A novel approach to the synthesis of semiorganic ammonium hydrogen oxalate oxalic acid dihydrate single crystal and its characterization

Article abstract of DOI:10.1007/s10973-020-09965-5

A different approach used in the synthesis of ammonium hydrogen oxalate oxalic acid dihydrate (NH₄H₃(C₄O₈)·2H₂O) single crystals is presented. The crystals are synthesized using solvent evaporation technique. The analysis of the synthesized NH₄H₃(C₄O₈)·2H₂O single crystals has been carried out through measurements such as single-crystal X-ray diffraction (SCXRD), powder XRD, UV–Vis and photoluminescence spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric and differential thermal analysis (TG–DTA), Vickers microhardness measurements and?current versus voltage (I–V) characteristics. The evaluation of the molecular structure of the crystals using SCXRD shows that they belong to the triclinic crystal system with space group P-1. The structural properties of the crystals are also evaluated using powder XRD measurements on their finely crushed powder. These measurements corroborate the results of SCXRD and confirm the triclinic system of the crystals. The optical properties like absorbance and emission of these crystals are determined using UV–Vis and photoluminescence spectroscopy, respectively. These measurements show that the crystals exhibit substantial emission in the blue region. The thermal stability and the decomposition mechanism of the crystals is studied using TG–DTA analysis. The mechanical strength of the crystals is determined using Vickers microhardness technique. The photoresponse of the NH₄H₃(C₄O₈)·2H₂O crystals is analyzed from light-dependent I–V characteristics, and it is observed that the crystals exhibit photoresistive behavior.

Full text of DOI:10.1007/s10973-020-09965-5

Journal of Thermal Analysis and Calorimetry
https://doi.org/10.1007/s10973-020-09965-5
A novel approach to the synthesis of semiorganic ammonium
hydrogen oxalate oxalic acid dihydrate single crystal and its
characterization
1
2
1
K. Mahendra · Jean Maria Fernandes · N. K. Udayashankar
Received: 8 January 2020 / Accepted: 15 June 2020
©
Akadémiai Kiadó, Budapest, Hungary 2020
Abstract
A diferent approach used in the synthesis oꢀ ammonium hydrogen oxalate oxalic acid dihydrate (NH H (C O )·2H O) sin-
4
3
4
8
2
gle crystals is presented. The crystals are synthesized using solvent evaporation technique. The analysis oꢀ the synthesized
NH H (C O )·2H O single crystals has been carried out through measurements such as single-crystal X-ray difraction
4
3
4
8
2
(
SCXRD), powder XRD, UV–Vis and photoluminescence spectroscopy, Fourier transꢀorm inꢀrared spectroscopy, thermo-
gravimetric and diferential thermal analysis (TG–DTA), Vickers microhardness measurements and current versus voltage
I–V) characteristics. The evaluation oꢀ the molecular structure oꢀ the crystals using SCXRD shows that they belong to the
(
triclinic crystal system with space group P-1. The structural properties oꢀ the crystals are also evaluated using powder XRD
measurements on their ꢁnely crushed powder. These measurements corroborate the results oꢀ SCXRD and conꢁrm the
triclinic system oꢀ the crystals. The optical properties like absorbance and emission oꢀ these crystals are determined using
UV–Vis and photoluminescence spectroscopy, respectively. These measurements show that the crystals exhibit substantial
emission in the blue region. The thermal stability and the decomposition mechanism oꢀ the crystals is studied using TG–DTA
analysis. The mechanical strength oꢀ the crystals is determined using Vickers microhardness technique. The photoresponse
oꢀ the NH H (C O )·2H O crystals is analyzed ꢀrom light-dependent I–V characteristics, and it is observed that the crystals
4
3
4
8
2
exhibit photoresistive behavior.
