3. STUDIES ON POTASSIUM LEAD BROMIDE
SINGLE CRYSTALS
3.1 INTRODUCTION
Ternary alkali lead halide single crystals have become important because of
their potential applications in acousto-optic and opto-electronic devices .Lead bromide
crystals hold much promise in applications for acouto-optic devices in signal
processing and optical spectrum analyzing systems. Single crystals of this material
have favourable acousto-optical properties, the most significant of which are its a)
spectral transmission range, (b) photo-elastic co-efficient, (c) acousto-optic figure of
merit, (d)acoustic velocity and (e) acoustic attenuation , although its use has been
hampered by difficulties in growing crystals of high optical quality. Recently, it has
been found that ternary alkali halide single crystals can be grown by the melt method
and they become important due to their potential applications. Monoclinic KPb2Br5
(KPB) is among the most promising bromide host materials because this material
possesses an incorporation of Nd3+, Tb3+, Dy3+ and Er3+ doping ions and provides
better homogeneity and quality of doped single crystals [57]. The crystal structure of
KPB, (having spacegroup P21/c, lattice parameters a=8.854(2) Å, b=7.927(2) Å ,
c=12.485(3) Å , β=90.05(3)Å and Z=4), is shown in Figure 3.1 [124]. Complex
polyhedral coordination by bromine atoms was found for both potassium and lead
atoms. An important step towards practicality was made when the rare-earth-doped
alkali-lead halide crystals MPb2Hal5 (M = Rb,K and Hal = Cl, Br) were identified as
promising new low-phonon-energy host materials for mid-IR applications.
44
The present investigation deals with the growth of lead bromide and potassium
bromide mixed crystals by slow evaporation technique. The grown crystals (expected
to be KPb2Br5, KPbBr3, K2PbBr4 and K3PbBr5) were subjected to powder X-ray
diffraction (PXRD), single crystal XRD, AAS, EDAS, SEM, TGA/DTA, UV-Vis-NIR
spectral and electrical (both AC and DC) measurements. The results of these
experiments are reported and discussed in this chapter.
Figure 3.1: The crystal structure of KPb2Br5 single crystal
45
3.2 GROWTH OF SINGLE CRYSTALS
Analytical reagent (AR) grade samples of Lead Bromide (PbBr2), and
Potassium Bromide (KBr) along with double distilled water were used for the growth
of Potassium Lead Bromide single crystals. Lead Bromide and Potassium Bromide
were taken in the ratios 1: 0.5, 1:1, 1:2 and 1:3 dissolved in double distilled water and
maintained at 80o C for about 60 minutes with continous stirring to ensure
homogenous temperature and concentration over the entire volume of the solution.
Temperature as low as 80o C was maintained in order to avoid decomposition of the
salt. The supersaturated solutions were filtered using 4 micro watman filter paper.
Then the filtered solutions were kept for free evaporation. Clear tiny needle like
crystals were obtained in about 20 days. A photograph of the grown crystals is shown
in Figure 3.2.
3.3 CHARACTERIZATION
The powder X- ray diffraction (PXRD) analysis was carried out using an Xray
powder
diffractometer
(PANalytical)
with
scintillation
counter
and
monochromated CuKα (λ = 1.54056 Å) radiation. The samples were scanned over the
2θ range 10 - 70° at a rate of one degree/minute. The single crystal XRD data were
collected using an automated 4-circle diffractometer (Enraf Nonius CAD4). Atomic
absorption spectra were recorded using Perkin Elmer spectrophotometer. The UV-VisNIR spectrum was recorded in the range of 190 - 900 nm using a Shimadzu UV-2400
PC spectrometer. SEM and EDAS analysis were carried out to study the morphology
and elemental compositions .The thermo gravimetric analysis (TG) of the crystal was
carried out using an Universal V4.1 DTA Instruments, in the temperature range from
50 to 700o C in nitrogen atmosphere at a scanning rate of 10 K/min.
46
The AC conductivity, dielectric constant and dielectric loss of the samples were
determined to an accuracy of ± 2% using an LCR meter (Agilent 4284A) with five
different frequencies (100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz) at various
temperatures ranging from 40 –150°C. The measurement of DC electrical conductivity
was done using the conventional two-probe technique using a million megohm meter
for temperatures ranging from 40 – 150 °C. The crystals grown are needle shaped ones
with small thickness. So, crystal portion with sufficient size cannot be out and polished
for the use of electrical measurements. Hence, in order to make the electrical
measurements, we have made pellets of the grown crystals and used as the sample for
the AC and DC electrical measurements. The flat surfaces of the pellet were coated
with graphite to have a good conductive surface layer.
47
Figure 3.2: Photograph of the sample crystals grown
[From left are: KPb2Br5 K PbBr3, K2PbBr4 and K3PbBr5 ]
48
3.4 RESULTS OBTAINED
3.4.1 Single Crystal XRD Analysis
It is observed from the single crystal XRD data that all the crystals crystallize
in the orthorhombic system except KPbBr3. The KPbBr3 crystal belongs to the
monoclinic system .The single crystal XRD data for the samples prepared are
presented in Table 3.1.
