Facile Synthesis, Magnetic and Electric Characterization of Mixed

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Facile Synthesis, Magnetic and Electric Characterization of Mixed Valence La0.75K0.25AMnTiO6 (A = Sr and Ba) Perovskites Ganghua Zhang,†,‡ Haijie Chen,‡ Zheming Gu,† Peizhi Zhang,§ Tao Zeng,*,† and Fuqiang Huang*,‡ †

Shanghai Key Laboratory of Engineering Materials Application and Evaluation, Shanghai Research Institute of Materials, Shanghai 200437, P. R. China ‡ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China § The Ministry of Powder Materials, Shanghai Research Institute of Materials, Shanghai 200437, P. R. China S Supporting Information *

ABSTRACT: A new series of mixed valence perovskites, La0.75K0.25AMnTiO6 (A = Sr and Ba) nanocubes, have been synthesized by a mild hydrothermal route. From powder X-ray diffraction (XRD) analysis, the crystal structure of La0.75K0.25AMnTiO6 was solved as the orthorhombic symmetry (space group Pbnm) with a random A-site or B-site arrangement. The phase purity of the products was confirmed by ICP, SEM, and EDS analyses, and the oxidation states of the B-site metal atoms were determined to be +4 for Ti and +3/+4 for Mn from XPS results. The soft ferromagnetic behavior is present in both samples. The Curie point TC has been detected as high as 309 K in La0.75K0.25SrMnTiO6 and 217 K in La0.75K0.25BaMnTiO6. A Griffiths phase can be observed at the high temperature region, which is related to the magnetic inhomogeneity induced by the existence of short-range ferromagnetic clusters. The resistivity measurements indicate that the semiconducting properties of the samples can be depicted better by variable range hopping (VRH) due to a diluted double-exchange interaction of Mn3+-O-Mn4+ with Ti4+ doping.

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in manganite perovskites,15−17 up to now, most efforts have been devoted to adjusting the stoichiometric ratio18 or vacancies19 of Ln1−xAxMnTiO6. However, alkaline-metal doping is another efficient route to tune the properties of manganites to desired functionalities, such as enhanced ferromagnetism in La1−xNaxMnO3 (TC ∼ 327 K)20 and increased magneto-resistance in La1−xKxMnO3.21 Moreover, especially for substituting the alkaline-earth cations due to the similar ionic radius, alkaline-metal doping can not only maintain the structural stabilization of the parent compounds but also introduce the fascinating properties, such as superconducting (Ba1−xKx)Fe2As2 with the highest TC ∼ 38 K in hole doped iron arsenide superconductors.22 Therefore, unique physical properties would of course be expected in the alkalinemetal doped La-A-Ti-Mn-O system. On the other hand, there are still some debates on the structure and magnetism in the existing literature on the Sr1−xAxTiMnO6 system,10,18,19 which is presumably caused by different synthesis methods or nonstoichiometric components. Commonly, Ti-doped manganites were synthesized by a “liquid-mix” method6,10,19 and a high-temperature solid-state reaction method.7−9,18 However, the nonstoichiometric components and crystal defects usually exist in the products

INTRODUCTION Perovskite-type manganites Ln1−xAxMnO3 (Ln for rare earth element; A for Ca, Sr, and Ba) have attracted considerable scientific and engineering interest due to their intriguing magnetic, electrical, and colossal magnetoresistance (CMR) properties.1−3 In order to gain insight into their unique physical properties, extensive studies have been devoted to the substitution of Mn.4−6 Among these, Ti-doped Ln1−xAxMnO3 systems became very popular since the discovery of high permittivity,7 large tunable magnetocaloric,8 and room-temperature MR effects.9 More interesting findings include a metal− insulator (M−I) transition at 210 K and a ferromagnetic transition with TC ∼ 380 K in LaSrMnTiO6,10 even higher than the highest TC perovskite manganite (∼365 K in La2/3Sr1/3MnO3).11 Normally, the ordered B-site arrangement leads to high magnetic transition temperature and enhanced ferromagnetic interaction,12,13 but no B-site ordering of Mn and Ti was found in LaSrMnTiO6 by neutron powder diffraction (NPD).10 Therefore, it is unusual for long-range magnetic ordering to occur at such high temperature in LaSrMnTiO6. Actually, nearroom-temperature ferromagnetism (TC ∼ 280 K) has been also observed in La2MnNiO614 compound without B-site ordering. Thus, more in-depth studies should be carried out on such Tidoped system and new room-temperature ferromagnetic materials can be expected by ion doping. In view of the great influence of the oxidation state on the structure and properties © XXXX American Chemical Society

