3
GLYCINE DOPED MgSO4.7H2O AND NiSO4.7H2O
CRYSTALS
NiSO4.7H2O and MgSO4.7H2O are hydrogen bonded crystals having a wide
range of applications in various fields. In an attempt to understand the effect of
glycine as an impurity on the properties of NiSO4.7H2O and MgSO4.7H2O crystals,
we have grown by the free evaporation method at room temperature and characterized
pure and glycine added NiSO4.7H2O (NSH) and MgSO4.7H2O (MSH) single crystals.
The
grown
crystals
were
characterized
chemically,
structurally,
optically,
mechanically and electrically. The results obtained in the present study are reported
herein and discussed in this Chapter.
3.1.
Crystals Grown
Figure 19 shows the photograph of the pure and glycine added NiSO4.7H2O
and single crystals grown in the present study. All the six crystals grown are found to
be stable in atmospheric air, greenish in colour and shining. Crystals up to a
maximum of 2 cm length were obtained. A rough thermal test carried out on all the
six grown crystals indicates that at about 85°C, the crystals lose slowly the shining
green colour and release the water of hydration with increasing temperature. Results
obtained through AAS analysis indicate the presence of Na atom at a level of 648
ppm.
Figure 20 shows the photograph of the pure and glycine doped MgSO4.7H2O
single crystals grown in the present study. All the six crystals grown are transparent
without any colour and are found to be stable in atmospheric air. Crystals upto a
55
maximum of 2 cm length were obtained. The TGA – DTA patterns observed in the
present study for pure and maximum concentrated glycine doped MgSO4.7H2O
crystals are shown in Figures 21 and 22. The TGA curves show that there is a rapid
weight loss starting at about 70 °C and continuing up to about 260 °C. This weight
loss may be due to the removal of water molecules. The patterns observed for the
glycine added MgSO4.7H2O crystals are very similar to that observed for the pure
MgSO4.7H2O crystal. So, the crystals grown in the present study can be considered to
be thermally stable up to 70 oC only. The peaks observed in the DTA patterns endorse
this. Results obtained through AAS analysis indicate the presence of natural metallic
impurities at a level of Na-486 ppm, K-264 ppm and Ca-172 ppm.
3.2. Density and Lattice Variation
The (indexed) PXRD patterns recorded are shown in Figures 23 and 24 for the
pure and glycine doped
NiSO4.7H2O and MgSO4.7H2O crystals. Appearance of
strong and sharp peaks confirms the crystalline nature of the sample crystals grown.
The indexed PXRD data obtained are given in Appendix C (Tables AC1-AC12). The
average densities and lattice parameters obtained are provided in Tables 1 and 2
respectively for the pure and glycine doped NiSO4. 7H2O and MgSO4.7H2O crystals.
The average density values and lattice parameters observed in the present study for
the pure NiSO4. 7H2O and MgSO4. 7H2O crystals agree well with the literature
values, confirming the identity of the substance. The observed change of density and
lattice volume caused by the glycine (impurity) addition indicates that the impurity
(dopant) molecules have entered into the NiSO4.7H2O and MgSO4.7H2O crystal
matrixes. Moreover, it can be seen that the density and lattice volume vary further
with the increase in impurity concentration.
56
Figure 19: Photograph showing the pure and glycine doped NiSO4.7H2O
single crystals grown [From left-Pure NiSO4.7H2O and 0.2, 0.4, 0.6,
0.8 and 1.0 mole% glycine doped NiSO4.7H2O]
Figure 20: Photograph showing the pure and glycine doped MgSO4.7H2O single
crystals grown [From left-Pure MgSO4.7H2O and 0.2, 0.4, 0.6, 0.8
and 1.0 mole% glycine doped MgSO4.7H2O]
57
Figure 21: TG-DTA patterns observed for the pure MgSO4.7H2O crystal
Figure 22 : TG-DTA patterns observed for the 1.0 mole% glycine added
MgSO4.7H2O crystal
58
Figure 23: The PXRD patterns observed for the pure and glycine doped
NiSO4.7H2O crystals
59
Figure 24: The PXRD patterns observed for the pure and glycine doped
MgSO4.7H2O crystals
60
Table 1: Average densities and lattice parameters observed for the pure and
glycine doped NiSO4.7H2O crystals.