3.4.2 Powder X-ray Diffraction Analysis
X-ray diffraction data were collected from powder samples using an automated
X-ray powder diffractometer. The reflections were indexed using a homely designed
two theta software [125,126]. Figures 3.3-3.6 show the indexed XRD patterns.
49
Table 3.1: Single crystal XRD data for potassium lead bromide crystals grown in
the present study
Crystallographic
data
KPb2Br5
KPbBr3
K2PbBr4
K3PbBr5
a (Å)
4.702
12.134
4.685
4.703
b (Å)
8.002
4.317
7.991
8.032
c (Å)
9.469
12.357
9.450
9.493
α(º)
90
90
90
90
β(º)
90
100.83
90
90
γ(º)
90
90
90
90
Volume (Å3)
356.2
636
353.8
358.6
Crystal system
orthorhombic
monoclinic
50
orthorhombic orthorhombic
51
52
53
54
3.4.3 Atomic Absorption Spectra
The
AAS measurements were
carried out using a
Perkin Elmer
spectrophotometer to determine the K and Pb atom contents in the grown crystals. The
AAS results are given in Table 3.2, which reveal the presence of K+ and Pb2+ ions in
the crystals.
3.4.4 Energy Dispersive X-ray Absorption Spectra
The EDAS spectra observed are shown in Figures (3.7-3.10). Results are
summarized in Table 3.3. The dominant peaks correspond quite well to the energies of
lead and bromine while a small hemp at 3.2 keV corresponds to K line of potassium
(reported in the EDAS international chart), giving a clue that lead is dominant over
potassium in the crystals grown.
55
Table 3.2: Atomic absorption spectral data
Atomic content (ppm)
Sample
Pb
K
KPb2Br5
569290
122
KPbBr3
564784
134
K2PbBr4
567966
170
K3PbBr5
561985
199
Table 3.3: Energy dispersive X-ray absorption spectral data for potassium lead
bromide crystals
Atomic % of
Sample
Pb
K
Br
KPb2Br5
21.69
0.63
75.68
KPbBr3
35.92
0.47
63.62
K2PbBr4
31.38
0.23
68.39
K3PbBr5
20.63
0.28
79.09
56
57
Figure 3.7: EDAS spectrum for K Pb2Br5
58
Figure 3.8: EDAS spectrum for KPbBr3
59
Figure 3.9: EDAS spectrum for K2PbBr4
60
Figure 3.10: EDAS spectrum for K3PbBr5
3.4.5 Scanning electron microscopic pictures
The quality of the grown crystals can be inferred to some extent by observing
the surface morphology of the cut and polished crystals. The SEM image of all the 4
crystal samples observed are shown in Figures 3.11-3.14. It is observed from SEM
photographs that all the crystals are free from cracks and significant visible inclusions.
They have rod like morphology.
3.4.6 UV- Visible Absorption Spectra
The observed UV- Visible spectra for the four grown potassium lead bromide
crystals are shown in Figure 3.15. All the four crystals exhibit absorption edges at
nearly 370 nm and good transmittance in the visible region. The transmittance (T) in
the order of T for KPb2Br5 > T for K2PbBr4>T for K3PbBr5>T for KPbBr3.
61
Figure 3.11: SEM photograph of KPb2Br5 crystals
Figure 3.12: SEM photograph of KPbBr3 crystals
62
Figure 3.13: SEM photograph of K2PbBr4 crystals
Figure 3.14: SEM photograph of K3PbBr5 crystals
63
KPb2Br5
KPbBr3
K2PbBr4
K3PbBr5
absorption(arb.unit)
5
4
3
2
1
0
300
350
400
450
500
550
600
650
700
750
Wavelength(nm)
Figure 3.15: UV-Vis spectra observed for the grown crystals
64
3.4.7 Thermal Studies
The thermo gravimetric and differential thermal analysis [127-129] were
carried out for all the four crystals and the patterns observed are presented in Figures
3.16 to 3.19. The plots are marked with temperature against weight loss percentage.
The TGA patterns show that all the grown crystals were thermally stable up to 500oC.
The exothermic peak at 373oC for KPb2Br5 single crystal corresponds to the phase
transition [130]. For the remaining crystals the phase transitions occur at 372.6oC,
373oC and 368oC respectively.
65
66
Figure 3.16: TG / DTA pattern of KPb2Br5 single crystal
67
Figure 3.17: TG / DTA pattern of KPbBr3 single crystal
68
Figure 3.18 : TG / DTA pattern of K2PbBr4 single crystal
69
Figure 3.19: TG / DTA pattern of K3PbBr5 single crystal
3.4.8 Dielectric Parameters
The dielectric parameters, viz. the εr, tanδ and σac values obtained in the
present study for the pelletised samples are provided in Tables 3.4 – 3.15 and also
shown in Figures 3.20 to 3.31. They are found to increase with increasing temperature
for all the four crystals considered in the present study. The εr and tanδ values decrease
while σac value increase with the increase in frequency of the applied field. This
shows that all the four crystals grown exhibit the normal dielectric behavior.
