3. STUDIES ON POTASSIUM LEAD BROMIDE SINGLE CRYSTALSshodhganga.inflibnet.ac.in/bitstream/10603/17934/9/09... · 2018. 7. 9. · Potassium Bromide (KBr) along with double distilled

44

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.

45

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

46

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 X-

ray 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-Vis-

NIR 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.

47

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.

48

Figure 3.2: Photograph of the sample crystals grown

[From left are: KPb2Br5 K PbBr3, K2PbBr4 and K3PbBr5 ]

49

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.

50

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)

b (Å)

c (Å)

4.702

8.002

9.469

12.134

4.317

12.357

4.685

7.991

9.450

4.703

8.032

9.493

α(º)

β(º)

γ(º)

90

90

90

90

100.83

90

90

90

90

90

90

90

Volume (Å3) 356.2 636 353.8 358.6

Crystal system orthorhombic monoclinic orthorhombic orthorhombic

55

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.

56

Table 3.2: Atomic absorption spectral data

Sample

Atomic content (ppm)

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

Sample

Atomic % of

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

57

Fig

ure

3.7

: E

DA

S s

pect

rum

fo

r K

Pb

2B

r 5

58

Fig

ure

3.8

: E

DA

S s

pect

rum

fo

r K

Pb

Br 3

59

Fig

ure

3.9

: E

DA

S s

pec

tru

m f

or

K2P

bB

r 4

60

Fig

ure

3.1

0:

ED

AS

sp

ectr

um

fo

r K

3P

bB

r5

61

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.

62

Figure 3.11: SEM photograph of KPb2Br5 crystals

Figure 3.12: SEM photograph of KPbBr3 crystals

63

Figure 3.13: SEM photograph of K2PbBr4 crystals

Figure 3.14: SEM photograph of K3PbBr5 crystals

64

300 350 400 450 500 550 600 650 700 750

0

1

2

3

4

5

ab

so

rption

(arb

.un

it)

Wavelength(nm)

KPb2Br

5

KPbBr3

K2PbBr

4

K3PbBr

5

Figure 3.15: UV-Vis spectra observed for the grown crystals

65

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.

66

Fig

ure

3.1

6:

TG

/ D

TA

pa

tter

n o

f K

Pb

2B

r 5 s

ingle

cry

sta

l

67

Fig

ure

3.1

7:

TG

/ D

TA

patt

ern

of

KP

bB

r 3 s

ing

le c

ry

sta

l

68

Fig

ure

3.1

8 :

TG

/ D

TA

patt

ern

of

K2P

bB

r 4 s

ing

le c

ryst

al

69

Fig

ure

3.1

9:

TG

/ D

TA

pa

tter

n o

f K

3P

bB

r 5 s

ing

le c

ryst

al

70

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.

71

Table 3.4: The dielectric constants for KPb2Br5 crystal

Temp

( °C) εr with frequency

100 Hz 1 kHz 10 kHz 100 kHz 1 MHz

40 7.199 5.797 4.744 4.458 4.384 50 8.008 6.126 4.875 4.503 4.413 60 8.518 6.374 5.000 4.537 4.428 70 9.357 6.636 5.208 4.586 4.449 80 10.953 6.927 5.356 4.639 4.471 90 11.587 7.218 5.583 4.709 4.494

100 12.696 7.434 5.753 4.763 4.507 110 14.920 7.768 5.975 4.835 4.524 120 15.403 7.961 6.111 4.883 4.533 130 15.600 8.041 6.234 4.930 4.538

