CHAPTER 5

STUDY OF SUPERCONDUCTING

BEHAVIOR OF NbB2 COMPOSITES

106

5.1 SYNTHESIS AND CHARACTERIZATION OF BORON

EXCESS SAMPLES OF NIOBIUM DIBORIDE

5.1.1 INTRODUCTION

The discovery of high Tc,(Critical transition temperature) superconductivity in MgB2 [1]

has put an impetus in search for analogous behavior in similar systems. Although the

most similar candidates for such investigations seem to be the AlB2 type alkali, alkaline

or group III element diborides AB2 (A=Li, Na, Be, Al) but none of them is reported to

have superconductivity [2,3,4]. The group IVa Va & VIIa elements (Nb, Ta, Mo, Zr &

Hf) also form diborides isostructural to MgB2. These transition metal diborides are

studied theoretically by various groups [5-7] but the reports on experimental approach

towards the synthesis and characterization of diborides other than MgB2, are still scant.

Various controversial reports exist on the superconductivity and value of Tc for different

diborides. Hulm, Mathias and others [8,9] reported superconductivity in Boron deficient

alloys like NbB1.94, Cooper et al [10] found superconductivity in Nb- deficient

compositions, such as NbB2.5, but absence of superconductivity in stoichiometric NbB2.

Gasprov et al [11] and Kackzorowski et al [12] also reported no superconductivity in

NbB2. On the other hand, several reports exist on the presence of superconductivity in

NbB2 with different transition temperatures [13-17]. Specifically, Tc is found to vary

between 0.62 K and 9.2 K. This means that even the phase pure samples of NbB2 show

varying behavior. The clear analysis of existing literature highlights the point that the

samples are synthesized employing different synthesis methods in different reports. For

example, some groups synthesized NbB2 sample at high temperature and high pressure

conditions[18], some at high temperature with no pressure by solid state reaction route

and a few reports also exist where the sample is either synthesized by arc melting or by

some other mechano-chemical process [10, 19]. These different synthesis routes although

result in phase pure compound but the lattice parameters are different. In other words,

the synthesis conditions affect stress/strain in the unit cell and hence results in the

107

compressed/ stretched unit cell. The stretched lattice results in strong electron-phonon

interaction and hence helps in emergence of superconductivity. This was one of the

possible reasons behind the unique superconductivity of MgB2. Keeping this idea of

inducing strain in the lattice to get enhanced lattice parameters, the off stoichiometric

samples namely NbB2+x are synthesized. As expected, the excess of Boron in NbB2

samples results in the increase of lattice parameters. The role of stretched c parameter on

the superconductivity of NbB2 is described systematically.

5.1.2 EXPERIMENTAL

5.1.2.1 Synthesis of samples

The details of sample synthesis by solid state reaction route are mentioned in section 2.2

under Chapter 2 and both In-Situ and Ex-situ synthesis techniques are described in

section 1.3, Chapter 1. Briefly, the Polycrystalline bulk samples of NbB2+x (x = 0.0, 0.2,

0.4, 0.6, 0.8 & 1.0) were synthesized by Ex-situ solid-state reaction route. The

commercial NbB2 and Boron powders were taken as the precursor material. Both the

ingredients were taken in stoichiometric ratio according to the desired composition,

NbB2+x as [NbB2+x(B)]. The powders were grinded thoroughly for 2-3 h. The well

ground mixtures were palletized and encapsulated in iron tubes followed by sintering in a

tubular furnace at 1100oC in Argon flow for 20h. The ramp rate during heating was

maintained to be 10o/min. Then the samples were directly quenched to liquid nitrogen

temperature. Direct quenching to liquid nitrogen temperature or slow cooling does not

have much effect on phase purity of samples of NbB2.

5.1.2.2 Characterization of the samples

The characterizations of the samples were carried out by the following techniques:

5.1.2.2 .1 X-ray diffraction Studies

Details of X-ray diffraction technique are described in Chapter 2 under Section 2.3.1.1.

Briefly, the X-ray diffraction patterns of all the synthesized samples were recorded using

108

the synthesized samples in powder form and X-ray source consists of CuK radiation

(1.54Å). Rietveld analysis was done by Fullprof program so as to obtain lattice

parameters.

