Extensive Parallelism between Crystal Parameters and Magnetic

Article pubs.acs.org/IC

Extensive Parallelism between Crystal Parameters and Magnetic Phase Transitions of Unusually Ferromagnetic Praseodymium Manganite Nanoparticles Anustup Sadhu,† Hemant G. Salunke,‡ Sonnada M. Shivaprasad,§ and Sayan Bhattacharyya*,† †

Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER), Kolkata, Mohanpur 741246, India Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India § International Centre for Materials Science & Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India ‡

S Supporting Information *

ABSTRACT: The alterations in physical property across different space groups of the same material are sometimes conveniently reflected by the crystal structure as a function of temperature. However, mirroring the physical property and crystal parameters over a wide range of temperatures within the same space group is quite unusual. Remarkably, Rietveld analyses of the X-ray diffraction patterns of PrMn0.9O3 (ABO3) nanoparticles (NPs) with a constant Pnma space group from 300 to 10 K could successfully predict the four magnetic phases, viz. paramagnetic, antiferromagnetic (AFM), ferromagnetic (FM), and spin-glass-like ordering. The increase in Mn−O−Mn bond angles and tolerance factor leads to FM ordering below ∼100 K in usually AFM PrMn0.9O3 NPs. The concurrent decrease of lattice cell volume and Mn−O−Mn bond angles near the AFM to FM transition temperature (Tc) suggests that the AFM character increases just above Tc due to atomic deformations and reduced Mn−Mn separation. The predictions from crystal structure refinement were successfully verified from the cooling path of the temperature-dependent field-cooled magnetization measurements. A mechanism involving incoherent spin reversal due to competition between the neighboring spins undergoing antiparallel to parallel spin rotations was suggested. The structure− property parallelism was cross-checked with the A-site vacant Pr0.9MnO3.2 NPs.

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tions.5,10,11 One such condition is the temperature of measurement with which the electrical, magnetic, and optical properties vary because of the thermal changes in crystal lattice and local electronic environment.6,12 The structural changes relate mostly to the crystallographic phase transition or minor alterations in bond angles and lengths.13 The phase changes as a result of transformation of space groups at different temperatures are widely studied by techniques such as X-ray and/or neutron diffraction in bulk manganites.14−17 However, temperature variation of the lattice and strain parameters within the same space group synergistic with the property changes, for example, paramagnetic to AFM or insulating to metallic, are less studied, and almost all investigations so far are based on neutron diffraction experiments.18−20 Instances where the structure−property characteristics are exactly mirrored across a significant temperature range are also uncommon. For example, the magnetic moment of the manganites (A1−xA′xBO3, A = trivalent rare earth cation, A′ = divalent alkaline earth cation, B = Mn3+/Mn4+) is heavily dependent on

INTRODUCTION Undoped PrMnO3 (PMO) is a ABO3-type AFM manganite with almost 95% Mn3+ ions.1 The AFM undoped manganites can be converted to being FM by creating A- and B-site cation vacancies to induce double exchange by reducing the Jahn− Teller (J−T) Mn3+ ions at the expense of introduced Mn4+ species.2 Although ferromagnetism in vacancy-induced undoped LaMnO3 is common,2,3 so far only one report is available for FM PMO where oxygen excess resulted in extra Mn4+ ions.4 In particular, the manganite NPs show a distinct influence of the particle size on their crystal structure parameters and the magnetic properties.5−7 The NPs with a significantly higher fraction of surface atoms than their bulk counterparts demonstrate peculiarities in the observation of ferromagnetism in conventional AFM materials, low-temperature glassy behavior, anomalous lattice strain, and bond angles.5−8 Moreover, simultaneous structure−property correlations at the nanoscale has never been observed because of the complexities involved. The intrinsic physical properties depend on the crystal structure,9 morphology, and surface chemistry of the nanomaterials in addition to the property measurement condi© XXXX American Chemical Society