Keywords Ammonium hydrogen oxalate oxalic acid dihydrate · Synthesis · Single-crystal X-ray difraction · Negative
photoconductivity
Introduction
over the decades, a lot oꢀ efort has been made by research-
ers to design such highly eꢂcient materials used in these
devices [10–13]. Organic and inorganic crystals are impor-
tant classes oꢀ optoelectronic materials, which are widely
used in these applications due to their availability in abun-
dance [14, 15]. However, they possess their own merits and
demerits [16]. Organic materials exhibit excellent optical
properties, but are brittle in nature. Certain applications
require a smooth surꢀace oꢀ these materials, which can be
achieved by polishing them [17]. Nevertheless, polishing
organic materials is extremely challenging since they are soꢀt
materials having weak hydrogen bonds [18, 19]. Moreover,
they also require extreme care during device ꢀabrication due
to their poor thermal and mechanical stability. On the other
hand, inorganic materials possess excellent mechanical and
thermal stabilities, but moderate optical properties. These
drawbacks can be overcome by developing an alternate class
oꢀ materials known as metal–organic or semiorganic. They
Optoelectronic devices and their applications in high-speed
inꢀormation processing, optical communication, optical data
storage, etc., have pervaded our everyday lives to an enor-
mous extent [1–7]. The key enabling technology which pro-
vides a basis ꢀor innovation in these applications is a promi-
nent ꢁeld in photonics called nonlinear optics (NLO) [1, 8,
9
]. Various materials exhibit nonlinear optical behavior, and
*
K. Mahendra
mahendra.ph13ꢀ06@nitk.edu.in
1
Department oꢀ Physics, National Institute oꢀ Technology
Karnataka, Surathkal, P.O. Srinivasnagar, Mangalore,
Karnataka 575025, India
2
Department oꢀ Post Graduate Studies and Research
in Physics, St Aloysius College (Autonomous), Mangalore,
Karnataka 575003, India
Vol.:(0
1
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3

K. Mahendra et al.
possess excellent optical properties as in the case oꢀ organic
materials, and good mechanical and thermal stabilities oꢀ
inorganic materials [20–23].
source is a monochromatic wavelength 0.7107 Å oꢀ molyb-
denum Kα radiation, and data were collected at ambient tem-
perature (296 K). The molecular structure was solved using
direct method and ꢀurther reꢁnement oꢀ the structure was
perꢀormed using SHELXL-2007/2014 soꢀtware by means
oꢀ ꢀull matrix least square technique. Powder X-ray diꢀ-
ꢀractometer (PXRD) (Rigaku 600) with an X-ray source oꢀ
wavelength 1.5405 Å (CuKα) and scan range 10° to 50° with
a scanning speed oꢀ 2° per minute was used ꢀor the struc-
tural characterization oꢀ the synthesized crystal. Absorbance
spectrum over a spectral range oꢀ 200 nm to 900 nm was
recorded using Ocean Optics USB 4000 UV–Vis spectrom-
eter. Fundamental molecular vibrations were studied by
recording IR spectrum using Shimadzu FTIR Spectrometer
employing ATR method. Thermal studies oꢀ the synthesized
single crystals were carried out using PerkinElmer thermal
analyzer in nitrogen-inert atmosphere over a temperature
range oꢀ 30 °C to 800 °C with a heating rate oꢀ 10 °C per
minute. Photoluminescence spectra was recorded using Hor-
iba Scientiꢁc-Fluromax-4 spectrophotometer with excitation
wavelength 280 nm. Vickers microhardness technique was
employed to examine the mechanical strength oꢀ the sin-
gle crystals. Varying indenter load oꢀ masses 10 g, 25 g,
50 g, 100 g and 200 g was applied ꢀor a dwell time oꢀ 15 s
on the (1 0 0) plane oꢀ NH H (C O )·2H O crystals. Light-
Recently, Stash et al. have reported the synthesis oꢀ
semiorganic ammonium oxalate oxalic acid dihydrate
(
NH H (C O )·2H O) single crystal and its single crystal
4 3 4 8 2
XRD characterization [9]. The standard method they used
ꢀor synthesis is as ꢀollows:
Ammonium 2,3,5,6-tetraoxo-4-nitropyridinate aque-
ous solutions were boiled ꢀor a long time to obtain
+
−
NH ·C HO ·C H O ·2H O. Minimum quantity oꢀ water
4
2
4
2
2
4
2
was taken to dissolve ammonium 2,3,5,6-tetraoxo-4-nitro-
pyridinate single crystals. This solution was maintained in
a boiling water bath ꢀor a time duration oꢀ 30 min and then
kept at room temperature ꢀor evaporation. Finally, dark red
colored ammonium 2,3,5,6-tetraoxo-4-nitropyridinate single
crystals and colorless prismatic single crystals oꢀ ammo-
nium oxalate oxalic acid dihydrate were isolated. However,
to the best oꢀ our knowledge, there are no reports till date
on the structural, optical, thermal and electrical properties
oꢀ NH H (C O )·2H O single crystals. We report a diferent
4
3
4
8
2
approach and relatively simple method to synthesize ammo-
nium oxalate oxalic acid dihydrate (NH H (C O )·2H O)
4
3
4
8
2
single crystal. We also present a detailed experimental inves-
tigation oꢀ the synthesized crystals.
