Journal of Solid State Chemistry 290 (2020) 121597

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Journal of Solid State Chemistry

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Ba doping induced modifications in the structural, morphological and

dielectric properties of double perovskite La2NiMnO6 ceramics
Shah Aarif Ul Islam a, *, Farooq Ahmad Andrabi b, Fida Mohmed a, Khalid Sultan c, Mohd Ikram a,
Kandasami Asokan d
a
Department of Physics, National Institute of Technology Srinagar, 190006, J & K, India
b
Govt. Degree College (Boys) Pulwama, University of Kashmir, 192301, J & K, India
c
Department of Physics, Central University of Kashmir, Tullamullah, Ganderbal, 190015, J & K, India
d
Materials Science Division, Inter University Accelerator Centre, 110067, New Delhi, India

A R T I C L E I N F O A B S T R A C T

Keywords: We report the modifications in the structural, morphological and dielectric properties of La2NiMnO6 (LNMO) by
LNMO doping Ba at La site for three different proportions. These ceramics were successfully synthesized by the solid-
Monoclinic state route as revealed by the X-ray diffraction patterns of three compositions. The sample phases were
P21/n
confirmed by the Rietveld refinement which showed monoclinic symmetry having P21/n space group. The
Dielectric spectroscopy
scanning electron microscopy imaging of the pristine LNMO sample reveals ablated spheres, and this gets
Relaxation behaviour
modified to sugar-cube like structures after the Ba substitution at La site. In addition to modification in
morphology, a decrease in porosity is also noticed after Ba doping as calculated from the ratio of pore-pixels to the
entire pixel-count of the image which reveals the percent porosity. The substitution of Ba2þ in place of La3þ is
expected to bring about changes in the overall conduction mechanisms and also the dielectric relaxations in the
LNMO. The temperature-dependent (100K–400K) and frequency-dependent (1 kHz–2 MHz) dielectric studies
reveal that the Ba doping results in the decrease of dielectric constant and dielectric loss. This decrease is
attributed to conversion between Ni2þ to Ni3þ and Mn4þ to Mn3þ due to Ba doping resulting in an increase of
grain resistance as such decreasing possibility of most of the electrons to reach grain boundaries, which in turn
causes a decrease in polarization and thus resulting in the decrement in the value of dielectric constant. Low
dielectric materials are preferable and more reliable as they cause low heat dissipation and reduce any parasitic
capacitance, thus allowing faster switching speeds in the fabricated devices.

1. Introduction compounds of this family with a wide range of properties can be formed.
From last few decades, the exploration of double perovskites has gained
Transition metal oxides possessing double perovskite structure huge response for the reason that some compounds belonging to this
(A2BBʹO6) consist of materials with multifunctional properties as these family Sr2FeMoO6 [8], La2VRuO6 [9] were found to show outstanding
oxides possess properties extending from ferroelectric, ferromagnetic, physical properties like half metallicity and colossal magnetoresistance.
antiferromagnetic, semiconducting, dielectric as well as spintronic Additionally, the scope for diverse composition and structural arrange-
properties. These diverse properties exhibited by this class of materials ment of ions in these compounds leads to their applicability in desired
are associated with their structure, oxidation state of the transition metal areas. Consequently, the double perovskite compounds are regarded as
ions comprising them. Studies related to these double perovskite com- the source for devising new materials with desired applications.
pounds started in the early fifties (1950). Since then hundreds of these Double perovskite La2NiMnO6 (LNMO) belongs to manganite class
compounds have been synthesized and studied as they exhibit fascinating which is a unique material with potential applications in multiple areas.
electronic, structural, optical, electrical transport, dielectric and mag- The crystal structure of LNMO has been reported to be either monoclinic
netic properties [1–7]. Owing to the availability of a wide range of cat- with space group P21/n at low temperatures or rhombohedral with space
ions, A, B and Bʹ ions, that constitute the double perovskite oxides, many group R3 at high temperatures and these two phases even coexist in a

* Corresponding author.
E-mail address: aarif_1phd15@nitsri.net (S.A. Ul Islam).