System (with impurity
concentration in the
solution, mole %)
Density
(g/cc)
Lattice parameters
a(Å)
b(Å)
c(Å)
1.956
11.350
12.196
6.712
Volume
(Å3)
929.1
0.2
1.954
11.804
12.389
6.527
954.5
0.4
1.951
11.450
12.321
6.818
961.9
0.6
1.949
12.001
12.187
6.654
973.2
0.8
1.948
11.839
12.379
6.792
995.4
1.0
1.946
11.863
12.146
7.407
1067.3
Pure NSH
Glycine added NSH
61
Table 2: Average densities and lattice parameters observed for the pure and
glycine doped MgSO4.7H2O crystals.
System (with impurity
concentration in the
solution, mole %)
Density
(g/cc)
Lattice parameters
a(Å)
b(Å)
c(Å)
1.668
12.001
12.509
6.623
Volume
(Å3)
994.2
0.2
1.664
11.753
12.207
6.857
983.8
0.4
1.661
11.816
12.263
6.773
981.4
0.6
1.658
11.931
12.439
6.586
977.4
0.8
1.655
11.913
12.226
6.623
964.6
1.0
1.651
11.925
12.247
6.586
961.9
Pure MSH
Glycine added MSH
62
3.3. FTIR Spectra
The FTIR spectra recorded are shown in Figures 25 and 26 respectively for the
pure and glycine doped NiSO4.7H2O and MgSO4.7H2O crystals. The vibrational band
assignments are provided in Tables 3 and 4 respectively for the pure and glycine
doped NiSO4.7H2O and MgSO4.7H2O crystals. Significant difference could not be
observed for the doped crystals as the dopant concentrations considered in the present
study are small.
Four normal modes are present in the infrared region for the sulphate anion
(SO42-): a non-degenerate symmetric bending 1, a doubly degenerate symmetric
bending 2, and two triply degenerate symmetric stretching and bending 3 and 4
respectively [2]. Changes in protonation, metal complexation and solvation of SO42can modify S-O bond length and, as a result, may change the symmetry of the anion.
This leads to a shift in the vibrational bands to different wave numbers and causes the
degenerate vibrations to become non-degenerate.
The broad envelope around 3394-3198 cm-1 indicates the presence of water
and it belongs to free water symmetry stretch. The asymmetric stretch of water has
been observed at 1670-1634 cm-1. The bending mode of water has been observed at
around 460-420 cm-1. The symmetric stretch of sulphate (
3) appears at 1194-1105
cm-1. The bending modes of sulphate (
4) are positioned at around 752-730 and 654595 cm-1. The spectra observed for all the twelve grown crystals are similar to that
reported in the literature for MgSO4.7H2O [2,107,108] and ZnSO4.7H2O [27] crystals.
63
dƌĂŶƐŵŝƚƚĂŶĐĞ;Ă͘Ƶ͘Ϳ
Ͳϭ
tĂǀĞŶƵŵďĞƌ;Đŵ Ϳ
Figure 25: The FTIR spectra observed for the pure and glycine doped
NiSO4.7H2O crystals
64
dƌĂŶƐŵŝƚƚĂŶĐĞ;Ă͘Ƶ͘Ϳ
tĂǀĞŶƵŵďĞƌ;ĐŵͲϭͿ
Figure 26: The FTIR spectra observed for the pure and glycine doped
MgSO4.7H2O crystals
65
Table 3:
The FTIR spectral (vibrational) band assignments for the pure and
glycine doped NiSO4.7H2O crystals
Wave numbers (cm-1) observed for
0.2
mole%
glycine
added
0.4
mole%
glycine
added
3313
3198
3408
3379
3329
3358
3312
3286
3221
3213
1668
1634
1634
1670
1634
1400
1337
1400
1194
1111
NSH
0.6
mole%
glycine
added
1.0
mole%
glycine
added
3379
3356
3323
3358
3323
3213
1634
1664
1634
1670
1634
1398
1400
1400
1400
1194
1111
1136
1105
1192
1109
1109
1194
1111
752
752
750
752
750
730
654
606
644
606
608
608
608
640
595
463
460
450
450
445
450
3381
66
0.8
mole%
glycine
added
Band
assignment
Presence of
H2O
molecules
(free water
symmetric
stretch)
OH2 bending
mode
(asymmetric
stretch)
Combination
bond
3SO4
(symmetric
stretch)
4SO4
(bending