4
3
4
8
2
In the present work, we report the synthesis oꢀ
NH H (C O )·2H O single crystal using oxalic acid dihy-
dependent I–V studies were carried out using Keithley-
2410-C source meter. In these measurements, a customized
setup was used in order to characterize the crystal sample
in dark and illuminated conditions. The crystal sample was
polished at both the sides and placed in between a two-probe
sample holder. A 50 W halogen dichroic lamp was used as
white light source. An input voltage ranging ꢀrom −5 V to
+ 5 V was applied to the sample and the resulting current
was measured.
4
3
4
8
2
drate and ammonium dihydrogen phosphate, and the crystal
structure is solved to the R value oꢀ 0.0409. The synthesized
crystal is subjected to ꢀurther characterization using powder
XRD, UV–Vis and photoluminescence spectroscopy, ther-
mogravimetric and diferential thermal analysis (TG–DTA),
Fourier transꢀorm inꢀrared spectroscopy (FTIR), Vickers
microhardness measurements and I–V studies.
Material synthesis
Results and discussions
Salts oꢀ oxalic acid dihydrate and ammonium dihydrogen
phosphate obtained ꢀrom M/s. Merck were taken in 1:1
molar concentration and mixed in double distilled water. The
mixture was stirred ꢀor 4 h using a magnetic stirrer to obtain
clear saturated solution. This solution was then ꢁltered and
kept ꢀor slow evaporation at room temperature. Colorless
and transparent crystals were harvested aꢀter 3 weeks, and
these crystals were puriꢁed by multiple crystallization pro-
cess to enhance their quality.
Single‑crystal XRD (SCXRD) analysis
An asymmetric unit oꢀ NH H (C O )·2H O crystal con-
4
3
4
8
2
sists oꢀ one molecule oꢀ oxalic acid, one molecule oꢀ hemi
oxalate and two molecules oꢀ water. Ammonium transi-
tion in this crystallographic unit ꢀorms a cyclic tetramer
unit (Fig. 1) with oxalic acid and two molecules oꢀ water
via N–H–O (bond length 2.944 Å) and O–H–O hydrogen
bonds (bond lengths 2.834 Å and 2.901 Å). This interaction
ꢀ
urther continues, resulting in a linear 1D chain structure
Characterization techniques
oꢀ NH H (C O )·2H O. The 1D structure ꢀurther converts
4
3
4
8
2
to a 2D structure due to O–H–O hydrogen bond (O–H–O
oꢀ oxalic acid and oxygen oꢀ water molecule) and N–H–O
hydrogen bond. The packing view and 3D representation oꢀ
the NH H (C O )·2H O crystal are shown in Figs. 2 and 3,
Single-crystal X-ray difraction analysis using Apex II duo
Bruker difractometer coupled with CCD detector was used
to study the molecular structure oꢀ the crystal. The X-ray
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3
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8
2
1
3

A novel approach to the synthesis of semiorganic ammonium hydrogen oxalate oxalic acid dihydrate…
respectively. Complete inꢀormation about the crystal struc-
ture obtained ꢀrom the SCXRD analysis is shown in Table 1.
It can be seen ꢀrom the table that the crystal belongs to cen-
trosymmetric space group P-1 with triclinic crystal system.
