Characterization of paramagnetic KHo(WO4)2 nanocrystals: Synthesized by polymeric mixed-metal precursor sol-gel method

Article abstract of DOI:10.1016/j.jallcom.2011.07.074

Nanocrystalline KHo(WO₄)₂ (KHW) particles were successfully synthesized via conventional Pechini sol-gel method. Prepared precursor gel was calcined at 250, 550, 600, 650 and 700 °C, and the resulting samples were analyzed with TG-DTA, powder X-ray diffraction, FT-IR, Raman, FESEM, TEM, UV-Vis-NIR (diffuse reflectance spectrum (DRS)), fluorescence and vibrating sample magnetometer (VSM). Thermal degradation of derived gel was observed up to 400 °C and phase formation starts from 550 °C. The product phase formation at higher annealing temperature was investigated by means of powder XRD. Organic liberation in the samples with respect to temperature was analyzed using FT-IR spectrum. Raman spectrum reveals the formation of tungsten ribbons as well as the quality of the samples while increasing the calcination temperature. The nano size of the synthesized particles was confirmed with FESEM and TEM micrographs. Reflectance and emission studies reveal the corresponding absorption and emission properties of trivalent state holmium ion. Paramagnetic behavior of the derived KHW was confirmed with VSM results.

Full text of DOI:10.1016/j.jallcom.2011.07.074

Journal of Alloys and Compounds 509 (2011) 9890–9896
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Characterization of paramagnetic KHo(WO4)2 nanocrystals: Synthesized by
polymeric mixed-metal precursor sol–gel method
D. Thangarajua, A. Durairajana, S. Moorthy Babua,∗, Y. Hayakawab
a Crystal Growth Centre, Anna University, Chennai, Tamil Nadu 600 025, India
b RIE, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Shizuoka 432-8011, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 April 2011
Received in revised form 21 July 2011
Accepted 21 July 2011
Available online 9 August 2011
Nanocrystalline KHo(WO4)2 (KHW) particles were successfully synthesized via conventional Pechini
sol–gel method. Prepared precursor gel was calcined at 250, 550, 600, 650 and 700 ◦C, and the resulting
samples were analyzed with TG–DTA, powder X-ray diffraction, FT-IR, Raman, FESEM, TEM, UV–Vis-NIR
(diffuse reflectance spectrum (DRS)), fluorescence and vibrating sample magnetometer (VSM). Thermal
degradation of derived gel was observed up to 400 ◦C and phase formation starts from 550 ◦C. The product
phase formation at higher annealing temperature was investigated by means of powder XRD. Organic
liberation in the samples with respect to temperature was analyzed using FT-IR spectrum. Raman spec-
trum reveals the formation of tungsten ribbons as well as the quality of the samples while increasing
the calcination temperature. The nano size of the synthesized particles was confirmed with FESEM and
TEM micrographs. Reflectance and emission studies reveal the corresponding absorption and emission
properties of trivalent state holmium ion. Paramagnetic behavior of the derived KHW was confirmed
with VSM results.
Keywords:
Double tungstates
Paramagnetic materials
Sol–gel processes
Optical properties
Luminescence
© 2011 Elsevier B.V. All rights reserved.
and waveguide laser architectures. The stoichiometric level of Yb3+
dopant in KREW system was well investigated [2,3]. Monoclinic
1. Introduction
Monoclinic phase of potassium based rare earth double
tungstates, with general formula KRE(WO4)2(RE = Gd, Y and Lu)
(KREW) are well established materials for stimulated Raman scat-
tering (SRS). These materials exhibit high values of third order
nonlinear susceptibility. The compatibility of the lanthanide las-
ing ions in RE matrix sites make these crystals a good host for
frequency self-conversion lasers [1]. These tungstate hosts are
most efficient for stimulated emission with small pumping ener-
gies compared to any other inorganic laser hosts. Laser active ions
doped KREW matrix show high efficiency for stimulated emission
even at low value of pump energy with laser diode excitation.