3.4.9 The DC conductivities
Table 3.16 provides the σdc values obtained in the present study for the
pelletized samples. Also σdc values are shown in Figure 3.32. The DC electrical
conductivity (σdc) increases, in all the four crystals studied, smoothly with the
temperature increase through the temperature range considered in the present study. It
should be noted that the σdc values are more than the σac values at all temperatures for
all the four potassium lead bromide crystals studied in the present investigation.
70
Table 3.4: The dielectric constants for KPb2Br5 crystal
Temp
( °C)
40
50
60
70
80
90
100
110
120
130
140
150
100 Hz
7.199
8.008
8.518
9.357
10.953
11.587
12.696
14.920
15.403
15.600
16.242
16.771
1 kHz
5.797
6.126
6.374
6.636
6.927
7.218
7.434
7.768
7.961
8.041
8.192
8.334
εr with frequency
10 kHz
100 kHz
4.744
4.458
4.875
4.503
5.000
4.537
5.208
4.586
5.356
4.639
5.583
4.709
5.753
4.763
5.975
4.835
6.111
4.883
6.234
4.930
6.283
4.951
6.295
4.972
1 MHz
4.384
4.413
4.428
4.449
4.471
4.494
4.507
4.524
4.533
4.538
4.539
4.544
Table 3.5: The dielectric constants for KPbBr3 single crystal
Temp
( °C)
40
50
60
70
80
90
100
110
120
130
140
100 Hz
17.212
22.300
28.379
36.756
45.017
54.792
64.096
77.670
87.500
93.903
102.807
εr with frequency
1 kHz
10 kHz
100 kHz
7.499
4.947
4.397
8.182
5.187
4.437
8.976
5.443
4.492
9.892
5.738
4.559
10.937
6.045
4.632
12.392
6.372
4.721
13.874
6.752
4.809
15.637
7.185
4.926
16.959
7.471
5.013
17.983
7.712
5.083
19.196
8.037
5.190
150
109.689
21.429
8.936
71
5.607
1 MHz
4.266
4.294
4.317
4.363
4.390
4.419
4.444
4.473
4.495
4.517
4.546
4.682
Table 3.6: The dielectric constants for K2PbBr4 crystal
εr with frequency
Temp
( °C)
40
50
60
70
80
90
100
110
120
130
140
150
100 Hz
1 kHz
10 kHz
100 kHz
1 MHz
13.411
16.076
20.723
26.768
33.626
40.324
49.240
56.992
61.957
66.788
74.861
79.357
8.628
8.892
9.453
10.163
11.097
12.259
13.753
15.027
16.385
17.204
17.599
19.423
6.203
6.442
6.783
7.096
7.382
7.672
8.039
8.289
8.601
8.819
8.937
9.038
5.173
5.216
5.317
5.432
5.559
5.700
5.886
6.008
6.181
6.306
6.371
6.406
5.022
5.043
5.082
5.119
5.153
5.188
5.227
5.261
5.287
5.309
5.333
5.343
Table 3.7: The dielectric constants for K3PbBr5 crystal
Temp
( °C)
εr with frequency
40
100 Hz
18.692
1 kHz
8.544
10 kHz
6.324
100 kHz
5.232
1 MHz
5.079
50
60
22.719
27.792
9.191
10.048
6.589
6.867
5.309
5.418
5.098
5.125
70
80
90
31.979
40.648
48.268
11.128
12.453
14.290
7.113
7.374
7.688
5.529
5.659
5.823
5.158
5.179
5.213
100
110
54.120
61.198
16.079
18.356
7.988
8.411
5.958
6.147
5.239
5.284
120
130
66.568
70.970
20.161
21.197
8.724
8.994
6.267
6.378
5.314
5.338
140
150
77.448
85.364
22.872
23.862
9.351
9.421
6.358
6.571
5.377
5.394
72
Table 3.8: The dielectric loss factors for K Pb2Br5 crystal
Temp
( °C)
tanδ with frequency
40
100 Hz
0.236
1 kHz
0.169
10 kHz
0.090
100 kHz
0.023
1 MHz
0.005
50
0.308
0.181
0.109
0.031
0.006
60
0.363
0.187
0.125
0.039
0.008
70
0.442
0.206
0.143
0.048
0.01
80
0.526
0.232
0.159
0.059
0.012
90
0.671
0.283
0.173
0.071
0.016
100
0.805
0.311
0.18
0.081
0.019
110
0.815
0.356
0.192
0.095
0.023
120
0.844
0.382
0.196
0.104
0.026
130
0.940
0.403
0.203
0.114
0.029
140
0.952
0.410
0.208
0.116
0.031
150
1.034
0.428
0.218
0.118
0.032
Table 3.9: The dielectric loss factors for KPbBr3 crystal
Temp
( °C)
tanδ with frequency
40
100 Hz
1.668
1 kHz
0.686
10 kHz
0.254
100 kHz
0.053
1 MHz
0.008
50
1.762
0.814
0.309
0.070
0.011
60
1.809
0.958
0.360
0.089
0.014
70
1.936
1.115
0.418
0.110
0.018
80
1.993
1.283
0.476
0.134
0.022
90
2.094
