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CHAPTER 5

GEL GROWTH AND CHARACTERIZATION OF PURE AND

DOPED POTASSIUM HYDROGEN

TARTRATE CRYSTALS

5.1 Introduction

Metal tartrate compounds deserve special attention because of their many

interesting physical properties such as dielectric, piezoelectric, ferroelectric and

optical second harmonic generation (Quasim et al. 2009; Joshi et al. 2006; Sawant

et al.2011; Parekh et al. 2009). The characteristics of Potassium hydrogen tartrate

crystals are utilized for their use in transducers, linear and non-linear mechanical

devices (Suryanarayana et al.1988). They are also used in non-linear optical

devices, optical transmission characteristics, fabrication of crystal oscillators and

controlled laser emission (Bamzai et al.2008; Ram Kripal et al. 2008; Basharat

Want et al.2007; Arora et al. 2004. The vibrational spectra of pure potassium

hydrogen tartrate, lithium and potassium doped potassium hydrogen tartrate

crystals have been reported already (Quasim et al.2009).

The effect of dopants on various properties of single crystals is of great

interest from both solid state science as well as technological point of view.

Amongst the metal tartrates, potassium hydrogen tartrate is visualized to be of

great interest because of its interesting physical properties such as dielectric,

piezoelectric, ferroelectric and optical second harmonic generation. The addition

of magnetic ions like copper and iron with potassium hydrogen tartrate crystals

influences the growth kinetics, morpholology, shape, size, perfection, unit cell

dimensions, magnetic and thermal properties. Practically no information on copper

and iron doped potassium hydrogen tartrate crystals grown by gel technique is

available. The substituted ions that have been selected for the modified potassium

hydrogen tartrate are copper and iron. This is because the ionic radii of copper

 

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(0.73Å) and iron (0.645 Å) are smaller than that of potassium (1.38 Å). Hence

their entry into the pure potassium hydrogen tartrate crystal is quite probable. The

reason for doping two particular different transition metal ions Viz. copper and

iron in the pure potassium hydrogen tartrate crystal is that the ionic radii of

copper(0.73 Å) and iron (0.645 Å) are smaller than the ionic radius of potassium

(1.38 Å). Hence the dopant metal ions can replace the potassium ion. In the

present investigation a systematic and complete analysis of copper doped and iron

doped potassium hydrogen tartrate crystals have been done for the first time. The

vibrational spectral analysis of the pure and doped crystals is explained from FTIR

studies. The unit cell dimensions of pure and doped potassium hydrogen tartrate

crystals are found out from powder XRD analysis. Magnetic susceptibility and

thermal analysis of the samples have been made to find the effect of dopant on

pure crystals.

5.2 Experimental method of crystal growth

Single crystals of pure KHC4H4O6, copper doped and iron doped KHC4H4O6

have been grown by gel method at room temperature. This technique consists of

incorporating one reactant in the gelling mixture and then diffusing another

reactant into the gel. This leads to high supersaturation, the initiation of nucleation

and finally crystal growth. Silica gel (Sodium metasilicate solution) is used as the

growth medium. The required quantity of double distilled water is added with the

sodium meta silicate to obtain a specific gravity 1.05 gml-1. The required quantity

of tartaric acid (1M to 2M) is added to form the gel medium. The pH of the

mixture is 5. The gel setting time was found to be strongly dependent on pH and

environmental temperature. It would take about 24 hours for the gel to set in

summer (35-40˚C) whereas it would take even 14 days for the gel to set in winter

(10-15˚C). After confirming the gel setting, an aqueous solution of potassium

chloride of a particular molarity was poured over the gel carefully along the walls

of a test tube so as to avoid any gel breakage. The diffusion of K+ ions through the

 

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narrow pores of the silica gel lead to reaction between these ions and HC4H4O6-

ions present in the gel as lower reactant. The following reaction is expected to

take place leading to the formation of pure potassium hydrogen tartrate crystals:

KCl + C4H6O6 KHC4H4O6 + HCl

Table 5.1

Summary of the experiments of single crystal growth of pure and doped

Potassium hydrogen tartrate

Name of the crystal Constant parameters Variable parameters Results

Pure KHC4H4O6

Gel R.d. = 1.05

Gel pH =5

Gel age = 24 hours

L.R. concentration

= 2 Molar

U.R. concentration:

1 Molar

2 Molar

Silvery white

crystals of

orthorhombic

structure

Copper doped

KHC4H4O6

Gel R.d. = 1.05

Gel pH =5

Gel age = 24 hours

L.R. concentration

=2 Molar

U.R.concentration:

1 Molar (96% +4%

Cu2+)

2 Molar (96%+4%

Cu2+)

Bluish transparent

crystals of

orthorhombic

structure

Iron doped

KHC4H4O6

Gel R.d. = 1.05

Gel pH =5

Gel age = 24 hours

L.R. concentration

= 2 Molar

U.R. concentration:

1 Molar (96% +4%

Fe2+)

2 Molar (96% +4%

Fe2+)

Slightly brownish

transparent needle

shaped

orthorhombic

structure *All experiments performed at the temperature range 30-38°C ; Upper reactant (U.R)- Potassium chloride and lower reactant

(L.R.) – tartaric acid ; R.d. – Relative density*

 

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To grow doped potassium hydrogen tartrate crystals, potassium chloride solution

was first mixed with an aqueous solution of copper nitrate (having oxidation state

of +2) of a particular molarity. The diffusion of K1+ and copper ions though the

narrow pores of the silica gel lead to reaction between these ions and HC4H4O6-

ions present in the gel as lower reactant. The reaction leads to the formation of

copper doped potassium hydrogen tartrate crystals. Similar procedure was

followed to grow iron doped potassium hydrogen tartrate crystals (having

oxidation state of +2) using aqueous solution of iron sulphate. The summary of the

single crystals grown by gel method is shown in Table 5.1.

5.3 Results and discussion

5.3.1 Compositional analysis

The atomic and weight % of pure and doped KHC4H4O6 crystals are shown

in Table 5.2. It is observed that the dopant Cu2+ and Fe2+ has entered into the

lattice site of pure potassium hydrogen tartrate crystals. The entry of Copper and

Fe2+ ions into the lattice of pure potassium hydrogen tartrate might be due to the

fact that the ionic radii of Cu2+(0.73Å) and Fe2+(0.645 Å) is smaller and in close

approximation of that of K+(1.38 Å) ion.

Table 5.2

Atomic and weight % of pure and doped Potassium hydrogen tartrate crystals

Name of the crystal Element Experimental Theoretical Atomic % Weight % Atomic % Weight %

Pure crystals

KHC4H4O6

K 100 100 100 100 Cu 0 0 0 0 Fe 0 0 0 0

Cu2+ doped KHC4H4O6 K 95.60 92.56 96 93.65 Cu 4.40 7.44 4 6.35

Fe2+ doped KHC4H4O6 K 95.30 95.22 96 94.38 Fe 4.70 4.78 4 5.62

 

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Fig 5.1 Crystals grown in gel (a) pure KHC4H4O6 (b) Cu2+doped KHC4H4O6

(c) Fe2+ doped KHC4H4O6

 

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Doping of Cu2+ and Fe2+ ions into pure potassium hydrogen tartrate crystals

is observed to influence perfection, size, colour, morphology and internal

crystallographic feature. Pure potassium hydrogen tartrate crystals and copper

doped crystals were found to be more perfect (in terms of transparency and

morphological development). This might be due to the slower reaction nature of

K+ and copper ions with HC4H4O6- ions respectively and hence the size of the

crystals is small.

The Fe2+ doped potassium hydrogen tartrate crystals are less perfect due to

the higher reactive nature of Fe2+ ions with HC4H4O6- ions and hence the size of

the crystals are large. The photographs of the pure and doped crystals are shown

in Fig 5.1. The size of the pure KHC4H4O6 crystal is found to be 8 x 4 x 3 mm3

and the growth rate is slow. The size of the Cu2+ doped KHC4H4O6 is found to be

8.5 x 4.5 x 3 mm3 and the growth rate is slow. The size of the Fe2+ doped

KHC4H4O6 crystal is found to be 40 x 3 x 2 mm3 and the growth rate is fast.

