Structural, electrical, ferroelectric and mechanical properties with Hirshfeld surface analysis of novel NLO semiorganic sodium p-nitrophenolate dihydrate piezoelectric single crystal

Article abstract of DOI:10.1039/c5ra10501c

Novel non-linear sodium para-nitrophenolate dihydrate (SPNPD) single crystals were grown by the controlled evaporation method. Various functional groups and chemical bonding were identified by FTIR and Raman analysis. Hirshfeld surface and fingerprint plo

Full text of DOI:10.1039/c5ra10501c

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Structural, electrical, ferroelectric and mechanical properties with Hirshfeld surface
analysis of novel NLO semiorganic sodium pꢀnitrophenolate dehydrate
piezoelectric single crystal
Jyoti Dalal1, Nidhi Sinha1,2, Harsh Yadav1, Binay Kumar1*
1Crystal Lab, Department of Physics & Astrophysics, University of Delhi, Delhiꢀ7, India
2Department of Electronics, SGTB Khalsa College, University of Delhi, Delhiꢀ7, India
Abstract
Novel nonꢀlinear sodium paraꢀnitrophenolate dihydrate (SPNPD) single crystals were grown by controlled
evaporation method. Various functional groups and chemical bonding were identified by FTIR and Raman
analysis. Hirshfeld surface and fingerprint plot were drawn and analyzed for the intermolecular interactions
present in crystal structure. UVꢀvisible studies indicate the high transmittance of crystals in visible region
with a wide band gap 6.12 eV. In photoluminescence spectrum, a sharp broad emission peak centered at 525
nm indicates green emission. In dielectric study a broad peak with low dielectric constant value was
observed at 34 °C which may be due to ferroelectric to paraelectric transition. Piezoelectricity was
confirmed by determining the piezoelectric charge coefficient (d33 =2.24 pC/N). The hysteresis loop with
values of remnant polarization and coercive field as 2.67 ꢁC/cm2 and 7.43 kV/cm, respectively. Piezoꢀ
/ferroelectricity were reported for the first time in pꢀnitrophenolate based compounds. SHG efficiency of
this material was found to be 4.15 times of that of KDP. Mechanical strength was confirmed from Vicker’s
microhardness study and voids volume. Thus promising piezoelectric, ferroelectric behavior of material
along with high SHG efficiency and low dielectric constant, established SPNPD as a potential material for
transducer, optoelectronics and nonvolatile memory devices applications.
Keywords: Hirshfeld Surface analysis, dielectrics, piezoelectricity, ferroelectricity.
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ꢀ Introduction
In recent years there has been considerable interest on nonlinear optical materials because of their
applications in optical communications, optical computing, high speed information processing, electroꢀoptic
1–2
shutters and optical data storage.
Pure organic crystals have very large nonlinear susceptibilities
compared to inorganic crystals but their use in device fabrication is restricted by low optical transparency,
poor mechanical properties and the difficulties in the growth of large crystals.3ꢀ4 On the other hand,
inorganic materials have excellent mechanical and thermal properties but possess relatively modest optical
nonꢀlinearities because of the lack of delocalization in πꢀelectrons.5 Semiorganic materials combine the high
optical nonꢀlinearities of organics with good thermal stability and transmittance of inorganics. 6ꢀ7
Paraꢀnitrophenol is such an organic material which gives variety of derivatives with alkali metal
8ꢀ11
hydroxides.
In recent years, semiꢀorganic complex products of pꢀnitrophenol have been attracted for
NLO properties because of the presence of the delocalized πꢀ electrons in organic ligand pꢀnitrophenol
resulting in high nonlinear optical coefficients in this kind of materials. Sodium pꢀnitrophenolate dihydrate
(SPNPD·2H2O) is such a semiorganic non linear material in which sodium ion (Na+) is bonded to an
organic ligand (nitrophenoxy ion). In this crystal, the πꢀelectron cloud movement from donor to acceptor
results in intermolecular charge transfer interaction and hence makes it nonꢀcentrosymmetric. Crystals
having nonꢀcentrosymmetric structure exhibit both SHG and piezoelectric effects.12Although, SPNPD is a
well established nonꢀlinear optical material for long13, its piezoelectricity has not been reported yet and
therefore its possible applications in piezoelectric devices have not been exploited so far. This
piezoelectricity has not been reported in other pꢀnitrophenolate based compounds as well. 8ꢀ11 Furthermore,
crystals exhibiting both luminescence and non linear optical properties are used in solid state laser sources.
14ꢀ15
In addition to the NLO property, dielectric behaviour of material plays an important role in device
applications. Regarding the NLO device fabrication, materials should also have stringent property imposed
by operating frequency like low dielectric constant and required power levels under the electric field.16 Low
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dielectric constant means material supports an electrostatic field with minimal heat energy dissipation. Thus
lowꢀdielectric materials can lower the power consumption and decrease RC delay in microelectronics.17
Therefore in grown crystal, we have carried out the study of dielectric properties i.e. dielectric constant and
dielectric loss over a range of frequencies and temperatures to analyze their electrical properties.