Received: May 25, 2017

A

DOI: 10.1021/acs.inorgchem.7b01337 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Experimental, simulated, and deviation patterns resulting from the Rietveld refined XRD data at room temperature: (a) for La0.75K0.25SrMnTiO6 and (b) for La0.75K0.25BaMnTiO6. Insets show the enlarged view of (101) and (111) diffraction regions. order to investigate the oxidation states of Ti and Mn, X-ray photoelectron spectroscopy (XPS) was carried out on an Axis Ultra spectrometer. The binding energies were collected for Mn 2p, Ti 2p, K 2p, La 3d, Sr 3d, Ba 3d, and O 1s regions with the C1s reference of 284.8 eV. The magnetism measurements of La0.75K0.25AMnTiO6 were conducted on a quantum design physical properties measurement system (PPMS). The zero-field-cooling (ZFC) and field-cooling (FC) magnetization measurements were performed in 100 Oe, 1 kOe, 2 kOe, and 10 kOe magnetic fields from 2 to 400 K. For the resistivity measurements, the powder samples were first pressed into pills and then sintered at 1073 K for 10 h. The temperature-dependent resistivity, ρ(T), was also recorded on the PPMS by the standard fourprobe method.

obtained from the conventional solid-state reactions and “liquid-mix” method. Considering the mild reaction conditions, spontaneous crystallization and simple operations, a hydrothermal method has been adopted to synthesize various oxides with uniform particle size distribution, controlled particle morphology, and phase homogeneity.23−28 In this paper, Kdoped LaAMnTiO6 nanocrystals were prepared by a facile hydrothermal method. With the aim of revealing the effect of alkaline-earth cations on the structural and physical properties, the K-doping level was fixed to 0.25 with a typical Mn3+/Mn4+ ratio of 1:1. X-ray powder diffraction, ICP, SEM, EDS, and XPS were performed to determine the crystal structure, morphology, and oxidation states. Magnetic and electric measurements have been also studied. The rather high Curie transition temperature (TC) from paramagnetism achieved ∼309 K for La0.75K0.25SrMnTiO6. The semiconducting behavior of the samples could be explained better by the VRH model.

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RESULT AND DISCUSSION Commonly, in double perovsktie systems, the cation ordering relies on the significant difference between the ionic radii or the difference between their oxidation states greater than 2.29,30 Since the ionic radii and oxidation states of Ti4+, Mn3+, and Mn4+ are closer to each other, there is no direct indication of the cation ordering observed in double B mixed perovskite MnTi systems so far. Figure 1 shows the XRD results for as-grown La0.75K0.25AMnTiO6 at room temperature. We note that some debates remain on the structure of LaSrMnTiO6 (orthorhombic in Pbnm10 or rhombohedral in R3̅c18). In the present work, some additional Bragg peaks of (101) and (111) can be observed in both samples (shown as the insets in Figure 1). Thus, the XRD data can be indexed to the known orthorhombic structure of LaSrMnTiO6 .10 The crystal structures of La0.75K0.25AMnTiO6 are shown in Figure 2. The solved structure shows that these compounds are perovskite phases and exhibit a high degree of crystallinity. It can be found that the atomic arrangement of La/A/K and Mn/Ti are disordered on both A- and B-sites, which is consistent with the reported LaSrMnTiO6 structure solved by NPD data.10 The lattice parameters of the title compounds determined from XRD analyses are given in Table 1. The increase of the unit cell size is associated with the different ionic radii of Sr (1.18 Å) and Ba (1.35 Å). Compared with the reported LaSrMnTiO6

EXPERIMENTAL SECTION

La0.75K0.25AMnTiO6 was synthesized by a facile hydrothermal method. Briefly, 7.5 mL of La(NO3)3 (0.2 M), 10 mL of A(NO3)2 (0.2 M), 10 mL of MnCl2 (0.2 M), and 10 mL of Ti(SO4)2 (0.2 M) were initially added into a beaker by continuously stirring. Then, 60 g of KOH was weighed and added into this solution with stirring. When the reaction mixture was cooled to room temperature in the air, it was then transferred into an 80 mL Teflon-lined stainless steel autoclave with 70% filling and heat treated at 260 °C for 6 h. After the autoclave was cooled and depressurized, product was initially immersed into the diluted HNO3 (0.1 M) for 4 h and sonicleaning later with distilled water. Afterward, a fine dark crystalline powder was obtained at the bottom of the beaker. Powder XRD data were collected on a Rigaku D/Max-2000 diffractometer with Cu Kα radiation (λ = 1.5418 Å) at 40 kV, 100 mA at room temperature. The composition and morphology of the samples were checked by energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM) using a FEI Quanta 200F microscope operating at 3 kV. The chemical compositions of the samples were also confirmed by inductively coupled plasma (ICP) analysis performed on a PROFILE SPEC ICP instrument, which gives an approximate ratio of La:K:A:Mn:Ti ∼ 0.75:0.25:1.00:1.00:1.00. In B