5.1.2.2 .2 Resistivity measurements

Temperature dependent resistivity measurements on NbB2+x samples were carried out

using four-probe technique and the probe was inserted in a container having Helium

liquid so as to achieve temperature as low as 4.2 K. Resistivity measurements were

carried out in the temperature range 4.2-300 K. Details of this technique are given under

Section 2.3.2.1 in Chapter 2.

5.1.2.2 .3 Magnetization Studies

Details of magnetic characterization of samples are given in Chapter 2 in section 2.3.2.2

and section 2.3.2.3. Briefly, Magnetization measurements on NbB2 sample were carried

out on a SQUID magnetometer (MPMS-XL).

5.1.3 RESULTS AND DISCUSSION

To understand the diversities of reported superconducting Tc, the structural phases of

NbB2 with different Nb/B ratios are realized. X-ray diffraction patterns of NbB2+x (x=0.0,

0.2, 0.4, 0.6, 0.8 & 1.0) are shown in Fig. 5.1. All samples crystallize in P6/mmm,

hexagonal structure. The characteristic peaks for the pure NbB2 sample are indexed in

Fig. 5.1. No extra impurity peak is noticed in any sample. Systematic shift is observed in

(002) peak towards lower angle side with the increment in Boron content indicating the

increase in c parameter. The enlarged view is shown in the inset of Fig. 5.1. A single

(002) peak is obtained up to NbB2.8 i.e. for Nb0.71B2. Actually, the Boron plane is quite

rigid and doesn’t allow the extra Boron to be incorporated at interstitial site. Hence the

non-stoichiometry or Boron excess is is accommodated by metal deficiency. The extra

boron is incorporated into the niobium diboride phase creating metal vacancy in the

lattice as discussed in various theoretical studies [20, 21]. That’s why NbB2+x samples

can be regarded as Nb1-xB2 samples and hence NbB2.8 corresponds to Nb0.71B2. From X-

109

ray diffraction it is

observed that for x=1.0

sample i.e. NbB3.0,

instead of a single (002)

peak, a doublet is

obtained instead of a

single peak obtained for

x0.8 samples. It

indicates that in x=1.0

sample the two peaks in

doblet corresponds to

NbB2 and Nb1-xB2 phase.

It means that

NbB3(NbB2+x, x=1.0)

sample exists as a

mixture of stoichiometric

NbB2 phase and Nb

deficient NbB2 phases.

That’s why boron excess

samples can be said as

NbB2 composites

containing mixture of

different phases. It can also

be inferred from these

observations that Boron

cannot be incorporated in

the Niobium diboride

lattice after a limit and

multiphase samples are

obtained if boron content is

increased further.

Fig. 5.1 X- ray diffraction patterns of NbB2+x (x=0.0,

0.2, 0.4, 0.6, 0.8 & 1.0) samples. The inset shows the

shift of (002) peak in enlarged view.

20 30 40 50 60 70 55 56

(002)

x=1.0

x=0.8

x=0.6

x=0.4

x=0.2

x=0.0

2 (deg.)

NbB2+x

(111)

(201)

(200)

(102)

(110)(002)

(101)(100)

(001)

x=1.0

x=0.8

x=0.6

x=0.4

x=0.2

x=0.0

2 (in deg.)

20 30 40 50 60 70 80

NbB2

I (a

rb.

Un

its

)

P6/mmm

Nb (0,0,0)

B (1/3, 2/3,1/2)

a = 3.11032(13) Ao

c = 3.26396(17) Ao

2 (in deg.)

Fig. 5.2 (a) Rietveld refined plot for NbB2 sample. X-

ray experimental diagram (dots), calculated pattern

(continuous line), difference (lower continuous line)

and calculated Bragg position (vertical lines in

middle).

110

Diffraction patterns are fitted

using Rietveld analysis with the

hexagonal AlB2 structure model

and space group P6/mmm (No.

191). Fig. 5.2(a) and 5.2(b) shows

the Rietveld fitted diffraction

patterns for NbB2 and NbB2.4.