1.474
0.549
0.161
0.028
100
2.228
1.646
0.621
0.189
0.034
110
2.467
1.887
0.716
0.225
0.042
120
2.595
2.078
0.783
0.248
0.048
130
2.810
2.239
0.842
0.266
0.053
140
3.141
2.471
0.931
0.298
0.061
150
3.390
2.703
1.273
0.350
0.075
73
Table 3.10: The dielectric loss factors for K2PbBr4 crystal
Temp
( °C)
tanδ with frequency
40
100 Hz
0.705
1 kHz
0.288
10 kHz
0.202
100 kHz
0.061
1 MHz
0.010
50
0.819
0.370
0.226
0.080
0.012
60
0.926
0.469
0.244
0.099
0.016
70
0.942
0.572
0.264
0.118
0.021
80
0.955
0.683
0.321
0.138
0.026
90
0.982
0.773
0.352
0.157
0.031
100
1.042
0.893
0.357
0.182
0.039
110
1.126
0.981
0.395
0.209
0.047
120
1.304
1.077
0.433
0.220
0.055
130
1.398
1.129
0.456
0.230
0.059
140
1.450
1.154
0.459
0.236
0.061
150
1.503
1.203
0.466
0.239
0.066
Table 3.11: The dielectric loss factors for K3PbBr5 crystal
Temp
( °C)
40
tanδ with frequency
1 kHz
10 kHz
100 kHz
0.414
0.209
0.072
100 Hz
0.775
50
0.803
0.506
0.229
0.090
0.013
60
0.876
0.597
0.255
0.111
0.018
70
0.878
0.674
0.282
0.128
0.023
80
0.900
0.754
0.320
0.146
0.028
90
0.908
0.833
0.371
0.166
0.036
100
0.924
0.878
0.418
0.182
0.043
110
1.101
0.954
0.480
0.204
0.052
120
1.125
0.986
0.531
0.217
0.059
130
1.210
1.019
0.556
0.229
0.065
140
1.264
1.065
0.613
0.246
0.073
150
1.366
1.090
0.631
0.250
0.076
74
1 MHz
0.010
Table 3.12: The AC electrical conductivities for K Pb2Br5 crystal
Temp
( °C)
40
50
60
70
80
90
100
110
120
130
140
150
100 Hz
0.094
0.137
0.172
0.230
0.320
0.432
0.568
0.676
0.723
0.816
0.860
0.964
σac (x 10-7 mho/m ) with frequency
1 kHz
10 kHz
100 kHz
0.545
2.375
5.703
0.617
2.955
7.764
0.663
3.476
9.841
0.760
4.142
12.243
0.894
4.736
15.222
1.136
5.372
18.594
1.286
5.760
21.456
1.538
6.380
25.546
1.691
6.662
28.242
1.802
7.038
31.257
1.868
7.269
31.942
1.984
7.632
32.631
1 MHz
12.191
14.727
19.703
24.743
29.836
39.994
47.628
57.875
65.552
73.188
78.254
80.878
Table 3.13: The AC electrical conductivities for KPbBr3 crystal
Temp
( °C)
σac (x 10-7 mho/m ) with frequency
40
100 Hz
1.597
1 kHz
2.861
10 kHz
6.989
100 kHz
12.961
1 MHz
18.979
50
2.185
3.704
8.913
17.272
26.272
60
2.855
4.782
10.898
22.233
33.610
70
3.958
6.134
13.339
27.893
43.675
80
4.990
7.804
16.003
34.522
53.714
90
6.381
10.159
19.456
42.273
68.813
100
7.942
12.701
23.320
50.554
84.031
110
10.657
16.410
28.611
61.643
104.478
120
12.628
19.599
32.535
69.140
119.996
130
14.675
22.393
36.115
75.193
133.135
140
17.959
26.381
41.612
86.016
154.211
150
20.681
32.214
63.267
109.147
195.304
75
Table 3.14: The AC electrical conductivities for K2PbBr4 crystal
σac (x 10-7 mho/m ) with frequency
Temp
( °C)
100 Hz
1 kHz
10 kHz
100 kHz
1 MHz
40
0.526
1.382
6.969
17.551
27.928
50
0.732
1.830
8.097
23.208
33.659
60
1.067
2.466
9.204
29.276
45.219
70
1.402
3.233
10.418
35.650
59.785
80
1.786
4.215
13.178
42.669
74.518
90
2.202
5.270
15.019
49.770
89.451
100
2.854
6.830
15.961
59.574
113.367
110
3.569
8.199
18.210
69.839
137.522
120
4.493
9.814
20.712
75.629
161.729
130
5.193
10.802
22.365
80.658
174.220
140
6.037
11.295
22.815
83.623
180.917
150
6.633
12.995
23.425
85.144
196.134
Table 3.15: The AC electrical conductivities for K3PbBr5 crystal
σac (x 10-7 mho/m ) with frequency
Temp
( °C)
100 Hz
1 kHz
10 kHz
100 kHz
1 MHz
40
0.806
1.967
7.351
20.949
28.250
50
1.015
2.587
8.392
26.576
36.857
60
1.354
3.336
9.738
33.449
51.305
70
1.562
4.171
11.155
39.359
65.976
80
2.035
5.222
13.124
45.954
80.656
90
2.438
6.620
15.864
53.762
104.368
100
2.781
7.851
18.571
60.311
125.287
110
3.747
9.739
22.452
69.738
152.827
120
4.165
11.056
25.765
75.630