Received: April 4, 2016

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DOI: 10.1021/acs.inorgchem.6b00815 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Mn3+−O−Mn3+/Mn3+−O−Mn4+/Mn4+−O−Mn4+ bond angles, where flatter angles facilitate ferromagnetism by double exchange of the electrons and narrower angles are the result of J−T distortion leading to AFM superexchange interactions.21 The relative fluctuation of the Mn−O−Mn angles and Mn−O bond lengths across temperatures is reflected through changes in the lattice strain and cell volume, the parameters being largely governed by NP size.5−7 Only a few reports have demonstrated a break in the continuous slope of the temperature variation of the lattice parameters and Mn−O bond lengths at the magnetic transition temperature of doped manganite by synchrotron XRD and neutron diffraction for bulk manganite.13,19,20,22 However, there has been no report on extensive structure−magnetic property parallelism in NPs. We used the laboratory X-ray diffraction (XRD) technique at variable temperatures to unravel the thermal variation of structural parameters that match at nearly every step with the magnetic transition temperatures in the PMO NP systems. In this study, PrMn0.9O3 NPs with 10% vacancy at the Mn3+ site was synthesized by a nonaqueous sol−gel route. PrMn0.9O3 showed FM character below ∼100 K. Rietveld refinement of the XRD pattern was used to elucidate the lattice parameters, cell volume, orthorhombic strain (OS), Mn−O−Mn angles, and Mn−O lengths at temperatures of 10−300 K. Our strategy allows the detection of thermal variation of structural parameters that alter hand-in-hand with the different magnetic transitions, and to our knowledge this long-range structure− magnetic synergistic trend was never observed earlier.

Figure 1. (a) FESEM image. (b) TEM image of PrMn0.9O3 NPs with high-resolution view of the selected area (open rectangle) and FFT pattern. (c) Rietveld analysis of the XRD pattern. Legends: diff (blue line; difference plot between the observed and calculated patterns), Obs (plus symbols; observed pattern), Calc (red line; calculated pattern), and bckgr (green line; background plot). (d) PrMn0.9O3 unit cell based on the obtained parameters.

Temperature Dependence of Structural Parameters. The structural parameters were obtained from the Rietveld analysis of the XRD patterns at different temperatures with decent goodness of fit (reduced χ2) and weighted profile (Figure S3 and Table S2). The thermal behavior of cell volume (V) is shown in Figure 2b and Table 1. The parameters of PrMn0.9O3 follow the relation b/√2 < c < a at all temperatures. As compared to b and c, the variation of parameter a is dramatic, whereby it shows a consistent decreasing trend at ≥Tc. From the variable trends of V, a, b, and c, four regions corresponding to the different magnetic behavior were identified, from 300 to 150 K, 150 K to ≥Tc, Tc to ∼50 K, and 50 to 10 K. The cell parameters and their thermal variations are significantly different from those observed for bulk PrMnO38,23 and La0.75Ca0.25MnO3.13 The parameter a is the most trend-setting parameter, and its increase from 300 to 150 K also increases V. Due to the partial absence of B-site cations, the center of the octahedron is replaced by a vacancy which results in negatively charged octahedra (VMnO63−). The negatively charged 6 O2− ions repel each other because there is no cation to stabilize them, which swells the octahedron relatively. From 300 to 150 K, a, b, and c parameters change by +0.20, −0.02, +0.03%, respectively. The + and − values indicate increase and decrease of a particular parameter, respectively. V shows a sharp dip at and above Tc (Table 1 and Figure 2b), which was never experienced earlier in any magnetically ordered system. The thermal variation of OS is shown in Figure 2c. The strain in the ac plane [OS∥ = 2(c − a)/(c + a)] is higher than that along the b axis [OS⊥ = 2(a + c − b√2)/(a + c + b√2)]. OS increases from 300 to 150 K in the paramagnetic regime, decreases near Tc, increases again up to 50 K, and decreases thereafter. In fact, the much lower OS’s over the entire temperature range in the NPs as compared to the bulk materials is a direct signature of high magnetic ordering in good agreement with the reported results for LaMnO3, PrMnO3, La0.5Ca0.5MnO3, and Pr0.5Ca0.5MnO3.23−25