mode)
4SO4
(stretching
vibration)
H2O (bending
mode,
asymmetric
stretch)
Table 4: The FTIR spectral (vibrational) band assignments for the pure and
glycine doped MgSO4.7H2O crystals
Wave numbers (cm-1) observed for
MSH
0.2
mole%
glycine
added
0.4
mole%
glycine
added
0.6
mole%
glycine
added
1.0 mole
%
glycine
added
3377
3319
3209
3377
3319
3211
3371
3315
3200
3394
3394
3383
3321
3211
1670
1670
1668
1668
1670
1670
1454
1398
1454
1400
1452
1400
1454
1400
1452
1400
1454
1400
1194
1113
1194
1113
1192
1111
1192
1111
1194
1113
1194
1113
752
752
750
750
750
750
652
606
652
606
650
606
650
606
652
606
652
606
460
430
445
420
450
420
445
425
450
430
450
420
67
0.8
mole%
glycine
added
Band
assignment
Presence of
H2O
molecules
(free water
symmetric
stretch)
OH2 bending
mode
(asymmetric
stretch)
Combination
bond
3SO4
(symmetric
stretch)
4SO4
(bending
mode)
4SO4
(stretching
vibration)
H2O (bending
mode,
asymmetric
stretch)
3.4. Optical Properties
The UV-Vis-NIR transmittance spectra observed (for the crystals dissolved in
water) are shown in Figures 27 and 28 respectively for the pure and glycine doped
NiSO4.7H2O and MgSO4.7H2O crystals. All the spectra are found to be similar and
they show wide transmission window with a small dip at around 390 nm in the UVVis-NIR region (from 210-1100 nm). This enables these crystals to be potential
candidates for opto-electronic applications. Efficient nonlinear optical (NLO) crystals
are expected to have optical transparency lower cut-off wavelengths between 200 and
400 nm [109]. From this it can be understood that the crystals grown in the present
study can be considered as promising NLO crystals.
The second harmonic generation (SHG) efficiencies (compared to that of
KDP) observed are provided in Table 5. Results obtained indicate that the crystals
grown in the present study are NLO active.
3.5
Mechanical Properties
The hardness behaviour and log P versus log d plots observed are shown in
Figures 29 and 30 respectively for pure and glycine doped NiSO4.7H2O and
MgSO4.7H2O crystals. The Hv and d values observed are given in Tables 6 and 7
respectively for pure and glycine doped NiSO4.7H2O and MgSO4.7H2O crystals.
Results obtained indicate that the Hv value increases with increasing load for all the
twelve crystals grown in the present study. The Hv value increases up to a load of 100
g, above which cracks start developing which may be due to the release of internal
stress generation with indentation.
68
Figure 27: The UV-Vis-NIR spectra observed for the pure and glycine doped
NiSO4.7H2O crystals
69
Figure 28: The UV-Vis-NIR spectra observed for the pure and glycine doped
MgSO4.7H2O crystals
70
Table 5: The SHG efficiencies observed for the pure and glycine doped
NiSO4.7H2O and MgSO4.7H2O crystals
System (with impurity concentration in
the solution, mole %)
SHG efficiency (in KDP unit) for
NiSO4.7H2O
MgSO4.7H2O
0.0 (Pure)
0.93
0.70
0.2
0.95
0.66
0.4
1.03
0.58
0.6
1.23
0.60
0.8
1.29
0.64
1.0
1.35
0.55
Table 8 contains the work hardening coefficients (n) estimated from the slopes
of the best-fitted straight lines of log P versus log d curves. The ‘n’ values observed in
the present study are all more than 2 which indicates that all the twelve crystals grown
belong to soft materials category and the experimental results obtained in the present
study follow the normal ISE (indentation size effect) trend [82].
