II. EXPERIMENTAL

The LNMO sample was prepared by the gel combustion method from the component nitrates and glycine. The amorphous material thus obtained was calcined at 923 K and then heated at 1523 K for 24 h in air. Pellets of the preformed sample of LNMO were annealed in static air (LNMO-A), flowing oxygen (LNMO-O), and N₂ (LNMO-N) atmosphere at 1123 K for 8 h. The phase characterization of these materials was carried out by powder x-ray diffraction (XRD) data recorded on a Panalytical X-pert pro diffractometer (The Netherlands) using Cu K_α radiation. The XRD data were collected in the two-theta range of 15°–90° with step width and time as 0.02° and 2.50 s, respectively. At higher temperature XRD data were recorded using an Anton Parr high temperature attachment (The Netherlands) with a platinum strip as sample holder and heater. The powder neutron diffraction (ND) data recorded at Dhruva Research Reactor, BARC, Mumbai, using neutron of wavelength 1.249 Å. The low temperature neutron diffraction patterns were recorded at several temperatures below ambient temperature using a closed-cycle refrigerator-based cryostat. At each temperature the sample was equilibrated for about an hour and diffraction data were collected between 10° and 140° in about 10 h. The observed XRD and ND data were analyzed by the Rietveld method using the Fullprof-2K software package.Reference Rodriguez-Carvajal³¹

The magnetic properties of the samples were investigated (between 5 and 300 K) using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, USA) in both field-cooled (FC) and zero-field-cooled (ZFC) condition. The hysteresis loops of the sample were also recorded at several temperatures. The dielectric properties of the samples in pellet form were studied at several frequencies using a Novacontrol Alpha AN impedance analyzer and Quatro nitrogen cryosystem (Germany) in the temperature range of 160–300 K at several frequency points. Cylindrical pellets (10 mm diameter and 2 mm thickness) of about 85% of theoretical densities were used for dielectric measurements. Silver paint was applied on the sample pellets for proper electrical contact and sandwiched between gold-plated electrodes in a parallel plate configuration for dielectric property measurements.

III. RESULTS AND DISCUSSION

The gray fluffy powder obtained after the gel combustion was amorphous. However, it turns dark gray and crystalline when heated at 1523 K for 24 h in air. The powder XRD pattern shows typical reflections attributable to the La₂NiMnO₆ without any other reflections attributable to any possible secondary phases. Further studies were carried out from this preformed LMNO powder. XRD patterns of the LNMO samples annealed in different atmospheres are shown in Fig. 1. For comparison, the XRD pattern of sample prepared at 1523 K is included in Fig. 1. The XRD pattern recorded after 1523 K and that obtained for the sample annealed at 1123 K in air do not show any significant difference and indicate no drastic change in the phase of the prepared sample. The annealing temperature is appreciable for causing any significant change in crystal structure and composition. In addition, the temperature around 1073 K has been used for observing cation ordered structure of La₂NiMnO₆ thin films due to appreciable cation migration. As such rare-earth perovskites crystallize either in the GdFeO₃-type orthorhombic lattice or LaCoO₃-type rhombohedral lattice with trivalent transition metal ions. In particular, manganates of lighter rare-earth elements have either orthorhombic or rhombohedral lattice depending on the method or preparation and oxygen stoichiometry. In the orthorhombic (Pnma) lattice the Ni and Mn are occupied statistically in 4b (1/2, 0, 0) sites and have two crystallographically distinct oxygen sites, namely 4c and 8d. However, in the cation ordered lattice the symmetry is lowered from Pnma to P2₁/n with Ni and Mn segregated to 2c (0,1/2,0) and 2d (1/2,0,0) sites. A rhombohedral feature is observed in the XRD data of LNMO-A sample (Fig. 1). However, the exact profile shape of the intense peak at 32.6° (Fig. 1) indicates an additional coexisting phase. The observed powder XRD data of the LNMO-A sample was analyzed by Rietveld refinement by considering the details of earlier reported cation disordered Pnma and Reference Dass, Yan and Goodenogh^16, Reference Blasco, Sanchez, Peraj-Cacho, Garacia, Subias and Campo¹⁸ or cation ordered P2₁/n and lattices.Reference Blasco, Garacia, Sanchez, Campo, Subias and Perz-Cacho^19, Reference Bull, Gleeson and Knight²⁴ The XRD pattern clearly demonstrates the coexistence of rhombohedral and orthorhombic phases, however, no significant differences are observed for the trials with cation ordered or disordered models. This can be attributed to the smaller difference in the x-ray scattering factor of Ni and Mn. Thus, further analysis of the sample was carried out from the powder ND data.