Ferroelectric materials have many optoelectronic applications such as capacitors, nonvolatile memory
devices, highꢀperformance gate insulators etc. Molecular surface of the grown crystal was computed on the
basis of Hirshfeld surface analysis to understand the intermolecular interactions and hardness of the crystal
was compared based on void volume estimation.18ꢀ19 Therefore, in the present work we have reported
structural characterization, Hirshfeld surface analysis, optical properties, photoluminescence, dielectric
behavior, piezoelectric charge coefficient (d33 pC/N), ferroelectricity, nonꢀlinear optical and mechanical
properties with calculated void volume in details for the first time. Due to the excellent non linear optical
property, strong PL emission, low dielectric constant value and good piezoꢀ/ferroelectric behaviour, the
grown SPNPD crystals can have technological applications in optoꢀelectronic and microelectronics devices.
ꢀ Experimental
Crystal growth and Characterization
Sodium pꢀnitrophenolate Dihydrate (SPNPD) single crystals were grown by slow evaporation solution
technique by taking sodium hydroxide and pꢀnitrophenol in the molar ratio of 1:1. These calculated salts
were dissolved in double distilled water. The resultant solution was stirred well at 45 °C for 4 hrs to ensure
the homogeneous temperature and concentration over the entire volume of the solution. The reaction which
takes place for the synthesis of sodium pꢀnitrophenolate dihydrate is as follows:
NO2ꢀC6H4ꢀOH + NaOH + H2O → NO2ꢀC6H4ꢀONa.2H2O
The resulting solution was filtered twice and finally it was kept in a constant temperature oil bath at 40 °C
with an accuracy of 0.01 °C. The purity of the synthesized salt was improved by performing
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recrystallization many times. Yellow colored transparent single crystals of size 9 x 7 x 2 mm3 were
harvested from the mother solution in 2ꢀ3 weeks as shown in Fig.1 (a). Grown SPNPD single crystals have
been characterized from single crystal Xꢀray diffraction data analysis using an Oxford Diffractometer with
graphite monochromatic MoKα radiation (λ = 0.7107 Å). Structure of grown crystal was solved by fullꢀ
matrix leastꢀsquare technique on F2 using SHELXꢀ97 program.20 Its morphology has been deduced from
BFDH and Modified BDFH theory.21 For confirming functional groups, the FTIR spectrum of grown crystal
was recorded in the middle infrared region of 400 and 4000 cmꢀ1 with PerkinꢀElmer spectrum BX
spectrophotometer using the KBr pellets while Raman spectra were recorded with FTꢀRaman
spectrophotometer in the range 400–3000 cmꢀ1 using excitation radiation at 514 nm with Arꢀion laser at
room temperature. The Hirshfeld surfaces and unit cell voids were generated using Crystal Explorer 3.1
programme.22 Fingerprint plots showing intermolecular interactions were derived from the Hirshfeld
surfaces. To know the suitability of crystal for optical applications, the transmission spectra were recorded
using an evolution 300 spectrophotometer in the region 200ꢀ1100 nm. During this optical analysis,
powdered sample was dissolved in distilled water therefore baseline corrections were performed for getting
required material data only. Photoluminescence (PL) measurements were taken on Varian Cary Eclipse
fluorescence spectrophotometer at room temperature by exciting the crystal at 450 nm. Dielectric
measurements were carried out by the impedance analyzer (Agilent E4980A) with sample holder (Agilent
Model 16048A) for a frequency range 100 Hz ꢀ2 MHz in the temperature range from RT to 58 °C. Crystals
were poled using a DC poling unit (electric field range 0–5 kV mm−1) by immersing them in silicon oil. The
piezometer charge coefficient (d33) measurement was done by PMꢀ300 piezometer system. For finding NLO
property, the grown crystal was subjected to second harmonic generation (SHG) test performed by the Kurtz
powder technique. In the experiment, crystals were ground into powder form with uniform particle size of
63 µm, then packed in a microcapillary tube and finally a Qꢀswitched Nd:YAG laser beam of wavelength
1064 nm was passed through it to get SHG efficiency. The second harmonic radiations generated in the
sample were focused by a lens and detected by a photomultiplier tube. This SHG output was converted into
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electrical signal and displayed on an oscilloscope which indicated the SHG efficiency of the sample. The
emission of green light confirmed the SHG property of material. The hysteresis loop was traced using an
automatic PꢀE loop tracer. The mechanical characterization of the SPNPD crystals was done by Vickers
microhardness test performed at room temperature. The indentations on crystals were made gently by loads
varying from 5 to 100 g for a dwell time of 10 s.
Fig.1. (a) Photographs of yellow colored transparent SPNPD single crystals grown at 40°C. This
good transparency indicates high crystal quality. (b) Morphology of the grown crystal obtained using
WinXMorph software.
Morphological Studies
The morphology importance (MIhkl) of the planes present in crystal can be solved by BFDH law which
states that the rates of growth of the given hkl face are equal to the reciprocal of the interplanar distance dhkl.
