Hydrothermal synthesis of doped lanthanum zirconate nanomaterials and the effect of V–Ge...

Accepted Manuscript

Title: Hydrothermal synthesis of doped lanthanum zirconatenanomaterials and the effect of V-Ge substitution on theirstructural, electrical and dielectric properties

Author: Muhammad Asim Farid Muhammad Adnan AsgharMuhammad Naeem Ashiq Muhammad Fahad EhsanMuhammad Athar

PII: S0025-5408(14)00428-0DOI: http://dx.doi.org/doi:10.1016/j.materresbull.2014.07.052Reference: MRB 7577

To appear in: MRB

Received date: 3-10-2013Revised date: 5-2-2014Accepted date: 26-7-2014

Please cite this article as: Muhammad Asim Farid, Muhammad Adnan Asghar,Muhammad Naeem Ashiq, Muhammad Fahad Ehsan, Muhammad Athar, Hydrothermalsynthesis of doped lanthanum zirconate nanomaterials and the effect of V-Gesubstitution on their structural, electrical and dielectric properties, Materials ResearchBulletin http://dx.doi.org/10.1016/j.materresbull.2014.07.052

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http://dx.doi.org/doi:10.1016/j.materresbull.2014.07.052

http://dx.doi.org/10.1016/j.materresbull.2014.07.052

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Hydrothermal synthesis of doped lanthanum zirconate nanomaterials and the effect of V-Ge

substitution on their structural, electrical and dielectric properties

Muhammad Asim Farid, Muhammad Adnan Asghar, Muhammad Naeem Ashiq*, Muhammad

Fahad Ehsan and Muhammad Athar*

Institute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800, Pakistan

*Corresponding author.E-mail: [email protected] (M. Athar);[email protected] (M. N. Ashiq), Tel.: +923234548374; fax: +92619210085.

Graphical abstract

Variation of dielectric constant with frequency for all the synthesized materials

Highlights

Hydrothermal method has been successfully employed to synthesize the zirconates.

XRD confirmed the formation of required phase.

Increased electrical resistivity makes these materials useful for microwave devices.

Dielectric Parameters of zirconates decrease with increasing frequency.

Dielectric Constant decreases with increasing substituents concentration.

ABSTRACT

A hydrothermal method was successfully employed for the synthesis of a series of vanadium and

germanium co-doped Pyrochlore Lanthanum Zirconates with composition La2-xVxZr2-yGeyO7

(where x, y = 0.0, 0.25, 0.50, 0.75 and 1.0). The XRD and FTIR analyses confirmed the

formation of single phase except vanadium and germanium substituted samples and the

crystallite sizes are in the range of 7-31 nm for V3+-Ge4+ substituted samples. The theoretical

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compositions are confirmed by the ED-XRF studies. The room temperature electrical resistivity

increase with the substituents concentration which suggests that the synthesized materials can be

used for microwave devices as such devices required highly resistive materials. Dielectric

properties were measured in the frequency range of 6 kHz to 1 MHz. The dielectric parameters

decrease with increase in frequency. The DC resistivity data is in good agreement with the

dielectric data.

Keywords: Nanostructures; X-ray diffraction; Electrical properties; Dielectric properties

1. Introduction

As the dimensions of materials are reduced to nanometer scale, the peculiar properties differing

from bulk materials become prominent. For the last two decades, the study of preparation and

properties of various nanostructured materials has attracted extensive attention due to the fact

that physicochemical properties of particles in the size range of 1–100 nm are dramatically

different from those of their bulk counterparts [1-3]. Pyrochlore type of rare earth zirconium

oxides (Re2Zr2O7, Re = rare earth) has received considerable attention in recent years [4, 5].

Great interests have been focused on the synthesis and investigation of the properties of

Re2Zr2O7, due to their excellent properties, such as low thermal conductivity, high thermal

expansion coefficient, metal–insulator transitions, geometrically frustrated Kagome-type spin

lattices, dielectric properties, high ionic conductivity, superconductivity, luminescence and

ferromagnetism [6-11]. Among these rare earth pyrochlore materials, La2Zr2O7 has been selected

due to its high melting point (2573 K), low thermal conductivity (1.56 Wm-1K-1), relatively low

thermal expansion coefficient (9×10-6 K-1), good chemical stability and no phase transformation

up to its melting point [12-13]. Recently, they have attracted attention as geometrically frustrated

magnets, exhibiting ‘spin ice’ behavior [14]. These materials can also be used for nuclear

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applications [15], sub-catalysts for automotive exhaust gases [16], high-temperature thermal-

barrier-coating materials [17], hosts for fluorescence centers [18] and oxidation catalysts [19].