71
(a)
(b)
Figure 29: The hardness behaviour (a) and log P versus log d plots (b) observed
for the pure and glycine doped NiSO4.7H2O crystals
72
(a)
(b)
Figure 30: The hardness behaviour (a) and log P versus log d plots (b) observed
for the pure and glycine doped MgSO4.7H2O crystals
73
Table 6:
The Vicker’s hardness numbers (Hv) and mean diagonal lengths of
indentations made (d) observed for the pure and glycine added
NiSO4.7H2O crystals.
Hv (kg/mm2) for load
d (ȝm) values for load
System
(impurity in mole % in the
solution)
25g
50g
100g
25g
50g
100g
a) For pure NSH
38.5
50.4
61.6
34.55
42.90
53.55
0.2
47.8
51.8
66.0
31.13
42.31
52.98
0.4
25.0
32.0
51.2
43.12
53.70
60.19
0.6
30.1
41.0
66.9
39.20
47.5
52.6
0.8
27.8
36.6
58.8
40.82
50.29
56.14
1.0
23.6
32.9
48.2
44.29
53.02
62.15
b) For glycine doped NSH
74
Table 7:
The Vicker’s hardness numbers (Hv) and mean diagonal lengths of
indentations made (d) observed for the pure and glycine added
MgSO4.7H2O crystals
System
(impurity in mole % in the
solution)
Hv (kg/mm2) for load
d (ȝm) values for load
25g
50g
100g
25g
50g
100g
a) For pure MSH
21.1
30.2
56.5
46.50
51.56
54.59
0.2
30.5
48.8
75.3
39.01
43.58
49.63
0.4
60.6
62.1
94.5
27.67
38.61
44.30
0.6
68.8
95.7
110.0
26.00
31.10
41.10
0.8
15.6
27.0
39.0
54.47
58.54
68.80
1.0
28.6
44.2
64.0
40.21
45.80
53.83
b) For glycine doped MSH
75
Table 8:
The work hardening coefficients (n) observed for the pure and
glycine doped NiSO4.7H2O and MgSO4.7H2O crystals
System (with impurity
concentration in the solution,
mole %)
Work hardening co-efficients (n) for
NiSO4.7H2O
MgSO4.7H2O
0.0 (Pure)
3.17
9.11
0.2
2.59
5.76
0.4
4.03
3.38
0.6
5.02
3.12
0.8
4.20
6.62
1.0
4.10
4.77
3.6. Electrical Properties
The electrical parameters, viz. dc, r, tan and ac values observed for the
pure and glycine doped NiSO4.7H2O and MgSO4.7H2O crystals are provided in
Tables 9-16 and also shown in Figures 31-38. It can be seen that dc, r, tan and ac
values increase with increase in temperature. However, no systematic variation is
observed with the impurity concentration (taken in the solution used for the crystal
growth) for all the above electrical parameters in the whole temperature range
considered in the present study. This is illustrated in Figures 39 and 40 respectively
for glycine doped NiSO4.7H2O and MgSO4.7H2O crystals.
76
Table 9: The DC electrical conductivities (ıdc) observed for the pure and
glycine doped NiSO4.7H2O crystals
Temp.
(°C)
ıdc x 10-6 (mho/m) values for
1: 0.0
1:0.002
1:0.004
1:0.006
1:0.008
1:0.010
35
0.241
0.224
0.448
0.120
0.478
0.202
40
0.258
0.251
0.487
0.129
0.498
0.221
45
0.271
0.251
0.532
0.140
0.553
0.238
50
0.356
0.324
0.620
0.229
0.777
0.320
55
0.457
0.456
0.887
0.313
1.037
0.467
60
0.640
0.648
1.311
0.476
1.555
0.723
65
0.941
0.884
1.893
0.744
2.764
1.104
70
1.333
1.458
2.839
1.191
4.146
1.735
75
2.000
2.244
4.259
1.832
5.923
2.698
80
4.000
4.167
10.022
3.307
11.056
4.857
77
Table 10: The DC electrical conductivities (ıdc) observed for the pure and
glycine doped MgSO4.7H2O crystals
Temp.