https://doi.org/10.1016/j.jssc.2020.121597
Received 11 April 2020; Received in revised form 18 July 2020; Accepted 18 July 2020
Available online 25 July 2020
0022-4596/© 2020 Elsevier Inc. All rights reserved.
S.A. Ul Islam et al. Journal of Solid State Chemistry 290 (2020) 121597

certain temperature range [10,11]. In this particular compound, the samples of La2-xBaxNiMnO6, (x ¼ 0, 0.3 & 0.5). The chemicals La2O3,
electronic charge, dielectric properties and spin can be regulated by the BaCO3, Mn2O3 and NiO (Sigma Aldrich with purity 99.9% or greater)
application of electric and/or magnetic fields [12–15]. The most were used as starting materials with the appropriate stoichiometric ratio
important aspect of this material is that above behaviours can be tuned in the solid-state reactions. The homogeneous mixing was carried in a
near the room temperature, thus proving to be effective for the use in vortexer for 10 min and later this mixture was thoroughly ground using a
spintronic applications [12,14,16]. The promising spintronic pestle and mortar. The thoroughly ground samples were calcined at
-applications of LNMO have been revealed by the discovery of large different temperatures for various durations followed by intermediate
magnetoresistance and magneto-capacitance effects by Rogado et al. grinding at the end of each calcination. Finally, the samples were heated
[12]. Previous magnetism studies revealed the near-room temperature at 1400  C for 36 h to obtain the desired polycrystalline La2-xBaxNiMnO6
ferromagnetic insulating nature in LNMO where the ferromagnetic powdered samples. The synthesized samples may likely be oxygen-
ordering is attributed to the super-exchange interaction arising between deficient as the heating was carried in the absence of oxygen supply in
the Mn–O–Ni orderly arranged bonds in the double perovskite structure the furnace chamber. These ceramics were again ground finely and
[13]. compressed into 10 mm diameter and 1–2 mm thick pellets using a hy-
In addition to that, the physical properties of these double perovskites draulic pellet press and applying a pressure of 5 Mega Pascal. Moreover,
in general and LNMO in particular remarkably change upon substituting 1–2% of polyvinyl alcohol (PVA) was added as a binding agent. The
the A site with a suitable rare-earth having different ionic radii and/or pellets were finally sintered in the air in alumina boats at temperature
different oxidation states. The change in properties can be accredited to 1450  C for 36 h in a (Nabertherm) programmable furnace, followed by
the change in B–O-Bʹ bond angles. The physical properties of double cooling to the room temperature at a rate of 120 per hour with in-
perovskite compounds are greatly influenced by the valences and ionic between annealing stages to yield the compact ceramics.
radii of A-site ions [17]. The replacement of La3þ by divalent cations like
Ca2þ, Sr2þ or some other alkali or alkaline earth metal could lead to the 2.2. Characterizations
crystal structure distortion, thereby affecting the different physical
properties (especially electrical and magnetic) of the material [18–20]. To determine the phase formed and to study the crystal structure,
Many investigations have been carried out on doping effects in A2BBʹO6 XRD pattern of sintered pellets was recorded using D8 Advance Bruker
(double perovskites) at the A site. For example, a corresponding increase diffractometer, having a source of Cu-Kα with the wavelength of 1.541Å.
and decrease in Tc has been reported in the colossal magnetoresistance At a scanning rate of 1 per minute, the XRD scan was taken from 20 to
materials Ba2FeMoO6 and Sr2FeMoO6 by doping with La and K respec- 80 with voltage 40 kV and current 40 mA. The study of microstructure
tively [21–24]. Another study revealed the polar behaviour introduced and morphology of the grains/particles was done by the scanning elec-
by the A-site Lu doping in the LNMO in which Lu doping leads to size tron microscope. The instrument used for obtaining the SEM micrographs
disorder at A-site while maintaining the same oxidation state [25]. The was JEOL JSM6490. Also, to quantify the composition of the ceramics the
doping of Sr ions in LNMO results in hole carriers, wherein the valence energy dispersive analysis of X-rays (EDAX) was done on the pelletized
states of Ni and Mn changes upon replacement of La3þ ions by Sr2þ ions samples to reveal the actual composition of the samples. The EDAX was
to retain charge neutrality within the compound [26]. Bongjae Kim et al., done on the Oxford ISIS300 attached with the SEM instrument. The
in 2010 reported that half-metallic properties are induced in the LNMO temperature-dependent dielectric study in the temperature range of
when doped with Sr and concluded that the effect is associated with the 100K–400K and dielectric dispersion study in the frequency range of 1
B-site antisite disorder [27]. Guo et al., in 2013 performed a study on the kHz to 2 MHz were performed on the LCR meter, Agilent E4980A.
different Sr-doping concentration levels on LNMO and while carrying the attached with a temperature controller, Lakeshore 325. These two were
studies on structure, valency and magnetic properties revealed that there interfaced through a Lab-View program.
is an augmentation of exchange bias effect and simultaneously
enhancement in the antisite disorder by increasing the doping levels 3. Results and discussions
[28]. Zhou et al., in 2010 in their study on Sr-doped LNMO revealed the
ferromagnetic ordering in LNMO arises due to the antisite defects 3.1. XRD analysis
induced by the Sr-doping [29].
Dielectrics comprise a technologically significant class of materials. The powder XRD pattern of La2-xBaxNiMnO6 (x ¼ 0, 0.3 & 0.5) ce-
Materials having high dielectric constant are very essential because they ramics are shown in Fig. 1(a). It is evident that the samples display better
possess applications in energy storage devices [30,31]. To have appli- crystallisation with well-defined diffraction lines.
cability in devices, a material should possess very low dielectric loss The Bragg peak having the highest intensity around 32 , assigned as
(dissipation factor) along with least dependence on temperature and the (020), (112) and (200) planes, observed in each composition
frequency such that its applicability is not limited to a certain range of confirms the double perovskite phase of all the samples [32]. The XRD
temperature or frequency. This study focuses on the structural, patterns confirm the single-phase monoclinic structure without any
morphological and dielectric studies of La2-xBaxNiMnO6 (x ¼ 0, 0.3 and noticeable secondary phase within the instrumental resolution for all the
0.5) synthesized through the solid-state reaction route. The effects of Ba samples. A representative Rietveld refined XRD pattern of LNMO is
doping mainly on the electrical transport properties of LNMO are ana- presented in Fig. 1(b). An excellent fit (Chi2 ¼ 1.17) is observed for the
lysed and discussed. Our earlier study on the effects of varying Sr doping monoclinic structure while performing the refinement [33], employing
content on the LNMO has revealed interesting results like enhancement the Pseudo-Voigt function to describe the shape of the peak. Hence all the
in structural stability and tuning of bandgap to the optimum level in this diffraction peaks correspond to the monoclinic structure and can be
material to have applications in solar cells [3]. Similarly, given the above indexed with the P21/n space group as revealed from the refinement
results, the varying amounts of Ba2þ substitution are expected to induce structural parameters. These unit cell parameters (a, b, c & β) along with
different charge carriers and thus modify the dipole interactions, which cell volume (V) obtained after the Rietveld analysis are listed in Table 1.
could greatly affect the dielectric behaviour of LNMO. The reliability factors like Rwp, Rp, Re, RBragg and (Chi2) are also tabulated.
The structural parameters viz. lattice constants, value β and cell volume
2. Experimental were also estimated using the software (SPuDs), Structure Prediction
Diagnostic Software [33] and were in good agreement with the values
2.1. Sample preparation obtained from the Rietveld analysis. As seen from the table there is a
considerably small deviation in the values obtained for the unit cell pa-
The solid-state reaction technique was used to synthesize the bulk rameters (a, b & c) for the three compositions of La2-xBaxNiMnO6 (x ¼ 0,