The synthesis of La2Zr2O7 has been achieved by many methods, including traditional solid-state

reaction, co-precipitation, sol–gel, moltan salt assisted method and hydrothermal methods [20–

29]. The hydrothermal method was used due to some advantages over other methods like

morphology and homogeneity control and kinetic control of reaction. The main advantage is the

low temperature range used during the reaction. The simultaneous substitution at La3+ as well as

at Zr4+ site with transition metals like Vanadium and Germanium can improve the electrical and

dielectric properties of the lanthanum zirconates. The double ions substitution philosophy in

La2Zr2O7 have not been reported previously. In the present work, synthesis of nanosized

vanadium and germanium co-doped lanthanum zirconates and the effect of vanadium and

germanium substitution on its structure have been reported. Moreover, DC electrical resistivity,

dielectric constant (έ) and dielectric loss (tan δ) have been studied for these nanoparticles

2. Materials and Methods

2.1 Experimental

The chemicals used for the synthesis of La2Zr2O7 and its derivatives are La(NO3)3·6H2O,

ZrOCl2·8H2O, VCl3, GeO2, C12H25NaO4S (sodium dodecyl sulphate) and NaOH. These

chemicals were obtained from Sigma-Aldrich and Merck and were used without further

purifications. The pyrochlore Lanthanum Zirconates (La2Zr2O7, Compound 1) was prepared by

hydrothermal method. For this purpose 1mmol (0.322g) ZrOCl2·8H2O and 1mmol (0.434g)

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La(NO3)3·6H2O were dissolved in 10 mL deionized water in a beaker. 1mmol (0.288g) of

sodium dodecyl sulphate (SDS) was added to the above mixture. This solution mixture was

stirred on a hot plate at 40°C for half an hour. After that NaOH solution (2M) was added drop-

wise to maintain the pH of the solution at 12, which resulted in precipitation due to the formation

of metal hydroxides. The precipitated mixture was further stirred for half hour. This mixture was

then poured into a 23-mL stainless steel autoclave. The autoclave was sealed and placed in oven

at 120°C for 8 hours [30-37]. After the room temperature cooling of the autoclaves the powdered

material produced in the autoclave was washed several times with deionized water and

centrifuged at each washing to remove the impurities. The synthesized powder materials were

dried at 100°C to remove the moisture contents. Vanadium and Germanium co-doped

Lanthanum Zirconates were synthesized by employing the same method as described for the

synthesis of compound-1. The initial composition of the reaction mixture before adding SDS was

maintained by varying the amounts of all the components as mentioned in the Table 1. In this

way four different compounds (2-5) were synthesized.

2.2 Characterization Techniques

Fourier transform infra-red (FTIR) spectrophotometer (8400 Shimadzu model) was used to record

the FTIR spectra of as-synthesized samples. Philips X’Pert 3040/60 diffractometer with Cu-Kα

radiation source was used to confirm the phase of the materials. The surface morphology of the

materials was studied by SEM analysis using SEM (JEOL, JSM-6490A). The energy dispersive

X-ray fluorescence (ED-XRF) analysis was performed by Horiba MESA-500 with 0-20keV energy

range to confirm the composition. The electrical resistivity was measured by the two point probe

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method using Keithley Source Meter-2400. Dielectric parameters were recorded by using LCR

meter (Waykner LCR 6600) in 6 kHz–1 MHz frequency range.

3. Results and Discussions

3.1 Powder X-Ray Diffraction Analysis

The powder XRD patterns of all synthesized compounds (1-5) are shown in (Figure 1). These

patterns were matched with standard pattern ICDD-01-071-2363. All the peaks matched

perfectly which confirm the production of single phase. A number of small peaks are appeared in

the XRD patterns when substituted with V-Ge. These peaks originate due to additional phase of

vanadium/germanium zirconates.

Various parameters calculated by using the Powder XRD data are presented in Table 2. The

crystallite size was calculated by the Scherrer’s formula (equation 1).

(1)

Where λ is X- ray wavelength and its value is 1.542 Å, β the half-peak width, θ is the Bragg

angle and K is a constant having value 0.9. The crystallite size is found in the range of 7-31nm.