The experimentally obtained

diffraction pattern matches with

the theoretically Rietveld

generated pattern. The differences

between the experimental and

calculated XRD patterns are very

small and are shown by a line

curve at the bottom of the graph. Bragg peaks obtained from Rietveld refinements are

shown by bars below the experimental and theoretical curves. It can be clearly seen that

all the Bragg peaks are obtained in the experimental data thus confirming the pure phase

formation of NbB2. It can also be noticed that no extra peak other than the Bragg peak of

NbB2 phase is found to exist in the experimental data of X-ray diffraction patterns.

x in

NbB2+x

a (Å) c (Å) Volume

(Å 3

)

c/a B/Nb

(stoichi-

ometric

ratios )

% age

of

metal

vaca-

ncy

B/Nb

(estimated

from

Rietveld

Fits)

0.0 3.11032(13) 3.26396(17) 27.345(2) 1.049 2.0 0 2.038

0.2 3.10132(13) 3.30509(18) 27.531(2) 1.066 2.2 9 2.184

0.4 3.10416(18) 3.32016(11) 27.706(1) 1.069 2.4 17 2.405

0.6 3.10187(10) 3.31951(14) 27.660(2) 1.070 2.6 23 2.626

0.8 3.10397(11) 3.31718(15) 27.678(2) 1.069 2.8 29 2.614

1.0 3.10246(10) 3.31961(11) 27.670(1) 1.070 3.0 33 2.674

20 30 40 50 60 70 80

NbB2.4

P6/mmm

Nb (0,0,0)

B (1/3,2/3,1/2)

a = 3.10416(18) Ao

c = 3.32016(11) Ao

2 (in deg.)

I (a

rb.

Un

its

)

Fig. 5.2 (b) Rietveld refined plot for NbB2.4

sample.

Table 5.1: Lattice parameters, cell volume, c/a values and B/Nb ratios for NbB2+x

samples with x = 0.0, 0.2, 0.4, 0.6, 0.8 & 1.0.

111

Rietveld analysis also calculates the exact lattice parameters of sample up to 3rd

place of

decimal and error is provided for 4th

and 5th

place of decimal. In this way, lattice

parameters are calculated for all the NbB2+x samples and are tabulated in Table 5.1. It is

observed that c parameter increases continuously with the increase in boron content up to

x=0.4 in NbB2+x samples. For pure NbB2, c = 3.26396(17)Å, which increases sharply to

3.30509(18)Å and 3.32016(11)Å for NbB2.2 & NbB2.4 samples respectively. Beyond that

parameter changes slightly in a random way and hence has reached the saturation value.

The occupancy factors are also calculated from the Rietveld analysis. As seen from Table

5.1, there is no considerable difference between the experimentally taken stochiometric

ratios and the Rietveld determined values up to NbB2.6 sample. After that the level of

Boron incorporation in the lattice or the extent of metal vacancy creation seems to be

saturated because the Rietveld determined B/ Nb ratio does not increase much after

x=0.6. It means that saturation limit of boron incorporation or metal vacancy creation

corresponds to NbB2.6 or Nb0.76B2. The percentage of metal vacancy can be calculated

mathematically from the stoichiometric ratios of the samples using the formula

% age of metal vacancy in NbB2+x sample = [1-{2/(2+x)}]*100

The calculated values of %age of metal vacancy in boron excess samples are tabulated in

Table 5.1. It can be seen from table 5.1 that maximum limit of metal vacancy creation is

approximately 23% corresponding to NbB2.6 sample. The fact of saturation limit of boron

incorporation is also confirmed by the saturation of c parameter values after NbB2.6 or

Nb0.76B2. Thereafter, the extra added Boron forms NbB2 phase along with the Nb1-xB2

phase as observed in terms of a doublet in case of NbB3(NbB2+x, x=1.0) shown in inset of

Fig. 5.1.

To have a clear idea, the lattice parameters a & c and the ratio c/a are plotted in Fig. 5.3

with varying Boron content. It is observed that c/a parameter changes exactly in the same

way as the lattice parameter c indicating the minor changes in a parameter. Thus, the

lattice expands in c-direction with the Boron excess. These structural changes are in

confirmation with other reports [18, 22, 23].

After synthesizing the samples in pure phase, the next step was to check for the existence

112

0.0 0.2 0.4 0.6 0.8 1.0

3.10

3.11

3.123.26

3.28

3.30

3.32

1.04

1.05

1.06

1.07

c /

a

La

ttic

e P

ara

me

ters

(Ao)

x in NbB2+x

c

a

c/a

Fig. 5.3 Variation of lattice parameters and c/a value

with the increasing Boron content in non-

stoichiometric Niobium boride.

