Crystalization, Spectroscopic and Optical Investigation of Potassium Hydrogenselenate (KHSeO₄) Single Crystals

Crystalization, Spectroscopic and Optical Investigation of Potassium Hydrogenselenate (KHSeO₄) Single Crystals

1. Abu El-Fadl*

   Physics Department, Faculty of Science Assiut University

   Assiut, Egypt

   A. M. Nashaat

   Physics Department, Faculty of Science Assiut University

   Assiut, Egypt

   Abstract: Single crystals of potassium hydrogenselenate were successfully grown by solution growth technique. The presence of functional groups in the crystal lattice was qualitatively analyzed by FTIR spectrum The optical absorption spectra were recorded for KHSeO4 single crystals along the a-, b-, and c-axes.. The dependence of the absorption coefficient () on the photon energy (h) was determined for the principal axes and the crystals show indirect allowed inter-band transitions. The absorption coefficient (), indirect optical energy gap (Eg), extinction coefficient (k), refractive index (n) and related parameters were found to be dependent on the measured axis. To explain the dispersion of the refractive index in the principal crystal axes the single oscillator model by Wemple-DiDomenico was used and the dispersion parameters were calculated.

   Keywords; KHSeO4 single crystals, FTIR Spectroscopy, Optical energy gap, Dispersion parameters, Optical constants.

   1. INTRODUCTION

      MHXO4 crystals (M=K, Rb, Cs and NH4, X=S or Se) has been widely studied because of its high proton conductivity, its phase transition behavior and the possible usage in fuel cells [1]. These crystals show various possible applications by changing alkaline metal site M and some of these crystals exhibit ferroelectric properties [2]. One member of the MHXO4 crystal family is the potassium hydrogenselenate (KHSeO4) crystal, which crystallizes in the orthorhombic system with Pbca space group [3]. The most interesting group in this crystal structure is the hydrogen selenite (HSeO4) ion, which is usually found to be distorted and arranged in a tetrahedral ions, these ions are connected to each other with short hydrogen bonds. Therefore, the crystal structure of KHSeO4 crystal contains simultaneously infinite chains and cyclic dimers of the hydrogen-bonded HSeO4 anions [3].

      FTIR spectroscopy of a compound provides more information than normally available electronic spectra. The presence or absence of absorption bands helps in predicting the presence of functional groups in the compound.

      The investigation of the fundamental absorption edge can provide extensive information about the band structure of crystals, disorder effects, the character of electronphonon interaction and existence of exciton and their role in the absorption mechanism. This can provide also information about the mechanism of the optical transition.

      Optical constants define how light interacts with a material. The determination of these optical constants is expected to

      provide more physical information about the spectral dependence of optical parameters (such as refractive index, extinction coefficient, real and imaginary dielectric constant and absorption coefficients) which are essential in characterizing materials used in the fabrication of optoelectronic devices [4].

      Our goal in this study is to investigate the optical parameters for KHSeO4 crystal along the a-, b- and c-axes and explain the dispersion of the refractive index in terms of the Wemple-DiDomenico single-effective-oscillator model. Another goal is to study the vibration bands for this crystal by Fourier transform infrared (FTIR) spectroscopy.

   2. EXPERIMENTAL

      Saturated solutions of H2SeO4 mixed with K2SeO4 were prepared. The obtained solution was allowed to evaporate at room temperature by conventional isothermal method. Crystals up to 2 to 3 mm in dimensions could be grown over the period of 72 hours. Best crystals were collected and used as seed crystals. 250 ml of saturated solutions were taken in the crystal growth apparatus fabricated in our laboratory and placed on the platform of a constant temperature bath with an accuracy of

      ±0.1°C and was allowed to equilibrate at 45°C. Optically good quality seed crystals were suspended firmly inside the solutions using a nylon thread. The jars containing the solution were covered by a transparent polythene sheet to prevent the evaporation of the solvent. The growth runs were initiated by reducing the temperature at the rate of 0.2°C/day. After several weeks, the solutions lead to nearly perfect single crystals. Their chemical syntheses are reproducible, and the crystals obtained in this way are pure, having appropriate sizes and stable under normal conditions of temperature and humidity. The formula of this material was determined by chemical analysis and confirmed by structural refinement.