174.366
130
4.776
12.013
27.811
81.227
192.962
140
5.443
13.547
31.878
86.984
218.312
150
6.485
14.465
33.060
91.356
227.988
76
100 Hz
1kHz
10kHz
100kHz
1MHz
18
16
14
εr
12
10
8
6
4
40
60
80
100
120
140
160
o
Temperature( C)
Figure 3.20: Temperature dependence of dielectric constant
for KPb2Br5 crystal for various frequencies
100 Hz
1kHz
10kHz
100kHz
1MHz
100
80
εr
60
40
20
0
40
60
80
100
120
140
160
o
Temperature( C)
Figure 3.21: Temperature dependence of dielectric constant
for KPbBr3 crystal for various frequencies
77
100 Hz
1kHz
10kHz
100kHz
1MHz
80
70
60
εr
50
40
30
20
10
0
40
60
80
100
120
140
160
o
Temperature( C)
Figure 3.22: Temperature dependence of dielectric constant
for K2PbBr4 crystal for various frequencies
100 Hz
1kHz
10kHz
100kHz
1MHz
90
80
70
ε
r
60
50
40
30
20
10
40
60
80
100
120
140
160
o
Temperature ( C)
Figure 3.23: Temperature dependence of dielectric constant
for K3PbBr5 crystal for various frequencies
78
100 Hz
1kHz
10kHz
100kHz
1MHz
1.0
0.8
tanδ
0.6
0.4
0.2
0.0
40
60
80
100
120
140
160
o
Temperature( C)
Figure 3.24: Temperature dependence of dielectric loss factor
for KPb2Br5 crystal for various frequencies
B
B
B
B
B
3.5
3.0
2.5
tanδ
2.0
1.5
1.0
0.5
0.0
40
60
80
100
120
140
160
o
o
Temperature(
C)
Temperature(
C)
Figure 3.25: Temperature dependence of dielectric loss factor
for KPbBr3 crystal for various frequencies
79
1.6
100 Hz
1kHz
10kHz
100kHz
1MHz
1.2
tan δ
0.8
0.4
0.0
40
60
80
100
120
140
160
o
Temperature( C)
Figure 3.26: Temperature dependence of dielectric loss factor
for K2PbBr4 crystal for various frequencies
1.5
100 Hz
1kHz
10kHz
100kHz
1MHz
1.2
tan δ
0.9
0.6
0.3
0.0
40
60
80
100
120
140
160
o
Temperature( C)
Fig 3.27: Temperature dependence of dielectric loss factor
for K3PbBr5 crystal for various frequencies
80
100 Hz
1kHz
10kHz
100kHz
1MHz
90
80
70
σac
60
50
40
30
20
10
0
40
60
80
100
120
140
160
o
Temperature( C)
Figure 3.28: The AC electrical conductivities (x10-7 mho/m)
for K Pb2Br5 crystal for various frequencies
100 Hz
1kHz
10kHz
100kHz
1MHz
210
180
150
σac
120
90
60
30
0
40
60
80
100
120
140
160
o
Temperature( C)
Fig 3.29: The AC electrical conductivities (x10-7 mho/m)
for KPbBr3 crystal for various frequencies
81
210
100 Hz
1kHz
10kHz
100kHz
1MHz
180
150
σac
120
90
60
30
0
40
60
80
100
120
140
160
o
Temperature( C)
Fig 3.30: The AC electrical conductivities (x10-7 mho/m)
for K2PbBr4 crystal for various frequencies
250
200
σac
150
100
50
0
40
60
80
100
120
140
160
o
Temperature( C)
Fig 3.31:The AC electrical conductivities (x10-7 mho/m)
for K3PbBr5 crystal for various frequencies
82
Table 3.16: The DC electrical conductivities for potassium lead bromide crystals
σdc ( x 10-5 mho / m ) for
KPbBr3
K2PbBr4
Temperature
(o C)
K Pb2Br5
40
50
4.990
5.106
7.885
7.900
7.769
7.809
7.764
7.814
60
5.166
7.935
7.950
8.156
70
5.196
7.955
7.965
8.191
80
5.271
7.975
7.990
8.226
90
5.402
7.990
8.035
8.246
100
5.533
8.005
8.819
8.256
110
5.668
8.015
9.317
8.538
120
5.759
8.020
9.382
8.749
130
6.975
8.030
9.533
8.809
140
7.417
8.040
9.568
8.920
150
7.548
8.091
9.875
9.312
K3PbBr5
KPb2Br5
KPbBr3
K2PbBr4
K3PbBr5
10
9
σdc
8
7
6
5
40
60
80
100
120
140
160
o
Temperature( C)
Figure 3.32: The DC electrical conductivities (x10-5 mho/m)
for potassium lead bromide crystals
83
3.5 DISCUSSION
All the four single crystals (KPb2Br5, KPbBr3, K2PbBr4 and K3PbBr5 as per the
initial composition considered for crystallization) grown are of needle shape. The
grown crystals show considerable transparency and mechanical and thermal stabilities.