Article

Inorganic Chemistry

Figure 2. Crystal structure of La0.75K0.25AMnTiO6 viewed along the [100] direction. Manganese/titanium coordination octahedra are indicated in gray.

Figure 3. SEM images and EDS results of the nanocrystalline samples: (a) for La0.75K0.25SrMnTiO6 and (b) for La0.75K0.25BaMnTiO6.

Table 1. Rietveld Refined Crystallographic Parameters from the XRD Data of La0.75K0.25AMnTiO6a

4+

O6. The XPS measurements were carried out to check the oxidation states of Mn and Ti (Figure 4). The XPS results

sample La0.75K0.25SrMnTiO6 a (Å) b (Å) c (Å) V (Å3) atom La/K/A

5.497(1) 5.484(1) 7.778(1) 234.48 site 4c

x 0.004(2) y 0.003(1) z 0.25 Mn/Ti 4b x 0.5 y 0 z 0 O(1) 4c x 0.012(1) y 0.473(1) z 0.25 O(2) 4d x 0.729(2) y 0.259(3) z 0.011(1) selected bond lengths (Å) and angles (deg) Mn/Ti−O(1)×2 1.951(2) Mn/Ti−O(2)×2 1.904(1) Mn/Ti−O(2)×2 1.989(4) ∠Mn/Ti−O(1)−Mn/Ti 170.47(2) ∠Mn/Ti−O(2)−Mn/Ti 171.51(1) Rwp 0.0784 Rp 0.0559 χ2 1.18 a

La0.75K0.25BaMnTiO6 5.628(1) 5.561(1) 7.968(1) 249.37 0.001(1) −0.005(1) 0.25 0.5 0 0 0.008(3) 0.502(2) 0.25 0.736(1) 0.277(1) 0.018(1)

g 1

1

1

1

Figure 4. XPS spectra of La0.75K0.25AMnTiO6: (a, b) for Mn 2p and (c, d) for Ti 2p.

1.992 (4) 1.939(7) 2.041(2) 177.36(1) 167.41(1) 0.0865 0.0650 1.26

revealed that the binding energy of Ti 2p3/2 (458.1 eV for La0.75K0.25SrMnTiO6 and 458.0 eV for La0.75K0.25BaMnTiO6) corresponds to that of previously reported Ti4+ oxidation state31 and the binding energy of Mn 2p3/2 (642.3 eV for La0.75K0.25SrMnTiO6 and 642.2 eV for La0.75K0.25BaMnTiO6) is an intermediate value between those of Mn3+ and Mn4+.32 Therefore, the valence of Ti atoms is tetravalent, whereas that of the Mn atoms is a mixed Mn3+/Mn4+, which is consistent with the charge neutrality calculated for the title compounds. The XPS spectra of K 2p, La 3d, Sr 3d, Ba 3d, and O 1s qre given in Figure S1. The FC and ZFC magnetic measurements were performed under various external fields as shown in Figure 5a,b. A sharp increase of magnetization can be observed below a certain temperature in both ZFC and FC curves. The magnetization shows almost a saturated value in the FC curves at low temperature, which properly suggests the presence of the ferromagnetic behavior. The ZFC and FC curves are separate from each other, indicating an inhomogeneous magnetic state at the low temperature region for the ZFC case, which is also observed in related ferromagnetic manganites.6,18,33 It is worth

Space group is Pbnm (No. 62) and g is the occupation factor.

structure,10 the introduction of K atoms induces a slight increase in the unit cell volume because of the larger summed ionic radii (r(K+) = 1.38 Å; r(La3+) = 1.03 Å; r(Mn3+) = 0.64 Å; r(Mn4+) = 0.53 Å; r(Ti4+) = 0.61 Å). The crystallographic parameters for La0.75K0.25AMnTiO6 are listed in Table 1. As shown in SEM photographs (Figure 3), the samples present a cubic-like morphology with a particle-size distribution around 20−50 nm. The uniform compositions are confirmed by EDS analysis (see Figure 3), which shows all five elements in the selected regions with a constant ratio. According to the rule given for the alkaline-metal doped manganites,20,21 the formula 2+ + 3+ 4+ of our compounds should be given as La3+ 0.75K0.25A Mn0.5Mn0.5C