FIG. 1.

Powder XRD patterns of LNMO samples (Cu K_α radiation). [Indices of observed peaks are marked, subscripts m and r indicate monoclinic (M) and rhombohedral (R) phase].

The powder ND data of the LNMO-A sample were analyzed by Rietveld refinement similar to the powder XRD data. We have considered the cation ordered monoclinic (P2₁/n) and rhombohedral lattices as well as cation disordered orthorhombic (Pnma) and rhombohedral lattices for the refinement of the ND data. The background of the neutron diffraction data was modeled using linear interpolation of selected points and the Bragg peak was modeled using the pseudo-Voigt profile function. A clear evidence of cation ordering is observed from the intensity matching of the neutron diffraction profile with those calculated by ordered and disordered models. Thus, we limited our further studies to the cation ordered P2₁/n and models only. The scale factor and unit cell parameters were refined in the initial cycles followed by profile and other structural parameters. After getting a proper match in the profile, the positional parameters and thermal parameters were refined. The distribution of the Ni and Mn in the octahedral 2c and 2d sites of the P2₁/n and 3a and 3b sites of lattices are carried out using the constraint of Ni:Mn = 1:1 in both phases. The quantitative analysis of the rhombohedral and monoclinic phase was obtained from the full structural refinement of the powder ND data. The goodness of the refinements was observed by the residuals (R values) and difference plots. The weight fraction of the coexisting rhombohedral and monoclinic (P2₁/n) in the LNMO-A sample are 69(5)% and 31(4)%, respectively. The refined unit cell parameters for the two phases observed in LNMO-A are: a = 5.497(1), c = 13.216(4) Å, V = 345.9(1) ų, and Z = 3 (for Rhombohedral, phase); a = 5.432(3), b = 5.485(3), c = 7.739(4) Å, β = 89.71(6)°, V = 230.6(2) ų, and Z = 4 (for Monoclinic, P2₁/n phase). Refined structural parameters for LNMO-A as observed from powder neutron diffraction data are given in Table I. The typical Rietveld refinement plots of powder neutron diffraction and XRD data of the LNMO-A sample are shown in Figs. 2 and 3, respectively.

FIG. 2.

Rietveld refinement plot of powder neutron diffraction data of LNMO-A sample. Vertical lines indicate Bragg positions for rhombohedral phase (upper) and monoclinic (P2₁/n). phase (lower). (Rp = 6.01, Rwp: 7.72, and χ² = 3.46). (Indices of selected peaks are only marked) : a = 5.497(1) and c = 13.216(4) Å, V = 345.9(1) ų. Weight fraction = 69(5)% P2₁/n: a = 5.432(3), b = 5.485(3), c = 7.739(4) Å, β = 89.71(6)°, and V = 230.6(2) ų. Weight fraction = 31(4)%

FIG. 3.

Rietveld refinement plot of powder XRD data of LNMO-A sample. Vertical lines indicate Bragg positions for rhombohedral . phase (upper) and monoclinic (P2₁/n) phase (lower). (Indices of selected peaks are only marked. Subscripts m and r indicate monoclinic and rhombohedral phases).

TABLE I.

Structural parameters of LNMO-A at 300 K (from powder neutron diffraction data).

B_ov = 1.1(1) Ų.