2
S.A. Ul Islam et al. Journal of Solid State Chemistry 290 (2020) 121597

Fig. 1. (a) The XRD profiles of La2-xBaxNiMnO6 (x ¼ 0, 0.3 & 0.5) ceramics. (b) illustrative Rietveld refinement of LNMO.

0.3 & 0.5). This small change introduced in the parent LNMO indicates
Table 1 that Ba is substituted at La-site and also does not bring about any phase
Refined unit cell, structural parameters obtained from the Rietveld refinement of change or structural instability in the original LNMO structure.
XRD patterns. The average bond lengths of Mn–O/Ni–O and average bond angles of
Parameters Sample Composition

La2NiMnO6 La1.7Ba0.3NiMnO6 La1.5Ba0.5NiMnO6 Table 2

Refined values of bond angles and bond lengths of La2-xBaxNiMnO6 (x ¼ 0, 0.3 &
a (Å) 5.5289  0.0172 5.5342  0.0421 5.54617  0.0726
0.5).
b (Å) 5.4877  0.0187 5.4918  0.0474 5.4974  0.0891
c (Å) 7.7732  0.0096 7.7845  0.0082 7.8021  0.0074 Parameters Sample composition
β ( ) 89.9757  0.1127 89.9873  0.1462 90.0127  0.1317
V (Å) 235.845 237.810 238.321 Bond Angle in deg. La2NiMnO6 La1.7Ba0.3NiMnO6 La1.5Ba0.5NiMnO6
Rwp 17.8 18.5 16.7 Mn–O1–Ni 157.6 157.1 156.4
Rp 13.4 12.8 11.4 Mn–O2–Ni 163.5 162.7 162.1
Re 15.2 13.4 12.6 Mn–O3–Ni 160.9 160.5 159.7
RBragg 11.4 12.8 10.7 Average Mn–O–Ni 160.7 160.1 159.4
Chi2 1.17 1.38 1.32 Average bond length (Å) (Å) (Å)
Mn–O 1.896 1.912 1.917
Ni–O 2.048 2.027 2.011