(°C)
ıdc x 10-6 (mho/m) values for
1: 0.0
1:0.002
1:0.004
1:0.006
1:0.008
1:0.010
35
0.099
0.083
0.542
1.049
0.483
0.180
40
0.120
0.089
1.532
1.758
0.797
0.202
45
0.146
0.096
3.608
2.782
1.366
0.223
50
0.177
0.106
6.956
3.877
1.895
0.267
55
0.211
0.132
11.787
4.800
2.392
0.330
60
0.276
0.168
17.404
6.718
3.054
0.417
65
0.430
0.223
27.236
9.500
4.306
1.112
78
Table 11: The dielectric constants (İr) observed for the pure and glycine doped
NiSO4.7H2O crystals
Temp.
(°C)
İr values for
1: 0.0
1:0.002
1:0.004
1:0.006
1:0.008
1:0.010
35
11.391
11.784
16.117
6.658
5.355
3.745
40
11.391
11.784
16.117
7.365
5.355
3.745
45
11.391
12.554
16.913
7.365
5.355
3.745
50
11.391
12.554
16.913
8.073
5.355
3.745
55
11.391
12.554
16.913
8.073
6.807
4.660
60
12.191
12.554
17.709
8.073
6.807
5.575
65
12.191
12.554
17.709
8.780
8.258
5.575
70
12.990
13.224
17.709
8.780
8.258
6.490
75
14.589
13.224
18.504
8.780
8.258
6.490
80
16.187
13.224
18.504
9.487
11.162
6.490
79
Table 12: The dielectric constants (İr) observed for the pure and glycine doped
MgSO4.7H2O crystals
Temp.
(°C)
İr values for
1: 0.0
1:0.002
1:0.004
1:0.006
1:0.008
1:0.010
35
7.403
6.487
5.598
5.267
2.359
3.514
40
7.802
8.842
5.895
5.978
3.21
4.073
45
8.203
10.393
6.058
6.215
4.16
4.352
50
8.308
13.884
6.517
6.452
4.747
4.631
55
8.603
16.892
6.977
6.689
5.012
4.911
60
9.021
20.259
7.437
6.926
6.527
5.469
65
9.404
23.626
8.357
7.163
7.617
5.748
80
Table 13: The dielectric loss factors (tan į) observed for the pure and glycine
doped NiSO4.7H2O crystals
Temp.
(°C)
tan į values for
1: 0.0
1:0.002
1:0.004
1:0.006
1:0.008
1:0.010
35
0.19
0.20
0.25
0.14
0.09
0.09
40
0.19
0.20
0.25
0.15
0.09
0.09
45
0.19
0.21
0.26
0.15
0.09
0.09
50
0.19
0.21
0.26
0.16
0.09
0.09
55
0.19
0.21
0.26
0.16
0.10
0.10
60
0.20
0.21
0.27
0.16
0.11
0.11
65
0.20
0.21
0.27
0.17
0.11
0.11
70
0.21
0.22
0.27
0.17
0.12
0.12
75
0.23
0.22
0.28
0.17
0.12
0.12
80
0.25
0.22
0.28
0.18
0.13
0.12
81
Table 14: The dielectric loss factors (tan į) observed for the pure and glycine
doped MgSO4.7H2O crystals
Temp.
(°C)
tan į values for
1: 0.0
1:0.002
1:0.004
1:0.006
1:0.008
1:0.010
35
0.32
0.54
0.56
0.55
0.35
0.47
40
0.4
0.75
0.58
0.56
0.88
0.55
45
0.44
0.86
0.67
0.62
1.4
0.66
50
0.51
0.95
0.86
0.68
1.76
0.72
55
0.64
1.07
1.37
0.73
1.82
0.84
60
1.08
1.22
1.68
0.84
1.91
1.2
65
1.72
1.35
1.83
1.2
2.39
1.98
82
Table 15: The AC electrical conductivities (ıac) observed for the pure and
glycine doped NiSO4.7H2O crystals
Temp.