140 16.242 8.192 6.283 4.951 4.539 150 16.771 8.334 6.295 4.972 4.544

Table 3.5: The dielectric constants for KPbBr3 single crystal

Temp



40 17.212 7.499 4.947 4.397 4.266

50 22.300 8.182 5.187 4.437 4.294

60 28.379 8.976 5.443 4.492 4.317

70 36.756 9.892 5.738 4.559 4.363

80 45.017 10.937 6.045 4.632 4.390

90 54.792 12.392 6.372 4.721 4.419

100 64.096 13.874 6.752 4.809 4.444

110 77.670 15.637 7.185 4.926 4.473

120 87.500 16.959 7.471 5.013 4.495

130 93.903 17.983 7.712 5.083 4.517

140 102.807 19.196 8.037 5.190 4.546

150 109.689 21.429 8.936 5.607 4.682

72

Table 3.6: The dielectric constants for K2PbBr4 crystal

Temp

( °C)

εr with frequency


40 13.411 8.628 6.203 5.173 5.022

50 16.076 8.892 6.442 5.216 5.043

60 20.723 9.453 6.783 5.317 5.082

70 26.768 10.163 7.096 5.432 5.119

80 33.626 11.097 7.382 5.559 5.153

90 40.324 12.259 7.672 5.700 5.188

100 49.240 13.753 8.039 5.886 5.227

110 56.992 15.027 8.289 6.008 5.261

120 61.957 16.385 8.601 6.181 5.287

130 66.788 17.204 8.819 6.306 5.309

140 74.861 17.599 8.937 6.371 5.333

150 79.357 19.423 9.038 6.406 5.343

Table 3.7: The dielectric constants for K3PbBr5 crystal

Temp



40 18.692 8.544 6.324 5.232 5.079

50 22.719 9.191 6.589 5.309 5.098

60 27.792 10.048 6.867 5.418 5.125

70 31.979 11.128 7.113 5.529 5.158

80 40.648 12.453 7.374 5.659 5.179

90 48.268 14.290 7.688 5.823 5.213

100 54.120 16.079 7.988 5.958 5.239

110 61.198 18.356 8.411 6.147 5.284

120 66.568 20.161 8.724 6.267 5.314

130 70.970 21.197 8.994 6.378 5.338

140 77.448 22.872 9.351 6.358 5.377

150 85.364 23.862 9.421 6.571 5.394

73

Table 3.8: The dielectric loss factors for K Pb2Br5 crystal

Temp

( °C) tanδ with frequency


40 0.236 0.169 0.090 0.023 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



40 1.668 0.686 0.254 0.053 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

74

Table 3.10: The dielectric loss factors for K2PbBr4 crystal

Temp



40 0.705 0.288 0.202 0.061 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



40 0.775 0.414 0.209 0.072 0.010

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

75

Table 3.12: The AC electrical conductivities for K Pb2Br5 crystal

Table 3.13: The AC electrical conductivities for KPbBr3 crystal

Temp

( °C) σac (x 10

-7 mho/m ) with frequency


40 1.597 2.861 6.989 12.961 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

Temp

( °C) σac (x 10



40 0.094 0.545 2.375 5.703 12.191

50 0.137 0.617 2.955 7.764 14.727

60 0.172 0.663 3.476 9.841 19.703

70 0.230 0.760 4.142 12.243 24.743

80 0.320 0.894 4.736 15.222 29.836

90 0.432 1.136 5.372 18.594 39.994

100 0.568 1.286 5.760 21.456 47.628

110 0.676 1.538 6.380 25.546 57.875

120 0.723 1.691 6.662 28.242 65.552

130 0.816 1.802 7.038 31.257 73.188

140 0.860 1.868 7.269 31.942 78.254

150 0.964 1.984 7.632 32.631 80.878

76

Table 3.14: The AC electrical conductivities for K2PbBr4 crystal

Temp

( °C) σac (x 10



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

Temp

( °C) σac (x 10



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

77

40 60 80 100 120 140 160

4

6

8

10

12

14

16

18 100 Hz 1kHz 10kHz 100kHz 1MHz

εε εε r

Temperature(oC)