5.3.2 Optical studies

Fig 5.2 shows the FTIR spectrum for pure, Cu2+ doped KHC4H4O6 and

Fe2+ doped KHC4H4O6 crystals. The observed vibrational frequency and their

assignment are presented in Table 5.3. FTIR spectrum reveals the presence of

O-H bonds, C-O bond, C-H bond, C-C bond and carbonyl C=O bonds [Suthar et

al.2006; Ramachandran et al.2007; Valarmathi et al.2010; Vijayabhaskaran et

al.2011]. From the spectrum, it was found that although the radiations are

absorbed at the same frequency by all the three crystals, the percentage of

transmittance of Copper doped KHC4H4O6 crystal is higher than the percentage of

transmittance of pure and iron doped KHC4H4O6 crystals. This may be due to the

absence of IR absorbing impurities in copper doped KHC4H4O6 crystals.

 

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Fig 5.2 FTIR spectrum of (a) pure KHC4H4O6 (b) Cu2+ KHC4H4O6

(c) Fe2+ doped KHC4H4O6

 

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The number of peaks in Cu2+ and Fe2+ doped potassium hydrogen tartrate

crystals have been decreased when compared to pure crystals. In the frequency

range 1100 cm-1 to 1400 cm-1 there are three extra peaks(1162.39 cm-1,1212.44

cm-1,1337.12 cm-1) found in the pure potassium hydrogen tartrate crystals. This

confirms the +2 oxidation state in the case of copper and iron doped potassium

hydrogen tartrate crystals.

Table 5.3

FTIR assignments for pure and doped Potassium hydrogen tartrate crystals

5.3.3 Structural analysis

The powder X-ray diffractograms of pure and doped potassium hydrogen

tartrate crystals are shown in Fig 5.3. The crystallinity of both pure and doped

crystals is quite clear from diffractograms because of the occurrence of sharp

peaks at specific Bragg’s angles. From the diffractograms, it is clear that the entry

of copper and iron atoms into the modified composition of potassium hydrogen

tartrate crystals lead to shift in the positions of peaks. The indexed XRD data for

pure and doped potassium hydrogen tartrate crystals are shown in Table 5.4.

Wave number (cm-1) Assignments

Pure KHC4H4O6 Copper doped

KHC4H4O6 Iron doped KHC4H4O6

3315.43 3324.40 3320.72 γ(OH) of acid

1568.52 1603.23 1563.85 γas(COO-) 1413.90 - 1407.64 γs(COO-) 1337.12 - - OH plane bending 1212.44 - 1213.66 βCH

1068.00 - 1065.48 δ (C -H) + π (C-H)

904.20 904.01 904.30 γ(C-C)

572.17 to 485.48 581.96 to 405.75 568.19 to 470.09 Metal –oxygen stretching

 

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Fig 5.3 Powder X-ray diffractograms of (a) pure KHC4H4O6 (b) Cu2+doped KHC4H4O6 (c) Fe2+ doped KHC4H4O6 crystals

Calculation of cell parameters reveals that both pure and doped crystals belong to

orthorhombic crystal system having space group P212121. A comparison of

crystallographic data of pure and doped crystals are given in Table 5.5. It is clear

that doping has brought about a change in the cell dimensions due to the change in

bond lengths resulting into a change in cell volume.

 

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Table 5.4

Indexed XRD data for pure and doped Potassium hydrogen tartrate crystals.

Pure KHC4H4O6 Cu 2+ doped KHC4H4O6 Fe2+ doped KHC4H4O6

hkl 2θ hkl 2θ (°) hkl 2θ (°) 102 23.30 102 23.44 012 24.46 112 23.75 120 24.41 121 24.91 120 24.71 111 27.69 113 28.00 113 28.03 300 31.00 122 28.30 300 31.30 132 36.29 300 31.48 132 36.72 321 37.69 321 34.34 410 37.98 410 38.36 104 38.10

Table 5.5

Comparative unit cell parameters for pure and doped Potassium hydrogen

tartrate crystals

Chemical formula Inter axial

angles Unit cell dimensions

(Å)

Unit cell volume

(Å3)

Pure KHC4H4O6 α = β = γ = 90o a = 9.625, b =8.545,

c = 10.425 857.41

(K)0.96(Cu)0.04H C4H4O6 α = β = γ = 90o a = 9.845, b = 10.045,

c = 7.995 790.64

(K)0.96(Fe)0.04H C4H4O6 α = β = γ = 90o a = 9.945, b =7.932,

c = 8.925 704.03

 

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5.3.4 Magnetic properties

The pure and doped KHC4H4O6 crystals are finely ground, crushed and the

resulting powders were packed in a Gouy tube of known magnetic susceptibility.