(°C)
ıac x 10-6 (mho/m) values for
1: 0.0
1:0.002
1:0.004
1:0.006
1:0.008
1:0.010
35
1.203
1.309
2.239
0.518
0.267
0.187
40
1.203
1.309
2.239
0.614
0.267
0.187
45
1.203
1.465
2.444
0.614
0.267
0.187
50
1.203
1.465
2.444
0.717
0.267
0.187
55
1.203
1.465
2.444
0.717
0.378
0.259
60
1.355
1.465
2.657
0.717
0.378
0.341
65
1.355
1.465
2.657
0.829
0.505
0.341
70
1.516
1.629
2.657
0.829
0.505
0.433
75
1.865
1.629
2.879
0.829
0.505
0.433
80
2.249
1.629
2.879
0.949
0.806
0.433
83
Table 16: The AC electrical conductivities (ıac) observed for the pure and
glycine doped MgSO4.7H2O crystals
Temp.
(°C)
ıac x 10-6 (mho/m) values for
1: 0.0
1:0.002
1:0.004
1:0.006
1:0.008
1:0.010
35
0.132
0.195
0.174
0.161
0.046
0.092
40
0.174
0.369
0.190
0.186
0.157
0.125
45
0.201
0.497
0.226
0.214
0.324
0.160
50
0.236
0.734
0.312
0.244
0.465
0.185
55
0.306
1.005
0.532
0.272
0.507
0.229
60
0.542
1.375
0.695
0.324
0.693
0.365
65
0.900
1.774
0.851
0.398
1.013
0.633
84
Figure 31: The DC electrical conductivities (ıdc) observed for the pure and
glycine doped NiSO4.7H2O crystals
85
Figure 32: The DC electrical conductivities (ıdc) observed for the pure and
glycine doped MgSO4.7H2O crystals
86
Figure 33: The dielectric constants (İr) observed for the pure and glycine doped
NiSO4.7H2O crystals
87
Figure 34: The dielectric constants (İr) observed for the pure and glycine doped
MgSO4.7H2O crystals
88
Figure 35: The dielectric loss factors (tan į) observed for the pure and glycine
doped NiSO4.7H2O crystals
89
Figure 36: The dielectric loss factors (tan į) observed for the pure and glycine
doped MgSO4.7H2O crystals
90
Figure 37: The AC electrical conductivities (ıac) observed for the pure and
glycine doped NiSO4.7H2O crystals
91
Figure 38: The AC electrical conductivities (ıac) observed for the pure and
glycine doped MgSO4.7H2O crystals
92
(a)
(b)
93
(c)
(d)
Figure 39: Impurity concentration dependences of ıdc (a), İr (b), tan į (c) and
ıac (d) at 35, 60 and 80 oC observed for NiSO4.7H2O signal crystals
94
(a)
(b)
95
(c)
(d)
Figure 40: Impurity concentration dependences of ıdc (a), İr (b), tan į (c) and
ıac (d) at 35, 50 and 65 oC observed for MgSO4.7H2O signal crystals
96
The temperature dependence of all the four electrical parameters considered
indicates the normal dielectric behaviour of all the twelve crystals studied. This can be
understood on the basis that the mechanism of polarization is similar to the
conduction process. The electronic exchange of the number of ions in the crystal gives
local displacement of electrons in the direction of the applied field, which in turn
gives rise to polarization.
The electrical conduction in dielectrics is mainly a defect controlled process in
the low temperature region. The presence of impurities and vacancies mainly
determine this region. The energy needed to form the defect is much larger than the
energy needed for its drift. The conductivity of the crystal in the higher temperature
region is determined by the intrinsic defects caused by the thermal fluctuations in the
crystal [110].
The conduction region considered in the present study seems to be connected
to mobility of vacancies. If the probability of occupation of an interstice is , then the
probability of finding a vacant neighbour site is (1-). Even for very high
concentrations, of the order of 1020 cm-3, does not exceed 10-2 so that in real cases
with concentration of interstitials of the order of 1015 to 1020 cm-3, (1-) 1 [110].
Electrical conductivity of NiSO4.7H2O and MgSO4.7H2O crystals may be
determined by the proton transport within the framework of hydrogen bonds. A
combination of the following two mechanisms may be considered. The first
mechanism is identical to the conductivity mechanism in ice also containing hydrogen
bonds. According to the second mechanism, conductivity is associated with the
incorporation into the crystal lattice of impurities and the formation of corresponding
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defects in ionic crystals. The proton conduction may be accounted for by motion of
protons accompanied by a D defect (excess of positive charge). Migration of these
defects may only modify electric polarization and may not change the charge at an
electrode [110]. The motion of defects occurs by some kind of rotation in the bond
with defects. The speed of displacement v= Ȟa, where a and Ȟ are the distance and
frequency respectively of the jump from one bond to the other.