The crystallite size in the present work is much smaller as compared to reported earlier 50 nm

[38]. The crystallite size is observed to increase with the substituents might be due to phase

transition [39, 40].

The lattice constant (a), cell volume (V), X-ray density (ρx-ray), bulk density (ρm) and porosity (P) are

calculated through the equations (2-6) by using XRD data.

(2)

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(3)

(4)

(5)

Where Z is the number of formula units in a unit cell which is 8 for cubic pyrochlore, M is the molecular

mass of the sample, NA is the Avogadro’s number, m is the mass of the pellet, h is the height of the pellet

and r is the radius of the pellet. The values of lattice constant (a) are found to be decreased slightly

from 10.7 to 10.5 and the cell volume (V) from 1245 to 1156 Å3 with the vanadium/ germanium

concentration. The smaller ionic radii of V3+ (0.59 Å)+ and of Ga4+(0.53 Å) as compared with

1.061 Å of La3+ and 0.72 Å of Zr4+ cause a decrease in values of cell volume as well as lattice

constant. The values of X-ray density for the substituted materials are also decreased with an

increased vanadium/ germanium concentration which is due to smaller atomic masses of the

vanadium and germanium as compared to lanthanum and zirconium. The calculated lattice strain

is decreased from 12.7 ×10-1 to 3.4 × 10-1. The variations in lattice constant, cell volume and

lattice strain with substituents indicate that the V and Ge are doped at the lanthanum zirconates

lattices. The crystallinity is increased with the substituents which are indicated by the sharpening

of peaks in the XRD patterns.

3.2 Energy Dispersive X-ray Fluorescence (ED-XRF) Analysis.

The elemental compositions of compounds 1-5 as determined by ED-XRF are shown in Table 3. It

shows that the theoretical composition and experimental values are comparable. As the amount of

V3+ and Ge4+ ions are increased the concentrations of La3+ and Zr4+ are decreased. The XRF spectrum

of compound 5 is shown in (Figure 2).

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3.3 FTIR Analysis

FTIR spectra of compounds 1-5 are shown in supplementary information (Figures S1-S5). The

FTIR spectrum of compound 1 shows absorption peaks at 530 and 490 cm-1 which corresponds

to Zr-O and La-O vibrational modes, respectively [41]. The FTIR Spectra of the compounds 2 -5

in which V and Ge were doped show the peaks at 830 and 600 cm-1, which can be related to the

vibrational modes of V-O bonds.

3.4 Scanning Electron Microscopy Analysis.

The SEM images of compounds 1, 3 and 5 are shown in Figures 3 (a-c). The effectiveness of

Vanadium and Germanium addition as substituents regarding the morphology of the particles is

evident from the SEM images. The particle size is increased with the increase in substituent

concentration and some of the particles are agglomerated.

3.5 Electrical Measurement

The resistivity calculations were done by equation as given

(7)

Where R is resistance, A is area of prepared pellets and L shows the length.

Figure 4 shows the temperature dependence of electrical resistivity of compounds 1-5 in the

temperature range of 303-673K. The electrical resistivity of compound 1 first increases with the

increase in temperature up to 372 K showing metallic behavior (Region I). Then it decreases

with the increase in temperature exhibiting semiconductor properties (Region II). Vanadium and

germanium substituted at the La and Zr sites show the same resistivity trend. Region I possesses

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the electronic conduction and region II comprises ionic conduction involving oxygen ions at

higher temperature (Arrhenius behavior). Such kind of temperature dependence of resistivity is

mainly due to the occurrence of phase transition, cation migration, cations re-ordering, presence

of impurities, structure disorders and magneto-transport effect.

In the present case, the presence of two different phases i.e. lanthanum and vanadium zirconate

may cause this type of resistivity behavior. The other possibility is structure disorder which can

be related to the change in rA/rB and this may be responsible for such behavior of resistivity. The

temperature for maximum value of electrical resistivity (ρ) is known as metal to semiconductor

transition temperature (TM-S). The value of TM-S increases with the increase in V and Ge

concentration up to x = 1.0 as shown in figure 4 which may be due to presence of second phase.

The variation of room temperature resistivity with V-Ge content is shown in figure 5. It is clear

from the Figure that the resistivity is increased by increasing V-Ge content. It has been reported

that the resistivity for germanium (46×106 µΩ-cm) is greater than that of zirconium (40µΩ-cm).