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RESULTS AND DISCUSSION Structural Characterization. The oxygen nonstoichiometry of PrMn1−xO3+δ was found to be δ = 0 ± 0.01 by gravimetric analyses (Table S1, Supporting Information), and inductively coupled plasma-mass spectrometry (ICP-MS) analysis confirmed the final stoichiometry to be PrMn0.9O3. The interconnected ∼35 nm diameter NPs are observed from the Fourier transform scanning electron microscope (FESEM) image in Figure 1a, whereas the interplanar spacing and indexed selected area electron diffraction (SAED) pattern from the transmission electron microscope (TEM) micrograph (Figure 1b) shows the NPs to be highly crystalline. Elemental mapping shows the homogeneity of individual elements throughout the NPs (Figure S1). The Rietveld refined room-temperature XRD pattern (Figure 1c) corroborates the orthorhombic crystal structure with Pnma space group (Figure 1d). The space group remains unaltered between 300 and 10 K (Table 1). Magnetic Characterization. Figure 2a shows the zerofield-cooled (ZFC) and field-cooled (FC) curves at 100 Oe applied field. ZFC was measured upon warming the sample, and FC was recorded while cooling. A transition onset (Tc) at 100 K in the ZFC−FC curves indicates the dominant FM character below 100 K. The unsaturated field (H) dependent magnetization (M) plot at 5 K (Figure 2a, inset) reveals inherent AFM fractions. With 5 T applied field, the magnetic moment reaches 32 emu/g in addition to a high coercivity of 848 Oe, which is in good agreement with the FM nature. The FM ordering is further supported by the change in nature of the M−H plots above and below 100 K. At 120 K, the straight line plot transits to the “S”-shaped curve at 100 K, and this curved feature increases at 80 K (Figure S2). The AFM charge ordering above 250 K is absent similar to other manganite nanostructures.5 B


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5.5162(±1) 7.6811(±1) 5.4434(±1) 230.64 5.51978(±1) 7.6823(±1) 5.4426(±1) 230.79

The numbers within parentheses indicate standard deviation. a

250 200

5.5232(±2) 7.6809(±2) 5.4427(±1) 230.90 5.527(±1) 7.6796(±2) 5.4417(±1) 230.97

150 125

5.5252(±1) 7.6783(±1) 5.4418(±2) 230.87 5.525(±2) 7.679(±1) 5.4413(±2) 230.85

110 100

Table 2 lists the Mn−O−Mn bond angles (θ) that represent the extent of FM ordering. XRD refinement results showed that the bond angles are far more flatter than other reports,4 as well as those reported in the Pr1−xCaxMnO3 (x = 0.3, 0.49) series.7 The sharp dip in Mn−Oac−Mn and Mn−Ob−Mn bond angles at ≥Tc is shown in Figure 2d. In fact, Mn−Oac−Mn shows a similar trend of a. The higher the parameter a, there will be more drag on the (Mn−O)ac bonds and the Mn−Oac−Mn bond angles will be flatter. However, the temperature variation of Mn−Ob−Mn bond angles does not match the trend of b. Again, the lowest Mn−O−Mn angles are observed at 100 K slightly above Tc. An important parameter to understand the trends in distortion of the MnO6 octahedra is cos ϕ, where ϕ (= 180° − θ) is the bending of Mn−O−Mn angle due to cooperative rotations of the MnO6 octahedra. The magnetic transition temperatures are proportional to the square of cos ϕ due to the spin−spin interactions such as the eg orbital σ-bond superexchange along the b axis and semicovalent exchange in the ac plane.26 The plots of cos2 ϕ as a function of temperature similarly display a sharp dip at Tc (Figure S4). The Goldschmidt tolerance factor (t) of the perovskite manganites provides an additional indication of the evolution of FM character when t becomes close to 1.23,26 Generally, t = (rPr + rO)/[√2(rMn + rO)], where rPr, rMn, and rO represent the radius of Pr3+, Mn3+, and O2− ions, respectively. A slightly modified version based on the bond lengths is t = Pr−O/√2(Mn−O).26 Bulk AFM PrMnO3 has t ≈ 0.896,23 whereas PrMn0.9O3 NPs show t ≈ 0.99, which again corroborates its dominant FM character. Interpretation of the Thermal Behavior of Structural Parameters. From the structural divergences with temperature (Tables 1 and 2), the region from 300 to 150 K can be correlated to the paramagnetic region where the electron spins are thermally governed in the absence of external magnetic field. While decreasing from 300 to 150 K, the highest intensity Bragg reflection shifts to lower 2θ by 0.10°. Because of the steepest rise of parameter a, both OS⊥ and OS∥ increase in this region and the electron spins remain randomly oriented in the absence of magnetic field. The region of 150 K to Tc is dominated by AFM ordering of the spins, with preferential canted antiparallel arrangement in the absence of any field. In earlier reports, pristine PrMnO3 and Pr1−xCaxMnO3 (0 < x < 1) with comparative fractions of Mn3+ and Mn4+ ions are perfectly AFM and have a Néel temperature (TN) of 99 and 135−170 K, respectively.8 In PrMn0.9O3, TN is arguably at 150 K. FM ordering supersedes AFM ordering at ≤ Tc. The high magnetization in PrMn0.9O3 NPs is the result of their nanostructured morphology and cation vacancies precisely tuned by the nonaqueous sol−gel route. The vacancies are responsible for partial conversion of the Mn3+ to Mn4+ ions, which is the key to their FM double-exchange interactions. Analysis of the XPS spectra of Mn 2p3/2 and 2p1/2 levels (Figure 3) shows 32% Mn4+ with respect to Mn3+ ions. Creating the Mn vacancies in PrMn0.9O3 however decreases the Mn4+ fraction, reducing its moment. The AFM ordering is driven by the short-range disordered Mn3+−O−Mn3+ and Mn4+−O− Mn4+ superexchange interactions, which is prominent with higher Mn vacancies.27 The region below ∼50 K can be ascribed to the glassy behavior of the spins.6 The high anisotropy of the frozen spin-glass-like region was suggested by the existing bifurcation of the ZFC and FC curves even at a high applied field of 1 T (Figure S5).6