of superconductivity in

NbB2+x samples. Hence,

the magnetic susceptibility

measurements were

carried out. M-T plots for

all the samples are shown

in Fig. 5.4. It is observed

that the pure NbB2 sample

doesn’t give diamagnetic

signal confirming that

pure NbB2 is non-

superconductor. NbB2.2

sample gives a very weak

diamagnetic signal at a

temperature of about 8.9

K, which can only be seen

in the enlarged view

shown in the lower inset of

Fig. 5.4. The samples with

further higher Boron

content i.e. NbB2+x with

x0.4 show considerable

diamagnetic signal at their

respective transition

temperature in the range

10-11 K. The transition

temperature is defined at

the onset of diamagnetic

signal. The upper inset of

Fig. 5.4 shows the field

cooled and zero field cooled magnetization curves for one of the composition NbB2.4.

4 6 8 10 12 14 16

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

4 6 8 10 12

-0.0016

-0.0012

-0.0008

-0.0004

0.0000

NbB2.2

NbB2

NbB2.2

NbB2

M (

em

u/g

)

T (K)

4 6 8 10 12 14

-0.20

-0.15

-0.10

-0.05

0.00

H = 10 Oe

NbB2.4

ZFC

FC

M (

em

u/g

)

T (K)

NbB2+x

ZFC

M (

em

u/g

)

T (K)

x=0.0

x=0.2

x=0.4

x=0.6

x=0.8

x=1.0

Fig. 5.4 Magnetization vs temperature

measurements showing the transition temperature

for NbB2+x samples with x=0.0, 0.2, 0.4, 0.6, 0.8 &

1.0. Lower inset shows the enlarged view for NbB2

and NbB2.2 samples. Upper inset is the FC and ZFC

magnetization plots for NbB2.4 sample.

113

0.0 0.2 0.4 0.6 0.8 1.027.2

27.3

27.4

27.5

27.6

27.7

27.8

27.9

0

1

2

3

4

5

6

7

8

9

10

11

Vo

lum

e[(A

o)3

]

x in NbB2+x

Volume

T

c (

K)

Tc

Fig. 5.5 Cell Volume and superconducting transition

temperature at different Boron contents.

This sample has a sufficient superconducting volume fraction. Hence, it is observed that

stoichiometric NbB2 sample is non-superconducting while all boron excess samples are

found to be superconducting. Thus, Superconductivity is induced in Niobium diboride

composites, NbB2+x with non-stoichiometry created by extra boron content or by lesser

niobium content.

In order to have a clear

picture of variation of

transition temperature with

Boron content, the exact

values of Tc’s are plotted in

Fig. 5.5 along with the plot

of volume of the cell(shown

in Table 5.1) with increasing

boron content. The transition

temperature increases

continuously from 0 to 11 K

with increasing born content

up to x = 0.6 sample i.e

transition temperature is

highest for NbB2.6 sample.

With further increment in

boron content, transition

temperature decreases

slightly. The cell volume

also shows exactly the same

behavior as the transition

temperature, Tc with

increasing Boron content. It

means that induction of

superconductivity and hence

0 50 100 150 200 250 300

0

100

200

300

400

500

600

700

0 100 200 300

90

100

110

120

Non- Superconducting

NbB2

(

-cm

)

T (K)

NbB2.4

(

-cm

)

T (K)Fig. 5.6 Variation of Resistivity with temperature

for NbB2.4 sample. The inset shows the same for

NbB2.

114

the transition temperature in NbB2+x samples is related with the cell dimensions or the

lattice parameters. The lattice stretchening in c-direction increases the electron phonon

interaction and hence induces superconductivity in the non-stoichiometric Niobium

diboride samples.

The Resistivity vs temperature measurement (-T) is also carried out for further

confirmation of existence of superconductivity in NbB2+x samples. Fig. 5.6 shows the -T

plots for NbB2 and NbB2.4 sample. The main panel shows the -T measurement for

NbB2.4 sample while the inset shows the same for NbB2 sample. The NbB2.4 sample

shows a sharp transition with a Tc onset of 7.5 K. On the other hand the stoichiometric

NbB2 sample just shows metallic behavior from 300 K to about T=80 K. With further

decrease in temperature, resistivity becomes almost constant and shows no

superconducting transition down to 5 K. Thus -T measurement is in confirmation with

the M-T measurement showing that only Boron excess sample is superconducting

although Tc onset obtained from magnetization measurement for NbB2.4 is comparatively

higher.

Hence, it is confirmed that

superconductivity is

introduced in NbB2 by

increasing Boron content

or by creating Nb vacancy.