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S.A. Ul Islam et al. Journal of Solid State Chemistry 290 (2020) 121597

Mn–O–Ni obtained after atomic position refinements are listed in in the porosity of the samples with the Ba doping, consistent with earlier
Table 2. With increasing Ba doping concentration, it is seen that the reports on perovskite and spinel oxides [38,39]. The decrease in porosity
average length Mn–O almost remains constant, while average bond is also confirmed from the SEM analysis as explained in the following
length Ni–O and average bond angle Mn–O–Ni are found to decrease. The section.
bond lengths Mn–O and Ni–O obtained for LNMO are found to be 1.896 Å
and 2.048 Å respectively. From the Shannon ionic radii data, the bond
lengths of configurations, Mn4þ-O2-/Ni2þ-O2- and Mn3þ-O2-/Ni3þ-O2- are 3.2. SEM analysis
1.88 Å/2.04 Å and 1.95 Å/1.995 Å respectively [34].
By comparing these refined values with the values of Shannon ionic Fig. 3(a–c) shows the SEM micrographs of the three compositions of
radii data, it can be concluded that the Mn4þ-O2-/Ni2þ-O2- configuration Ba doped LNMO. Along with these micrographs, on the right side are the
is more favourable. Hence, the synthesized ceramics possess the tetra- EDAX spectra representing the elemental composition and the histogram
valent Mn and bivalent Ni ions i,e. Mn with oxidation state (þ4) and Ni with Gaussian fit representing the average grain size of each composition.
with oxidation state (þ2). There is also a partial conversion of bivalent Ni Before taking the SEM images, the pelletized samples were coated with
(Ni2þ) to trivalent Ni (Ni3þ) and tetravalent Mn (Mn4þ) to trivalent Mn 80 nm thick layer of gold on the top side to make the sample surface
(Mn3þ) introduced in the LNMO by subsequent Ba doping as indicated by conducting and thus evade any accumulation of charges. The micro-
the decrease in the values of Ni–O bond lengths in the doped samples. graphs show the near-spherical grains in the pure LNMO sample, while
Similar results have been observed in the previous study by Yuqiao Guo the morphology gets modified to the ice-cube like structures by subse-
et al. in Ca doped LNMO [35]. quent doping with Ba ions. The grain size calculations show that grains of
The average crystallite sizes of three compositions were calculated variable sizes ranging from 0.40 μm to 3.02 μm are present in each
using the Debye–Scherer’s equation for the crystallite size estimation: composition. However, the average grain sizes calculated for each
composition are listed in Table 3. The grain size calculation was per-
Kλ formed on the Image-J software. From Table 3 it can be seen that the
D¼
βhkl cosθ average grain size gets modified and decreases upon Ba doping, being
largest in the pure LNMO. This reduction in grain size may be attributed
Where D corresponds to the crystallite size, K is a number, a constant
to the variation in the ionic radii of the substituted Ba2þ ions than the
equal to 0.9 and called as the shape factor, λ is the X-ray wavelength used,
replaced La3þ ions, which causes stress or strain in the lattice and thus
βhkl is the full-width half-maximum of Bragg peak corresponding to the
hampers the process of grain growth. The porosity in the samples can be
Bragg angle (θ). The values of the averaged crystallite size thus obtained
estimated while assuming the pores having a uniform distribution
are listed in Table 3. As seen from the table the average crystallite size
throughout [40]. By calculating the pore-area percentage in the micro-
decreases with the Ba doping.
graphs, these porosities present in each composition can be estimated.
Another important aspect to evaluate the material porosity is to
The pixels in the photographs comprising the pore-areas can be added
perform the calculations of X-ray density and bulk density and hence
along with the pixels of the entire photograph in a Photoshop software.
calculate the porosity from the obtained values. We calculated X-ray
The ratio of these pore-pixels to the entire pixel-count corresponds to the
density using the formula [36]:
pore percentage and thus the porosity can be estimated. The porosity is
Z:M seen decreasing with the increasing Ba doping content, being least in the
ρðxrayÞ ¼ sample with x ¼ 0.5. The decrease in porosity may be accredited to the
NA :Vunit cell
formation of smaller grains as discussed above. These smaller grains thus
and calculated the bulk density by the formula: formed lead to effective stacking within the larger grains and thus bring
about a considerable decrease in the porosity. The values of the porosities
m
ρðbulkÞ ¼ calculated based on the above procedure are listed in Table 3.
V
The typical EDAX spectra of La2-xBaxNiMnO6 (x ¼ 0, 0.3 & 0.5)
From the values of x-ray density and bulk density, the percent samples are shown in Fig. 2 (bottom right against each SEM micrograph).
porosity of all three ceramics was calculated using the formula [37]: Each spectrum confirmed the existence of the constituent elements (i,e.
La, Ba, Ni, Mn and O) constituting the respective sample compositions.
ρðxrayÞ
P¼1  The quantitative proportion of each element obtained from the EDAX
ρðbulkÞ
spectrum is nearly equal to the expected stoichiometric proportion in
In the above formulas, Z stands for the total number of atoms each composition. The percentage weight proportion, as well as the
constituting a unit cell, which is 2 for a monoclinic structure, M is the atomic proportion of each constituent element in the respective compo-
molar mass in grams, NA is the Avogadro’s constant and V, the volume of sitions, is represented in Table 4.
the constituent unit cell. Likewise, in the next formula, m is the mass and
V is the volume of the cylindrical pelletized composition. The values
obtained from the three equations are presented in Table 3. It is noticed 3.3. Dielectric analysis
that both the X-ray density as well as the bulk density values increase
with the Ba doping concentration. There is also a corresponding decrease Dielectric spectroscopy (DS) is one of the most important experi-
mental techniques employed to explore the electrical transport behaviour
of the material by analysing its frequency dependence of dielectric
Table 3 phenomena [41]. This is used to quantify the electric and dielectric
The values obtained for bulk/x-ray density, porosity, crystallite size and average
properties of a material packed between conducting electrodes, as the
grain size of La2-xBaxNiMnO6 (x ¼ 0, 0.3 & 0.5).
function of applied frequency [42]. DS is a well-known technique
Parameters Sample composition particularly in the areas of materials science, physics, chemistry, colloidal
La2NiMnO6 La1.7Ba0.3NiMnO6 La1.5Ba0.5NiMnO6 science, polymers and pharmaceutics [43]. The dielectric spectroscopy
X-Ray density(g/cm3) 06.75 06.82 06.91 has been performed to discuss the dielectric formalism of the sample
Bulk density(g/cm3) 05.62 05.91 06.10 compositions under investigation, i.e. La2-xBaxNiMnO6 (x ¼ 0, 0.3 and
Porosity (%) 16.74 13.33 11.71 0.5) and to study the frequency-dependent and temperature-dependent
Crystallite size (nm) 33.81 29.44 25.36 dielectric parameters of interest like dielectric constant, dielectric loss
Avg. Grain size (μm) 02.86 01.14 0.72
and ac conductivity.