Article

Inorganic Chemistry

Figure 5. ZFC and FC magnetization of the nanocrystalline particles under various magnetic fields: (a) for La0.75K0.25SrMnTiO6 and (b) for La0.75K0.25BaMnTiO6. The 1/χ vs T curves under various magnetic fields: (c) for La0.75K0.25SrMnTiO6 and (d) for La0.75K0.25BaMnTiO6. Dashed lines show the fitting results with the Curie−Weiss law. The data with H = 100 Oe are plotted in the insets. The Curie temperature TC and Griffiths temperature TG are indicated by arrows. The 1/χ(T) curves redrawn in the log−log form under various magnetic fields: (e) for La0.75K0.25SrMnTiO6 and (f) for La0.75K0.25BaMnTiO6. A linear fitting is shown by a solid line. Isothermal magnetization curves taken at 2 and 300 K: La0.75K0.25SrMnTiO6 (g) and La0.75K0.25BaMnTiO6 (h). The inset shows the visual images of nanoparticles in ethanol (left) and the room-temperature response to an external magnetic field (right).

noting that the low temperature separations between ZFC and FC are reduced gradually with increasing magnetic fields and finally disappeared at 1 T, which confirms the magnetic heterogeneity in both samples. During the FC process, the ferromagnetic spins could be aligned by the applied field.

However, under the ZFC process, the random orientation of the ferromagnetic clusters should be caused by the local anisotropy field at low temperatures. Thus, the magnetization of ZFC is much lower than that of FC because the low fields (≤1 kOe) are not sufficient to overcome the effect of the local D


Article

Inorganic Chemistry

Figure 6. Temperature-dependent resistivity ρ(T) curves (a, b), the plots of ln ρ vs T−1 fitting to TA model (c, d), ln(ρ/T) vs 1/T fitting to SPH model (e, f), and ln ρ vs T−1/4 fitting to VRH model (g, h) for La0.75K0.25SrMnTiO6 and La0.75K0.25BaMnTiO6, respectively.

anisotropy field. Increasing external fields could overturn the local anisotropy field and align the ferromagnetic spins, which merges the separation between the ZFC and FC. Ferromagnetism in the present samples can be readily interpreted in terms of the double exchange between Mn3+/Mn4+ cations. As previously reported in Mn-Ti mixed perovskite systems,10,18 the disordering of Mn and Ti atoms dilutes the ferromagnetism and leads to the inhomogeneous spin structures. However, considering the empty eg orbitals of Ti4+ cations, the nextnearest-neighbor exchange interactions could be properly generated between Mn3+ and Ti4+ cations. This is definitely

the origin of the ferromagnetism for stoichiometric LaSrMnTiO618 without the contribution of the double-exchange interaction of Mn3+-O-Mn4+. As shown in the 1/χ vs T plots at 100 Oe of as-prepared samples (see the insets of Figure 5c,d), an abrupt downturn below the temperature TGP (∼345 K for La0.75K0.25SrMnTiO6 and ∼274 K for La0.75K0.25BaMnTiO6) can be interpreted properly as the existence of the Griffiths phase (GP) which is always reported in the ferromagnetic perovskites with inhomogeneous magnetic states.34,35 In order to confirm this scenario, the magnetization measurements χ(T) were conE


Article

Inorganic Chemistry Table 2. Fitting Parameters of La0.75K0.25AMnTiO6 from High Temperature Resistivity Data VRH modela

TA model compounds/parameters Sr Ba a

EA (eV) 0.231 0.556

ρ0 (Ω·cm)

T0 (K)

ρ0 (Ω·cm)

0.169 0.043

3.822 × 10 6.782 × 109 8

−12

3.570 × 10 6.469 × 10−15

α−1 (nm)

N(Ef) (eV−1 cm−3)

1.729 0.138

1.063 × 1023 1.178 × 1025

The detailed calculations for the VRH model are given in the Supporting Information.