5.52494(±1) 7.67826(±1) 5.4416(±1)) 230.84 5.5246(±1) 7.6782(±1) 5.4423(±1) 230.86

90 50

5.5255(±1) 7.6782(±1) 5.4436(±2) 230.95 a b c V

40 10

5.524(±1) 7.6775(±2) 5.4429(±1) 230.84

T

Table 1. Variation of Unit Cell Parameters (a, b, c) and Cell Volume (V) of PrMn0.9O3 at Different Temperaturesa

5.526(±1) 7.6764(±1) 5.4409(±1) 230.80

300

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Figure 2. (a) Plot of M as a function of temperature (T) at an applied field of 100 Oe for PrMn0.9O3 NPs. (Inset) M−H plot at 5 K, with an enlarged view showing the coercivity of 848 Oe. Parallelism between crystal structure and magnetic transitions: (b) ZFC curve at 100 Oe applied field and cell volume (V) versus T. (c) Orthorhombic strain (OS) as a function of T. (d) Variation of Mn−O−Mn bond angles with T. ac and b indicate Mn− Oac−Mn and Mn−Ob−Mn bond angles, respectively. The shaded rectangles are indicators of the AFM to FM transition regions, where sharp changes in the crystal structure parameters are observed.

Corroborative Evidence of AFM Ordering. One of the key challenges is to obtain direct experimental proof of the AFM ordering from magnetic measurements that can correlate the interpretations from XRD Rietveld analysis. Since the XRD measurements were performed in the absence of a magnetic field, the correlations are most suitable with temperaturedependent magnetization plots with the lowest applied fields possible (Figure 4a). Large applied fields and high FM moments can overshadow the weaker AFM signatures, which are strongly dependent on the measurement conditions applied. Although the intense FM transition is distinct at ∼100 K irrespective of the applied field, the AFM transition was only detected during field-cooled cooling (FCC) at low fields (Figure 4a−c) and not with ZFC or FC warming (ZFCW or FCW) processes. During FCC, the antiparallel ordering of the spins can be easily achieved from a random paramagnetic state, whereas during ZFCW and FCW, the memory effect of the strong parallel orientation of the FM spins does not allow their spontaneous reorientation into the antiparallel arrangement. The AFM transition is clearly visible from the lower inset of Figure 4a when the applied field is as low as 100 Oe. The antiparallel ordering of the spins starts from 210 K, peaks at ∼150 K, and dominates until Tc. With 200 Oe applied field (Figure 4b), a small peak at ∼200 K is observed instead of the broad AFM transition since the higher field could only detect the very weak onset of AFM transition far from Tc. With further increase of applied field to 1000 Oe, the transition is not visible and only a small difference between the FCW and the FCC curves is observed in the AFM region (Figure 4c and lower inset). The peak at ∼200 K has similarities with the AFM charge ordering usually observed in manganites.25 However, this peak vanishes with 1000 Oe applied field, which implies its association with a transition from the paramagnetic to a weak AFM state. When the field is as high as 2000 Oe, the FCW and