Morphology of grown crystals with crystal planes and directions is shown in Fig.1 (b) which has been
obtained using WinXMorph software.23 The BFDH law is applicable in many cases in predicting
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morphology, but in addition, it is also dependent on the interfacial angles with adjacent planes. Thus,
instantaneous growth rate of hkl face can be expressed by modified BFDH formula as given below 20
where
are the normal growth rates of the hkl, h1k1l1 and h2k2l2 faces and their values
are equal to reciprocal of their interplanar distances accordingly,1/dhkl according to BFDH law, α and γ are
the interfacial angles for given hkl plane. The growth rates and their corresponding Morphological
importances (MI) of the (010), (111) and (123) face were computed form BFDH law and modified BFDH
laws which are summarized in Table1 and these directly reflects the corresponding area of the crystal faces
according to their weightages. It has been found that the experimentally observed morphology of the grown
crystal is closely matching with the predictions of both simple BFDH and modified BFDH law which means
that the growth rates of the adjacent planes significantly contribute in the growth rate of the SPNPD crystal
plane.
Table 1 Morphological importance of SPNPD crystal faces computed from BFDH and modified BFDH
laws.
Faces ( hkl)
dhkl(Å)
Calculated
relative growth importance by
rates
Morphological
dhkl/dt
Morphological
importance by
modified
from BFDH law
BFDH law
BFDH law
010
111
123
19.6850
4.5787
2.0075
1.000
4.299
9.8057
1.000
0.233
0.102
0.7165
0.5489
0.2692
1.000
1.305
2.661
ꢀ
Results and discussion
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Single crystal XRD analysis and structural description
The structural analysis of grown crystal was carried out by single crystal Xꢀray diffraction analysis. For
this characterization a good single crystal with dimensions 0.5 mm x 0.5 mm x 0.5 mm was chosen. The
structure was deposited to CCDC (No. 1403345). It was observed that grown crystal belongs to the
orthorhombic system with nonꢀcentrosymmetric space group Ima2. The observed cell parameters agree well
with the reported values. 24ꢀ27 The details of the crystal data and refinement parameters of the studied crystal
are listed in Table 2.1. The molecular structure of grown crystal with the atom numbering scheme has been
depicted in Fig.2. Fig.3. shows the packing of the molecules arranged in net pattern. In the structure,
sodium cation is bonded with the oxygen atom (O2) of nitro group (NO2) and also attached with the four
oxygen atoms of water molecules (Fig.2). Thus sodium ion exists in a six fold coordinated geometry based
on an octahedron. Another significant fact is that phenolic oxygen (O1) atom has strong interaction with
hydrogen atoms of water molecules which are attached with sodium atom. Thus, intermolecular hydrogen
bonding interactions {O…H (W)} takes place between the nonꢀcoordinated oxygen (O1) atom and the
hydrogen atoms of water molecules (Fig.3) which minimizes energy and hence helps in stabilization of
crystal structure. Further, the atomic coordinates of the nonꢀhydrogen atoms with their equivalent
displacement parameters for SPNPD compound are listed in Table. 2.2. Bond lengths and bond angles of
atoms present in the material are presented in Tables 2.3ꢀ2.4. Also, the hydrogen bonds present in the crystal
structure are listed in Table.2.5.
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Table 2.1 Single crystal data and structure refinement of SPNPD crystal.
Empirical Formula
Formula weight
C6H8O5NNa
197.12
Temperature
293K
Wavelength
0.71073 Å (Mo Kα radiation)
Orthorhombic, Ima2
6.8849(6) Å
Crystal system, space group
Cell length, a
Cell length, b
19.6618(1) Å
Cell length, c
6.4413(5) Å
Volume
871.95(12) Å3
0.5 x 0.5 x 0.5 mm3
3.3ꢀ29.3°
Crystal size
Theta range for data collection
Limiting indices
Refinement method
Data/restraints/parameters
Goodnessꢀofꢀfit on F2
R[F2 > 2σ(F2)], wR(F2)
ꢀ9 ≤ h ≤ 8,ꢀ26 ≤ k ≤ 26,ꢀ8 ≤ l ≤ 8
Fullꢀmatrix leastꢀsquares on F2
1087/1/85
0.797
0.0295,0.0854
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Table 2.2 Atomic coordinates ( x 104) of the nonꢀhydrogen atoms and their equivalent isotropic
displacement parameters (Å2 x 103) for SPNPD.