Large absorption and emission cross sections observed in KREW
when doped with other lanthanides, add special significance for
better laser action. Good optical, elastic and magnetic nature of
these double tungstate materials draw attention of researchers
towards industrial application especially in medicine and military
areas. Complete replacement of non-laser active rare-earth ions
with laser active ions like Yb, Er, Ho and Dy in KREW system show
high crystalline quality and have excellent optical properties. Heav-
ily doped solid-state laser materials were used to prepare thin disk
phase of KHo(WO4)2 (KHW) belongs to the family of KREW with
promising applications like solid state laser and cooling agent in
adiabatic demagnetization for very low temperatures. This mate-
rial crystallizes in monoclinic structure with C2/c space group and
˚
˚
˚
unit cell parameters are a = 10.6403 A, b = 10.3445 A, c = 7.5515 A
and ˇ = 130.7◦. Rare earth Ho3+ ions have transitions at various
wavelengths in the infrared, visible and ultraviolet region because
of many meta stable energy levels [4]. Trivalent holmium has 14
laser channels in the wavelength range from 0.55 to 3.9 ␮m, in that,
5
the infrared emission in 5I7 → I8 transition at 2 ␮m and visible
emission (green) in 5S2 → I8 transitions at 0.55 ␮m are impor-
5
tant wavelength emissions for applications in medicine, eye-safe
remote sensing, coherent Doppler velocimetry and fluid mechan-
ics [5]. Stoichiometric KHW was successfully grown by Top Seeded
Solution Growth (TSSG) [4,6]. Nanocrystalline form of this double
tungstate was synthesized for application as sensitizing host for
phosphors, upconversion host for laser [7] and transparent laser
ceramics [8–10]. In the literature, only a few of KREW (RE = Gd, Yb,
Eu) [11,12] and NaRE(WO4)2 (La, Gd, Tb and Al) [13–16] materials
were reported. In this present communication, KHW nanocrystal-
lites obtained by conventional Pechini polymeric complex sol–gel
synthesis method and its characterization are presented for the first
time. Derived KHW powders were characterized with structural,
optical, vibrational, morphological and magnetic analysis.
∗
Corresponding author. Tel.: +91 442 235 8333; fax: +91 442 235 2774.
E-mail address: smoorthybabu@yahoo.com (S.M. Babu).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.07.074

D. Thangaraju et al. / Journal of Alloys and Compounds 509 (2011) 9890–9896
9891
2. Experimental procedures
The reagents KNO3 (Alpha Aesar), Ho2O3 (dissolved in HNO3) (Alpha Aesar),
ammonium para tungstate (CDH, India), citric acid and ethylene glycol were used
as starting materials. Analytical grade reagents and acids were used.
2.1. Gel preparation
The precursors in the form of nitrates and tungstate sources were dissolved
in 50 ml of deionized water independently. After the complete dissolution was
achieved, the citric acid was mixed in each of this clear solution of nitrates and
tungstates. The mixed solution is clear without any precipitation. Citric acid reacts
with metal ions in the mixed solution and forms the citrate complex in the solution.
Then, these individual citrate complexes were mixed together. The mixed solution
was stirred for about 45 min using magnetic stirrer to have a thorough mixing of
individual citrates. Then, ethylene glycol was added to the homogenously mixed
citrate solution to polymerize the citrate complex. The pH value of the completely
mixed solution was adjusted to 3.5 by adding ammonia solution. This mixed aqueous
solution was heated at 80 ◦C with uniform stirring for 4 h to enhance the polyester-
ification reaction. Silicone oil bath with resistive coil heater was used for uniform
heating of the mixture. The ratio of metal ions, citric acid and ethylene glycol was
maintained as 0.9:1:1. The advantages of using lower citric acid ratio are that, the
acidic pH was enough to ionize the total amount of [COOH]− for chelation. High
degree of chelation was responsible for high uniformity of metallic constituents
in the homogeneous solution [17,18]. Microwave oven heating was introduced for
the fast evaporation of superfluous liquid molecules in the solution to control the
degradation of polymer in a reversible reaction, which leads to the fast extraction
of gel. Light brown coloured semitransparent gel was obtained after the complete
evaporation of the liquid species present in the solution.