These experiments were repeated five times and the change in weight was

calculated for the given magnetic field.

Table 5.6

Change in mass with respect to applied magnetic field for (a) pure KHC4H4O6

(b) Cu2+ doped KHC4H4O6 and (c) Fe2+ doped KHC4H4O6 crystals

Name of the crystal Magnetic field in Kilogauss

Mass in Kilograms

Pure KHC4H4O6

1 0.2669

2 0.2667

3 0.2665

4 0.2661

5 0.2656

Cu2+ doped KHC4H4O6

1 0.1781

2 0.1780

3 0.1778

4 0.1775

5 0.1772

Fe2+ doped KHC4H4O6

1 0.3486

2 0.3485

3 0.3483

4 0.3480

5 0.3475

 

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The readings in Gouy balance was recorded when the values became

steady. These values are given in Table 5.6. The magnetic susceptibility of the

samples are found out by using the equation mg = χ H2, where ‘m’ is the

mass of the substance; ‘A’ is the area of cross section of the glass tube; ‘H’ is the

magnetic field between the pole-pieces and ‘χ’ magnetic susceptibility of the

substance. A graph is drawn between m and H2 and the slope gives A /2g. Hence

the susceptibility ‘χ’ can be calculated. This is shown in Fig 4.4. The slope is

found out at the linear region of the graph. The magnetic moment ‘µ’ of pure and

doped KHC4H4O6 crystals is found out using the formula µ = 2.828

(χ x T )1/2 BM, where ‘T’ is the room temperature in terms of Kelvin. The

susceptibility and magnetic moment of the pure and doped crystals are given in

Table 5.7.

Fig 5.4 Graph between ‘m’ and ‘H2’ for (a) pure KHC4H4O6 (b) Cu2+ doped

KHC4H4O6 (c) Fe2+ doped KHC4H4O6 crystals

 

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Table 5.7

Susceptibility ‘χ’ and magnetic moment ‘µ’ for pure and doped crystals

5.3.5 Thermal analysis

The thermal behavior (TGA) of pure and doped KHC4H4O6 crystals are

shown in Fig 5.5. Thermograms of pure KHC4H4O6, Cu2+ doped KHC4H4O6 and

Fe2+ doped KHC4H4O6 crystals shows that there is no loss in weight up to 250˚C,

235˚C and 245˚C respectively. Hence the materials are thermally stable, which

indicates no possibility of co-ordinated water molecules in these crystals. The

first stage of decomposition in the case of pure KHC4H4O6 crystals starts from

254˚C and continues up to 310.5˚C resulting in weight loss of about 57% which

indicates the major decomposition of the material. The second stage of

decomposition starts from 786˚C and continues up to 1000˚C resulting in weight

loss of about 16.2% which indicates the minor decomposition of the material.

Comparison of the observed and calculated weight losses suggests chemical

formula for the grown crystal to be KHC4H4O6. In the case of Cu2+ doped

KHC4H4O6 , the first stage of decomposition starts from 240˚C and continues up

to 320˚C resulting in the weight loss of about 56.9% which indicate the major

decomposition of the material. The second stage of decomposition starts from

Name of the crystal Magnetic susceptibility

( χ ) x10-6 emu

Magnetic moment ( µ ) BM

Pure KHC4H4O6 crystals 26.5908 2.525

Cu2+ doped KHC4H4O6 20.4545 2.215

Fe2+ doped KHC4H4O6 22.3636 2.316

 

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785˚C and continues up to 1000˚C resulting in the weight loss of about 16.1%

which indicates the minor decomposition of the material. Similarly, in the case of

Fe2+ doped KHC4H4O6 the first stage of decomposition starts from 255˚C and

continues up to 325˚ C resulting in the weight loss of about 57.1% which indicates

the major decomposition of the material. The second stage of decomposition starts

from 779˚C and continues up to 1000˚C resulting in a weight loss of about 15.9%

which indicates the minor decomposition of the material. The decomposition

process of pure and doped KHC4H4O6 crystals are shown in Table 5.8.