The highly resistive substituent causes the increase in resistivity. The decrease in the lattice

strain also supports the increase in resistivity due to decrease in defects (Table 1). Because of

very high resistivity these materials can be used in microwave devices, which usually involve the

high frequency radiations and also for switching applications in electronic devices.

3.6 Dielectric Measurement

The dielectric constant was determined by using the following formula.

(7)

Where C is the capacitance of the pellet in faraday, d is the thickness of the pellet in meter, A is the

cross-sectional area of the flat surface of the pellet and εo is the constant of permittivity of free

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space.

The dielectric tangent loss factor was calculated using the relation

(8)

Where δ is the loss angle, ƒ is the frequency, Rp is the equivalent parallel resistance and Cp is the

equivalent parallel capacitance. The relation given below gave dielectric loss factor.

(9)

Where, tanδ is dielectric loss factor, έ is the dielectric constant, and ἔ is the dielectric loss.

Dielectric constant, which is a measure of extent of polarization of a material, depends upon the

structure of the materials, composition and synthesis method. Frequency dependence of

dielectric constants, dielectric loss and dielectric loss factor for compounds 1-5 are shown in

Figs. 6-8, respectively. At low frequency region, high value of dielectric constant is observed,

which is found to decrease with increase in frequency. At very high frequency range, the values

of dielectric constant become constant. The dielectric loss and dielectric loss factor also shows

the same frequency dependence. This effect of frequency on dielectric constant of compounds 1-

5 is explained on the basis of polarization mechanism. The dielectric materials polarize via one

of the three mechanisms i.e. electronic, ionic or orientational polarization. The electronic

polarization and conduction mechanism are similar [42]. The relaxation time for the polarized

material is 10-11 s, therefore at higher frequencies (>1011 Hz) the dipole orientation cannot keep

up with the alternating field and thus dielectric constant become constant at higher frequencies

[43]. This effect of very high frequency on dielectric phenomena has been predicted by

Maxwell-Wagner [44].

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The compositional dependence of dielectric constant, dielectric loss and dielectric loss factor at 6

kHz, 60 kHz and 1 MHz is shown in Table 4. The dielectric constant values found to be

decreased with increased concentration of vanadium and germanium. The maximum value of

dielectric constant (2.54×103) is observed for compound 1 and the minimum value of dielectric

constant (0.82×103) is observed for compound 5 at 6 kHz frequency. This can be explained by

the conduction mechanism. As the La and Zr are replaced with V and Ge respectively, the

resistivity of the system is increased. This increase in resistivity is attributed to decreased

polarization. Dielectric loss and dielectric loss factor of compound 1-5 also decreased with

increased concentration of vanadium and geranium. The maximum value of dielectric loss

(18.34×103) and dielectric loss factor (7.211954) is observed for compound 1 at 6 kHz

frequency. Similarly Compound 5 shows minimum dielectric loss (3.74×103) and dielectric loss

factor value (4.554745) at 6 kHz frequency. The increase in dc electrical resistivity while

decrease in dielectric constant with substituents suggests that these materials can be used for

applications in microwave devices as such devices require highly resistive materials.

Conclusion

The hydrothermal method has been employed for the synthesis of V-Ge substituted lanthanum

zirconates. The XRD analysis confirmed the formation of pyrochlor structure in the undoped

sample while V-Ge doped materials showed some impurity peaks along with pyrochlor phase.

The XRF confirmed the composition of the synthesized materials. The room temperature dc

resistivity increase with the V-Ge content which is beneficial for their in the microwave devices.

The dielectric constant, dielectric loss and dielectric loss factor decrease with the frequency as

well as with the substituent concentration. The dc resistivity and dielectric data are in good

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agreement with each other. These materials can be used in microwave devices as the electrical

resistivity increase while dielectric constant decreases with the V-Ge content.

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Figure 1: Indexed Powder XRD pattern of Compounds 1-5

Figure 2: XRF Spectrum of Compound 5

Figure 3: SEM Image of (a) compound 1, (b) compound 3 and (c) compound 5

Figure 4: Effect of temperature on the electrical resistivity of compounds 1-5

Figure 5: Variation of room temperature resistivity with substituents of compounds 1-5

Figure 6: Effect of frequency on the dielectric constant of compounds 1-5

Figure 7: Effect of frequency on the dielectric loss of compounds 1-5

Figure 8: Effect of frequency on the dielectric loss factor of compounds 1-5

Table 1: Composition of reactants for the compounds (2-5).