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S.A. Ul Islam et al. Journal of Solid State Chemistry 290 (2020) 121597

Fig. 2. (a–c) Scanning electron microscopy images of La2-xBaxNiMnO6 (x ¼ 0, 0.3 & 0.5), along with the respective EDAX spectra on then right and histograms
depicting the average grain sizes.

3.3.1. Frequency dependence of dielectric phenomena decrease is seen in the pristine sample, while in case of doped samples the
Fig. 3(a) displays the frequency dependence of relative permittivity corresponding decrease is comparatively less. Other studies have re-
(dielectric constant) of La2-xBaxNiMnO6 (x ¼ 0, 0.3 and 0.5) in a fre- ported a similar trend in the variation of dielectric constant with fre-
quency range (1 kHz - 2 MHz). In all the three compositions, the value of quency based on the Maxwell-Wagner dielectric model [44,45]. This
dielectric constant reduces with the increase in frequency, the drastic phenomenon can be elucidated on the grounds of different mechanisms

5
S.A. Ul Islam et al. Journal of Solid State Chemistry 290 (2020) 121597

Fig. 3. (a) Frequency-dependent behaviour of dielectric constant (εʹ) (b) dielectric loss ðtan δÞresponse with increasing frequency and (c) the frequency-dependent ac
conductivity of La2-xBaxNiMnO6 (x ¼ 0, 0.3 and 0.5) ceramics.