The presence of vacancies

in the Niobium sub-lattice

of NbB2 brings about

considerable changes in the

density of states in the near

Fermi region and gives rise

to a peak in the density of

states [24]. The increase in

the DOS (density of states)

at fermi level corresponds

-2 -1 0 1 2

-6

-4

-2

0

2

4

6

-2 -1 0 1 2

-0.030

-0.015

0.000

0.015

0.030 NbB2.2

NbB2

M (

em

u/g

)

H (kOe)

5 K

M (

em

u/g

)

H (kOe)

x=0.0

x=0.2

x=0.4

x=0.6

x=0.8

x=1.0

Fig. 5.7 Magnetization hysteresis loops (M-H) for

NbB2+x samples with x=0.0, 0.2, 0.4, 0.6, 0.8 & 1.0.

Inset shows the enlarged view for NbB2 and NbB2.2

samples.

115

to the increase in transition temperature with Boron excess.

Magnetization hysteresis loops (M-H) are shown in Fig. 5.7 for all synthesized samples in

both the increasing and decreasing field directions at 5 K. Pure NbB2 sample doesn’t

show any negative moment, rather a paramagnetic signal is given which can be seen in

the enlarged view in the inset of Fig. 5.7. NbB2.2 sample gives weak negative moment

with the field and possess a hysteresis in increasing and decreasing field directions which

is not visible in the main graph but can be seen in the inset. All other samples with higher

Boron content i.e samples with x0.2 show considerable magnetic moments in opposite

direction of field and can be seen in the main graph of Fig. 5.7. Since the magnetization

measurements are taken in both increasing and decreasing field directions, two curves are

seen for one particular sample in both the quadrants of Fig. 5.7 thus forming the M-H

hysteresis loop. The magnetization M(H) grows (as usual) slowly with H and then falls to

near zero moment value, and further grows again in a common way. It is clear from the

M-H plots that the non-stoichiometric Niobium boride samples are Type-II

superconductor and the results are supported by existing report on this compound [25].

Although, it can be seen that

the magnetization hysteresis

loop closes very soon at

applied magnetic field of up

to 2 kOe.

To have a clear vision of

lower critical field, Hc1 and

upper critical field, Hc2 for

NbB2+x samples, the enlarged

view of first quadrant of Fig.

5.7 is shown in Fig. 5.8.

The Hc1 is taken as the

inversion point from where

the diamagnetic moment

0.0 0.5 1.0 1.5 2.0-5

-4

-3

-2

-1

0x=1.0

x=0.8

x=0.4

Hc2

Hc1

NbB2+x

M (

em

u/g

)

H (kOe)

x=0.6

Fig. 5.8 Enlarged view of M-H loops for NbB2+x

samples with x=0.4, 0.6, 0.8 & 1.0. The Hc1 & Hc2

values are marked by arrows.

116

starts decreasing or otherwise the field starts penetrating through the sample. Hc2 is taken

as the field value at which the diamagnetic signal of the sample vanishes or otherwise the

applied field completely penetrates through the sample. The lower critical field value Hc1

increases with increasing Boron content and is found to be maximum for the x=0.6

sample i.e. Hc1 = 592 Oe for NbB2.6 sample. With further increase in Boron content, Hc1

value decreases slightly. On the other hand, the upper critical field, Hc2 is same for 0.4

x0.8 samples and the value is about 2000 Oe while it is 1600 Oe for x=1.0 i.e NbB3

sample.

5.2 SYNTHESIS AND CHARACTERIZATION OF Mg

SUBSTITUTED NbB2 SAMPLES

5.2.1 INTRODUCTION

In the previous section (5.1), it is observed that Boron excess samples result in increase

of c parameter or in other words, result in lattice stretchening in c direction. The c/a value

increases from 1.05 to 1.07 with Boron excess. This lattice stretchening increases the

electron phonon interaction and hence induces superconductivity in the non-

stoichiometric NbB2+x samples. An alternative way to induce stress in the lattice is to

substitute a slightly bigger metal element at Nb site which can accommodate the same

space group. Secondly, substituting a less valent element will create holes in the lattice. It

results in increase of hole states at Fermi level and hence lowers the Fermi level as

compared to that of NbB2. Both the factors are helpful in inducing superconductivity. Nb

is tetra valent in NbB2. Mg is divalent and MgB2 itself is stable in the same space group

P6/mmm as that of NbB2. Moreover, the ionic radius of Mg is also greater than that of

Nb. So, substituting Mg at Nb site is one of the best options.

