Inﬂuence of Ruthenium Doping on the Crystal Structure and Magnetic Properties of Pr 0.67 Ba 0.33 Mn 1–x Ru x O 3 Manganites

: This study reports the structural, morphological, and magnetic properties of ruthenium doping at the manganese site in Pr 0.67 Ba 0.33 MnO 3 manganites. Rietveld reﬁnement X-ray powder di ﬀ raction (XRD) data show that Pr 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 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 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 samples, respectively.


Introduction
Doped perovskite manganite compounds with the general chemical formula A 1−x M x MnO 3 (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 Mn 3+ and Mn 4+ .A strong spin phonon interaction arises from deformation of the MnO 6 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 Mn 3+ /Mn 4+ 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 Mn 4+ ion [8].Vabitha et al. reported that Ru is a new potential candidate that induces a metal-to-insulator transition in Nd 0.5 Ca 0.5 MnO 3 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 0.67 Ba 0.33 MnO 3 should be investigated.This work presents the results of investigations into synthesized crystalline samples of Pr 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 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 0.67 Ba 0.33 Mn 1-x Ru x O 3 has not been reported.

Materials and Methods
Pr 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 were synthesized through a solid-state reaction method.Stoichiometric amounts of high-purity (>99.9%)praseodymium oxide (Pr 6 O 11 ), barium carbonate (BaCO 3 ), ruthenium oxide (RuO 2 ), and manganese oxide (Mn 2 O 3 ) 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 cm 2 .Subsequently, the pellets were sintered in air at 950 • C for 24 h.The phase identification and crystallinity of the Pr 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 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 = 1.5406Å) 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.

Structural Analysis
Figure 1 displays the XRD patterns of Pr 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 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 Mn 2 O 3 , whereas Figure 1b shows small impurity peaks at 35 • and 60 • , which were identified as Mn 3 O 4 , as indicated by the (#) symbol in the XRD patterns.The appearance of a secondary phase in the Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 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 0.67 Ba 0.33 MnO 3 [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 Tables 1 and 2, 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), O 1 was fixed at the 4c site (x, 0.25, z), and O 2 was fixed at the 8d site (x, y, z).The unit cell volume slightly increased from 235.91 to 236.78 Å 3 after the Ru doping.The increase in cell volume can be ascribed to the larger ionic radii of Ru 4+ (0.62 Å) compared to that of Mn 4+ (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: 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.
Crystals 2020, 10, x FOR PEER REVIEW 3 of 11 hkl planes and match the previously reported data of Pr0.67Ba0.33MnO3[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 Tables 1 and 2, 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 Å 3 after the Ru doping.The increase in cell volume can be ascribed to the larger ionic radii of Ru 4+ (0.62 Å ) compared to that of Mn 4+ (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: 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 CuKα 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 0.67 Ba 0.33 MnO 3 , which shows an octahedral MnO 6 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 ABO 3 -type perovskite structure occupied by Pr/Ba cations was surrounded by 12 oxygen ions, as shown in Figure 3b.The octahedral MnO 6 formed by the position of Mn ions at the B site was surrounded by six oxygen ions.For the Pr 0.67 Ba 0.33 MnO 3 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 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 , 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 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 are listed in Table 3. Figure 4 shows the coordination polyhedral for the Mn and the distance between Mn and all the neighboring oxygen.six oxygen ions.For the Pr0.67Ba0.33MnO3sample, 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 Pr0.67Ba0.33Mn0.9Ru0.1O3,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 Pr0.67Ba0.33MnO3and Pr0.67Ba0.33Mn0.9Ru0.1O3are listed in Table 3. 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 Mn 4+ (0.53 Å ) and Ru 4+ (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 Pr0.67Ba0.33MnO3and Pr0.67Ba0.33Mn0.9Ru0.1O3.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 Pr0.67Ba0.33Mn0.9Ru0.1O3were 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.

FTIR Spectra
Figure 5 depicts the broad absorption band obtained through FTIR for the Pr0.67Ba0.33MnO3and Pr0.67Ba0.33Mn0.9Ru0.1O3samples 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.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 MnO 6 octahedron in the Pnma perovskites, in which the distortion was driven by the increase in the ionic radii size between Mn 4+ (0.53 Å) and Ru 4+ (0.62 Å).Introducing Ru at the Mn site will cause a distortion in the MnO 6 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 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 .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 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 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.

FTIR Spectra
Figure 5 depicts the broad absorption band obtained through FTIR for the Pr 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 samples recorded within a wavenumber range of 400-2000 cm −1 .The stretching vibration mode (v 1 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 MnO 6 octahedron.This formation confirms the presence of the MnO 6 octahedron in the perovskite structure, which is in agreement with the XRD results.

Morphology
The surface morphology of the Pr 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 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 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 were ~2.4 and ~1.1 µm, respectively.