      NICOLET FTIR 6700 spectrometer was used to record the FTIR spectra in the range 400 – 4000 cm1 by the KBr pellet method to study the functional groups of the samples.

      The grown crystal was cut into thin plates along the a-

      , b- and c-axes. The plates were transparent and clear from any noticeable defects. Optical transmittance (T) and reflectance

      (R) were recorded at room temperature using Shimaduzu UV- VIS-2101 PC dual beam scanning spectrophotometer in the energy range 2.1-6.4 eV. The incident unpolarized light was nearly perpendicular to (100), (010) and (001) crystal planes.

   3. RESULTS AND DISCUSSION

1. Fourier Transform Infrared Spectroscopy

   The spectra carried out in the range 400-4000 cm-1 of as grown KHSeO4 crystals and crystal annealed for 1 and 2 hours at 150 C as shown in Fig. 1 have been assigned in Table. 1 and all the peaks have been assigned to the corresponding functional groups. The peaks observed at the range (2923- 1631) cm-1 are assigned to the symmetric vibration of O-H while the in-plane bending of O-H is assigned to the peaks observed in the wavenumber range (1424-1261) cm-1. The symmetric and asymmetric vibrations of Se-O2 are assigned to

   4000 3500 3000 2500 2000 1500 1000 500

   100

   80

   60

   40

   % Transmittance

   20 As grown

   0

   100

   80

   60

   40

   20

   the peaks located at (965-914) and (884-877) cm-1,

   respectively. Se-O-OH asymmetrical vibration is assigned to the peak observed near 825 cm-1. SeO4 symmetrical vibration and Se-OH symmetric vibration appear at 711 cm-1 and (421-

   404) cm-1 respectively. The peak appeared at 687.22 cm-1 with medium intensity is assigned to O-H degenerate deformation.

   0

   100

   80

   60

   40

   Annealed at 150 oc for 1h.

   The assignment of the functional groups is in good agreement with the literature [5, 6].

   TABLE 1. ASSIGNMENT OF THE FTIR ABSORPTION BANDS OF KHSeO4

   20 Annealed at 150 oc for 2h.

   0

   4000 3500 3000 2500 2000 1500 1000 500

   Wavenumber (cm-1)

   SINGLE CRYSTAL.

   Wavenumber (cm-1)

   Fig. 1. FTIR spectrum for as grown and annealed KHSeO4 single crystals.

2. Optical absorption coefficient ()

   As Annealed grown at 150C		Annealed at 150C	Assignment The optical behavior of a material is generally utilized to
   	for 1h.	for 2h.		coefficient . The absorption coefficient () spectra for
   2923.54	2925.3	2923.62	O-H symmetric vibration	different crystallographic axes are calculated from the /td>
   2452.88	2362.89	2453.91
   1631.66	1631.73	1631.59		(1/ d ) Ln T , where d is the crystal thickness and T is
   1424.37	1418.53	1424.6	O-H in-plane bending	the optical transmittance.
   1261	1261.36	1265.7		The spectral distribution of the absorption coefficient () for
   956.74	956.77	965.81	Se-O2 symmetric vibration	KHSeO4 crystals along the principal crystallographic directions
   914.54	919.7	923.75	were plotted in the 4.0-5.4 eV portion of photon energy as
   884.13	883.55	877.39	Se-O2 asymmetric vibration shown in figure (Fig.2-i). It is observed that is in continuous
   825.12	825	826.16	Se-O-OH asymmetrical vibration	increasing with increasing h, near the absorption edge
   737.49	736.87			exhibits a steep rise and a straight line relationship is observed
   711.29	711.03	687.22	Se-OH symmetric vibration O-H degenerate deformation	in the high region, it also shows that the magnitude of and h shape of the dependence at all energies are different for
   		553.84		different crystal axis. On the other hand, its value for the b-axis
   421.53	417.59	404.02	SeO4 symmetrical vibration	is higher than that for the other two axes.