Fig. 1. TG–DTA analysis curve of derived KHW gel.
105 and 222 ◦C a weight loss of 8.63% was noticed. A drastic weight
loss of around 32.28% was observed in the C stage between 222 and
340 ◦C. In the final stage (D) between 340 and 555 ◦C the weight loss
was about 5.58%. During first and second stages of decomposition,
two strong endothermic peaks were observed at 103 and 219 ◦C;
this may be due to the liberation of superfluous water and NH3
elimination from the gel matrix [19]. Two exothermic peaks were
observed at 266 and 284 ◦C and attributed to the decomposition of
citric acid and ethylene glycol. The high amount of heat flow at 266
and 284 ◦C was due to the bond breaking of citric acid–ethylene
glycol polymers associated with metal ions and evolution of high
amount of gases such as CO2 [20]. The broad exothermic peak at
405 ◦C reveals the degradation of residual organics present after the
degradation of citrate polymer [21]. The broader exothermic peak
between 450 and 600 ◦C indicates that the compound formation
was initiated during that stage. After the final stage, no significant
weight loss has occurred.
2.2. Temperature treatment
The calcination process of the derived gel was performed with a resistive fur-
nace (super kanthal wounded). The calcinations temperature was controlled with
Eurotherm temperature controller coupled with the furnace. Prefiring of the samples
was performed in glass beaker and platinum crucible was used for high temperature
calcinations. The temperature treatment process was carried out with the following
procedures: Temperature of the as prepared gel was gradually increased to 110 ◦
C
for further drying and the sample was retained at this temperature for 5 h. The dried
gel samples appeared as whitish hard flakes. These resin flakes were prefired up to
250 ◦C and the blackish prefired powder was obtained. This black powder sample
was further annealed in air at different temperatures like 550, 600, 650 and 700 ◦
C
at a heating rate of 50 ◦C/h in the resistive furnace. While increasing the annealing
temperature towards 700 ◦C, the powder turns as white in colour. Different temper-
atures for calcination were attempted to understand the phase formation nature of
the crystalline KHW from the amorphous gel.
The powder XRD patterns of KHW samples calcined at different
temperatures 250, 550, 600, 650 and 700 ◦C are shown in Fig. 2.
The pattern corresponding to the gel prefired at 250 ◦C indicate the
amorphous nature of the sample with very low intense unidenti-
fied bunch of peaks at around 30◦. The powder pattern of samples
calcined at 550 ◦C confirms the formation of KHW phase. In addi-
tion, intense peaks corresponding to the intermediate oxides, like
Ho2WO6 (JCPDS Card PCPDF No: 23-1110), Ho2O3 (JCPDS Card
PCPDF No: 89-3849) and K2CO3 (JCPDS Card PCPDF No: 70-0292)
were also observed. Intensity of these satellite peaks correspond-
ing to impurities becomes weaker, when the samples are calcined
at 600 ◦C. The KHW phase evolution was partially completed in this
temperature, but the intensity of the peaks corresponding to this
phase increases for further calcination at 650 and 700 ◦C [22,23].