Keeping all these key points in mind, we have synthesized Nb1-xMgxB2 samples by

simple solid-state reaction route at ambient pressure in order to induce an increase in c

parameter. The structural and superconducting characterizations are carried out

systematically for all the samples. The impact of Mg substitution at Nb site on the

117

superconducting behavior of NbB2 is studied in detail.

5.2.2 EXPERIMENTAL

The details of sample synthesis by solid state reaction route are mentioned in section 2.2

under Chapter 2 and both In-Situ and Ex-situ techniques are described in section 1.3,

Chapter 1. Briefly, the Polycrystalline bulk samples of Nb1-xMgxB2 were synthesized by

Ex-situ solid-state reaction route. The commercial NbB2 and MgB2 powders were taken

as the precursor material. Both the ingredients were taken in stoichiometric ratio

according to the desired composition, Nb1-xMgxB2 as [(1-x)NbB2+x(MgB2)]. The pellets

were enclosed in soft iron tubes and then sealed in Quartz tube up to a vacuum of 10-5

torr. The sealed quartz tubes were sintered at 1100oC for 20 hours followed by natural

cooling to room temperature. The heating rate was approximately 10oC/min. It is to be

mentioned here that vaccum annealing can be done to avoid the continuous argon flow.

The X-ray diffraction patterns of all the synthesized samples were recorded using the

synthesized samples in powder form. Rietveld analysis was done by Fullprof program so

as to obtain lattice parameters. Magnetization measurements on NbB2 sample were

carried out on a SQUID magnetometer (MPMS-XL), details of which are given in Chapter

2 in section 2.3.2.2 and section 2.3.2.3 while details of X-ray diffraction technique are

given in Chapter 2 under Section 2.3.1.1.

5.2.3 RESULTS AND DISCUSSION

The X-ray diffraction patterns for the pure and Mg substituted NbB2 samples are shown

in Fig. 5.9. The characteristic peaks are indexed in the Fig. and no impurity peak of

considerable intensity is noticed. The Mg substituted samples are also phase pure like

pristine sample except a small intensity MgO peak. A shift in the (002) peak towards the

lower angle side with increasing Mg content is observed which can be seen clearly in

inset of Fig. 5.9. In order to confirm the exact phase formation and to determine the

lattice parameters precisely, the Rietveld analysis is done on all the samples. The

118

Rietveld refined diffraction pattern for pristine sample is shown in Fig. 5.10(a). The

experimentally obtained

diffraction pattern matches

well with the theoretically

Rietveld generated pattern.

The minor difference

between the experimental

and calculated XRD

patterns is shown by a line

curve at the bottom of the

graph. Bragg peaks

obtained from Rietveld

refinements are shown by

bars below the

experimental and

theoretical curves.

The lattice parameters for

this vacuum annealed NbB2

sample are found to be a =

3.11002(8)Å and

c=3.26453(11)Å with c/a

value of 1.05. The lattice

parameters values are almost

same to the NbB2 sample

synthesized under Ar flow,

shown in Fig. 5.2(a). Fig.

5.10(b) and 5.10(c) show the

observed and fitted pattern

for Mg substituted samples,

Nb0.80Mg0.20B2 and

20 30 40 50 60 70 80

55 56 57

(002)

x=0.0

x=0.20

x=0.40

2 (deg.)

MgO*

*

*

(201)

(200)

(102)(111)

(110)

(002)

(101)(100)

(001)

I (a

rb.

Un

its)

x=0.40

x=0.20

x=0.0

Nb1-x

MgxB

2

2 (deg.)Fig. 5.9 X- ray diffraction patterns of Nb1-xMgxB2

(x = 0.0, 0.20 & 0.40) samples in the angular

range 20o 2 80

o. Inset shows the enlarged

view of shift in (002) peak.

20 30 40 50 60 70 80

I (a

rb. units

)

P6/mmm

Nb (0,0,0)

B (1/3,2/3,1/2)

a = 3.11002(8) Ao

c = 3.26453(11) Ao

NbB2,vacuum annealed

1100oC, 20h

2 (in deg.)