The photograph oꢀ the synthesized NH H (C O )·2H O sin-
4
3
4
8
2
gle crystal is shown in Fig. 4.
Powder XRD analysis
Powder XRD analysis was carried out on the ꢁne powder
obtained by crushing the synthesized single crystals using
motor and pestle. Sharp peaks are observed in the XRD pat-
tern as shown in Fig. 5, indicating crystalline nature oꢀ the
sample. The SCXRD data are ꢁrst analyzed using Mercury
soꢀtware, and the Miller indices obtained are used to index
the crystals. X-pert HighScore soꢀtware is then used ꢀor the
analysis oꢀ powder XRD data. It is ꢀound that the crystals
belong to triclinic crystal system, which is in good agree-
ment with the results obtained ꢀrom single crystal XRD data.
The obtained XRD patterns are ꢀurther reꢁned using Xpert
Highscore Plus soꢀtware to obtain unit cell parameters and
shown in Table 2.
Fig. 1 Asymmetric unit oꢀ NH H (C O )·2H O single crystal
4
3
4
8
2
UV–Vis analysis
UV–Vis spectroscopy in the range 200–900 nm was used
to obtain the absorption spectrum oꢀ the crystals. In the
UV region, the crystals show strong absorbance, whereas
in the visible region they show no absorbance, as observed
in Fig. 6a. The absorption peak oꢀ the crystals is observed
around 290 nm, corresponding to the UV region oꢀ the elec-
tromagnetic spectrum.
The optical bandgap (E ) oꢀ the crystals is associated
g
Fig. 2 Packing view oꢀ NH H (C O )·2H O single crystal
4
3
4
8
2
with the incident photon energy as shown in the equation
2
(
αhν) =A(hν−E ) [24], where A is a constant, α is the opti-
g
cal absorption coeꢂcient and hν is the photon energy. This
Fig. 3 3D representation oꢀ
NH H (C O )·2H O single
4
3
4
8
2
crystal
O–H–O hydrogen bond
N–H–O hydrogen bond
1
3

K. Mahendra et al.
Table 1 Crystal inꢀormation, data collection and reꢁnement details
.0 × 103
3
Crystal structure details
NH H (C O )·2H O
4 3 4 8 2
2
2
1
.5 × 103
.0 × 103
.5 × 103
CCDC number
Molecular ꢀormula
Molecular weight
Crystal system
1,547,828
C H N O10
4
11
466.28
Triclinic
P 1̄
Space group
a/Å
b/Å
c/Å
α/°
β/°
γ/°
6.3409 (10)
7.2278 (9)
10.5586 (18)
94.143 (6)
100.248 (6)
97.779 (5)
469.50 (12)
1
1.649
0.169
296 (2)
2340
1748
1
.0 × 103
.0 × 102
5
0.0
10
20
30
40
50
Volume/Å3
Z
2
θ°
Density/g/cm3
Fig. 5 Powder XRD pattern
−
1
µ (MoKα)/mm
T/K
Reꢃns collected
Unique rꢃns
Table 2 The reꢁned unit cell parameters oꢀ the NH H (C O )·2H O
4
3
4
8
2
single crystals
Parameter reꢁned
R1 (I>2σ)
164
0.0409
0.1053
0.732
Crystal structure details
AHOOD
Compound
Ammonium hydrogen
oxalate oxalic acid
dihydrate
wR (I>2σ)
2
GOF
Formula
Space group
NH H (C O )·2H O
P-1
4
3
4
8
2
Density
1.63 g/cc
6.2382
7.2056
10.5095
94.6059°
a/Å
b/Å
c/Å
α
β
γ
100.0921°
97.0821°
461.46
3
Volume/Å
Crystal system
Triclinic
absorbance) oꢀ the crystals in the visible region ꢀavors their
use in optoelectronic industry applications.