Growth of high quality crystals with uniform composition is of great
importance for high performance devices manufacturing. Among the requirements to
crystal properties, well-defined composition, macro- and micro- uniformity should be
mentioned in the first instance.
For example, in electronic and optoelectronic
applications the quality of the active epilayers often depends directly on the chemical
homogeneity of the substrate. In case of quasibinary solid solutions (A1-xBx)1-sX1+s,
the composition is characterized by the mole fraction x (which defines the energy band
gap) and the deviation from stoichiometry δ (which influences the carrier
concentration) [133]. It should be noted that in the case of lead chalcogenides, the
deviation from stoichiometry can be effectively controlled by a post -growth annealing
under Pb or chalcogen vapour, whereas the x value should be fixed during the growth
process. Axial or radial segregation, both at the macroscopic and the microscopic
scale, is one of the major factors limiting the yield of bulk crystals grown from the
melt or from the vapour. Besides, it should be mentioned that essential axial and radial
segregation causes noticeable increase of the dislocation density in the grown crystals.
The crystals of alloys are frequently subjected to serious distillation-like (i.e.,
thermodynamically imposed) segregation [134] leading to essential variation in
composition between the initially and finally grown fragments of the crystals, which
restricts the applicability of the obtained materials for the device manufacturing.
84
Shtanov and Yashine [133] have illustrated using (Pb1-xSnx)1-δSe1+δ solid solutions as
an example the application of T-x-y phase diagram for the control of the crystal
composition of alloy crystals during Bridgman growth.
The alloying of two or more metals has always been systematically used in
order to modify and improve the properties of the metallurgical materials. The mixing
of ionic solids has been equally investigated in the purpose of obtaining new materials
with specific properties. A very important situation that is special to ionic crystals
arises when these crystals are doped (or added) with impurities. The behavior depends
on the valence state of impurity ions. When an ion like Ca2+ replaces a Na+ ion in
NaCl crystal it results in the creation of a positive ion vacancy or a negative ion
interstitial. Anion impurities also produce corresponding charge compensating point
defects. Whether an impurity ion goes to substitutional position or interstitial position,
is determined by the ionic radius of the doped (or added) ion and also on the electronic
configuration of the ion. If the impurity ion behaves in the same way as the lattice ion,
a wide range of solubility may be possible. To describe this, the term ‘mixed crystal’ is
used. It should be realized, however, that the impurity ions are all distributed at
random throughout the lattice so that the term ‘solid solution’ is more appropriate.
Two compounds or elements are said to form a continuous solid solution if a
single lattice parameter as measured by X-ray powder diffraction patterns, can be
assigned to the solid solution at all compositions. In the continuous solid solutions of
alkali halides, Retger’s law (additivity of molar volumes) [135] and Vegard’s law
(linear variation of lattice parameter with composition) [136] are closely followed as
indicated by X-ray diffraction studies.
85
Potassium and lead halides are soluble in water. It is possible to grow, in
certain cases, mixed crystals by evaporation of aqueous solution. However, the melt
technique is the commonly employed technique to grow mixed crystals.
Tobolsky [137] showed that for ionic crystals like alkali halides, complete
miscibility is possible only above a particular temperature given by T=4.5δ2, where δ
being the percentage deviation in the lattice parameter. As per this, alkali halide
solutions have got only limited miscibility at room temperature.
Vertical Bridgman technique (melt technique) is mostly used for growing
single crystals of alkali lead halides and alkali halides. At temperatures nearer to the
freezing point, the crystals are observed to be fairly transparent. When the crystals are
cooled from high temperature to the room temperature in a relatively short time the
transparency of the crystals is found to be reduced and becoming white. This is partly
due to the introduction of thermal defects since the rate of cooling is high.
Transparency can be improved by reducing the rate of cooling and consequently
reducing the introduction of thermal defects. In this situation, growth of crystals by
the solution methods at near ambient temperatures can be considered to be useful.