Fig. 5.10 (a) Rietveld refined plot for NbB2

119

20 30 40 50 60 70 80

P6/mmm Nb0.60

Mg0.40

B2

a = 3.10003(10) إ

c = 3.31994(14) إ

Nb, Mg (0,0,0)

B (1/3,2/3,1/2)

I (a

rb. U

nits)

2 (in deg.)

Fig. 5.10 (c) Rietveld refined plot for

Nb0.60Mg0.40B2

Fig. 5.10 (b) Rietveld refined plot for Nb0.80Mg0.20B2

20 30 40 50 60 70 80

a = 3.10242(20) Ao

c = 3.29363(24) Ao

Nb, Mg (0,0,0)

B (1/3,2/3,1/2)

Nb0.80

Mg0.20

B2P6/mmm

I (a

rb.

un

its

)

2 (Deg.)

Nb0.60Mg0.40B2 respectively. The atomic positions taken are Nb (0, 0, 0), Mg (0, 0, 0) and

B (1/3, 2/3, 1/2) in P6/mmm space group (No. 191). There is a close agreement between

the experimentally observed and the theoretical Rietveld generated pattern. The

difference between the two is

drawn at the bottom. The

Bragg position is also marked

above the difference line. All

expected Bragg reflections

are obtained with a minor

peak of MgO whose intensity

increases with increase in Mg

content. The variation of

lattice parameters is noticed

with the increase in Mg

amount. The exact values of

rietveld refined lattice

parameters, c/a value along

with cell volume are

tabulated in Table 5.2. Both a

and c parameters changes

monotonically with the Mg

content. The parameter ‘a’

decreases slightly but c

parameter undergoes a sharp

increase with Mg content. c/a

value also increases

continuously indicating that

lattice is stretched in c

direction. The monotonic

changes in lattice parameters

are in confirmation with the

120

earlier report on Nb1-xMgxB2 [26].

To have a clear idea, the lattice parameters a & c and the ratio c/a are plotted in Fig. 5.11

with varying Mg amount in

Nb1-xMgxB2. The main panel

shows the variation of lattice

parameters with Mg content

while c/a value is plotted in the

inset. Both the c parameter

and c/a increases sharply while

‘a’ decreases with the increase

in Mg content. Thus, it is

observed that the lattice

expands in c-direction.

Magnetic susceptibility

variation with temperature (M-

T) for Nb1-xMgxB2 samples is

shown in Fig. 5.12. Pure NbB2 sample does not show any diamagnetic signal below 5 K

while Nb0.80Mg0.20B2 shows a clear diamagnetic signal at transition temperature of about

9.5 K in zero field-cooled measurement. Transition temperature increases with the

increase in Mg content. For Nb0.60Mg0.40B2 sample, Tc is 10 K. The inset shows the field

cooled and zero field cooled magnetization curves for Nb0.80Mg0.20B2 sample. In both the

situations, the diamagnetic signal is obtained at the same temperature TcDia

= 9.5 K. ZFC

moment is more than FC because of shielding contribution. Considerable diamagnetic

moment in field cooled case confirms the existence of bulk superconductivity in this

Sr. No. x in Nb1-xMgxB2 a (Å) c (Å) c/a Volume(Å3)

1 0.0 3.11002(8) 3.26453(11) 1.050 27.345(1)

2 0.20 3.10242(20) 3.29363(24) 1.062 27.466(3)

3 0.40 3.10003(10) 3.31994(14) 1.071 27.631(2)

Table 5.2 Rietveld refined parameters for Nb1-xMgxB2 samples(x=0.0-0.40)

Fig. 5.11 Variation of Lattice parameters and c/a

ratio with Mg content, x in Nb1-xMgxB2.

0.0 0.1 0.2 0.3 0.4

3.10

3.11

3.123.26

3.28

3.30

3.32

a

c

0.0 0.1 0.2 0.3 0.4

1.050

1.055

1.060

1.065

1.070c/a

c/a

x in Nb1-x

MgxB

2

x in Nb1-xMgxB2

La

ttic

e p

ara

me

ters

121

5 6 7 8 9 10 11 12

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

Nb0.60

Mg0.40

B2

Nb0.80

Mg0.20

B2

NbB2

4 6 8 10 12 14-0.030

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

Nb0.80

Mg0.20

B2

H=10Oe

FC

ZFC

M (

em

u/g

)

T (K)

ZFC

H=10 Oe

M

(e

mu

/g)

T (K)

Fig. 5.12 Magnetization –Temperature

measurements showing the transition temperature

for Nb1-xMgxB2 samples with x = 0.0, 0.20 & 0.40.

