Mn-O2-Mn (˚)	157.5	159.2
<Mn-O-Mn> (˚)	158.1	158.8
(Ba/Pr1)-O1 (Å)	2.9124	2.8976
(Ba/Pr1)-O1 (Å)	2.6037	2.6496
(Ba/Pr1)-O1 (Å)	3.0416	3.0944
(Ba/Pr1)-O1 (Å)	2.4523	2.4401
2× (Ba/Pr1)-O2 (Å)	2.6925	2.6864
2× (Ba/Pr1)-O2 (Å)	2.4499	2.4544
2× (Ba/Pr1)-O2 (Å)	3.1145	3.1269
2× (Ba/Pr1)-O2 (Å)	2.7912	2.7937
<(Ba/Pr1)-O> (Å)	2.7634	2.7671

. Figure 4 shows the coordination polyhedral for the Mn and the distance between Mn and all the neighboring oxygen.

The influence of the Ru content on unit cell volumes and parameters a, b, and c is shown in Table 2. The values of a and c for the lattice parameters were almost unchanged and were smaller than the value of b. The increase in the lattice parameter b was due to the tilting scheme of the MnO₆ octahedron in the Pnma perovskites, in which the distortion was driven by the increase in the ionic radii size between Mn⁴⁺ (0.53 Å) and Ru⁴⁺ (0.62 Å). Introducing Ru at the Mn site will cause a distortion in the MnO₆ octahedron. Consequently, the Mn–O–Mn bond angle and the Mn–O and Ru–O bond lengths increase (Table 3) [21,22,23]. Bond valence sum (BVS) calculations were performed for Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃. The valences of metal cations, Pr, and Ba were calculated according to the sum of all individual bond valences. The valence of the ions was 2.812 for Pr and 3.961 for Ba. The valences for Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ were slightly smaller than expected, where the valence of Pr was 2.679 and Ba was 3.751, which was probably due to the structural changes caused by Ru substitution at the Mn site.

3.2. FTIR Spectra

Figure 5 depicts the broad absorption band obtained through FTIR for the Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ samples recorded within a wavenumber range of 400–2000 cm⁻¹. The stretching vibration mode (v₁ mode) for both samples was observed at a wavenumber of around 600 cm⁻¹, corresponding to Mn–O and O–Mn–O deformations [24,25,26]. This finding indicates that both samples contained metal–oxygen bonds corresponding to the Mn–O–Mn bond length, which was confirmed by the internal motion causing the stretching mode. The absorption bands at around 1050 cm⁻¹ correspond to stretching of the MnO₆ octahedron. This formation confirms the presence of the MnO₆ octahedron in the perovskite structure, which is in agreement with the XRD results.

3.3. Morphology

The surface morphology of the Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ was investigated using field emission scanning electron microscopy (FESEM) (Figure 6). The images show that the particle size distribution is almost homogeneous. According to the images, the grain sizes of Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ were ~2.4 and ~1.1 μm, respectively.

3.4. AC Susceptibility Measurement

Figure 7a,b show the temperature dependence of the AC magnetic susceptibility of the Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ compound on the real part, χ’, and imaginary part, χ”. The Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ compound exhibited paramagnetic (PM) to ferromagnetic (FM) transition, where the Curie temperature (T_C) increased with Ru substitution from 130 to 153 K. The Curie temperature T_C values were determined by the minimum point of differentiation, as shown in the dχ’/dT versus temperature curve in the inset graph in Figure 7a. The increase of the transition temperature Tc was presumably due to the presence of double exchange (DE) interaction involving Mn³⁺ and Mn⁴⁺ ions. Doping with Ru at the Mn site does not change the electron concentration in the compound, but Ru destroys the order of Mn³⁺ and Mn⁴⁺, and affects the magnetic interaction between Mn³⁺−O−Mn⁴⁺ and Mn³⁺−O−Ru⁴⁺. This change considerably affects the value of T_C. These results are also consistent with recent observations in compounds Pr_0.6Ca_0.4Mn_1-xNi_xO₃ and Pr_1-xNd_xMnO₃ [27,28]. On the other hand, χ” showed a similar behavior, with the maximum peak broadening and shifting towards a high temperature after the doping of Ru. In χ”, a sharp peak was observed at 134 and 155 K for Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃, respectively. The negative χ” component for Pr_0.67Ba_0.33MnO₃ at ~200 K can be attributed to the coexistence of metastable and stable phases in the magnetic materials [29]. This curve also shows a small shoulder at around 100 K, which indicates that secondary magnetic phase transitions or domain wall interactions exist in the sample.

4. Conclusions

In this study, the structural and magnetic properties of crystallized Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃ were synthesized using a solid-state reaction method. The samples were crystallized in a Pnma orthorhombic space group. The introduced Ru at the Mn site plays an important role in increasing the unit cell volume and results in a slight change in the Mn−O−Mn bond angle. The increase in the lattice parameter was caused by the tilting scheme of the MnO₆ octahedron. The absorption band obtained through FTIR revealed the stretching vibration mode for Mn−O and Ru−O bonds. The FESEM analysis results indicated that the particles in both samples had an almost homogeneous distribution. The paramagnetic to ferromagnetic transition was confirmed at T_c = 130 and 153 K for Pr_0.67Ba_0.33MnO₃ and Pr_0.67Ba_0.33Mn_0.9Ru_0.1O₃, respectively. These results suggest that a small amount of Ru doped at the Mn site could change the magnetic interaction between Mn−O−Mn.