Figure 3.20: Temperature dependence of dielectric constant

for KPb2Br5 crystal for various frequencies

40 60 80 100 120 140 160

0

20

40

60

80

100

100 Hz 1kHz 10kHz 100kHz 1MHz

εε εε r

Temperature(oC)


for KPbBr3 crystal for various frequencies

78

40 60 80 100 120 140 160

0

10

20

30

40

50

60

70

80

100 Hz

1kHz

10kHz

100kHz

1MHz

εε εε r

Temperature(oC)


for K2PbBr4 crystal for various frequencies

40 60 80 100 120 140 160

10

20

30

40

50

60

70

80

90 100 Hz

1kHz

10kHz

100kHz

1MHz

εε εε r

Temperature (oC)



79

40 60 80 100 120 140 160

0.0

0.2

0.4

0.6

0.8

1.0

tanδδδδ

Temperature(oC)

100 Hz 1kHz 10kHz 100kHz 1MHz

Figure 3.24: Temperature dependence of dielectric loss factor

for KPb2Br5 crystal for various frequencies

40 60 80 100 120 140 160

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Temperature(o C)

B B B B B

tan

δδ δδ

Temperature(o C)



80

40 60 80 100 120 140 160

0.0

0.4

0.8

1.2

1.6 100 Hz

1kHz

10kHz

100kHz

1MHz

tan δδδδ

Temperature(o C)



40 60 80 100 120 140 160

0.0

0.3

0.6

0.9

1.2

1.5 100 Hz

1kHz

10kHz

100kHz

1MHz

tan δδ δδ

Temperature(oC)

Fig 3.27: Temperature dependence of dielectric loss factor


81

40 60 80 100 120 140 160

0

10

20

30

40

50

60

70

80

90

σσ σσac

Temperature(oC)

100 Hz

1kHz

10kHz

100kHz

1MHz

Figure 3.28: The AC electrical conductivities (x10-7

mho/m)

for K Pb2Br5 crystal for various frequencies

40 60 80 100 120 140 160

0

30

60

90

120

150

180

210 100 Hz 1kHz 10kHz 100kHz 1MHz

σσ σσac

Temperature(oC)

Fig 3.29: The AC electrical conductivities (x10-7

mho/m)


82

40 60 80 100 120 140 160

0

30

60

90

120

150

180

210

Temperature(oC)

100 Hz

1kHz

10kHz

100kHz

1MHz

σσ σσac

Fig 3.30: The AC electrical conductivities (x10-7

mho/m)


40 60 80 100 120 140 160

0

50

100

150

200

250

Temperature(oC)

σσ σσac

Fig 3.31:The AC electrical conductivities (x10-7

mho/m)


83

Table 3.16: The DC electrical conductivities for potassium lead bromide crystals

Temperature

(o C)

σσσσdc ( x 10-5

mho / m ) for

K Pb2Br5 KPbBr3 K2PbBr4 K3PbBr5

40 4.990 7.885 7.769 7.764 50 5.106 7.900 7.809 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

40 60 80 100 120 140 160

5

6

7

8

9

10

σσ σσdc

Temperature(oC)

KPb2Br

5

KPbBr3

K2PbBr

4

K3PbBr

5

Figure 3.32: The DC electrical conductivities (x10-5

mho/m)

for potassium lead bromide crystals

84

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.

85

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.

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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

87

[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

88

stoichiometry is present in all these crystals grown. These compositions were

estimated as K3.088 Ba0.912Cl4.832.1.369H2O 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

222 11P . 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 PbBr42-

complex 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

89

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 cP /21 . 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

cP /21 ) 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.

90

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

91

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

92

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 *

21)100( , while in the neighbourhood

of the gliding mirror planes at (001)1/4 and *

43)001( 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

−+ +⇔BrPb

VVO 22 ,

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 electro-

neutrality 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

][][][2][][ 223 −+

++

−++=+ AMeVMeV

PbBr

,

93

where square brackets denote concentrations. Upon doping with monovalent

cations in concentrations well above those of the intrinsic lattice defects this relation

becomes

][][ +−

= MeVBr

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.

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