Table 4
The expected and experimental percentages of constituent elements in the La2-xBaxNiMnO6 (x ¼ 0, 0.3 & 0.5) ceramics as obtained from the EDAX analysis.
Composition (La2xBaxNiMnO6) Element EDAX Analysis

Weight (%) Atomic (%)

Expected Experimental Expected Experimental

x ¼ 0.0 La 56.99 57.24 20.00 19.83

Ni 12.04 12.60 10.00 08.28
Mn 11.27 09.89 10.00 10.91
O 19.70 20.27 60.00 60.97
x ¼ 0.3 La 48.49 47.38 17.00 16.73
Ba 08.46 07.39 03.00 02.85
Ni 12.05 13.23 10.00 08.19
Mn 11.28 10.65 10.00 09.59
O 19.71 21.36 60.00 62.63
x ¼ 0.5 La 42.82 44.25 15.00 16.83
Ba 14.11 14.03 05.00 05.66
Ni 12.06 13.31 10.00 09.86
Mn 11.29 10.33 10.00 09.44
O 19.72 18.08 60.00 58.21

of polarization that contribute to the effective dielectric constant of a the three compositions in the low-frequency range. While in the
material. high-frequency range, the dielectric loss stays constant being almost in-
There are various contributing polarization mechanisms, depending dependent of the frequency. In one of the conduction mechanisms, called
on the way a dielectric medium under investigation responds to the as (QDC) quasi-dc process, the dielectric loss tends to increase continu-
external field. Each of these mechanisms has a dominating role in its ously in the low-frequency region without showing any peak or coming
distinctive frequency range, which is inversely proportional to the time to saturation. Such behaviour has been observed by Zou et al. in their
duration of that mechanism/process. Generally, the mechanisms of di- study on dielectric properties of epoxy-nanocomposites [50]. Corre-
electrics can be classified as, starting from the low frequencies: space spondingly, owing to the non-presence of any dielectric -loss peak in all
charge or interfacial polarization (Ps), dipolar polarization (Pd), ionic the three compositions under investigation i.e. La2-xBaxNiMnO6 (x ¼ 0,
polarization (Pi) and electronic polarization (Pe) [43]. As far as the 0.3 and 0.5), the behaviour can be speculated to be a (QDC) quasi-dc
low-frequency region is concerned, almost all the polarization mecha- process.
nisms sum-up to the dielectric constant. While upon increasing fre- Fig. 3(c) shows the ac conductance variation of the different Ba-
quency, the individual contribution from different mechanisms tends to doping concentrations of LNMO. The ac conductivity is almost inde-
cease one by one and finally at large frequencies of 15 orders of pendent of the frequency in the lower range. Lin et al. have previously
magnitude, there is a contribution from only the electronic polarization. reported similar behaviour in their study on LNMO [51]. While in the
This accounts for the decreasing trend observed for dielectric constant high-frequency range the conductivity rises almost exponentially for the
with the increasing frequency [46]. As per Koop’s theory, at the micro- pristine sample, but in case of the sample with x ¼ 0.5 Ba-doping con-
scopic level, solids comprise of various conducting-grains which are centration, the rise in conductivity is very small even in the higher fre-
connected by grain boundaries [47]. An external field applied to a solid quency range [52]. The frequency dependence behaviour of σ ac of all the
cause polarization along these boundaries depending on the conductivity three curves can be correlated to the tunnelling of charge carriers across
of the sample and number of charge carriers available. The presence of the variable heights of energy barriers through the hopping mechanism
low dielectric constant at the high frequency can, therefore, be ascribed having short range. The tunnelling of hopping charge carriers across a
to the grain boundaries as these grains at high frequencies are limited to low height barrier may take lesser time than across the barrier with
possessing a small value of dielectric constant [48]. Thus, grain bound- greater height, hence at high frequency, the tunnelling will be more
aries have a significant effect in determining the dielectric response of favourable for the charge carriers owing to the generation of electronic
solids/ceramics. hopping (at high frequencies) and this, in turn, will favour the
The dielectric loss of a material is another significant parameter enhancement in conductivity at the higher frequencies [53]. Hence, the
quantifying the intrinsic energy dissipation of an electromagnetic wave, overall responsible mechanism for the ac conduction in Ba doped LNMO
which is parameterized as tan δ, the loss tangent [49]. It can also be ceramics can be considered as the electronic hopping.
defined as the lag in a degree of polarization of the material with that of
the applied field. Fig. 3(b) shows the frequency dependence of dielectric 3.3.2. Temperature dependence of dielectric phenomena
loss parameter ðtan δÞtaken at room temperature for the three composi- The temperature-dependent dielectric response of La2-xBaxNiMnO6
tions of Ba doped LNMO. The loss tangent decreases considerably in all (x ¼ 0, 0.3 and 0.5) compositions are represented in Fig. 4(a–c)

6
S.A. Ul Islam et al. Journal of Solid State Chemistry 290 (2020) 121597

Fig. 4. (a–c) The temperature dependence of dielectric constant of Ba doped LNMO ceramics at various frequencies ranging from 1 kHz to 2 MHz and (d) comparative
temperature-dependent behaviour of the three compositions at low and high frequencies.