FCC curves are perfectly identical without any AFM signature (Figure 4d and lower inset). On the contrary, the behavior of FCW and FCC curves below 30 K is proportionate with increasing field strength (upper insets of Figure 4a−d). The different paths followed by the FCW and FCC curves below 25 K indicate the spin arrangement to be different from a pure parallel FM arrangement. The moment in FCW is slightly lower than the FCC curve, since in FCW the frozen spins are more anisotropic to temperature increase and results in randomly oriented frozen spins with lesser moment. In the FCC curve, the spins freeze uniformly along the direction of applied field, resulting in higher FM moments. The consistent hump at ∼25 K is likely the onset of spin freezing reminiscent of a glassy transition.6,28−30 Certainly, the unusual flattening below ∼50 K (Figure 2a) and difference in FCC and FCW curves in that region are strong indicators of the existence of spin-glass-like feature. Alternating currennt susceptibility measurement is a powerful tool to validate the spin-glass-like characteristics. The real part of ac magnetic susceptibility (χ′) measurements of PrMn0.9O3 NPs is plotted over the temperature range 5−300 K at 1 Oe applied dc magnetic fields and over the ac magnetic frequency range from 117 Hz to ∼5 kHz in Figure 5a. A distinct peak (Tf) is observed with the peak shifting from 26 to 29.9 K with increasing applied frequency from 117 to 4993 Hz. This shift is obvious in spin-glass systems,31,32 and the dynamic property is validated by fitting with phenomenological Volger− Fulcher law for interacting NPs31−33 τ = τ0 exp[Ea /k(Tf − T0)]

(1)

where τ = 1/f, f = frequency of measurement, τ0 = 1/f 0, f 0 = a characteristic frequency, Ea = activation energy, and T0 = a fitting parameter which formally represents a transition temperature to an equilibrium state and is related to an D


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The numbers within parentheses indicate standard deviation. a

300 250

161.0(±3) 167.7(±1) 164.35 2.2931(±2) 1.9304(±1) 1.5961(±2)

200

167.5(±2) 169.38(±2) 168.44 2.3789(±2) 1.9316(±2) 1.5210(±1)

150

169.13(±2) 173.91(±2) 171.52 2.2674(±2) 1.9286(±2) 1.6160(±1)

125

159.0(±3) 172.07(±1) 165.35 2.0559(±2) 1.9494(±2) 1.8361(±1) 156.82(±4) 171.66(±1) 164.24 2.2829(±2) 1.9597(±2) 1.6001(±2)

110

interaction energy of the system. The fitted parameters are Ea/k = 262 K, τ0 = 2.86 × 10−13 s, and T0 = 15 K with χ2 = 1.52 × 10−8 (Figure 5b). The resultant τ0 of the order of 10−13 is in good agreement with spin-glass systems.30−33 Explanation with Respect to Spin Ordering. The concurrent dip in the temperature-dependent plots of V, Mn−O−Mn bond angle, and OS at ≥ Tc (Figure 2) resembles the most interesting synergism between the magnetic and the structural transitions. As observed in Table 2 and Figure 6, the lowest Mn−O−Mn bond angle around Tc signifies that the Mn−Ol, Mn−Om, and Mn−Os bond lengths deviate the most from the average Mn−O bond length. The Mn−Oac−Mn bond angle decreases by 5.1% from 150 to 100 K and again rises up to 50 K by 7.1%. Since the Mn4+ ions are randomly interspersed over the Mn3+ sites,34 the Mn3+−O−Mn3+ and Mn4+−O−Mn4+ superexchange interactions and Mn3+−O−Mn4+ doubleexchange interactions are widely dispersed within the lattice. The random movement of bonds at temperatures above Tc decreases at temperatures below Tc, resulting in the concurrent dip of the structural parameters due to the highest dynamic J− T distortion at ≥Tc.13 Also, transition from the AFM to a FM state commences at Tc by noncollinear spin-flip transition over a wide temperature range of 50−100 K. This is largely commensurate with the incoherent atomic displacements relative to the lattice averaged positions around Tc. When the PMO NPs are cooled in the absence of a magnetic field, the magnetic behavior can be understood by the schematic in Figure 6. The paramagnetic region from 300 to 150 K consists of random electron spins (black arrows). The spins start to align in an antiparallel arrangement from 150 K (TN) onward. The AFM ordering increases with decreasing temperature due to restriction in the thermal spin flips from their antiparallel orientation. Accordingly, the anisotropic AFM ordering becomes maximum at just above Tc, supported by the temperature-dependent behavior of various crystal parameters which decrease from 150 K to Tc (Table 1). The lattice parameters are also a measure of Mn−Mn separation.35 When the Mn−O−Mn bond angle decreases due to AFM superexchange interactions, the Mn−Mn separation also decreases, corroborating the increase of AFM character until ≥Tc (Figure S6). The progressive antiparallel alignment of the spins with decreasing temperature from 150 K to Tc is shown in Figure 6 at every 10 K interval. In the absence of an external field, the relative orientation of the spins at the same temperature in neighboring NPs will be different at both the AFM and the FM