U(eq) is defined as one third of the trace of the orthogonalized Uij tensor
Atoms
Na(1)
C(5)
O(1)
O(2)
N(1)
C(3)
C(2)
O(3)
C(1)
C(6)
C(4)
O(4)
X
Y
Z
U(eq)
27(1)
33(1)
26(1)
64(1)
42(1)
40(1)
23(1)
56(1)
40(1)
46(1)
39(1)
30(1)
ꢀ2500
ꢀ2500
ꢀ2500
ꢀ2500
ꢀ2500
ꢀ2500
ꢀ2500
ꢀ2500
ꢀ2500
ꢀ2500
ꢀ2500
ꢀ69
ꢀ7284(1)
ꢀ5256(1)
ꢀ3217(1)
ꢀ6147(1)
ꢀ5972(1)
ꢀ4355(1)
ꢀ3870(1)
ꢀ6381(1)
ꢀ4109(1
ꢀ4798(1)
ꢀ5045(1)
7364(1)
ꢀ7079(1)
ꢀ3716(3)
ꢀ2456(2)
ꢀ5989(4)
ꢀ4151(4)
ꢀ1264(3)
ꢀ2853(3)
ꢀ2730(4)
ꢀ4911(3)
ꢀ5326(3)
ꢀ1674(3)
ꢀ9627(1)
Table.2.3 Bond lengths for SPNPD
Atoms
Length
Na(1)ꢀO(2)
Na(1)ꢀO(4)#1
Na(1)ꢀO(4)
Na(1)ꢀO(4)#2
Na(1)ꢀO(4)#3
Na(1)ꢀO(3)#4
Na(1)ꢀNa(1)#4
2.3437(19)
2.3491(10)
2.3491(10)
2.4032(10)
2.4032(10)
2.6567(15)
3.3305(4)
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Na(1)ꢀNa(1)#3
C(5)ꢀC(6)
3.3305(4)
1.373(3)
1.379(3)
1.435(2)
1.3071(18)
1.233(3)
1.219(3)
1.381(3)
1.400(3)
1.407(2)
2.6567(15)
1.381(3)
2.4032(10)
C(5)ꢀC(4)
C(5)ꢀN(1)
O(1)ꢀC(2)
O(2)ꢀN(1)
N(1)ꢀO(3)
C(3)ꢀC(4)
C(3)ꢀC(2)
C(2)ꢀC(1)
O(3)ꢀNa(1)#3
C(1)ꢀC(6)
O(4)ꢀNa(1)#4
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Table.2.4 Bond angles for SPNPD
Atoms
angle
O(2)ꢀNa(1)ꢀO(4)#1
O(2)ꢀNa(1)ꢀO(4)
105.82(5)
105.82(5)
90.86(5)
94.48(5)
159.42(3)
86.79(3)
159.42(3)
88.28(5)
171.64(9)
79.88(5)
79.88(5)
79.58(5)
79.58(5)
122.19(7)
46.17(2)
46.17(2)
124.21(3)
124.21(3)
66.17(6)
87.32(7)
O(4)#1ꢀNa(1)ꢀO(4)
O(2)ꢀNa(1)ꢀO(4)#2
O(4)#1ꢀNa(1)ꢀO(4)#2
O(4)ꢀNa(1)ꢀO(4)#2
O(4)ꢀNa(1)ꢀO(4)#3
O(4)#2ꢀNa(1)ꢀO(4)#3
O(2)ꢀNa(1)ꢀO(3)#4
O(4)#1ꢀNa(1)ꢀO(3)#4
O(4)ꢀNa(1)ꢀO(3)#4
O(4)#2ꢀNa(1)ꢀO(3)#4
O(4)#3ꢀNa(1)ꢀO(3)#4
O(2)ꢀNa(1)ꢀNa(1)#4
O(4)#1ꢀNa(1)ꢀNa(1)#4
O(4)ꢀNa(1)ꢀNa(1)#4
O(4)#2ꢀNa(1)ꢀNa(1)#4
O(4)#3ꢀNa(1)ꢀNa(1)#4
O(3)#4ꢀNa(1)ꢀNa(1)#4
O(2)ꢀNa(1)ꢀNa(1)#3
O(4)#1ꢀNa(1)ꢀNa(1)#3
O(4)ꢀNa(1)ꢀNa(1)#3
O(4)#2ꢀNa(1)ꢀNa(1)#3
O(4)#3ꢀNa(1)ꢀNa(1)#3
O(3)#4ꢀNa(1)ꢀNa(1)#3
Na(1)#4ꢀNa(1)ꢀNa(1)#3
C(6)ꢀC(5)ꢀC(4)
C(6)ꢀC(5)ꢀN(1)
C(4)ꢀC(5)ꢀN(1)
N(1)ꢀO(2)ꢀNa(1)
O(3)ꢀN(1)ꢀO(2)
O(3)ꢀN(1)ꢀC(5)
131.20(3)
131.20(3)
44.85(2)
44.85(2)
84.32(6)
150.49(4)
121.52(15)
119.70(18)
118.78(18)
123.66(17)
122.42(19)
120.1(2)
O(2)ꢀN(1)ꢀC(5)
117.5(2)
C(4)ꢀC(3)ꢀC(2)
O(1)ꢀC(2)ꢀC(3)
O(1)ꢀC(2)ꢀC(1)
C(3)ꢀC(2)ꢀC(1)
N(1)ꢀO(3)ꢀNa(1)#3
C(6)ꢀC(1)ꢀC(2)
C(5)ꢀC(6)ꢀC(1)
121.99(19)
121.76(16)
120.81(15)
117.43(16)
140.42(16)
120.71(18)
119.79(18)
C(5)ꢀC(4)ꢀC(3)
118.56(18)
88.98(3)
Na(1)ꢀO(4)ꢀNa(1)#4
Symmetry transformations used to generate equivalent atoms:
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#1 ꢀxꢀ1/2,y,z ; #2 x+0,ꢀyꢀ3/2,z+1/2
#3 ꢀxꢀ1/2,ꢀyꢀ3/2,z+1/2 ; #4 ꢀxꢀ1/2,ꢀyꢀ3/2,zꢀ1/2
Table.2.5 Hydrogen bond geometry (Å)
DꢀH···.A
DꢀH
H···A
D···A
DꢀH···A
C6ꢀH6···O2i
0.93
2.394
2.687
98.04
C4ꢀH6···O3i
0.93
2.434
2.033
2.033
2.004
2.004
2.957
2.715
2.785
2.785
2.811
2.811
3.265
97.40
O4ꢀH4A···O1ii
O4ꢀH4A···O1iii
O4ꢀH4B···O1iv
O4ꢀH4B··O1v
O4ꢀH4B··O1vi
0.786
0.786
0.824
0.824
0.824
159.98
159.98
166.40
166.40
104.75
Symmetry codes: (i) x,y,z ; (ii) ꢀx,ꢀyꢀ1,+zꢀ1; (iii) x+1/2,ꢀyꢀ1,+zꢀ1;
(iv) x+1/2,+yꢀ1/2,+zꢀ1/2; (v) ꢀx,+yꢀ1/2,+zꢀ1/2; (vi) x,ꢀyꢀ1/2ꢀ1,+z+1/2
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Fig. 2 An ORTEP view of the SPNPD crystal structure with the atom numbering scheme. Carbon and
oxygen atoms are denoted by black and red color ellipsoid balls while white color balls are for
hydrogen atom. Sodium cation strongly interacts with the oxygen atoms of the nitro (NO2) group and
water molecules.