The XRD pattern recorded for samples calcined at higher temper-
ature (700 ◦C) has no peaks corresponding to intermediate and
residual oxides. All the peaks observed for 700 ◦C calcined KHW
powders were indexed. All significant peaks observed for KHW
were slightly shifted towards higher angles compared to KGW
(JCPDS Card PCPDF No: 89-6489), this may be due to lower ionic
2.3. Characterization
Thermal decomposition and phase formation behavior of derived gel was ana-
lyzed by thermogravimetric and differential thermal analyzer using TA instruments
(Q600 SDT and Q20 DSC) with N2 gas atmosphere. Phase analysis was carried out
using BRUKER D8 Advance X-ray diffractometer using CuK␣ radiation at room tem-
perature in the range between 20 and 80◦ at a step rate of 0.02◦. FT-IR analysis
(functional group identification) was carried out using spectrum one FT-IR spec-
trometer in the range of 400–4000 cm−1 with KBr pellet technique. Vibrational
characteristics of the as synthesized gel as well as samples annealed at different
temperature were carried out using Laser Raman spectrophotometer (Model Aspire
785L). The experiment was carried out at room temperature. FESEM analysis were
performed using ZEISS SUPRATM 40 Field Emission Scanning Electron Microscope
and TEM analysis were made using JEOL JEM 2100 High Resolution Transmission
Electron Microscope (HRTEM) with in situ Energy Dispersive X-ray Analysis (EDS)
in order to obtain the surface morphology, particle dimension and elemental anal-
ysis. UV–Vis-IR absorption (diffuse reflectance) study was performed with CARY
5E UV–Vis-NIR spectrophotometer for 700 ◦C annealed KHW powder in the range
between 185 and 700 nm. Fluorescence analysis was carried out using JASCO FP-
6300 spectrofluorometer with 450 nm excitation wave length. Lakeshore VSM 7410
vibrating sample magnetometer was used to analyze the magnetic nature of the
sample calcined at 700 ◦C.
3. Results and discussion
radii of Ho3+ compared to Gd3+
.
The FT-IR spectra for synthesized gels and KHW samples
calcined at different temperature are shown in Fig. 3. Several
absorption bands were observed for the polymeric citrate resin
and evaporation of polymer species was clearly observed while
increasing the calcination temperature from 250 to 700 ◦C. The
absorption peaks observed at 3000–3750 cm−1 were attributed to
the stretching vibration of water, citric acid and hydroxyl groups
The TG–DTA curve recorded for derived precursor gel and
powders annealed at different temperatures 250, 550, 600, 650
and 700 ◦C are illustrated in Fig. 1. The thermo-gravimetric curve
reveals the four stages of decomposition that takes place in the gel
(A, B, C and D stage). In the first stage (A) between 40 and 105 ◦C
a weight loss of about 6.2% was observed. In the B stage between

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Fig. 4. Comparative Raman spectrum of derived KHW gel and calcined at different
temperatures (250, 550, 600, 650 and 700 ◦C).
Fig. 2. Comparative powder X-ray diffraction patterns of KHW powders derived at
confirm the conversion of citric acid to citrate salt [24]. The band
at 1759 cm−1 indicates the C O stretching mode of ester which
was formed between citric acid and ethylene glycol [25]. The peaks
corresponding to asymmetric stretching of COO− were observed at
1630 and 1558 cm−1 which may be due to unidentate and a bridg-
ing complex (forms between the citric acid and metal ions) [24]. The
band at 1398 cm−1 was attributed to the symmetric COO− stretch-
ing. This indicates the bridge formation between citric acid and
metal ions (i.e.) potassium and gadolinium ions [26]. Out of plane
bending of the O–H was observed at 951 cm−1 and the peak appears
as a little hump near the tungstate bands. The samples calcined at
250 and 550 ◦C indicate the presence of carbonates with a peak at
1398 cm−1. The band at 1613 cm−1 was present even after calcina-
tion at 700 ◦C for 4 h. The reason for the band even at this higher
temperature may be due to strong chelating ability of citric acid
that can stabilize the different metal ions within one molecule or
complex [25]. The sample annealed at 700 ◦C, shows the absorption
peaks corresponding to the appearance of (WO4)2−, which illus-
trates the formation of monoclinic KHW. The observed absorption
bands at mid IR region 925, 774, 636 and 479 cm−1 provides some
information about tungstates. The bands at 925 and 837 cm−1 were
assigned for stretching of W–O and WOW respectively. The bands
corresponding to stretching of double bridge WOOW were observed
different temperatures (250, 550, 600, 650 and 700 ◦C).
(O–H). This broad peak becomes weaker with increase in calcina-
tion temperature and the band disappears at higher temperature,
(i.e.) more than 550 ◦C. In the gel spectrum, the bands localized at
1500–1700 cm−1 and 1300–1485 cm−1 are assigned to asymmet-
rical and symmetrical stretching vibrations of carboxylate groups
respectively. The bands related to carboxylate stretching modes are
observed at 1759, 1613, 1562 and 1398 cm−1 of the gel. These bands
at 774, 636 and 479 cm−1
Vibrational characteristics of these nanocrystals were investi-
gated to analyze the bending modes of tungstate chains [W2O8]−4
.