AC Susceptibility Measurement
Figure 7a,b show the temperature dependence of the AC magnetic susceptibility of the Pr 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 compound on the real part, χ', and imaginary part, χ".The Pr 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 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 Mn 3+ and Mn 4+ ions.Doping with Ru at the Mn site does not change the electron concentration in the compound, but Ru destroys the order of Mn 3+ and Mn 4+ , and affects the magnetic interaction between Mn 3+ −O−Mn 4+ and Mn 3+ −O−Ru 4+ .This change considerably affects the value of T C .These results are also consistent with recent observations in compounds Pr 0.6 Ca 0.4 Mn 1-x Ni x O 3 and Pr 1-x Nd x MnO 3 [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 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 , respectively.The negative χ" component for Pr 0.67 Ba 0.33 MnO 3 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.

Conclusions
In this study, the structural and magnetic properties of crystallized Pr0.67Ba0.33MnO3and Pr0.67Ba0.33Mn0.9Ru0.1O3were synthesized using a solid-state reaction method.The samples were

Figure 2 .
Figure 2. Rietveld refinement of X-ray diffraction for (a) Pr0.67Ba0.33Mn0.9Ru0.1O3and (b) Pr0.67Ba0.33MnO3.Blue solid lines are observed data, the solid red line is the calculated pattern, and the pink line is the difference.Tick marks indicate the allowed Bragg reflections.The peaks of the impurity phase are schematized by (*) and (#).

Figure 2 .
Figure 2. Rietveld refinement of X-ray diffraction for (a) Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 and (b) Pr 0.67 Ba 0.33 MnO 3 .Blue solid lines are observed data, the solid red line is the calculated pattern, and the pink line is the difference.Tick marks indicate the allowed Bragg reflections.The peaks of the impurity phase are schematized by (*) and (#).

Figure 3 .
Figure 3. (a) The crystallographic structure for Pr0.67Ba0.33Mn0.9Ru0.1O3.Green-colored balls represent the Ba, yellow-colored balls represent the Pr, purple balls represent the Mn, white balls represent the Ru, and red-colored balls represent the O.(b) Structure of Pr0.67Ba0.33MnO3,where the Pr and Ba are 12-fold coordinated and Mn is 6-fold coordinated with the polyhedral.

Figure 3 .
Figure 3. (a) The crystallographic structure for Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 .Green-colored balls represent the Ba, yellow-colored balls represent the Pr, purple balls represent the Mn, white balls represent the Ru, and red-colored balls represent the O.(b) Structure of Pr 0.67 Ba 0.33 MnO 3 , where the Pr and Ba are 12-fold coordinated and Mn is 6-fold coordinated with the polyhedral.

FigureFigure 5 .Figure 5 .
Figure 7a and b show the temperature dependence of the AC magnetic susceptibility of the Pr0.67Ba0.33MnO3and Pr0.67Ba0.33Mn0.9Ru0.1O3compound on the real part, χ', and imaginary part, χ''.The Pr0.67Ba0.33MnO3and Pr0.67Ba0.33Mn0.9Ru0.1O3compound exhibited paramagnetic (PM) to ferromagnetic (FM) transition, where the Curie temperature (TC) increased with Ru substitution from 130 to 153 K.The Curie temperature TC values were determined by the minimum point of differentiation, as shown in the dχ'/dT versus temperature curve in the inset graph in Figure 7(a).The increase of the transition temperature Tc was presumably due to the presence of double exchange (DE) interaction involving Mn 3+ and Mn 4+ ions.Doping with Ru at the Mn site does not change the

FigureFigure 6 .
Figure 7a and b show the temperature dependence of the AC magnetic susceptibility of the Pr0.67Ba0.33MnO3and Pr0.67Ba0.33Mn0.9Ru0.1O3compound on the real part, χ', and imaginary part, χ''.The Pr0.67Ba0.33MnO3and Pr0.67Ba0.33Mn0.9Ru0.1O3compound exhibited paramagnetic (PM) to ferromagnetic (FM) transition, where the Curie temperature (TC) increased with Ru substitution from 130 to 153 K.The Curie temperature TC values were determined by the minimum point of differentiation, as shown in the dχ'/dT versus temperature curve in the inset graph in Figure 7(a).The increase of the transition temperature Tc was presumably due to the presence of double exchange (DE) interaction involving Mn 3+ and Mn 4+ ions.Doping with Ru at the Mn site does not change the

Table 1 .
Structural parameters for Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 obtained from Rietveld refinement analysis.

Table 2 .
Lattice parameters, unit cell volume, and goodness of fit for Pr 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 obtained from Rietveld refinement.

Table 3 .
Bond angles and bond lengths for Pr 0.67 Ba 0.33 MnO 3 and Pr 0.67 Ba 0.33 Mn 0.9 Ru 0.1 O 3 obtained from Rietveld refinement.