When the temperature of the crystal is increased there is a possibility of
weakening of the hydrogen bonding system due to rotation of the hydroxyl ions in
water molecules. This results in an enhanced conduction in these materials.
The mechanism of electrical conductivity in alkali and silver halide crystals is
usually the motion of ions and not the motion of electrons. This has been established
by comparing the transport of charge with the transport of mass as measured by the
material plated out on electrodes in contact with the crystal [111].
It is assumed that the conductivity of ice is determined by the simultaneous
presence of positive and negative ions and orientational defects-vacant hydrogen
bonds (L-defects) and doubly occupied hydrogen bonds (D-defects). Other possible
defects are vacancies and defect associates [112].
The experimental data and especially the character of the temperature
dependence of conductivity allowed to understand that the conductivity of KDP
crystals is determined by both thermally generated L-defects and the foreign
impurities incorporated into the lattice and generating L-defects there [112]. When
performing measurements, Lokshin [113] (in the case of KDP crystals) assumed that
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HPO42- ions are also responsible for the formation of vacant hydrogen bonds (Ldefects). Therefore, the pH value of the initial solution, which determines its ionic
composition, can be one of the most important factors that affects crystal
conductivity, because the concentration of HPO42- ions in the solution at some pH is
higher by several orders of magnitude than the concentration of any other impurity
[114].
From the above knowledge, it is understood that the proton transport depends
on the generation of L-defects. Hence, the increase of conductivity with the increase
in temperature observed for glycine doped NiSO4.7H2O and MgSO4.7H2O crystals in
the present study can be understood as due to the temperature dependence of the
proton transport. Also, the conductivity increases smoothly through the temperature
range considered in the present study; there is no sharp increase that would be
characteristic of a super-protonic phase transition [115].
In order to understand whether the conductivity in these crystals can be
considered to be protonic or not, as an illustration, we have analysed
values observed for the pure NiSO4.7H2O single crystal.
the
dc
Plot between ln dc and
1000/T (shown in Figure 41) is found to be nearly linear. So, the DC conductivity
values were fitted to the Arrhenius relation:
dc = odc exp [-Edc/(kT)] ,
(14)
where odc is the proportionality constant (considered to be the characteristic constant
of the material), k is the Boltzmann constant and T is the absolute temperature. The
DC activation energy (Edc) was estimated using the slope of the best fitted line plot.
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The estimated Edc value for pure NiSO4.7H2O crystal is 0.572 eV. The low activation
energies observed suggests that oxygen vacancies may be responsible for conduction
in the temperature region considered in the present study.
Figure 41: Plot between ln ıdc and 1000/T for the pure NiSO4.7H2O crystal
The DC electrical conductivity is easily calculated [94] to be :
dc = Ne2a2 / (kT),
(15)
where is a mean jump time, perhaps different from the dipolar orientation but still
given by an equation like:
1/ = 1/o exp (-Edc/(kT)),
(16)
where a is the distance of a jump. The factor 1/o = o (nearly equal to 2fD where fD is
the Debye frequency) is the ionic vibrational frequency around its equilibrium
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position and exp(-Edc/(kT)) is the statistical Boltzmann factor. A jump is attempted
with each vibration, but only a fraction succeeds, depending on the (activation) energy
Edc required in order to squeeze through the barrier to neighbouring equilibrium
position. N stands for the number of perfect bonds or the number of charges per unit
volume. The frequency 1/o 1013 s-1. Also 1/ 1011 s-1 and 1/ will be very much
smaller than this at temperatures much below the melting temperature [94].
The Debye (cutoff) frequency (fD) available in the literature [22] for the pure
NiSO4.7H2O (determined at 25 °C) is 3.247 x 1012 s-1. The 1/o and 1/ values
(estimated using equation (16) with the above fD and Edc values) are found to be 2.040
x 1013 s-1 and 4.411 x 103 s-1 respectively. These values compare well with those
expected by the above model. Also, the fD value compares well with the frequency of
the mode (2.5 x 1012 s-1 at 27 °C) [110] assigned to oscillation modes of protons.