ducted under different applied fields. As shown in Figure 5c,d, the abnormal downturn in 1/χ(T) curves can be suppressed gradually with increasing the applied field. At 10 kOe, the susceptibility χ(T) data above TC obey the typical Curie−Weiss law. The 1/χ(T) for both samples under various magnetic fields were redrawn in Figure 5e,f in the log−log form. Normally, the χ(T) data for Griffiths phase can be expressed as χ−1 ∝ (T− TC)1−λ (λ for the magnetic susceptibility exponent).36 Calculated from the slopes of the fitting lines at 100 Oe, the exponent λ’s are about 0.94 for La0.75K0.25SrMnTiO6 and 0.97 for La0.75K0.25BaMnTiO6 in the expected range of 0 ≤ λ ≤ 1.36 A noticeable transient region can be observed between the high temperature paramagnetic and Griffiths phases in 100 and 1000 Oe curves. However, such a transient region disappeared in the 10 kOe curve. With increasing external field, the suppression of this kind of transient region can be interpreted by the increasing paramagnetic background and/or the saturated ferromagnetic component which could hide the weak ferromagnetic signal.34 Therefore, the existence of the Griffiths phase should be the origin of the abnormal downturn in 1/ χ(T) curves. According to the Curie−Weiss (CW) law, the 1/ χ(T) data at temperatures above TGP can be linear fitted with a positive θ, which reveals the ferromagnetic nature in our samples. The Curie constant (C) and the effective magnetic moment (μeff) were obtained from the slopes of the fitted lines (2.26 emu·K/mol and 4.25 μB/f.u. for La0.75K0.25SrMnTiO6 and 2.17 emu·K/mol and 4.17 μB/f.u. for La0.75K0.25BaMnTiO6). With the formula of μeff = gJ[S(S + 1)]1/2 μB, the theoretical spin-only magnetic moments are calculated as 3.87 μB for the Mn4+ ion (3d3, high-spin state, S = 3/2, gJ = 2) and 4.90 μB for the Mn3+ ion (3d4, high-spin state, S = 2, gJ = 2). Since La3+, K+, Sr2+, Ba2+, and Ti4+ are all nonmagnetic, μeff’s for these cations are zero. Thus, the experimental μeff’s of La0.75K0.25AMnTiO6 are very close to the calculated one with the formula: μcal = [3/ 4 μ(La3+)2 + 1/4 μ(K+)2 + μ(A2+)2 + μ(Ti4+)2 + 1/2 μ(Mn3+)2 + 1/2 μ(Mn4+)2]1/2 = 4.42 μB/f.u., which is also confirmed the 1:1 ratio of Mn3+:Mn4+ in both samples. From the obtained minima in the dM/dT curves (Figure S2), the Curie temperatures (TC) were defined as 309 K for La0.75K0.25SrMnTiO6 and 217 K for La0.75K0.25BaMnTiO6. The lower TC induced by Ba-doping can be associated with the different degree of octahedral distortion induced by the A-site doping. As listed in Table 1, Ba-doping leads to the shortened Mn−O bond lengths and decreased Mn−O(2)−Mn angle. The Mn−Mn exchange interactions are usually weakened by the longer bond length and smaller bond angle, which results in a lower Curie temperature TC as illuminated in many A-site doping perovskites.37,38 Although lower than the TC (∼380 K) reported in Sr0.94La0.94MnTiO6,10 the TC of La0.75K0.25SrMnTiO6 is still much higher than room temperature. The strong room-temperature response to an external magnetic field is shown as the inset of Figure 5g. The as-prepared La0.75K0.25SrMnTiO6 nanoparticles were easily homogeneously dispersed into ethanol (left) and isolated by a magnet within several minutes (right). After the removal of the magnet, these