.
The recorded spectra are compared in Fig. 4 and the corresponding
tungsten oxide vibrational modes were matched with the results
available for polycrystalline samples of KGW [27]. As synthesized
gel to calcination of the powders up to 550 ◦C, there is no evidence
of peaks for the formation of double bridges of tungsten oxide in the
samples throughout the experimental range. When the calcination
temperature was increased to 600 ◦C, low intensity peaks related
to the formation of low temperature phase of KHW were appeared.
The intensity of the peaks gradually increases with increase in the
calcination temperature values. For 700 ◦C calcined samples, the
Fig. 3. Comparative FT-IR spectrum of derived KHW gel and calcined at different
temperatures (250, 550, 600, 650 and 700 ◦C).

D. Thangaraju et al. / Journal of Alloys and Compounds 509 (2011) 9890–9896
9893
Fig. 5. TEM micrographs and in situ EDS spectrum of 650 ◦C calcined KHW powder.
sharp and intense Raman peaks are centered at 760 and 900 cm−1
,
the stretching vibrations of (WOOW). The bands centered at 430,
368 and 287 cm−1 were attributed to the bending vibrations of
(WOW), (W–O) and (WOOW) respectively. The peak appearing at
308 cm−1 indicates the out of plane bending of (WOW). The bands
below 250 cm−1 are attributed to the translational lattice vibrations
with additional peaks at 681, 747 and 803 cm−1. Two units of KHW
make two polymeric ribbons coupled by the glide plane and C2 axis
in the polymeric matrix; so that, the two KHW molecules form four
complete WO6 octahedral, two single WOW bridges and one double
(WOOW) bridge. The peak observed at 900 cm−1 was attributed to
the stretching in-plane mode of (W O) at terminal oxygen. Vibra-
tional peak at 760 cm−1 represents the stretching in-plane mode of
WOOW with coupling vibrations as well as the stretching in-plane
mode of W–O. The bands centered at 681 and 803 cm−1 represents
the stretching vibrations of W–O. The peak at 525 cm−1 represents
of K+ and Ho3+
.
TEM and in situ EDS analysis were performed to estimate the
elements and the particle size of the powder derived at 650 ◦C cal-
cination. The typical TEM image with EDS spectrum are shown
in Fig. 5. KHW particles were particulate with different particle
dimensions, which varies between 10 and 40 nm. EDS spectrum
Fig. 6. FE-SEM micrographs of 700 ◦C calcined KHW powder for 30 min.

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D. Thangaraju et al. / Journal of Alloys and Compounds 509 (2011) 9890–9896
Fig. 7. FE-SEM micrographs of 700 ◦C calcined KHW powder for 4 h.
Fig. 8. Schematic representation of morphology changes from gel to calcinations at different temperatures.
on the single nano particle shows the presence of all the elements
of KHW along with Cu. The presence of Cu is due to the copper grid
in this analysis. Outline of all particles in the TEM image resem-
ble one uniform structure. Surface micrographs with FE-SEM were
recorded for KHW samples calcined at 700 ◦C (for 30 min) and (4 h)
and are as shown in Figs. 6 and 7, respectively. The external mor-
phology and particle size was clearly seen in these micrographs. It
can be clearly identified that as the calcination time increases the
size of the particle also increase. The surface morphology of 700 ◦C
for 30 min calcined particles appeared as relatively spherical shape
with various particle sizes between 50 and 150 nm. The individ-
ual contorted sphere particles were bonded together with soft and
hard agglomeration of primary particles. Presence of porosity in
the derived KHW powder may be due to flow of ammonia and CO2
during the degradation of organic species present in the precursor
gel. When the calcination time was increased at that same temper-
ature 700 ◦C the particle size increases from nanometers to micron
size [28–30]. The micron sized particles were quadrangular with
well angular facets and surface of the particles are very smooth
and flatten still some secondary particles were waiting to fuse on
the surface of the particles. The morphology changes with different
calcination temperature are represented in Fig. 8.