The cation distribution as observed from the powder neutron diffraction data indicates a significant amount of antisite disorder in the 2c and 2d sites of monoclinic phase as well as 3a and 3b sites of the rhombohedral phase. The typical disordering in the Ni and Mn sites are about 26% in the rhombohedral phase and 35% in the monoclinic phase. It can be mentioned here that earlier structural refinement by Lin et al.Reference Lin, Chen and Liu²⁸ and Booth et al.Reference Booth, Fillman, Whitaker, Abanti Nag, Tiwari, Ramanujachary, Gopalakrishnan and Lofland¹⁷ from powder XRD data indicates fully cation ordered monoclinic P2₁/n lattice for LNMO. However, from powder neutron diffraction data, Bull et al.Reference Bull, Gleeson and Knight²⁴ have reported about 15% antisite disorder monoclinic LNMO at ambient temperature and transforms to the rhombohedral phase with a similar order of cation disordering at a higher temperature. Thus, it can be concluded that the cation ordering in the LNMO is governed mainly by the preparation condition and that remains unchanged under postannealing, whereas the fully cation disordered orthorhombic and/or rhombohedral lattices have been reported in powder XRD studies of Blasco et al.Reference Blasco, Sanchez, Peraj-Cacho, Garacia, Subias and Campo¹⁸ Thus, the conclusion about the fully cation ordering reported by Lin et al.Reference Lin, Chen and Liu²⁸ and Booth et al.Reference Booth, Fillman, Whitaker, Abanti Nag, Tiwari, Ramanujachary, Gopalakrishnan and Lofland¹⁷ need to be considered with the limitation of XRD.

Using the refined structural parameters and occupation numbers obtained from the ND studies, the XRD patterns of samples annealed in oxygen (LNMO-O) or inert (LNMO-N) atmosphere were analyzed by the Rietveld method in a similar manner. The refined unit cell parameters and phases observed in these two samples are given in Table II. The Rietveld refinement plots of powder XRD data of these two samples are shown in Figs. 4 and 5. It is observed that on annealing in oxygen atmosphere the fraction of monoclinic phase increases [weight fractions of monoclinic and rhombohedral phases in LNMO-O are 50(1)% and 50(1)%]. However, in LNMO-N, a phase separation to La₂NiO₄, La and Ni deficient perovskite type and NiO is observed.

FIG. 4.

Rietveld refinement plot of powder XRD data of LNMO-O sample. Vertical lines indicate Bragg positions for rhombohedral phase (lower) and monoclinic (P2₁/n) phase (upper). (Indices of selected peaks are only marked. Subscripts m and r indicate monoclinic and rhombohedral phases.)

FIG. 5.

Rietveld refinement plot of powder XRD data of LNMO-N sample. Vertical lines indicate Bragg positions for monoclinic (P2₁/n) phase (upper), orthorhombic La₂NiO₄ (Bmab) phase (middle), and cubic, NiO (Fm3m) phase (lower). Indices of selected peaks are only marked. (Subscripts m, o, and c indicate monoclinic, orthorhombic, and cubic phases.)

TABLE II.

Unit cell parameter of LNMO sample treated in different atmospheres (from powder XRD data).

Furthermore, the temperature variations of structural changes of the LNMO-A sample were investigated by ND (below ambient temperature) and XRD (above ambient temperature) studies. All the variable temperature diffraction studies were carried out in static air. Typical powder diffraction data of LNMO-A at some temperatures are shown in Figs. 6 and 7. Similar to the ambient temperature diffraction data, the neutron diffraction data recorded at a lower temperature were analyzed by the Rietveld method. The refined unit cell parameters of these two phases show a feeble decreasing trend with lowering in temperature. The refined unit cell parameters at different temperatures and the corresponding quantitative phase analysis data are summarized in Table III. It is observed from Table III that the monoclinic phase fraction increases as the temperature is lowered. The diffraction data recorded at 473 K show significant amounts of rhombohedral phase, while those recorded at and above 673 K show only rhombohedral phase. The monoclinic to rhombohedral phase transformation of LNMO is in agreement with that reported earlier (648 KReference Bull, Gleeson and Knight²⁴). The unit cell parameters of the LNMO sample were determined at different temperatures by the Lebail profile refinement method and they are given in Table IV. The XRD pattern recorded at ambient temperature after cooling from 1073 K [shown in Fig. 7, as 300 K (RTC)] is similar to that observed at 300 K before heating. Thus, it is inferred that the monoclinic and rhombohedral phase formation depend on the preparation condition. The coexistence of these two phases is related to the competing relative stability of the two phases.Reference Blasco, Sanchez, Peraj-Cacho, Garacia, Subias and Campo¹⁸ The environment-dependent phase evolution is further reflected in the magnetic and dielectric properties of LNMO as explained below.