155.57(±3) 171.78(±1) 163.36 2.098(±2) 1.939(±2) 1.7896(±1)

100 90

Figure 3. XPS spectrum of the Mn 2p level of PrMn0.9O3 NPs. Deconvoluted peaks 1 and 1′ at 641.6 and 653.3 eV, respectively, correspond to Mn3+ state and 2 and 2′ at 642.6 and 654.3 eV, respectively, correspond to Mn4+ state, both at the core of the NPs. Peaks 3 and 3′ at 644.4 and 656.2 eV, respectively, are the Mn3+/Mn4+ partially coordinated to O2− ions at the NP surface.

165.0(±2) 172.45(±1) 168.73 2.1895(±2) 1.9213(±1) 1.6963(±2) 173.72(±2) 175.54(±1) 174.63 2.0944(±2) 1.9224(±2) 1.8007(±2)

50 40

171.0(±2) 175.3(±1) 173.15 2.1583(±2) 1.925(±2) 1.7224(±1)

10

171.25(±1) 169.96(±1) 170.61 2.1456(±2) 1.9205(±2) 1.7467(±1)

T

Mn−Oc−Mn Mn−Oab−Mn Mn−O−Mn Mn−Ol Mn−Om Mn−Os

Table 2. Bond Angles and Bond Lengths of PrMn0.9O3 at Different Temperaturesa

158.9(±1) 167.36(±1) 163.15 2.0582(±2) 1.932(±1) 1.8405(±1)

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Figure 4. FCC (black) and FCW (gray) plots of PrMn0.9O3 at different cooling fields of (a) 100, (b) 200, (c) 1000, and (d) 2000 Oe. (Top and bottom insets) Enlarged view of the low-temperature (75 K) regions, respectively.

to the increasing Mn−Mn separation. Below this temperature, magnetic degeneration takes place due to the freezing of local AFM regions comprising of Mn3+−O−Mn3+ and Mn4+−O− Mn4+ within the FM Mn3+−O−Mn4+ matrix with different orientations representing the glassy state.37−40 Likewise, with the increase in AFM domains below ∼50 K, the lattice parameters also decrease. In the presence of a minimum applied magnetic field of 100 Oe, the temperature-dependent magnetization plots in Figure 4a are the result of mutual alignment of the spins between neighboring NPs at a particular temperature, which also matches the trend observed from the temperaturedependent structural parameters. Generalization of the Observations with A-Site Vacant Pr0.9MnO3.2 NPs. The long-range parallelism between crystal structure and magnetic property in B-site vacant PrMn0.9O3 NPs is further generalized with the A-site vacant Pr0.9MnO3.2 NPs. Though the stoichiometry-directed synthesis protocols are similar, the particle size of Pr0.9MnO3.2 NPs is only ∼12 nm (Figure 7a) as compared to ∼35 nm for PrMn0.9O3 NPs (Figure 1a). The smaller size of Pr0.9MnO3.2 NPs facilitates overoxidation at the surface active sites, which leads to higher oxygen nonstoichiometry and a Mn4+/Mn3+ ratio of 48% (Figure 7b). The higher Mn4+ fraction results in a larger magnetization of 54.2 emu/g at an applied field of 5 T, although with persistent unsaturation attributed to the presence of AFM fractions (Figure 7c, inset). The crystal structure parameters such as V, Mn−O−Mn, OS, and cos2 ϕ display similar thermal variations to that of PrMn0.9O3 NPs (Figure 7d−f, Figures S4 and S6). Pr0.9MnO3.2 follows b/√2 ≈ c < a at 50−200 K and c < a < b/√2 at 250−300 K. From 300 to 150 K, a, b, and c parameters change by +0.45%, −0.33%, + 0.18%, respectively. Both Mn−Ob−Mn and Mn−Oac−Mn drop by 5.2% from 150 to 100 K and again increase by 5.6% up to 50 K. The characteristic dip of the parameters around Tc is thus irrespective of the A-site or B-site vacancy. However, the AFM