Fig. 4 Photograph oꢀ NH H (C O )·2H O single crystal
4
3
4
8
2
FTIR analysis
relation is known as the Tauc relation and a graph oꢀ (αhν)2
versus photon energy (hν) gives the Tauc plot (Fig. 6b). The
optical bandgap can be estimated by extrapolating the linear
portion oꢀ this graph. As seen in Fig. 6b, the optical bandgap
value ꢀor NH H (C O )·2H O single crystals is ꢀound to be
Figure 7 shows the FTIR spectrum used to study the
ꢀundamental molecular vibrations oꢀ the synthesized
NH H (C O )·2H O single crystals. The absorption in
4
3
4
8
2
−
1
the plot oꢀ FTIR spectrum at 3437 cm represents the
stretching ꢀrequency which can be assigned to O–H stretch-
ing and conꢀirms the water molecules in the structure
oꢀ NH H (C O )·2H O [26, 30, 31]. The absorption at
4
3
4
8
2
4
.03 eV. The bandgap energy is very close to the bandgap
values (4.09 eV–5.01 eV) ꢀor other oxalate crystals reported
in literature [25–29]. This makes them suitable ꢀor wide
bandgap applications. Moreover, the high transmittance (low
4
3
4
8
2
−
1
3137 cm can be assigned to NH stretching vibration [31].
Asymmetric and symmetric stretching vibration oꢀ C=O is
−
1
−1
represented by the absorption at 1706 cm and 1278 cm ,
1
3

A novel approach to the synthesis of semiorganic ammonium hydrogen oxalate oxalic acid dihydrate…
Fig. 6 a UV–Vis absorption, b
(b)
(
a)
Tauc plot
3 × 105
0.6
2
× 105
0
0
.4
.2
ν
1 × 105
α
0.0
4.03eV
0
200 300 400 500 600 700 800 900
3.50
3.75
4.00
4.25
4.50
Wavelength/nm
hν/eV
Table 3 FTIR assignments oꢀ NH H (C O )·2H O single crystal
4
3
4
8
2
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
Wave number/cm−1
Assignment
3137
NH stretch
C=O asymmetric stretch
1
706
1624
402
1278
105
732
−
NH asymmetric stretch+COO bending
3
−
1
NH Stretch+COO bending
4
C=O symmetric stretch
C–C stretch
O–C–O stretch
1
In the present work, thermal analysis was carried out on
NH H (C O )·2H O single crystals using TG and DTA in
4
000
3500
3000
2500
2000
1500
1000
500
4
3
4
8
2
Wavenumber/cm–1
order to determine their melting point, decomposition tem-
perature, purity and crystallinity. The characterization was
perꢀormed in nitrogen atmosphere on the crystal compound
oꢀ 6 mg. Figure 8 shows the TG and DTA plots oꢀ the syn-
thesized crystals. It is observed that NH H (C O )·2H O has
Fig. 7 FTIR spectrum oꢀ NH H (C O )·2H O single crystal
4
3
4
8
2
4
3
4
8
2
−
1
respectively [26, 32]. Likewise, absorption at 1624 cm
a 4.5% loss in mass starting at 71 °C, indicating the difu-
sion oꢀ adsorbed water molecules through the crystal [26,
42]. The process oꢀ dehydration continues till anhydrous
NH H (C O )·2H O compound is obtained and the mass
corresponds to the asymmetric stretching oꢀ NH and COO
3
bending [33–35]. NH stretching and COO bending leads to
4
−
1
absorption at 1402 cm [33–38], whereas the C–C vibra-
4
3
4
8
2
tions [33, 35] and O–C–O stretching [30, 36, 37] leads to
loss is also observed in steps oꢀ 14% and 6% (Fig. 8). Major
mass loss is witnessed beyond 156.4 °C due to the ꢃash
point oꢀ NH H (C O )·2H O crystal, with 76% mass loss
−
1
−1
absorption at 1105 cm and 732 cm , respectively. All the
assignments in the FTIR spectra are given in Table 3.