FIG. 6.

ND plots of LNMO-A sample at different temperatures (recorded in air, λ = 1.249 Å). (Indices of selected peaks are only marked. Subscript m indicates monoclinic phase.)

FIG. 7.

Selected peaks of XRD patterns of LNMO-A sample at different temperature (recorded in air, λ = Cu K_α). (Indices of peaks are marked in figure. Subscripts m and r indicate monoclinic and rhombohedral phases). (RTC, indicate the XRD data recorded after cooling the sample to ambient temperature after recording the high temperature data.)

TABLE III.

Unit cell parameter of LNMO-A sample at different temperatures (from powder neutron diffraction data).

TABLE IV.

Unit cell parameter of LNMO-A sample at different temperatures (from powder XRD data).

* Data recorded after cooling to ambient temperature (300 K, RTC).

The influence of the annealing atmosphere on the magnetic properties of LNMO was studied by temperature- and field-dependent magnetization. The temperature-dependent magnetization of the sample recorded in FC and ZFC conditions are shown in Fig. 8. Around 275 K, a weak indication of ferromagnetic transition is observed in the case of the LNMO-A sample, while a sharp rise in magnetization is observed in the LNMO-O sample. Ferromagnetic transition temperatures (T _c) for the LNMO-O and LNMO-A samples obtained from the change in slope of variation of magnetization with temperature plots are 278 and 272 K, respectively. In addition to this transition, an additional ferromagnetic transition around 100 K is observed in the LNMO-A sample. Similar multiple ferromagnetic transitions have been reported in partially ordered La₂CoMnO₆ thin films.Reference Singh, Truong, Jandl and Fournier²⁶ A ferromagnetic hysteresis loop with small remanent magnetization and coercive field is observed at 250 K in the LNMO-O sample while all other samples do not show any hysteretic behavior (Fig. 9). The observed transition temperature is similar to the earlier reported T _c of LNMO.Reference Goodenough, Wold, Arnott and Menyuk^12, Reference Blasse^14, Reference Blasco, Sanchez, Peraj-Cacho, Garacia, Subias and Campo^18, Reference Blasco, Garacia, Sanchez, Campo, Subias and Perz-Cacho¹⁹ Furthermore, it can be added here that the ZFC and FC for the LNMO-O sample show large separation compared to the LNMO-A sample. However, it is important to mention here that the magnetization behavior of LNMO-A and LNMO-O samples show significantly different values of magnetization at 5 K for an applied field of 100 Oe, namely Ms ~4.5 emu/g for LNMO-O and Ms ~1.0 emu/g for LNMO-A. Earlier studies of Blasco et al.Reference Blasco, Sanchez, Peraj-Cacho, Garacia, Subias and Campo^18, Reference Blasco, Garacia, Sanchez, Campo, Subias and Perz-Cacho¹⁹ on the magnetic properties of LaNi_0.5Mn_0.5O_3+δ, indicate a ferromagnetic transition temperature in between 265 and 285 K depending on the oxygen stoichiometry. Furthermore, they have mentioned that the Ni/Mn ratio is sensitive for the structure and magnetic properties. The compositions with Ni/Mn >1 preferentially form structure with T _c < 265 K while compositions with Ni/Mn <1 form orthorhombic (Pnma) structure with T _c < 200 K. In this study the observed transition temperature of 275 K is in agreement with the Ni/Mn = 1 stoichiometry reported by Blasco et al.Reference Blasco, Garacia, Sanchez, Campo, Subias and Perz-Cacho¹⁹

FIG. 8.

Temperature dependent (FC and ZFC) magnetizations of LNMO-O, LNMO-A, and LNMO-N samples. (Inset shows magnified image of LNMO-A data.)

FIG. 9.

Magnetic hysteresis loops of LNMO-O, LNMO-A, and LNMO-N samples. (Inset shows magnified image at lower applied field.)