   determine its optical constants for example the absorption

   transmittance data, the relation used is given by

   The IR spectrum of heat treated samples (Fig. 1) indicated that there is no change in the frequency of most of the bands. A decrease in intensity of most bands has been recorded. Some sharp peaks become broad and some peaks almost vanished with increasing the annealing duration like the peaks around 737 cm-1, 711 cm-1 and 421 cm-1 while peaks appeared stronger like the peaks centered at 1631.73 cm-1, 687.22 cm-1 and 553.84 cm-1. The broad hump near 3400 cm-1 increased in intensity with increasing the annealing duration and the small peaks near 2400 cm-1 decreased in intensity. The splitting in the peaks near 650 cm-1 decreased and the peaks begin to overlap. There is also a decrease in intensity of the peaks near 1300 cm-1.

   The optical absorption spectra reveal that no absorption peaks recorded in the range of photon energy (1.4- 6.5 eV) and the absence of absorption in the visible region clearly indicate that the grown crystal can be used for optoelectronic applications.

3. Optical band gap

   The fundamental absorption edge is one of the most important features of the absorption spectrum of a material. The increased absorption near the edge is caused by the transition of electrons from the valence band to the conduction band. The optical band gap Eg of KHSeO4 crystals can be determined from the dependence of absorption coefficient on the photon energy. The direct or indirect nature of optical transition between parabolic bands can be studied using the Tauc's expression [7]:

   g

   B(h E )r

   h

   (1)

4. Real and imaginary parts of the refractive index

   The refractive index is explained in terms of real and

   Where B is the proportionality factor and Eg is optical band- gap. Both values of B and Eg are constants or photon energy independent, the index r depends on the type of electronic transitions and assumes values of 1/2, 2, 3/2, and 3 for allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively. For the KHSeO4, value of r = 1/2 validates (1). A good fit of the experimental points with the above equation means that non-direct electronic transitions are the only mechanism responsible for the photon absorption

   imaginary parts. The real part of the refractive index (n) or

   simply the refractive index which describes the oscillations of the incident radiation in the crystal, while the imaginary part

   (k) known as the extinction coefficient which describes the attenuation of the incident radiation as it propagates in the crystal. The values of (n) and (k) can be determined from the absorption coefficient and reflection (R) spectra (figure of R spectra not included). The Reflectance of the surface (R) is the written in terms of refractive index (n) as [8]:

   inside KHSeO4 crystals.

   The values of (h)1/2 were calculated and plotted against

   (n 1)2 k 2

   R (n 1)2 k 2

   (2)

   h. Fig. 2-ii shows a typical Tauc's plot. The straight portion of the curve was then extrapolated and its intersection on the abscissa was determined as shown in the inset of Fig. 2-ii. This

   While the extinction coefficient k is related to the absorption coefficient by:

   value corresponds to the energy gap (Eg). Table 2 contains the data of optical band gap (Eg) which represents that, the value of

   k 4

   Eg for the c-axis is greater than its value for the other two axes while the spectra of (h)1/2 preserves the same behavior as that of the absorption coefficient .

   Fig. 2. The spectra of (i) absorption coefficient and (ii) (h)1/2 with respect to the principle crystallographic axes of KHSeO4 single crystal.

   By tuning the optical energy gap (Eg) and tailoring the absorption coefficient () of the materials, we can active the desired material which is suitable for the fabrication of various optoelectronic devise.

   The dependence of refractive index (n) on the photon energy is shown in Fig. 3-i and it is seen that the refractive index measured in the b-axis direction has higher values than those measured in the other crystallographic directions. Fig. 3- ii shows the plot of extinction coefficient (k) as a function of photon energy h.

   Fig. 3. The spectra of (i) refractive index n and (ii) extinction coefficient k with respect to the principle crystallographic axes of KHSeO4 single crystal.

   From the graph, it is clear that extinction coefficient (k) measured in the a-axis direction has higher values than those measured along the other two crystallographic directions unlike the refractive index which has higher values in the b-axis direction. Thus, the extinction coefficient (k) and refractive index (n) depend on the photon energy and the crystallographic direction. Fig. 3 may introduce that for this crystal the three crystallographic axes have different behaviors with the incident radiation as the radiation is attenuated most in the a-axis as indicated from the extinction coefficient values while the propagation of the incident radiation described by the refractive index is smoother in the b-axis direction.