4
3
4
8
2
occurring at 223.6 °C which is the melting point oꢀ the com-
pound. The volatilization oꢀ gases conꢁned in the material
occurs at 167.9 °C and is in good agreement with the ꢃash
point oꢀ C H NO ·1/2 H O [31]. At 223.6 °C, molecules oꢀ
Thermal analysis
Thermal analysis oꢀ a crystal is extremely crucial to investi-
gate its viability in several technological applications. Ther-
mogravimetric and thermal decomposition analysis meas-
urements oꢀ diferent oxalates oꢀ strontium, uranium, uranyl,
barium, thorium, etc., and their composites have been per-
2
5
4
2
NH , CO, CO and H O gases are obtained and the material
3
2
2
decomposes. The amount oꢀ material remaining evaporates
at 259.8 °C and the corresponding mass loss is 3.4%. The
reactions occurring during the thermal characterization are
shown in Eqs. 1 and 2.
ꢀ
ormed intensively by researchers due to their potential
applications in nuclear industry and medicine [39–41].
1
3

K. Mahendra et al.
Fig. 8 TG-DTA plots oꢀ
10
0
1
00
DTA
TGA
NH H (C O )·2H O single
4
3
4
8
2
14%
crystal
6%
8
6
4
2
0
0
0
0
0
–
–
–
10
20
30
7
6%
3
.4%
– 40
0
100 200 300 400 500 600 700 800
Temperature/°C
0
100 200 300 400 500 600 700 800
Temperature/°C
.0 × 105
Photoluminescence spectrum
2
Photoluminescence spectrum oꢀ NH H (C O )·2H O sin-
4
3
4
8
2
gle crystals is shown in Fig. 9. It can be observed ꢀrom the
1
1
5
.5 × 105
.0 × 105
.0 × 104
ꢁ
gure that the crystal has emission oꢀ substantial intensity
in the blue region and the peak at 360 nm (UV region) cor-
responds to the deꢀormation oꢀ O–C=O bond. These results
corroborate with the UV–Vis measurements. Emission in
blue region has also been reported by Dalal and Kumar in
2
016 ꢀor lithium nitrate oxalate monohydrate single crystals
[
25].
Vickers microhardness measurements
0.0
3
50
400
450
500
Hardness oꢀ a material is the amount oꢀ resistance ofered by
the material to local deꢀormation caused by indentation or
scratching [43]. The mechanical strength oꢀ our crystals was
tested using Vickers microhardness technique. The Vickers
Wavelength/nm
Fig. 9 Photoluminescence spectrum oꢀ NH H (C O )·2H O single
4
3
4
8
2
crystal
microhardness number is represented by H and is evaluated
v
2
using the ꢀormula H =1.8544 P/d [44], where P is the applied
v
indenter load in kg and d is the average diagonal length oꢀ the
indent in mm. The computed microhardness values are given
in Table 4. Figure 10 shows the graph oꢀ microhardness num-
ber (H ) versus applied load (P) oꢀ the NH H (C O )·2H O
NH H C O ⋅ 2H O → NH H C O + 2H O ↑
2
4
3
4
8
2
4
3
4
8
(1)
ꢀ
ꢁ
2
NH H C O
2
→ 2CO ↑ + 2CO ↑ + NH ↑ + 2H O ↑
2
4
3
4
2
3
v
4
3
4
8
2
(2)
single crystals. It can be observed ꢀrom the ꢁgure that the
microhardness value oꢀ the crystals decreases ꢀrom 94.65 kg/
2
2
mm to 72.12 kg/mm as the load is increased ꢀrom 10 g to
2
00 g. This can be attributed to the phenomenon oꢀ indentation
Table 4 Various parameters
Sl. no.
Load P/g
Hardness H / Yield Strength Elastic stifness
Fracture tough-
−3/2
Brittle-
v
related to microhardness
−2
−2
kg mm
σ /kg mm
constant (C )/Pa ness K /g µm
ness B /
y
11
c
i
−1/2
µm
1
2
3
4
5
10
25
50
100
200
94.65
79
74
72.3
72.12
31.55
26.33
24.66
24.1
2872.18
2093.37
1867.04
1792.63
1784.83
–
–
–
–
–
–
–
–
24.04
0.04969
1453.41
1
3

A novel approach to the synthesis of semiorganic ammonium hydrogen oxalate oxalic acid dihydrate…
Fig. 10 Log P versus Log d
100
2.4
2
2
1
.2
.0
.8
9
9
8
8
7
7
5
0
5
0
5
0
1.6
1
1
1
.4
.2
.0
n = 1.85
0.8
0
50
100
150
200
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
Load P/g
log d
size efect (ISE) [45], which is normally observed in single
crystals. The hardness also depends on the elastic stifness
the parameters decrease with increase in applied load, sug-
gesting normal indentation size efect behavior in the synthe-
sized NH H (C O )·2H O single crystal. The crack ꢀormed
(
C ) parameter which provides inꢀormation about the molecu-
11
4
3
4
8
2
lar bond strength in crystals. For diferent loads applied, C
during the indentation process is shown in Fig. 11.