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Fig. 3 Crystal packing diagram in which carbon, oxygen and nitrogen atoms are represented by black,
red, blue colors, respectively, while bigger grey color atom represents Sodium atom. The hydrogen
bonding interactions are shown by dotted green color lines which take place between phenolic oxygen
atom and hydrogen atoms of water molecules.
Vibrational Spectroscopy
Vibrational spectroscopy is an important tool in indentifying the functional groups present in the material.
Observed FTIR and Raman spectra are depicted in Fig. 4(a) and (b), respectively. In FTIR spectra the broad
asymmetric band centered at 3301cmꢀ1 can be assigned to the OꢀH stretching modes of the two hydrogen
bonds which shows that compound contain water molecules. The infrared bands observed at 493, 704 cmꢀ1
correspond to the torsion vibrations of the ring. IR and Raman bands observed at 646, 1115, 1587 and 646,
1110, 1270 cmꢀ1 is due to the ring bending or stretching and stretching vibrations of CꢀNO2 and CꢀO
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respectively. Out of plane symmetric deformation in NO2 and out of plane bending in CꢀH vibrations are
observed at 758 and 829, 853 cmꢀ1, respectively, in IR spectra. In IR and Raman spectra bands observed at
990, 1172 and 1170 cmꢀ1 respectively are due to the inꢀplane bending vibrations of CꢀH bond. In IR and
Raman spectra, rocking vibrations of NO2 are observed at 545 and 548 cmꢀ1 respectively while out of plane
bending vibrations of CꢀH are present at 835 and 860 cmꢀ1 in Raman spectra. Symmetric and antisymmetric
bands of NO2 are observed at 1347 and 1487 cmꢀ1 in Raman and IR spectra respectively. IR and Raman
bands at 1301, 1462 and 1409,1524,1579 cmꢀ1 respectively are attributed to the stretching or bending
vibrations of the ring. Antisymmetric vibrations of NO2 are observed at 1469 cmꢀ1 in Raman spectra. Thus
vibrational frequencies observed in FTIR and Raman spectra are found to be in good agreement with the
reported values.28
Fig.4. (a) FTIR spectra of grown sodium pꢀnitrophenolate dihydrate crystal. (b) Raman spectra of grown
crystal. These spectra identify OꢀH, NO2 and benzene ring vibrations present in material.
Hirshfeld Surface analysis
Hirshfeld surfaces visualize intermolecular interactions by colorꢀcoding having short or long contacts
which explore the properties of all intercontacts within crystal structure.18 These surfaces of a molecule in
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the crystal are constructed based on the electron distribution which is calculated as the average sum of the
spherical atoms electron densities.29 We use the geometric function dnorm plotted onto the Hirshfeld surface.
This normalized contact distance dnorm based on de (distance from a point on the surface to the nearest
nucleus external to the surface) and di (distance from a point on the surface to the nearest nucleus internal to
the surface) and the van der Wall radii of atoms internal or external to the surface (rivdw/ revdw) identifies the
regions of particular importance to the intermolecular interactions which is given by following equation:
vdw
dnorm = di ꢀ ri vdw/ ri vdw + de – re vdw/ re
(1)
where ri vdw and re vdw are Van der Walls radii of two atoms internal and external to the molecular surface.
The value of dnorm is positive or negative depending upon the intermolecular contacts shorter or longer than
rvdw, respectively.
Hirshfeld surfaces mapped with different properties i.e. dnorm, curvedness and shape index is very useful tool
for visualizing intermolecular interactions and their contribution towards the crystal packing behavior of
molecules.29The Hirshfeld surfaces of all the molecules mapped with above mentioned properties are shown
in Fig. 5ꢀ7.