Compounds ZrOCl2·8H2O La(NO3)3·6H2O GeO2 VCl3

La1.75V0.25Zr1.75Ge0.25O7 (2) 1.75mmol (0.564g)

1.75mmol (0.759g)

0.25mmol (0.026g)

0.25mmol (0.039g)

La1.50V0.50Zr1.50Ge0.50O7 (3) 1.50mmol (0.483g)

1.50mmol (0.651g)

0.50mmol (0.052g)

0.50mmol (0.078g)

La1.25V0.75Zr1.25Ge0.75O7 (4) 1.25mmol (0.402g)

1.25mmol (0.542g)

0.75mmol (0.078g)

0.75mmol (0.117g)

LaVZrGeO7 (5) 1mmol (0.322g)

1mmol (0.434g)

1mmol (0.104g)

1mmol (0.157g)

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Table 2: Crystallite size, lattice strain, lattice constant, cell volume, bulk and x-ray density for

the zirconate materials.

Parameters 1 2 3 4 5

Crystallite sizes (D) nm ± 1 7.21 15.14 21.51 27.4 30.9

Lattice Strain 1.27 0.83 0.43 0.39 0.34

Lattice constant (a) Å ± 0.01 10.7 10.6 10.5 10.5 10.5

Cell volume (v) Å3 ± 0.1 1245 1218 1198 1175 1156

Bulk density (ρm) g cm-3 ± 0.01 4.45 4.01 3.96 3.93 3.87

X-ray density (ρx-ray) g cm-3 ±0.01 6.86 5.92 5.74 5.56 5.35

Table 3: Elemental composition of compounds (1-5) as determined by XRF analysis.

Element 1 2 3 4 5 O 19.47 20.23 21.61 21.65 23.94

V - 2.37 4.88 8.26 11.98

Ge - 3.39 6.71 11.87 13.97

Zr 31.78 29.36 26.57 23.06 23.40

La 48.75 44.55 40.25 35.17 26.72

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Table 4: Dielectric constant (έ), Dielectric loss factor (ἔ) dielectric loss (tanδ) of compounds 1-5.

Parameters Frequency 1 2 3 4 5

Dielectric constant (×103)

6kHz 2.54 2.123 1.53 0.96 0.82

60KHz 0.63 0.59 0.49 0.34 0.33

1MHz 0.42 0.35 0.325 0.30 0.29

Dielectric loss factor

6kHz 7.21 6.75 6.09 4.72 4.55

60KHz 2.17 2.13 1.95 1.78 1.79

1MHz 1.82 1.75 1.58 1.56 1.45

Dielectric loss (×103)

6kHz 18.34 14.34 9.34 4.57 3.74

60KHz 1.37 1.25 0.96 0.61 0.60

1MHz 0.77 0.63 0.52 0.47 0.43

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1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0

2 T heta (deg.)

2

3

4

222

5

662

800-2

07711

622

440

51140

0

202

201

200

1

Inte

nsity

(a.u

)

Figure 1

of 24

Accep

ted

Man

uscr

ipt

Figure 2

of 24

Accep

ted

Man

uscr

ipt

Figure 3

of 24

Accep

ted

Man

uscr

ipt

350 400 450 500 550 600 6500

2

4

6

8

10

12

Res

istiv

ity (O

hm c

m)

(1) (2) (3) (4) (5)

T(K)

Figure 4

of 24

Accep

ted

Man

uscr

ipt

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

0

2

4

6

8

10

Roo

m te

mpe

ratu

re re

sist

ivity

(Ohm

cm

)

V, Ge Content

Figure 5

of 24

Accep

ted

Man

uscr

ipt

8 9 10 11 12 13 140

1

2

3 (1) (2) (3) (4) (5)

Ln F / Hz

Die

lect

ric C

onst

ant ×

10

3

Figure 6

of 24

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ted

Man

uscr

ipt

8 9 10 11 12 13 140

2

4

6

8

10

12

14

16

18

20 (1) (2) (3) (4) (5)

Ln F / Hz

Die

lect

ric L

oss

× 10

3

Figure 7

of 24

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ted

Man

uscr

ipt

8 9 10 11 12 13 14

2

4

6

8 (1) (2) (3) (4) (5)

Die

lect

ric L

oss

Fact

or

Ln F / Hz

Figure 8

Hydrothermal synthesis of doped lanthanum zirconate nanomaterials and the effect of V–Ge...

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