respectively at selected frequencies ranging from low to high. As can be thermally activated hopping mechanism of charge carriers leading to an
seen from the plots, there is a flat region at low temperatures depicting increase in dielectric contribution from all mechanisms. While a very less
the almost temperature-independent dielectric constant region increase (nearly temperature-independent behaviour) is noticed in all
(100K–150K) in case of the LNMO. This flat region starts depleting with samples at large frequencies unfolding the fact that at higher frequencies
the increasing Sr doping content and almost becomes negligible for the most of the polarizing mechanisms are absent thus there is a minor effect
composition with Ba content i.e. x ¼ 0.5. The flat region starts shifting of the thermally activated hopping process on the overall value of the
from low temperature to the higher temperature range (370K–400K) in dielectric constant [55]. Moreover, due to Ba doping, as the conversion,
the sample with x ¼ 0.5. This shifting maybe because of the change in the Ni2þ to Ni3þ and Mn4þ to Mn3þ takes place, the grain resistance increases
charge state of Ni and Mn with Ba doping. Since Ba2þ ions are substituted thus decreasing the possibility of most of the electrons to reach grain
for La3þ, to preserve the charge neutrality in the sample, the Ni2þ and boundaries, which in turn causes a decrease in polarization and thus
Mn4þ change to Ni3þ and Mn3þ respectively as also revealed by the resulting in the decrement in the value of dielectric constant as the
change in (Mn–O & Ni–O) bond lengths from XRD analysis. The newly doping percentage is increased.
created ions respond differently to the applied field and temperature The plot of dielectric loss tan δ vs temperature for the synthesized
conditions, thus causing a shift in the flat dielectric constant region. ceramics i,e. La2-xBaxNiMnO6 (x ¼ 0, 0.3 and 0.5) is shown in Fig. 5. The
Moreover, the dielectric constant (εʹ) in between the extreme loss tangent is plotted at various selected frequencies ranging between 1
temperature-ranges is seen increasing rapidly, with the slope denoting kHz and 2 MHz. From the plots, it is seen that the dielectric loss has a
the rate of increase for each composition. A thermally stimulated relax- least frequency dependence up to a temperature of 125K. However, the
ation mechanism may be responsible for the step alike surge exhibited in frequency dispersion starts increasing at a temperature of 125K and on-
the values of dielectric constant first two compositions. Since at lower wards and also attains a peak value especially in the LNMO. There is a
temperature, there is least contribution of charge carriers to the effective clear upward shift in the peak position of tan δ, as the temperature in-
mobility, as the thermal energy does not suffice, thus the charge carriers creases, accompanied with ease in the full width at half maximum
have a minor contribution towards the polarization resulting in a low (FWHM) as well as the peak height with the increasing frequency. This
value of dielectric constant. However, as temperature rises, there is a rise depicts the relaxation dielectric behaviour present in the samples with x
in the hopping-rate of charge carriers due to the thermal energy supplied ¼ 0 and 0.3 [56–58]. There is also shifting of the dielectric-loss peak at
by temperature, and these carriers respond effectively to the applied field higher frequencies towards the higher temperature and this supports the
[54]. Hence, a higher polarization is set up which leads to an increased thermally stimulated relaxation mechanism as discussed in the previous
dielectric constant at high temperatures. paragraph. Similar results have been reported in other studies on mul-
The comparative behaviour of temperature-dependent dielectric tiferroics [59,60].
constant for the three compositions is also shown in Fig. 4(d). The Further, the loss peak intensity appearing in the pure LNMO is seen
comparison is done at, both the lower frequency i,e. 1 kHz as well as at decreasing with the Ba doping as seen in the sample with x ¼ 0.3 and
the higher frequency i,e. 2 MHz, thus depicting the relative frequency finally the loss peak disappears in the sample with higher Ba doping
dependence of the compositions. A drastic decrease in the value of content i,e. x ¼ 0.5. This means that the relaxation behaviour is lost in the
dielectric constants is seen for all compositions at the 2 MHz than at the 1 LNMO with higher doping of Ba. The possible reason could be the con-
kHz as already discussed above. At higher frequencies, various polarizing version of Ni2þ to Ni3þ and Mn4þ to Mn3þ which takes place after the Ba-
mechanisms lose their importance thus leading to an effective decrease in doping as suggested by the change in corresponding bond lengths
the dielectric constant. At the low frequency, there is also a sharp in- revealed by Rietveld analysis. The Ni3þ and Mn3þ so created respond
crease in all compositions with the increasing temperature, owing to the poorly to the hopping mechanism and hence result in the decrease in the