   It is understood that the higher value of photon energy will enhance the optical efficiency of the material. Hence, by tailoring the photon energy, one can achieve the desired material for optical device fabrication. Meanwhile the refractive index (n) and the extinction coefficient (k) are the basis for any optical constants.

5. Dispersion characterizations

   To explain the dispersion of the refractive index of KHSeO4 single crystal Wemple and DiDomenico (WDD) model is used which is based on single oscillator formula [9, 10]:

   The values of static refractive index no are calculated and recorded in Table. 2. The values of dispersion parameters and the optical moments gatheredin Table 2., are strongly agree with Wemple [9] and DiDomenico [10].

   d

   n2 (h ) 1 E

   Eso

   /[E2

   (h )2 ]

   so

   Which can be rewritten in the form:

   so d d so

   1/(n2 1) (E / E ) [(1/ E E ) /(h )2 ]

   Where Ed is the oscillator strength or dispersion energy and Eso is the single oscillator energy or average energy gap. Factor Ed which is the mean energy of transition of the lone-pair state to the conduction band state depends on the imaginary part of dielectric constant (i) whereas Eso does not. Due to this reason Ed is very nearly independent of Eso, and Eso is related to the bond energy of chemical bonds present in the system. For oscillator parameters to be calculated a graph of (n2-1)-1 and (h)2 is drawn (Fig. 4-i) and the linear portion of the curve were up to the proximity of the band edge, where such a linear fit is valid near the optical energy gap is fitted to a straight line with

   Fig. 4. (i) Plot of (n2-1)-1 versus (h)2, (ii) the best fit of (n2-1)-1 versus (h)2 around the energy gap with respect to the principle crystallographic axes of KHSeO4 single crystal.

   TABLE 2. NORMAL DISPERSION PARAMETERS FOR KHSeO4 SINGLE CRYSTAL.

   a slope 1/EsoEd and an intercept Eso/Ed as depicted in Fig. 4-ii. The dispersion parameters Ed and Eso are calculated and listed in Table 2. Eso is considered a good approximation to an average energy gap; it varies in proportion to the Tauc band-

   gap Eg: Eso2Eg [11]. The dispersion plays a significant role

   Physical quantity

   Optical energy gap Eg (eV)

   Value

   a-axis b-axis c-axis

   4.559 4.486 4.580

   with respect to optical communication and spectral dispersion

   Refractive index n 1.3141 1.3779 1.2802

   [12].

   The moments of optical dispersion spectra M-1, and M-3, can be

   Extinction coefficient k.

   Single oscillator

   4.573210-5 4.243210-5 2.905410-5

   evaluated using the relationships [13]:

   energy Eso (eV) 6.7966 6.0313 7.6644

   2 M

   2 M 3

   (6)

   Dispersion energy

   Eso

   1

   M 3

   and

   Ed 1

   M 3

   Ed (eV) 2.7086 2.4205 3.1371

   The zero-frequency refractive index (static refractive index) is obtained using (4), by putting h=0, i.e. based on the

   Moment of the optical dispersion spectra M-1 (eV)2

   0.3985 0.4013 0.4093

   expression:

   n2 1 Ed

   (7)

   Moment of the

   optical dispersion spectra M-3 (eV)-2

   8.627210-3

   11.032210-3

   6.967710-3

   E

   o Static refractive

   so index no

   1.1826 1.1838 1.1871

   The dispersion of the refractive indexes in anisotropic KHSeO4 single crystals is refined. It is found that values of the refractive indexes depend on the crystal axes and also depend on the

   opt.

   nck

   and

   2

   ele. 2k opt.

   (9)

   radiation energy. This dependence is very important in using crystals as working elements of optical and quantum electronic devices.