nanoparticles can be readily redispersed by slight shaking. Form the isothermal magnetization (MH) curves of the title compounds at 2 and 300 K (Figure 5g,h), the hysteresis is nearly not observed, indicating the existence of a soft ferromagnetism as reported in other Ti-doped manganite perovskites.6,10,18 Commonly, the nanocubes could lead to the isotropic magnetic properties due to the isotropic growth without any dominated crystallographic plane. As a comparison, additional magnetic hysteresis loops at 2 K for the La0.75K0.25SrMnTiO6 ceramic sample sintered at 1173 K for 12 h have been measured to detect the magnetic anisotropy (as shown in Figure S3). The broadening hysteresis loops of the bulk sample can be definitely attributed to the size effect. By turning the direction of the bulk sample located in PPMS perpendicular or parallel to the applied field direction, the shapes of the hysteresis loops are obviously different, which confirms the magnetic anisotropy in the cuboid ceramic sample. The intrinsic hysteresis at 300 K again confirms the roomtemperature ferromagnetism in La0.75K0.25SrMnTiO6. The magnetic moments obtained from the MH curves at 2 K are 1.2 μB per Mn atom for La0.75K0.25SrMnTiO6 and 0.7 μB per Mn atom for La0.75K0.25BaMnTiO6, respectively. Because of the random distribution of Mn and Ti species in the B-site, the double-exchange interaction between Mn4+ and Mn3+ must be diluted by Ti4+ cations. Therefore, the experimental saturated magnetic moments are clearly lower than the theoretical one (3.5 μB per Mn atom) expected from the sample stoichiometry. Before the resistivity measurements, the purity of the ceramic samples was checked by the powder XRD (Figure S4). The temperature-dependent resistivity ρ(T) curves are shown in Figure 6. From Figure 6a,b, all the samples exhibit semiconducting behaviors. Although an MR enhancement has been reported in the Ti-doped La-Sr-Mn-O system, the introduction of K atoms makes the M−I transition disappear in the present work. Generally, the electrical transport behavior of the semiconductors could be described by three models. One is the classical thermal activation (TA) model with the expression as ρ = ρ0·exp(EA/kBT), where ρ0, EA, and kB are the infinite temperature resistivity, activation energy, and the Boltzmann constant, respectively. The second one is the small-polaron hopping (SPH) model with the expression as ρ = AT exp(EA/ kBT), where A is a constant.39 The third one is the variablerange hopping (VRH) model with the expression as ρ = ρ0 exp(T0/T)1/4, where ρ0 and T0 are the residual resistivity and characteristic temperature, respectively.40 As shown in Figure 6c−h, the transport properties of both samples have been studied by TA, SPH, and VRH models. Compared to the SPH model, the fitting result of TA model seems much better for both samples. Thus, the activation energy EA can be estimated from the slope of the linear curves using the TA model data (∼0.231 eV for La0.75K0.25SrMnTiO6 and ∼0.556 eV for La0.75K0.25BaMnTiO6, respectively). Compared with other models, the VRH model fits well our data corresponding to those previously reported in related perovskite-type systems.4,41,42 Actually, the double exchange of the Mn3+-OF


Inorganic Chemistry

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Mn4+ prefers to form small polarons. However, according to above magnetic results, the random magnetic clusters resulting from the disordered arrangement of Mn and Ti atoms weaken the double-exchange interaction and make the eg electron hopping between the clusters. Although there is somewhat contribution of the double-exchange interaction, the random magnetic distribution is a dominant effect for the electrical transport behaviors in both samples. Thus, the semiconducting behavior of our samples can be interpreted better by the VRH model. Calculated from the resistivity data in the high temperature region, the fitting parameters with SPH and VRH models are presented in Table 2.

ACKNOWLEDGMENTS This work was supported by the NSF of China Grant No. 51402341, Program of Shanghai Technology Research Leader Grant No. 17XD1420300, and LSD Project Grant No. 2016Z04.

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CONCLUSIONS New perovskite-type La0.75K0.25AMnTiO6 (A = Sr and Ba) nanocrystals have been successfully synthesized via a convenient hydrothermal method. These compounds crystallize in an orthorhombic symmetry with space group Pbnm, and the disordering of Mn and Ti atoms is detected from the XRD analysis. The soft ferromagnetic behaviors were observed in both samples at low temperature. The different magnetic behaviors between these two samples could be associated with the octahedral distortion affected by A-site doping. A Griffiths phase can be observed at high temperature due to the magnetic heterogeneity. The TC of La0.75K0.25SrMnTiO6 is lower than the one reported in LaSrMnTiO6 but still much higher than room temperature. The resistivity shows a semiconducting feature, and the electronic transport can be depicted better by the VRH model due to the formation of magnetic clusters induced by the random distribution of Ti and Mn atoms. Our study offers a new room-temperature ferromagnetic material, which is useful for the investigation of some unique physical properties of perovskites. The present work also reveals that hydrothermal synthesis may offer a feasible route to explore new functional materials. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01337.

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REFERENCES

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Article

Additional XPS analysis, the dM/dT curves, XRD patterns of annealed samples, additional MH measurements, and detailed calculations for VRH model (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.Z.). *E-mail: [email protected] (F.H.). ORCID

Tao Zeng: 0000-0001-6794-2228 Fuqiang Huang: 0000-0001-7727-0488 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. G


Article

Inorganic Chemistry

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