The red regions in the Hirshfeld surface mapped over a dnorm range of ꢀ0.75 to 1.10 (Fig. (5.a)) represent the
dominant interactions taking place in the crystal. The contributions of different intermolecular interactions
present in the crystal structure are provided by two dimensional pictures of fingerprint plots derived from
the Hirshfeld surfaces. The Hirshfeld surfaces and the associated fingerprint plots have been generated using
Crystal explorer. The 2ꢀD fingerprint plot of de vs di as shown in Fig. 5 (b) summarizes all intermolecular
contacts experienced by molecules in SPPNPD crystal. Particular atom pair close contacts have been
highlighted by decomposing fingerprint plots.30 In Fig.5(c,d) the O···H (di > de) and H···O (de > di)
intermolecular contacts are highlighted in red region on the dnorm surface. These interactions (O···H/ H···O)
appear as large spikes pointing towards lower left in the 2ꢀD plot comprising 8.2% and 6.3%, respectively to
the total area of Hirshfeld surface of molecules which reveals that these interactions are around the rvdW
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separation. The C···H contacts corresponding to the πꢀdonor and πꢀacceptor contribute very less (0.9%) to
the total Hirshfeld surface area.
Fig. 5 (a) Hirshfeld surface mapped with dntorm for structure. (b) Twoꢀdimensional fingerprint plot for
SPNPD molecule. (c,d) Front and back views of O···H and H···O intermolecular interactions in SPNPD
molecule. (e,f) Fingerprint plots of O···H and C···H interactions showing percentages of contacts
contributed to the total Hirshfeld surface area of the molecules.
The shape index (S) represents the local morphology of any given surface in terms of color coded
information i.e. hollow (red) and bumps (blue) 31 and can be expressed as:
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S= 2/π tanꢀ1 (k2 + k1/ k2 ꢀ k1)
Where k2 and k1 are two principal curvatures of the surface, k1≤ k2.
Curvedness (C) is defined as the root mean square curvature of the surface and is given by the following
equation32:
2
2
C= 2/π ln(k2 + k1 /2)
Fig.6 (a) and (b) illustrates how shape index and curvedness surfaces are used to identify planar stacking
(π···π) interaction arrangements. On the same region of shape index surface, the presence of red and blue
triangles shown by purple color ellipse in Fig.6 (a) indicates that the π···π interaction is almost identically
present in the crystal structure. Blue triangles represent convex region which is formed due to the carbon
atoms present in benzene ring of the molecule inside the surface while red triangle having concave regions
is due to the carbon atoms of π stacked molecule above it. This pattern of red and blue triangles on the
region of both sides of molecule determines the way in which molecules overlap and make contact with
each other and also shows how adjacent molecules are related by translation. The mapping of curvedness on
Hirshfeld surface (Fig.6 (b)) shows flat green region separated by blue edges. This clearly visible flat
regions as shown by purple color lines on curvedness surface is another characteristic of π···π stacking
interaction.
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Fig. 6 (a) Hirshfeld surface of SPNPD molecule mapped with shape index. The presence of red and blue
triangles has been shown by purple color ellipse in which red and blue color represent the bumps and hollow
regions on the shape index surfaces. (b) Hirshfeld surface mapped with curvedness for identifying the planar
(green) and curved (blue edge) regions in SPNPD molecule for planar stacking interactions.
The contribution of π···π stacking interactions (C···C) was further investigated by Hirshfeld surfaces as
shown in Fig.7. These decomposed fingerprints have enabled separate contributions of different
intermolecular interactions present in crystal structure. The C···C contacts contribute 2.9% to the total
Hirshfeld surface area which is very less. H···H interactions are reflected in the distribution of scattered
points in the 2ꢀD fingerprint plot which have a relatively significant contribution 19.9% to the total
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Hirshfeld surface area of the molecules This indicates that stronger H···H bonds exist between the SPNPD
molecules.
Fig.7. Two dimensional Fingerprint plots and Hirshfeld surfaces of H···H and C···C interactions with
19.9% and 2.9%, respectively.These C···C interactions contribute to the total intermolecular interactions
area for π···π stacking of the SPNPD molecules.
Optical transmission study
For any crystal, the optical absorption or transmittance window and cut off wavelength are very important
parameters in providing information about its optoꢀelectronic transitions. UVꢀvisible studies give important
structural information because absorption of UV and visible light results in the promotion of electrons in π
and σ orbital from the ground state to excited energy states.33 Fig. 8(a) shows the recorded UV–visible
transmittance spectra of the grown crystals which revealed that the ‘lower cutꢀoff’ in the case of SPNPD
samples is around 450 nm and there is very high transmittance (~99 %) in the region 480ꢀ110 nm which
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shows that crystal is free of defects such as precipitates and inclusions. In general, absorption peak of
benzene is observed around 200 nm. In case of 4ꢀnitrophenol, absorption peaks are strongly shifted because
of the presence of hydroxyl and pꢀsubstituted nitro groups in benzene ring.34 Although this crystal has a
lower cutꢀoff at 450 nm, it is still better compared to those of potassium pꢀnitrophenolate, guanidinium pꢀ
nitrophenolate crystal having lower cutꢀoffs at 510 nm and 505 nm, respectively.8ꢀ9 The very low absorption
in the entire region of 480ꢀ110 nm enables it to be a good candidate for optoꢀelectronic and NLO
applications. Below 280 nm the absorbance raises which may be due to the electronic transition πꢀ π* taking
place in benzene ring of SPNPD material. The appearance of such absorption peak in lower wavelength
region in pꢀnitrophenolate based compound has been reported by other researcher as well. 8
The optical constants of the material play an important role in fabricating optical devices. The dependence
of the optical absorption coefficient on the photon energy helps in determining the nature of optical
transitions of electrons which take place in material.