assigned transitions are mentioned in Fig. 9. Eight apparent transi-
tion peaks centered at 361, 387, 419, 451, 468, 487, 540 and 642 nm
are in accordance with the transitions of 3H6, 5G4, 5G5, 3K8 + 5G6,
5F2, 5F3, 5F4 + 5S2, 5F5 from 5I8 [31,32]. The absorption band at
451 nm looks broad and intense, which was used as the excitation
wavelength for emission analysis. Emission spectra of derived KHW
powder at various calcination temperatures were recorded and
compared in Fig. 10. Emission spectrum was collected with 451 nm
excitation wavelengths and was recorded in the range between 470
and 700 nm. In emission spectra, three strong bands centered at
489, 546 and 671 nm were observed and these radiative emissions
5
5
5
correspond to the transitions of 5F3 − I8, 5F4, 5S2 − I8 and 5F5 − I8,
5
respectively [33]. The electric dipole transition of 5F4, 5S2 − I8 yield
the green emission at 546 nm. Increasing the calcination temper-
ature increases the peak intensities of 546 nm emission [34]. The
gel sample has no emission in the whole range and the sample pre-
fired at 250 ◦C shows very low intense emission at this excitation.
The emission at red region (671 nm) was originated by the transi-
tion of 5D0–7F1 levels. This very low intensity peak was centered at
5
489 nm and corresponds to the weak transition of 5F3 − I8.
Vibrating sample magnetic analysis was performed for KHW
calcined at 700 ◦C and sol–gel synthesized KGW samples in the
range 0–2 T at room temperature. The plot of magnetization versus
magnetic field observed for these two samples was compared and
is shown in Fig. 11. Magnetization measurement of the sample
The absorption nature of the samples calcined at 700 ◦C was
examined by diffuse reflectance spectral study. Within the exper-
imental range several absorption bands were observed and the

D. Thangaraju et al. / Journal of Alloys and Compounds 509 (2011) 9890–9896
9895
indicates that there is no saturation magnetization up to 2 T, which
confirms the paramagnetic behavior [35] of these two samples.
The magnetization per gram of KHW (34.53 emu/g at 2 T) was high
when compared to KGW (16.72 emu/g at 2 T).
4. Conclusions
Nano crystallites of KHW were successfully synthesized using
conventional Pechini polymeric complex sol–gel method and the
derived powders were characterized. Thermal analysis of the
derived gel shows that the gel degrades up to 450 ◦C and parti-
cle formation starts around 575 ◦C. Improvement in crystallinity
at higher calcination temperature was observed in powder X-
ray diffraction pattern. The presence of mixture of oxides in
the pre-fired samples was clearly noted and the peaks were
indexed correspondingly. FTIR spectrum of derived gel confirms
the metal coordinated citrate formation and the formation of poly-
mer between ethylene glycol and metal citrates and the liberation
of organics at higher temperature was clearly seen. Raman spec-
trum recorded at different calcination temperature reveals that
W–O tungsten oxide double bridge was formed at 600 ◦C and inten-
sity of the peaks was gradually increased for calcinations at higher
temperature. The TEM and FESEM analysis reveals that the individ-
ual nano particles were formed at 650 ◦C and this individual nano
particles fuse together and form well aligned particles. The absorp-
tion and emission studies indicate the applicability of the derived
material for laser devices. The magnetization value of the KHW was
almost doubled when compared to pure KGW.
Fig. 9. Absorption (diffused reflectance) spectrum of KHW powder calcined at
700 ◦C.
Acknowledgements
The authors sincerely thank DST (SR/S2/LOP-0017/2008) and
DRDO (ERIP/ER/0803700/M/01/1179), Government of India for the
financial support. One of the authors (D. Thangaraju) sincerely
thanks Jawaharlal Nehru Scholarships for the financial support
towards Doctoral Studies.
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