11
can be computed ꢀrom Wooster’s relation as ꢀollows [46].
Negative photoconductivity
(7∕4)
C
= H
v
(3)
1
1
I–V plots oꢀ NH H (C O )·2H O single crystals in the illu-
4
3
4
8
2
The calculated C values are shown in Table 4.
1
1
minated and dark conditions are shown in Fig. 12. A linear
increment in the dark current and photocurrent with respect
to the applied electric ꢁeld is observed. At any particular
value oꢀ applied electric ꢁeld, the dark current is measured
to be superior than the photocurrent. This phenomenon is
termed as negative photoconductivity, and its occurrence
in the crystals may be due to the decrease in the charge car-
rier numbers or their liꢀetime upon illumination [55]. When
the sample is irradiated using white light, recombination
oꢀ holes and electrons takes place, thereby decreasing the
amount oꢀ mobile charge carriers and their liꢀetime, ulti-
mately leading to negative photoconductivity. According to
the well-known Stockmann model [56, 57], the ꢀorbidden
gap oꢀ a material has two energy levels corresponding to
two types oꢀ recombination centers. The position oꢀ one oꢀ
these levels is located in between the conduction band and
The relation between size oꢀ indentation and the load
applied is given by Meyer’s law [47–50]
n
P = Kd
(4)
where K is the material constant and n is the Meyers index.
The value oꢀ n computed ꢀrom the slope oꢀ log P versus log
d plot is 1.85, which conꢁrms the normal indentation size
efect (ISE) behavior observed in hard materials (n<2) [45].
The yield strength values shown in Table 4 are computed
using the ꢀollowing relation [51]
H
v
ꢀ
=
(5)
y
3
Other microstructural parameters such as ꢀracture toughness
K ) and brittleness (B ) oꢀ the crystal are also determined. The
(
c
i
ꢀ
racture toughness is calculated using the ꢀollowing expression
[
49, 52]
(3∕2)
K = ꢀ C
c
(6)
0
where C is crack length (61 μm) and β is taken as 7 ꢀor
0
Vickers indenter [53]
The relative measure oꢀ the material brittleness is obtained
ꢀ
rom the crack produced on the indented surꢀace and is related
to the hardness and ꢀracture toughness as ꢀollows [54].
H
v
Bi =
(7)
K
c
The calculated values oꢀ K and B are tabulated in
c
i
Table 4. From Table 4, it is observed that the values oꢀ all
Fig. 11 Indentation image at 200 g load
1
3

K. Mahendra et al.
the synthesized crystals, the melting point oꢀ the crystal is
Dark current
Photo current
1
1
5
0
5
0
5
ꢀ
ound to be 223.6 °C. Photoluminescence spectroscopy was
used to study the emission characteristics oꢀ the crystals, and
it shows that the crystal has maximum emission at 360 nm
along with substantial emission intensity in the blue region.
The crystals show negative photoconductivity with values
−
4
−1
−4
−1
6
.05×10 Ω m and 5.45×10 Ω m in dark and illu-
minated conditions, respectively. The results obtained in the
present work are in agreement with those reported in litera-
ture ꢀor other oxalates. The structural, optical, thermal and
electrical properties oꢀ the NH H (C O )·2H O crystals syn-
–
–
–
10
15
4
3
4
8
2
thesized using a diferent approach aꢂrm their suitability as
potential candidates in optoelectronics.
–
6
– 4
– 2
0
2
4
6
Voltage/V
Fig. 12 I–V curve in dark and illuminated conditions
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