Fig 5.5 TGA of (a) pure KHC4H4O6 (b) Cu2+ doped KHC4H4O6

(c) Fe2+ doped KHC4H4O6 crystals

 

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Fig 5.6 DSC of (a) pure KHC4H4O6 (b) Cu2+ doped KHC4H4O6

(c) Fe2+ doped KHC4H4O6 crystals

DSC is a thermoanalytical technique in which the difference in the amount

of heat required to increase the temperature of a sample and reference is measured

as a function of temperature. Both the sample and reference are maintained at

nearly the same temperature throughout the analysis. The result of a DSC

experiment is a curve of heat flux versus temperature. These curves may be

 

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exothermic or endothermic used to calculate enthalpies of transition. The glass

transition temperature, crystallization temperature and melting temperature can be

calculated. The DSC analysis was done between 0˚C to 1000˚C at a heating rate of

10˚C min-1 in nitrogen atmosphere and the DSC trace for pure and doped

KHC4H4O6 crystals are shown in Fig 5.6. The DSC curves show two endothermic

peaks in each case. The endothermic peaks at 285.27˚C and 836.08˚C show the

first and second stage of decomposition for pure KHC4H4O6 crystals. The

endothermic peaks at 286.68˚C and 841.67˚C shows the first and second stage of

decomposition for Cu2+ doped KHC4H4O6 crystals. Similarly, the endothermic

peaks at 290.46˚C and 849.57˚C shows the first and second stage of

decomposition for Fe2+ doped KHC4H4O6 crystals.

Table 5.8

Decomposition process of pure and doped KHC4H4O6 crystals

Temperature range

(o C)

Weight loss % Reaction

Observed Calculated

Pure KHC4H4O6

254-310.5

786-1000

57.0

16.2

57.27

15.9

KHC4H4O6 → KHCO2

KHCO2 → KO

Cu2+ doped KHC4H4O6

240-320

785-1000

56.9

16.1

57.4

15.82

(K)0.96(Cu)0.04 H C4H4O6 → (K)0.96(Cu)0.04 HCO2

(K)0.96(Cu)0.04 HCO2 → (K)0.96(Cu)0.04 O

Fe2+doped KHC4H4O6

255-325

779-1000

57.1

15.9

58.2 15.78

(K)0.96(Fe)0.04 H C4H4O6 → (K)0.96(Fe)0.04 HCO2

(K)0.96(Fe)0.04 HCO2 → (K)0.96(Fe)0.04 O

 

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5.4 Conclusion

Pure potassium hydrogen tartrate crystals and copper, iron doped potassium

hydrogen tartrate were grown as single crystals in silica gel medium. The optimum

conditions for better size and quality of crystals are : gel pH =5, gel age =24 hrs,

gel relative density = 1.05, upper reactant concentration = 1 molar, lower reactant

concentration = 2 molar, growth temperature = 33 to 38˚C. Entry of copper and

iron into the lattice of pure potassium hydrogen tartrate crystals as dopant

influences the size, perfection, morphology, transparency and internal crystal

structure. The FTIR spectroscopy reveals the presence of O-H bonds, C-O bond,

C-H bond, C-C bond and carbonyl C=O bonds. Doping of foreign ions Cu2+ and

Fe2+ into the pure crystal of potassium hydrogen tartrate affect the internal

crystallographic features. The unit cell dimensions of pure crystals are worked out

to be a = 9.625Å, b = 8.545Å, c = 10.425Å while that of copper doped crystals are

a = 9.845Å, b = 10.045Å ,c = 7.995Å and that of iron doped crystals are a =

9.945Å, b = 7.932Å, c = 8.925Å. The interfacial angles of pure and doped crystals

are α = β = γ = 90˚. The magnetic moment of pure potassium hydrogen tartrate

crystals = 2.525 BM, Cu2+ doped crystals = 2.215 BM and Fe2+ doped crystals =

2.316 BM. Thermal studies of pure and doped potassium hydrogen tartrate crystals

indicates no possibility of co-ordinated water molecules. There are two stages of

decomposition of the material. The first one is the major stage of decomposition

and the second one is the minor stage of decomposition which is confirmed

through DSC analysis.