Figure 5. Plot of χ′ as a function of temperature for PrMn0.9O3 measured at 1 Oe and different ac frequencies. (Inset) Fit to the Volger−Fulcher law according to eq 1.

regimes.36 Competition between the neighboring spins with different spin alignment likely gives rise to the incoherent spin reversal from the AFM to the FM state at ≥Tc. In manganites, the phase separation between FM and AFM regions is well known.37 Even for different lanthanide ions, the FM and AFM regimes vary, for example, in Sm0.5Sr0.5MnO3, Tc ≤ TN, whereas for Pr0.5Sr0.5MnO3, TN ≤ Tc.38 In the presence of smaller sized A-site cations like Pr/Ca or Sm/Sr, Tc remains below TN, where Tc represents a canted FM/AFM state.8,20 The high magnetization and significant coercivity at 5 K for PrMn0.9O3 indicate the low-temperature FM state below the AFM regime, duly supported by the alterations in structural parameters. At T ≤ Tc, a majority of the spins tends to align ferromagnetically. The sharp dip of various crystal parameters at close to Tc is due to this incoherence in the relative orientation of neighboring spins that shorten the bond angles and average Mn−Mn separation. The parallel alignment attained at Tc improves by lowering the temperature up to ∼50 K increasing the FM moment, which can also be correlated F


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Figure 6. (Top) Simultaneous change of average bond angle Mn−O−Mn and FCC loop at 100 Oe showing four different temperature regions with significant variations at the transition points. Here, Mn−O−Mn = (Mn−Oac−Mn + Mn−Ob−Mn)/2. (Bottom) Schematic representation of the magnetic behavior at four temperature zones: (right to left) from 300 to 150, 150 to 100 (Tc), Tc to 50, and 50 to 10 K. Below 150 K, the spin orientation is shown by alternate colors at 10 K intervals. Rounded rectangle regions represent neighboring NPs. The antiparallel or parallel orientation of the spins becomes more pronounced proceeding toward lower temperatures below Tc, whereas the relative orientation of the spins varies at neighboring NPs at the same temperature in the absence of an external magnetic field. Below 50 K, the AFM spins within the FM matrix are enclosed inside the dotted box.

Figure 7. Pr0.9MnO3.2 NPs: (a) TEM image with high-resolution view of the selected area (open rectangle) and FFT pattern. (b) XPS spectrum of the Mn 2p level. (c) ZFC-FC curves at an applied field of 100 Oe. (Inset) M−H plot at 5 K. (d) ZFC curve at 100 Oe applied field and cell volume (V) as a function of temperature. Temperatue dependence of (e) OS and (f) Mn−O−Mn bond angles. ac and b indicate Mn−Oac−Mn and Mn− Ob−Mn bond angles, respectively.

real part of ac susceptibility measurements. The most fascinating observation is the sharp consistent dip in the cell volume, lattice parameters, Mn−O−Mn bond angles, and OS at the transition temperature from the AFM to the FM state. This characteristic variation of the structural parameters near Tc was generalized with A-site vacant Pr0.9MnO3.2 NPs having higher FM moment. The nonsimultaneous rotation of the electron spins from the antiparallel to the parallel alignment at the transition temperature is supposed to cause incoherent atomic displacements and structural deformations implicating an increased AFM character. Such a near perfect correlation between the magnetic spin orientations with the structural changes has never been observed before within the same space group over a significant temperature range. We believe that this

transition of Pr0.9MnO3.2 could not be observed in either the ZFC or FC warming and cooling curves with 100 Oe applied field (Figure 7c and Figure S7), since the AFM character is largely suppressed by the high FM moments.