6. Optical constants

The dielectric constant can be expressed as a complex equation in the form =r – ii, where r is the real part generally relates to dispersion, while i is the imaginary part provides a measure of the dissipative rate of the wave in the medium. The real and imaginary parts of the complex dielectric constant are related to the refractive index and the extinction coefficient as:

Where is the incident wavelength and c is the velocity of

light. The energy dependence of the optical and electrical conductivities is illustrated in Fig. 6. It can be seen from the figure that the optical conductivity opt. increases with increasing the incident photon energy while the electrical conductivity ele. takes the opposite behavior in the same photon energy range.

r

n2 k 2

and

i 2nk

(8)

It is clear from (8), that the variation of r follows the same trend as refractive index (n) on photon energy h, whereas the variation of i mainly follows the behavior of the extinction coefficient k and the absorption coefficient () is the same as shown in Fig. 5.

2

Fig. 5. The spectral behaviour of (i) real r , and (ii) imaginary i, parts of

Fig. 6. The spectral behavior of (i) optical opt. and (ii) electrical ele. conductivities measured in the direction of the principle crystallographic axes of KHSeO4 single crystal.

The spectra of the optical conductivity opt. and the imaginary part of the dielectric constant i measured in the a- and b-axes exhibit a convergence near the value of the energy gap. This convergence results from that (8) and (9) depend on k and n with neither of them being the dominant term that resulted in a behavior in middle between the n and k spectra.

Lattice dielectric constant L and contribution of charge carriers (N) can be calculated by the fitting of the linear part of the relation [15]:

the dielectric constant with respect to the principle crystallographic axes of

o

KHSeO4 single crystal.

n2

N e2p 1

(10)

The optical conductivity is a measure of the frequency response of the material when irradiated with light and related to electrical conductivity by the following equations [14]:

L m* 4 2

Where e is electronic charge, c is the velocity of light and N/m* is the ratio of carrier concentration to effective mass; Fig. 7-ii shows the fitting of equation (10). The relation shows

that when the carrier concentration increases the energy gap decreases and so the refractive index increases.

Fig. 7. (i) Plot of n2 versus 2, (ii) the best fit of n2 versus 2 around the energy gap with respect to the principle crystallographic axes of KHSeO4 single crystal.

The electric susceptibility c can be calculated from the optical measurements according to the relation [16]:

TABLE 3. OPTICAL PARAMETERS FOR KHSeO4 CRYSTAL IN THE PRINCIPLE CRYSTALLOGRAPHIC DIRECTIONS.

effective mass N/m (m3.kg)-1

CONCLUSIONS

1. Good quality, highly transparent and well faceted large size single crystals of KHSeO4 were grown by solution growth technique.

2. FTIR spectra were measured for the as grown and annealed crystals and the functional groups were assigned. For annealed samples, some absorption peaks decrease or increase in intensities. There is a transformation of some sharp peaks to broad humps.

3. From the data of the absorption coefficient () the optical band gap Eg was deduced in the three crystallographic directions for samples of KHSeO4 single crystal. The type of transition was allowed indirect one.

4. The refractive index (n) values are calculated as a function of photon energy. The refractive index values have been fitted to the single oscillator Wemple DiDomenico (WDD) model. And the dispersion parameters were calculated in the normal dispersion region around the optical energy gap for the three crystallographic directions.

   0.17

   0.16

   0.15

   c 0.14

   0.13

   0.12

   r o

   a-axis b-axis c-axis

   4c

   n2 k 2

   (11)

5. Values of the extinction coefficient (k), the real and imaginary parts (of the dielectric constant and the optical and electrical conductivities (opt. & ele.) were estimated for KHSeO4 crystal along the principal crystallographic axes. The high magnitude of optical conductivity and the low extinction coefficient (10-5) confirms the presence of very high photo response nature of the material. This makes the material more prominent for device applications of various optoelectronics and photonic devices.

6. The calculated optical constants for KHSeO4 crystals clearly depend on the direction of crystallographic axis which confirms their anisotropic character.

0.11

3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4

h (eV)

(n2 k 2 )

c

o

4

(12)

Fig. 8. The spectral behavior of the electric susceptibility c in the principle crystallographic axes of KHSeO4 single crystal.

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