As mentioned earlier, both the LNMO-A and LNMO-O samples have coexisting rhombohedral and monoclinic phases. The increasing monoclinic phase in the oxygen annealed sample seems to be a responsible phase for the ferromagnetic behavior. The Ni²⁺–O–Mn⁴⁺ super exchange interactions as explained for the LNMO is a governing factor of the ferromagnetism in this sample. On the other hand, the sample annealed in the inert atmosphere shows only a weak fraction of ferromagnetism in addition to a sharp increase in magnetization (T _c) around 150 K. This feature is comparable to the magnetic properties of the cation disordered rhombohedral LaMnO_3+x phase (x ≥ 0.1).Reference Dass, Yan and Goodenogh¹⁶ It may be mentioned here that the magnetic property of LaMnO_3+δ is also strongly influenced by the δ value_, namely at δ = 0, LaMnO₃ shows exclusively antiferromagnetic interaction while a ferromagnetic interaction is favored for 0 ≤ δ ≤ 0.1.Reference Ritter, Ibarra, De Teresa, Algarabel, Marquina, Blasco, García, Oseroff and Cheong²¹ This transformation is accompanied with the reduction of Jahn–Teller distortion of the MnO₆ octahedra. The increasing ferromagnetic interaction has been attributed to the increasing cation vacancies and double exchange between Mn³⁺–O–Mn⁴⁺ of LaMnO_3+δ compositions. As explained earlier, the LNMO-N sample shows segregated La₂NiO₄ and NiO phases along with a monoclinic LNMO phase. Thus, the LNMO phase in this composition has a Mn excess stoichiometry. A comparison of the magnetic properties of the LNMO-O sample to those of LaNi_1–xMn_xO₃ compositions further confirms the Ni deficiency in this sample.Reference Dass, Yan and Goodenogh^16, Reference Blasco, Sanchez, Peraj-Cacho, Garacia, Subias and Campo¹⁸ Blasco et al.Reference Blasco, Sanchez, Peraj-Cacho, Garacia, Subias and Campo¹⁸ have reported a ferromagnetic transition temperature 152 to 156 K for LaNi_0.25Mn_0.75O₃-type orthorhombic composition. Dass et al.Reference Dass, Yan and Goodenogh¹⁶ have reported a frustrated ferromagnetic state in the cation-disordered LNMO sample quenched from 1400 °C. Furthermore, the authors have mentioned that similar cation disordering and magnetic properties are retained even after long annealing at a lower temperature.Reference Dass, Yan and Goodenogh¹⁶ Thus, one can conclude that the ordering of the Ni and Mn strongly depend on preparation and thermal history of the sample. In this study the sample prepared at 1473 K shows cation ordered monoclinic lattice, which is retained under all the annealing conditions. The electrochemical measurements show that the Ni²⁺–Mn⁴⁺ is favored by 0.8 eV over the Ni³⁺–Mn³⁺ combination in LNMO.Reference Paulsen, Thomas and Dahn³² Thus, the magnetic properties of the LNMO-O and LNMO-A samples are mainly controlled by the fraction of Ni²⁺–Mn⁴⁺ pair and oxygen stoichiometry.

The complex relative permittivities [ε*(ω) = ε′(ω) + i ε″(ω)] in the range of 160–300 K are extracted from the measured impedance data. The variations of a real part of relative permittivity with temperature for the LNMO samples are shown in Fig. 10; higher relative permittivity (~10⁴ at 500 Hz) is observed in the LNMO-A sample compared to others. A relaxor-like behavior with a broad peak in the plot of permittivity with temperature is observed in the LNMO-A sample [Fig. 10(a)]. The broad maximum (~275 K) of the permittivity of the LNMO-A sample is observed around the ferromagnetic transition temperature, which progressively shifts toward lower temperature with decreasing frequency. This observation is similar to that reported earlier by Lin et al.Reference Lin, Chen and Liu²⁸ The observed relative permittivity at 10 kHz of the air annealed sample was about two order higher than the oxygen-treated sample and about one order higher than the value reported earlier by Rogado et al. (relative permittivity = 600).Reference Rogado, Li, Sleight and Subramanian¹⁵ However, the LNMO-O sample shows a normal dielectric behavior as permittivity increases monotonically with the increase in temperature [Fig. 10(b)]. The temperature variations of the dielectric loss factor of the LNMO samples at various frequencies are shown in Fig. 11. A significantly higher loss factor is observed in the LNMO-A sample compared to LNMO-O. The temperature dependent relative permittivity of the LNMO-N sample shows quite different behavior compared to the LNMO-A and LNMO-O samples (Fig. 10). It is observed that at lower frequency the relative permittivity shows a sharp increase with temperature while an anomalous broad feature is observed around 225–250 K. The relative permittivity at 500 Hz rises to about 8000 around ambient temperature. The loss factor of the LNMO-N sample is also significantly higher compared to that of the LNMO-O sample. The magnitude of the relative permittivity of the LNMO-N and LNMO-A samples are comparable except for a clear relaxor-like behavior with a large relative permittivity in wider temperature of the LNMO-A sample.