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CONCLUSIONS The nonaqueous sol−gel method provides a way to prepare FM NPs of a usually AFM bulk manganite. We have shown perfect mirroring of the crystallographic lattice parameters with the magnetic transition temperatures in low-temperature FM PrMn0.9O3 NPs. The change in lattice parameters, OS, and Mn−O−Mn bond angles from 300 to 10 K is in agreement with the paramagnetic−AFM−FM transitions and a magnetically glassy state, as verified by Volger−Fulcher fitting from the G


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work paves the way to predict the transition temperatures of several other properties such as ferroelectric, magnetoresistive, and mechanical and optical properties by detailed refinement of laboratory X-ray diffraction patterns across temperatures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-9051167666. Fax: +91-33-25873020. Notes

EXPERIMENTAL SECTION

The authors declare no competing financial interest.

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Materials. All reagents were of the analytical-grade purity. Praseodymium(III) acetate hydrate (Pr(CH3COO)3·xH2O; Alfa Aesar, 99.9%), manganese(II) acetate tetrahydrate (Mn(CH3COO)2· 4H2O; Merck, ≥99.5%), and ethylene glycol (OHCH2CH2OH; Merck, ≥99%) were used without further purification. Synthesis. First, the hydrated acetate salts were heated in air at 170−210 °C to remove the water of crystallization and maintain exact stoichiometry for a completely nonaqueous sol−gel route. Thereafter, 0.61 and 0.50 g of anhydrous Pr(COOCH3)3 was dissolved separately in 130 mL of ethylene glycol by sonication for 1 h at 70 °C followed by the addition of 0.3 g of anhydrous Mn(COOCH3)2 according to the solution-phase stoichiometries of PrMn0.9O3 and Pr0.9MnO3, respectively. The solutions were vigorously stirred at 80 °C for 1 h and refluxed at 120 °C for 12 h, and the deep brown solutions were transferred in beakers. When the beakers were heat treated at 110 °C to evaporate the solvents, the sols turned into gels and finally brown powders. The brown powders were calcined at 700 °C for 4 h at 6 °C/ min to obtain the final products. Methods. FESEM images were recorded in a Carl Zeiss SUPRA 55VP FESEM. Elemental mapping was performed with the Oxford Instruments X-Max with INCA software coupled to the FESEM. TEM images were obtained by UHR-FEG-TEM, JEOL, JEM 2100 F model using a 200 kV electron source. The final stoichiometry of the products was obtained by ICP-MS analyses performed in a Thermo Scientific X-series with Plasma lab software. The variable-temperature XRD measurements were performed with a Rigaku (mini flex II, Japan) powder X-ray diffractometer having Cu Kα = 1.54059 Å radiation with the rate of 1°/min, and the sample powders were placed on a copper plate in a cryostat inside the diffractometer. Rietveld analysis of the diffraction patterns was performed by the General Structure Analysis System (GSAS) software, Los Alamos National Laboratory Report (2004). The 3-dimensional views of crystal structures were obtained using VESTA 3 software. X-ray photoelectron spectroscopy (XPS) studies were carried out on the samples mounted on copper stubs with silver paste using Al Kα radiation (1486.6 eV) in a commercial photoelectron spectrometer from VSW Scientific Instruments. The base pressure of the chamber was maintained around 5 × 10−10 mbar. Before XPS measurements, the samples were kept in vacuum to avoid moisture adsorption. The XPS data were fitted with fityk software. The magnetic measurements were performed using the Quantum Design PPMS system with a VSM magnetometer in the temperature range of 5−300 K and applied fields of 0−5 T. The temperature-dependent zero-field-cooled (ZFC) magnetization was measured by cooling the samples to 5 K under zero magnetic field, and after applying 100 Oe and 1 T field, the data was collected from 5 to 300 K. The field-cooled (FC) measurements were performed by cooling the samples in the presence of 1 T applied field.

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Article

ACKNOWLEDGMENTS A.S. thanks University Grants Commission (UGC), New Delhi, for his fellowship. The Science and Engineering Research Board (SERB) of Department of Science and Technology (DST), India, is duly acknowledged for the financial support under sanction no. SR/S1/PC-28/2011. S.B. thanks IISER Kolkata for the academic and research fund.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00815. (CIF) Gravimetric analysis, elemental mapping, magnetization plots at 80 and 120 K, XRD Rietveld refinement patterns at different temperatures, temperature dependence of cos2 ϕ, ZFC and FC plots at 1 T applied field, variation of Mn−Mn distance with temperature, FCC and FCW plots of Pr0.9MnO3.2 (PDF) H


Article

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Extensive Parallelism between Crystal Parameters and Magnetic

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