7
S.A. Ul Islam et al. Journal of Solid State Chemistry 290 (2020) 121597

Fig. 5. (a–c) The temperature dependence of dielectric loss (tanδ) of Ba doped LNMO ceramics at certain frequencies ranging from 1 kHz to 2 MHz and (d)
comparative temperature-dependent behaviour of the three compositions at low and high frequency.

relaxation process, thus the corresponding loss peak disappears in the in temperature especially in the sample with x ¼ 0.5. The change in
sample with x ¼ 0.5 having the maximum doping concentration leading oxidation states of Ni and Mn ions brought about by the divalent Ba
to the maximum probability of newly created ions. substitution at La site to maintain charge stability in the compound in-
Fig. 5(d) represents the comparative behaviour of Ba doped LNMO duces more dielectric loss into the system as the Ni3þ and Mn3þ ions
ceramics at the 1 kHz and 2 MHz. With an increase in Ba doping con- reluctantly respond to the hopping mechanism.
centration, the dielectric loss is seen increasing both at low as well as The plots depicting the ac conductivity variation with the increasing
high frequencies. Though the relaxation behaviour of dielectric loss temperature at designated frequencies for the synthesized ceramics i,e.
almost disappears, the dielectric loss increases continuously with the rise La2-xBaxNiMnO6 (x ¼ 0, 0.3 and 0.5) are shown in Fig. 6(a–c). The ac

Fig. 6. (a–c) The ac conductivity (σ ac )-response of La2-xBaxNiMnO6 (x ¼ 0, 0.3 and 0.5) ceramics to the increasing temperature for certain frequencies ranging
between 1 kHz and 2 MHz and (d) comparative response of the three compositions at low and high frequency.

8
S.A. Ul Islam et al. Journal of Solid State Chemistry 290 (2020) 121597

conductivity for these samples was calculated using the relationship [54]; CRediT authorship contribution statement

σ ac ¼ ωε0 ε tanδ Shah Aarif Ul Islam: Conceptualization, Data curation, Formal

In the above formula, ω is the applied angular frequency, ε0 stands for analysis, Methodology, Software, Writing - original draft, Writing - re-
the constant, free-space permittivity (having a value of 8.854  1012), ε view & editing, Software. Farooq Ahmad Andrabi: Writing - original
represents the relative permittivity, more commonly known as the draft, Writing - review & editing. Fida Mohmed: Writing - original draft,
dielectric constant and tanδ accounts for the dielectric loss of the ce- Data curation, Software. Khalid Sultan: Visualization, Investigation,
ramics under investigation. In the low-temperature range 100K–175K Software. Mohd Ikram: Supervision, Conceptualization, Writing - re-
almost a temperature-independent flat region could be seen in the three view & editing. Kandasami Asokan: Supervision, Conceptualization.
compositions. However, as the temperature increase beyond 175K (220K
for the sample with x ¼ 0.5), a sharp rise in conductivity is noticed in all
Declaration of competing interest
the samples at low as well as high frequencies. This rise in conductivity at
elevated temperatures can be ascribed to the drift-mobility enhancement
The authors declare that they have no known competing financial
of ions as a means of thermal activation [61,62]. This also justifies that
interests or personal relationships that could have appeared to influence
the hopping of charge carriers among the localized states is the main
the work reported in this paper.
conduction mechanism followed by these ceramics [53]. Earlier studies
reporting conductivity vs temperature behaviour are in agreement with
our results [46]. A good frequency dispersion can be seen in the pure Acknowledgements
LNMO sample as there is an appreciable variation of conductivity at
higher frequency (2 MHz) than at the lower frequency (1 kHz), which Authors SAUI and MI would like to thank the director, NIT Srinagar.
however is absent in the doped samples. Dr Saif A Khan (Scientist F, IUAC New Delhi), R C Meena (Scientist C,
Also, the comparative temperature-dependent ac conductivity IUAC New Delhi) and Dr Mukul Gupta (Scientist E, CSR Indore) are
behaviour at low frequency (1 kHz) and high frequency (2 MHz) of the acknowledged for providing SEM, Dielectric and XRD measurements
three compositions is depicted in Fig. 6(d). The comparison shows a respectively. Dr Shahnaz Majeed (Senr. Lecturer, UniKL RCMP Malaysia)
marked difference in the conductivity behaviour of pure and doped is acknowledged for useful discussions. SAUI would like to thank, Head
LNMO samples among which the pure sample shows a stronger tem- of the Department, Dr Prince A. Ganai for his encouragement and
perature as well as the frequency dependence. As evident, the conduc- support.
tivity (σ ac ) is showing an increasing trend for both the increasing
temperature as well as increasing frequency, this behaviour unfolds the References
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