The optical absorption coefficient (α) was calculated using the following relation:
α = (2.303/t) x log (1/T)
where, ‘t’ is the sample thickness and ‘T’ is the transmittance.
With the occurrence of a direct band gap, the crystal under study has an optical absorption coefficient (α)
which obeys the following relation for high photon energies,
(αhν)2 = A (hν ꢀ Eg)
Where ‘Eg’ is the optical band gap and ‘A’ is a constant which is nearly independent of photon energy. The
graph plotted between (αhν) 2 and photon energy (hν) has been shown in Fig.8 (b) and the optical band gap
2
was found to be 2.80 eV which has been evaluated by extrapolating the linear portion of (αhν) in the
photon energy axis. This band gap value indicates the large transparency range of material which makes it
more suitable for optical applications.
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Fig.8. (a) Optical transmission spectrum of SPNPD single crystal. (b) Plots of (αhν)2 function of hν with
evaluation of band gap of the material.
Photoluminescence
In solids, photoluminescence (PL) is the phenomenon in which electronic states of a solid are excited by
light of particular energy and the excitation energy is released as photons with different energy which is
related to the electronic structure of the material. Photoluminescence is a useful technique to identify the
impurities and find applications in lighting technologies. Generally photoluminescence phenomenon is
expected in aromatic molecules which contain multiple conjugated bonds leading to a high degree of
resonance stability. SPNPD crystal consists of phenoxy ion (benzene derivative) having delocalized πꢀ
electrons in C=C bonds due to which all C=C bonds can have different energy spacing between ground and
excited states. It means that there exist a large number of energy states between ground and excited states
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which are responsible for radiative recombination resulting in luminescence spectrum in grown crystal.
Photoluminescence (PL) spectrum of grown SPNPD crystal was recorded at room temperature by exciting it
at 450 nm as shown in Fig.9. A sharp broad emission peak centered at 525 nm was observed in the spectrum
which indicates green emission.35 This broad green emission peak is also related to deep level defect
transitions i.e. there is a contribution from deep holes in the energy levels. 36 This broadening in emission
peak is attributed to the presence of strong intermolecular hydrogen bonding interactions present in crystal
structure as evidenced in Hirshfield surface analysis. This hydrogen bonding is taking place between oxygen
of OH group (phenolic oxygen) attached to benzene ring and hydrogens of water molecules having attached
with sodium cation in crystal structure (as discussed earlier in Fig.3.) which causes the dominance of
radiationless pathways in decay process. Thus intermolecular interaction will have an important role in
crystal packing. Further, the asymmetric nature of the emission spectrum shows that the fluorescing moiety
present in crystal interacts differently with the neighbors. Thus grown crystals can be used to fabricate green
lasers. Such materials having broad emission peak can be used in new tunable laser system. 37
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Fig.9. Photoluminescence spectrum of SPNPD crystal showing green emission.
Dielectric Properties
Dielectric constant measurement is one of the basic electrical properties of solids which give information
about the nature of atoms, ions and their polarization mechanism present in a crystal. Basically, dielectric
materials enhance the capacitance between the plates of a condenser which can have useful applications in
capacitor technology. Dielectric studies of sodium pꢀnitrophenolate dihydrate crystal were carried out at
various frequencies and temperatures. For doing this, we selected good transparent crystal and both surfaces
of it were coated with high grade silver paste to make parallel electrodes. The dielectric constant of the
material is directly proportional to the polarization (charge displacement) which further depends on the
applied electric field and the responses of the different constituents (atoms, ions) of the solids. Fig. 10(a)
shows the variation of dielectric constant which decreases from 111 to 30 with increasing frequency at room
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temperature. The higher dielectric constant at lower frequencies is due to the contribution of all
polarizations i.e. electronic, ionic, dipolar and space charge polarizations. Further with increase in
frequency, the contribution of space charge polarization decreases resulting in decrease in dielectric constant
value. At lower frequencies, the dipoles follow the rapidly changing electric field which results in higher
dielectric constant value, while dipoles are unable to follow it at higher frequencies and hence leads to
reduction in dielectric constant value.
Now, the sample was heated from RT to 58 °C in the frequency range of 20 Hz to 2 MHz at a heating rate of
1 °C/ min. Fig. 10(b) shows the variation of dielectric constant with temperature at various frequencies. It is
observed from the figure that dielectric constant increases with increase of temperature and it attains a
maximum value at 34 °C and then it gradually decreases with further increase in temperature. Thus a
transition was observed at 34 °C which may be due to ferroelectric to paraelectric phase transition. It is
inferred that grown crystal is a ferroelectric material. The maximum value of dielectric constant obtained at
34 °C was found to be decreased with increasing frequency while the temperature of dielectric maxima
(34 °C) with different frequencies remains same showing the absence of any relaxation behavior. The value
of dielectric constant was found to be low which enhances SHG coefficient in material in accordance with
Miller’s rule.38 Also, the low dielectric constant makes the material a potential candidate for
microelectronics industries applications. The variation of dielectric loss with temperature at different
frequencies is shown in Fig.10 (c). The dielectric loss decreases with increasing frequency which indicates
that crystal has minimum defects and it can be used for photonic and electroꢀoptic devices applications.