Thus, the conduction in NiSO4.7H2O crystals can be considered to be protonic.
It is a known fact that glycine is a simple organic substance and is expected to
occupy mainly the interstitial positions. The density measurement shows a small
change of density with the increase of impurity concentration taken in the solution
used for the growth of single crystals. Moreover, the impurity concentrations
considered in the present study are small. So, the glycine molecules can be assumed to
replace the water molecules and ions (Ni2+ / Mg2+ and SO42-) to some extent in
addition to occupying the interstitials in the NiSO4.7H2O / MgSO4.7H2O crystal
matrix (or lattice) creating a disturbance in the hydrogen bonding system. As the
conduction in NiSO4.7H2O and MgSO4.7H2O crystals is protonic and mainly due to
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the water molecules and SO42- ions, the disturbance in the hydrogen bonding system
may cause the conductivity to vary nonlinearly with the impurity concentration.
The dielectric constant of a material is generally composed of four types of
contributions, viz., ionic, electronic, orientational and space charge polarizations. All
these may be active at low frequencies, the nature of variations of dielectric constant
with frequency and temperature indicates the type of contributions that are present in
them. Variation of r with temperature is generally attributed to the crystal expansion,
the electronic and ionic polarizations and the presence of impurities and crystal
defects. The variation at low temperature is mainly due to the expansion and
electronic and ionic polarizations. The increase at higher temperatures is mainly
attributed to the thermally generated charge carriers and impurity dipoles. Varotsos
[116] has shown that the electronic polarizability practically remains constant in the
case of ionic crystals. The increase in dielectric constant with temperature is
essentially due to the temperature variation of ionic polarizability.
It is interesting to note that the impurity addition (with some concentration)
leads to a reduction of dielectric constant significantly and consequently leads to low-
r value dielectric material which is gaining more importance now a days. Glycine
addition with 0.6, 0.8 and 1.0 mole% concentrations in the case of NiSO4.7H2O and
0.8 and 1.0 mole% concentrations in the case of MgSO4.7H2O leads to a reduction of
dielectric constant significantly. The disturbance caused due to glycine addition in the
hydrogen bonding system of the NiSO4.7H2O and MgSO4.7H2O crystal lattices may
be the reason for this.
Microelectronics industry needs replacement of dielectric materials in
multilevel interconnect structures with new low-dielectric constant (r) value
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materials, as an interlayer dielectric (ILD) which surrounds and insulates interconnect
wiring (schematically shown in Figure 42). Lowering the values of the ILD decreases
the RC delay, lowers power consumptions, and reduces ‘cross-talk’ between nearby
interconnects [117].
Metal components
Dielectric component
Figure 42: Schematic diagram of interconnect structure
Silica has r 4.0, in part as a result of the Si-O bonds. Several innovative
developments have been made for the development of new low- r materials to replace
silica. Reduction in r value has taken place (with non-porous and porous thin films)
but with several other problems. So, there is still a need for new low-dielectric
constant materials [117]. Goma et al [28] have reported reduction in r value in the
case of KDP added with 0.6 mole% urea. They observed at 40°C, r = 2.86 along aand 3.17 along c-directions. This illustrated that urea doping to KDP reduces the r
value. Also, Meena and Mahadevan [29] have found that L-arginine addition makes it
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possible for the KDP and ADP crystals to become low- r value dielectrics. Moreover,
material in the single crystal form would be very much interesting.
The present study indicates that glycine addition (1.0 mole %) to NiSO4.7H2O
reduces the r value from 11.391 to 3.745 in the temperature range of 35-60 °C. Also,
higher concentrated, viz.0.8 and 1.0 mole% glycine doping to MgSO4.7H2O reduces
the r value from 7.403 to 2.359 and 3.514 respectively at 35 oC. This shows that
NiSO4.7H2O and MgSO4.7H2O crystals become very interesting and more useful
when doped with glycine. So, glycine addition leads these crystals to become
potential materials useful in the microelectronics industry.
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