Inset shows the FC and ZFC magnetization plots for

Nb0.80Mg0.20B2 sample.

sample. Thus, superconductivity is induced with the Mg substitution at Nb site in

niobium diboride.

Magnetic measurements (M-

H) are also carried out with

continuously varying field at

a fixed temperature of 5 K in

both increasing and

decreasing directions of field.

Fig. 5.13 depicts the M- H

curves for Mg substituted

NbB2 sample while inset

shows the same for pristine

NbB2 sample. The NbB2

sample is slightly

paramagnetic and possesses a

magnetic hysteresis with

respect to the direction of

field. Paramagnetic nature at

5 K again confirms the

absence of superconductivity

in this sample. On the other

hand, both the Mg substituted

samples show clear

diamagnetic signals. If we see

in IVth quadrant, the

diamagnetic moment

develops with increasing field

and reaches to a maximum at

a field value, Hc1 of about 465

Oe & 520 Oe for x = 0.20 and

-2 -1 0 1 2

-1.8

-1.2

-0.6

0.0

0.6

1.2

1.8

-2 -1 0 1 2-0.02

-0.01

0.00

0.01

0.02

M (em

u/g)

H (kOe)

NbB2

1.Nb0.80

Mg0.20

B2

2.Nb0.60

Mg0.40

B2

M (

em

u/g

)

H (kOe)

2

1

5 K

Fig. 5.13 Magnetization hysteresis loops (M-H) for

Mg substituted Niobium diboride samples at 5 K.

Inset shows the same for pure NbB2 sample.

122

0.40 samples respectively. With further increment in field, the diamagnetic moment starts

decreasing gradually as generally happen in Type-II superconductor. The diamagnetic

signal vanishes at a field value of about 2000 Oe i.e. the sample remains no longer super

conducting with further increase in field. The magnitude of diamagnetic moment for

Nb0.60Mg0.40B2 sample is more than four times to that of Nb0.80Mg0.20B2 sample at its

peak, which shows the improved superconductivity with increasing Mg content.

5.3 SUMMARY AND CONCLUSIONS

The structural and superconducting behavior of non- stoichiometric boron excess,

NbB2+x samples and the Mg substituted NbB2 samples i.e. Nb1-xMgxB2 is studied

systematically.

Both, the Niobium vacancy in NbB2+x samples and the substitution of Mg at Nb

site in Nb1-xMgxB2 samples result in expanding of crystal lattice in c-direction,

thus, increasing the c/a ratio and the cell volume.

The upper limit to the metal vacancy creation in NbB2+x samples is observed to be

near 23% while Mg substitutes successfully at Nb site up to 40% in case of Nb1-

xMgxB2 samples.

These structural changes in NbB2+x and Nb1-xMgxB2 samples are accompanied

with the induction of superconductivity in both the systems. Resistivity and

Magnetization measurements confirm the induction of superconductivity in boron

excess and Mg substituted niobium diboride samples. The transition temperature

increases from 8.9-11 K with the increase of Niobium vacancy in NbB2+x samples

while increases from 9.5 to 10 K with increase in Mg content in Nb1-xMgxB2

samples.

The M-H hysteresis loops confirm the type-II superconductivity in both the

series. The lower critical field Hc1 increases with the increase in Boron content up

to x=0.6 sample in NbB2+x series with Hc1 592 Oe while decreases with further

increment in Boron content. The upper critical field value Hc2 is around 2000 Oe

for all superconducting NbB2+x samples except NbB3 with Hc2 1600 Oe.

123

Similarly, Hc1 of about 465 Oe & 520 Oe is observed for x = 0.20 and 0.40

samples respectively in Nb1-xMgxB2 series The upper critical field value Hc2 is

around 2000 Oe for both the superconducting Mg substituted samples.

The present investigations reveal that both the non-stoichiometry and the

substitution by less valency element like Mg can induce superconductivity in

NbB2 composites. The induction of superconductivity in these compounds is

directly related to the stretching of lattice in c-direction. That’s why improvement

of Tc is limited at 11 K for NbB2.4 sample with saturation to the increase in c-

parameter. Transition temperature may increase further if somehow lattice

stretchening in c-direction is enhanced further by some technique in niobium

diboride samples.

124

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