A3MX5.2H2O (where A is a univalent cation, M is a divalent metal and X is a
halogen) crystals exhibit unusual physical properties. They have attracted a great deal
of attention owing to the occurrence of varying stoichiometries in these compounds
[138]. A3MX5.2H2O crystals are closely related to A2MX4 and both represent the
largest known group of insulating crystals with structurally incommensurate phases
[139]. Byrappa et al [140] have mentioned that no detailed X-ray crystal structure
(refinement) is available for A3MX5.2H2O type crystals. However, Krishna kumar et al
86
[141], without giving any experimental details, have described in brief the crystal
structure of Na3BaCl5. 2H2O crystals. The structure described by them is as shown in
Figure 3.33. The Na3BaCl5. 2H2O crystals consist of metal ions such as Na and Ba,
Cl- ions and two H2O molecules. The chlorine atoms lie at the vertices of trigonal
bipyramidal geometery. Three Cl- ions form electrovalent bonds between the adjacent
Na+ and central Ba2+ ions. This bond is naturally the attractive electrostatic force
existing between positive and negative ions when they are brought into a closer
distance. The two H2O molecules are stacked diagonally up and down, which may
have a linkage with the adjacent Na+ ions.
Figure 3.33: Crystal structure of Na3BaCl5.2H2O
Manonmani et al [142,143,113] have attempted to grow from aqueons
solutions by the slow (free) evaporation of solution method single crystals of
(composition considered in the solution) K3BaCl5.2H2O, K3CaCl5.2H2O, and
Na3CaCl5.2H2O and characterize them. They have confirmed by experimental means
(XRD, TGA, AAS and FTIR and Raman spectroscopic measurements) that non
87
stoichiometry is present in all these crystals grown.
estimated
as
K3.088
Ba0.912Cl4.832.1.369H2O
These compositions were
for
K3BaCl5.2H2O,
K3.611Ca0.389Cl4.389.1.177H2O for K3CaCl5.2H2O and Na3.665 Ca0.335 Cl4.335.0.153H2O
for Na3CaCl5.2H2O. The variation of DC electrical conductivity with temperature
observed by them indicates that KCl-BaCl2 is a dielectric material while the others
(KCl-CaCl2 and NaCl-CaCl2) are ionic conductors. Less non stoichiometry retains the
dielectric nature (usual for ionic substances) and higher non stoichiometry leads to
ionic conductors.
Keller [144] has reported that orthorhombic symmetry is shown by single
crystals of K2PbBr4.H2O: a=8.537 Å, b=13.083Å,c=4.594Å. Z=2, space group
P 21 21 2 . He has demonstrated the analogy between the crystal structure of
K2PbBr4.H2O and KPb2Br5 by group – subgroup relations of space groups.
Iwadate et al [145] investigated the complex formation and ionic aggregation in
PbBr2-NaBr and PbBr2-KBr melts by Raman spectroscopy with supplementary use of
molecular orbital calculations (MO). Their results suggest that there existed PbBr42complex ions in the mixture melts, which might not form further clustering or
network.
Kusumoto et al [146] have mentioned that as PbBr2 hardly dissolves in water
(0.97g/100g water), it is not suitable for aqueous solution growth. So, they have grown
PbBr2 single crystals in silica gel and obtained the following results: i) Transparent
PbBr2 single crystals were obtained in a high-acidic gel, ii) sizable single crystals of
PbBr2 were also grown in the liquid placed over a gel because the gel barrier had the
task of slowing down the diffusion rate of reacting ions. Also, they have mentioned
88
that it was difficult for them to grow a PbBr2 crystal of optical high quality from the
melt even though they used a 99.999% purity material.
Rademaker et al [72] observed that the KPb2Br5 (KPB) crystal grown by the
Bridgman (melt) method is biaxial and has a monoclinic crystal structure with a space
group symmetry P 21 / c . From an X-ray single–crystal diffraction study of KPB, they
determined the lattice parameters to be a=9.256 (2) Å, b=8.365 (2) Å, c=13.025 (3) Å
and β=90.00 (3) , Z=4. These values were obtained for crystals evidencing substantial
micro twinning. For crystals with no twinning structures, the given lattice parameters
will change, but further research is needed to clarify this situation. Determined from
lattice constants, the density was found to be 5.62g/cm3 which matched with that
available in other literature, 5.60g/cm3 [91]. Rademaker et al [72] also have observed
a phase transition in KPB at a temperature of 249°C which matched with that of 242°C
reported in other literature [89,91].
Hommerich et al [147] have investigated KPb2Br5 (KPB) as a potential new
solid state laser host material. The fundamental absorption edge of KPB is located at
~400nm. At longer wavelength the transmission ranged between ~75-77% without
any significant absorption features.
According to Beck et al [90] KPb2Br5 (KPB) is monoclinic (space group
P 21 / c ) with an angle β very close to 90°. The unit cell parameters are a=9.264,
b=8.380, c=13.063 Å and β=90.06°; Z=4. Pb2+ ions occupy two non-equivalent lattice
sites of low symmetry, one site is a distorted octahedron and the second site is a
distorted trigonal prism.