FIG. 10.

Variation of real part of relative permittivity of (a) LNMO-A, (b) LNMO-O, and (c) LNMO-N with temperature.

FIG. 11.

Variation of loss factor (tanδ) of (a) LNMO-A, (b) LNMO-O, and (c) LNMO-N with temperature.

The relaxor-like behavior with high permittivity in the LNMO-A sample can originate from (i) interfacial polarization due to two coexisting phases in the sample, (ii) oxygen-ion vacancies in the lattice, and (iii) dynamic fluctuation of oxidation states of transition metal ions.Reference Lin, Chen and Liu^28–Reference Ikeda, Ohsumi, Ohwada, Ishii, Inami, Kakurai, Murakami, Yoshii, Mori, Horibe and Kito^30, Reference Cao and Randall^33, Reference West, Sinclair and Hirose³⁴ The variation of peak maxima can thus arise from the activation of charge carrier, due to dynamic fluctuation of charge on the Ni and Mn sites or oxygen-ion vacancies. The existence of Ni²⁺, Ni³⁺ and Mn⁴⁺, and Mn³⁺ in the LNMO sample has been suggested earlier.Reference Blasco, Sanchez, Peraj-Cacho, Garacia, Subias and Campo^18, Reference Blasco, Garacia, Sanchez, Campo, Subias and Perz-Cacho^19, Reference Asai, Sekizawa and Iida^23, Reference Bull, Gleeson and Knight²⁴ In this study, we have also observed similar coexistence of these valence states in the x-ray photoelectron spectroscopy (XPS) data of the LNMO-A sample. In addition, the presence of oxygen-ion vacancies may also lead to high permittivity and high loss behavior. Significantly higher loss character is observed in LNMO-A in LNMO-N samples compared to the LNMO-O sample (shown in Fig. 11). Comparison of the relaxor behavior of the LNMO-A sample with the charge-ordered electronic ferroelectric systems, like RFe₂O₄, RFeMO₄ (M = Co, Cu), and RGaCu₂O₄ (R = Yb and Lu) type materials suggests that the electronic exchange in the charge-ordered state may be the origin of the dielectric anomaly.Reference Yoshii, Ikeda, Matsuo, Horibe and Mori^29, Reference Ikeda, Ohsumi, Ohwada, Ishii, Inami, Kakurai, Murakami, Yoshii, Mori, Horibe and Kito³⁰ However, the more charge-ordered state in the LNMO-O sample as reflected from the magnetic properties show relatively lower relative permittivity compared to LNMO-A. The lower permittivity of the LNMO-O sample supports the annihilation of significant oxygen-ion vacancies. In addition, the presence of oxygen-ion vacancies may also lead to high permittivity and high loss behavior, which is observed in the LNMO-A sample. Thus, the relaxor behavior of the LNMO-A sample may not be solely from the multivalent characters of both coexisting phases, but most dominatingly from the oxygen-ion vacancies. The multiphasic sample of LNMO-N and associated defects give rise to higher permittivity with a distinct feature in temperature variation and higher loss behavior. Earlier, the normal dielectric behavior of the mononclinically ordered R₂NiMnO₆ was reported by Booth et al.Reference Booth, Fillman, Whitaker, Abanti Nag, Tiwari, Ramanujachary, Gopalakrishnan and Lofland¹⁷ The more charge-ordered state in the LNMO-O sample might be responsible for the magnetic properties instead of the dielectric properties. The lower permittivity indicates the annihilation of significant concentration of oxygen-ion vacancies in the LNMO-O sample.