3. studies on potassium lead bromide single · pdf file 2018. 7. 9. · potassium...

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  • 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.

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    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

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    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.

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    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

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    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

    F ig

    u r e

    3 .7

    : E

    D A

    S s

    p e ct

    ru m

    f o

    r K

    P b

    2 B

    r 5

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    F ig

    u r e

    3 .8

    : E

    D A

    S s

    p e ct

    ru m

    f o

    r K

    P b

    B r 3

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    F ig

    u r e

    3 .9

    : E

    D A

    S s

    p ec

    tr u

    m f

    o r

    K 2 P

    b B

    r 4

  • 60

    F ig

    u r e

    3 .1

    0 :

    E D

    A S

    s p

    ec tr

    u m

    f o

    r K

    3 P

    b B

    r 5

  • 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

    photogr

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