89
Lead bromide belongs to the orthorhombic symmetry class D2h and mmm
space group [148]. The lattice parameters are: a=8.0620(1)Å, b=9.53930(13) Å and
c=4.73480(6)Å. V=364.134Å3, Z=4, ρ=6.695gcm-1. PbBr2 exhibits extraordinary
properties, including a very large optical transparency range, an anomalously slow
longitudinal wave velocity in the [010] direction, a large birefringence and a high
figure of merit (M2-550, about twelve times higher than that of PbMoO4). Therefore
this material has good application potential, especially for infrared devices where large
diffraction efficiencies are needed. Crystals were grown by the vertical Bridgman
method.
Singh et al [49] observed that lead bromide crystals severely cracked during the
cool down period after the growth, due to destructive phase transformation. The
energy of phase transformation was suppressed by silver doping and large crystals
were grown from the melt. The acoustic attenuation constant, an important parameter
for the devices, was almost identical for doped (below 3000 ppm) and undoped
crystals.
In the present study, the results obtained through X-ray diffraction, AAS and
EDAS measurements indicate the absence of proper mixing of KBr and PbBr2 in all
the four potassium lead bromide crystals grown.
The grown crystals may be
considered as K+ doped PbBr2 single crystals. However, the thermal stability and the
temperature at which the phase transition occurs in all the four crystals studied are
similar. The phase transition occurs at ~370°C (see section 3.4.7) which is largely
deviated from that observed for KPb2Br5 crystals grown by the melt method (~245°C)
[16-18]. Singh et al [49] have presented a solid/solid phase transformation observed
by DTA in PbBr2 at 365°C. So, the results obtained in the present study through
90
thermal analysis also evidence the formation of KBr added PbBr2 crystals and not the
proposed mixed crystals. So, the chemical formulae used to represent the grown
potassium lead bromide crystals are not correct. However, we use here as the sample
representation. Since the initial composition used for the growth of crystal is the same.
The lattice parameters obtained in the present study for KPb2Br5, K2PbBr4 and
K3PbBr5 are nearly same with the orthorhombic crystal system. However, the lattice
parameters obtained for KPbBr3 are highly deviated and also with a different crystal
system (monoclinic). This may be due to lattice distortion which is evident from the
considerably lower Br- and higher Pb2+ contents when compared to the other three
crystals considered (see table 3.3).
The optical absorption edges observed for all the four potassium lead bromide
crystals grown in the present study are nearly 370 nm which is significantly less than
that observed for the melt grown KPb2Br5 (~400 nm) [147]. Like PbBr2 crystal, the
four crystals considered in the present study exhibit a large optical transparency.
Moreover, the transmittance observed is significantly more than that observed for
PbBr2 [148]. Even though they are not properly mixed potassium lead bromide
crystals, all the four single crystals grown in the present study exhibit superior optical
characteristics required for acousto-optical (AO) devices.
The large optical
transparency range of these crystals is very useful for wide band or multiple band AO
tuneable filters (AOTF) applications.
The intrinsic point defects in lead bromide are supposed to be either of the
Schottky or of the Frenkel type. Tubandt et al [149] concluded from transport
measurements that the electric current in lead bromide is carried exclusively by the
91
bromine ions. Therefore it is not necessary to consider the lattice defects in the lead
ion sub-lattice as charge carriers.
The crystal structure of lead bromide was
determined by Brackken and Harang [150] and by Nieuwenkamp [151] and shown a
coordination structure formed by a disturbed hexagonal packing of bromine ions
between which the lead ions are placed. These lead ions are surrounded by 9 bromine
ions at different distances (3.0 to 4.1 Å). In lead bromide the ions at interstitial sites
might occur only in the mirror planes (100)0 and (100)1* 2 , while in the neighbourhood
of the gliding mirror planes at (001)1/4 and (001)*3 4 bromine ions at 4.1 Å have left
enough space for ions with a radius of at most 0.94 Å.
The Pauling radii of bromine and lead ions are 1.95 and 1.21Å, respectively, so
we may disregard the occurrence of interstitial bromine and lead ions and so we
consider anion and cation vacancies to be the only intrinsic point defects in lead
bromide. According to a Schottky mechanism their thermal generation is given by
O ⇔V
Pb 2 +
+ 2V − ,
Br
where VPb2+, VBr- denote a missing lead ion at a lead ion site and a missing bromine
ion at bromine ion site, respectively, and O denotes the perfect lattice.
We assume that the foreign ions keep their normal valency states. The electroneutrality condition upon doping with monovalent cations Me+, divalent ions A2-, or
trivalent cations Me3+, according to the Koch and Wagner system is then given by
[V − ] + [ Me 3 + ] = 2[V 2 + ] + [ Me + ] + [ A 2 − ] ,
Br
Pb
92
where square brackets denote concentrations. Upon doping with monovalent
cations in concentrations well above those of the intrinsic lattice defects this relation
becomes
[V − ] = [ Me + ]
Br
All foreign ions have radii greater than 0.94Å, so in all cases the bromine ion
vacancies are to be considered to carry the electrical current in lead bromide [152]. In
the case of potassium doped PbBr2 crystals the K+ ions may not occur at interstitial
sites since the Pauling radius of the monovalent potassium ion is 1.51Å.
93