Overall, very low value of dielectric loss clearly reveals the usefulness of dielectric property of material. 39
After 5 KHz onwards, dielectric loss goes to very much small value which reveals that material can be used
for storing large number of charges without much energy loss.
Electrical conduction in crystal takes place when an electron jumps from lower energy state to higher energy
state. The variation of the ac conductivity (σac) with temperature at different frequencies is plotted in Fig.10
(d). The ac conductivity (σac) of grown crystal was calculated using the relation:
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σac = 2πf εoεrω tanδ
where εo is the permittivity of free space and ‘f’ is the frequency of the applied field. It is observed from the
figure that ac conductivity increases with increase in frequency which is attributed to the reduction in the
space charge polarization at higher frequencies. 40 With increase in temperature, more and more defects are
produced and the movements of these defects result in increased conductivity. Further, the sharp change in
conductivity at 34 °C supports the phase transition as observed in dielectric constant study (Fig. 10(b)). The
variation of conductivity with temperature shows that the conduction process is thermally activated.
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Fig.10.(a) Variation of dielectric constant with frequency at RT. (b) variation of dielectric constant with
temperature at various frequencies showing ferroelectric phase transition at 34 °C. Temperature dependence
of dielectric loss and Ac conductivity at various frequencies is shown in (c)ꢀ(d).
Poling and d33 measurements
Crystal symmetry plays an important role in piezoelectric effect which is defined as the ability of materials
to produce an electrical charge on applying mechanical stress. Piezoelectric studies were carried out on
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grown crystals. Firstly, the sample was poled by applying a moderate dc electric field of 1.5kV/mm for 25
min at room temperature because the poling process improves the net alignment of the dipoles of the
material in the direction of applied field. In general, Piezoelectric coefficient (dij) can be defined as:
dij = (∂Di/∂Tj)E
where D, T and E are the electric displacement, stress and electric field respectively. The variation of the
electric displacement ꢂD with external stress T under a constant applied electric field E is directly
proportional to the dipole movement. Larger is the polarizability, greater will be the piezoelectric coefficient
of the material. The piezoelectric coefficients can be reported in terms of d33, d15 and d31 etc. In the present
study, piezoelectric properties were confirmed by determining the piezoelectric charge coefficient, d33 pC/N
which can be defined as the generation of polarization in cꢀ direction when stress is applied in the same
direction. This d33 measurement was done along the poling direction of crystal axis across the basal surfaces
using PMꢀ300 Piezometer system by applying a tapping force of 0.25 N and tapping frequency of 110 Hz.
The piezoelectric coefficient of the crystal was found to be 2.24 pC/N which is in comparison with that of
Quartz (2.3 pC/N) and Triglycine sulphate crystal (TGS= 10.22 pC/N) 41ꢀ42 and hence SPNPD material finds
its applications in transducer devices, sensing microstructures on electronic microꢀ chips and electric voltage
sources. 43ꢀ44 In semiorganic crystals, the hydrogen bond and πꢀπ stacking interactions have an important
role in analyzing piezoelectric properties of crystal.45 These πꢀπ interactions present in crystal can screen the
Hꢀbond from the applied electric field.45 It means that the contribution of Hꢀbond is higher than πꢀsystem
along the poling direction of crystal axis as described in Hirshfield surface analysis (Fig. 5ꢀ7) resulting in a
significant d33 value. Thus piezoelectricity reported in grown crystal makes it a better candidate for device
applications among all reported pꢀnitrophenolate based compounds.8ꢀ11
Hysteresis Loop
Ferroelectricity property in materials can be defined as the presence of spontaneous electric
polarization which can be reversed by an applied electric field. The ferroelectric behaviour of grown crystal
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was confirmed by tracing P–E hysteresis loop at room temperature which is being reported for the first time.
Hysteresis loop of grown SPNPD crystal is shown in Fig.11. It is found that grown crystals can sustain a
switching ac electric field upto 10 kV/cm beyond which cracks are developed in them. So, we cannot apply
a field more than this value to achieve saturation polarization. Therefore, grown crystal does not show
saturation in polarization curve. The Ferroelectric loop has an oval shape which may be due to its Tc lying
just near the room temperature. From the hysteresis loop, the values of the remnant polarization (Pr) and
coercive field (Ec) are found to be 2.67 ꢁC/cm2 and 7.43 kV/cm, respectively. We define coercive field (Ec)
as the minimum applied electric field strength which is required for switching full remnanat polarization.
Till now ferroelectric behavior has not been reported in many other alkali pꢀnitrophenolate hydrate
8ꢀ11
compounds.
Due to ferroelectric behavior reported in grown crystal, this material can have many
optoelectronic applications such as capacitors, nonvolatile memory devices, highꢀperformance gate
insulators etc.
Fig.11 PꢀE hysteresis loop of grown crystal at room temperature
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