Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.7, pp.625-636, 2011 Printed in the USA. All rights reserved
Role of Strontium on the Crystallization of Calcium Hydrogen
Phosphate Dihydrate (CHPD)
K. Suguna
1, 2
, C. Sekar
Department of Physics, Sri Sarada College for Women, Salem -636 016, TN, India.
Department of Physics, Periyar University, Salem- 636 011, TN, India.
Department of Bioelectronics and Biosensors, Alagappa University,
Karaikudi-630003, TN, India.
*Corresponding Author:
Calcium hydrogen phosphate dihydrate (CHPD, CaHPO
· 2H2O) or brushite is found quite
frequently in urinary calculi (stones). Crystallization of brushite has been carried out in
sodium metasilicate (SMS) gel with and without adding ‘Sr’ as additive. In pure system,
dicalcium phosphate anhydrous (DCPA, CaHPO
) or monetite and hydroxyapatite (HA,
(OH)) grew along with brushite. The presence of Sr suppressed the formation of
HA and enhanced the number and size of monetite crystals and changed the morphology of
brushite crystals from needle shape to octopus-like shape. The samples were characterized by
powder & single crystal X-ray diffraction (XRD), scanning electron microscopy (SEM), X-
ray fluorescence spectroscopy (XRF), Fourier transform infrared spectroscopy (FTIR) and
thermal analyses (TG-DTA).
Keywords: Brushite, Crystal growth, Sr additive, SEM.
Calcium phosphates have been studied extensively because of their occurrence in normal and
pathological calcifications. Due to their excellent biocompatibility, it is a well-known
bioactive material suitable for bone and hard tissue replacement
. Hydroxyapatite (HA,
(OH), octacalcium phosphate (OCP,Ca
O)), tricalcium phosphate (β-
), dicalcium phosphate dihydrate or calcium hydrogen phosphate dihydrate
O), dicalcium phosphate anhydrous (DCPA, CaHPO4), tetracalcium
phosphate (TTCP, Ca
O) and amorphous calcium phosphate (ACP)
are different
626 K. Suguna and C. Sekar Vol.10, No.7
crystalline calcium phosphates that have applications in biological mineralization. Brushite
phase is mostly found in callus, bone, and kidney stones
Investigations of the urinary stones showed large number of trace elements including Cd, Pb,
Zn, Mg, Sr, Cr, Mn, Ni, Co, Cu, Au, Tl, Bi etc., along with the main constituents
. An
increase in the level of the trace element in the body fluid leads to the crystal deposition
which results in the development of kidney stones
Lundager Madsen
investigated the influence of 14 different di- and trivalent metal ions on
brushite formation and reported that some ions inhibit and some ions promote the formation
of brushite. Sekar et al.
reported that the fluoride addition reduces the size and total
number of brushite crystals. The presence of magnesium reported to inhibit the formation of
brushite crystals
. LeGeros
grew single crystals of brushite in silica gel, and reported that
the presence of Sr
and P
causes marked effect on the crystal habit. Addition of
changed the morphology from usual platelet to spiral aggregate and the presence of
led to the growth of small needle- shaped crystals.
The intake of Sr per day through food and fluids is 1.9 mg. In that the loss in urine is 0.34
mg, loss in feces is 1.5 mg ,loss in sweat is 0.02 mg, and hair is 0.2 x 10
Strontium renalate is a recommended medication for osteoporosis which is found to increase
the risk of stone formation in the patients
. These issues are important because, in the case
of kidney stones, the medical treatment depends strongly on the precise chemical phase and
on the morphology of the biological entities
. So the objective of the present study is to
explore the role of Sr on the crystallization and properties of CHPD in gel medium. Because
of viscous nature of the gel medium, it provides an in vitro model for crystallization of
2.1 Preparation
The crystallization was carried out in glass test tubes of 25 mm diameter and 150 mm length.
The chemicals used were AR grade SrCl
, CaCl
and orthophosphoric acid. The SMS gel was
prepared as described in the literature
. One of the reactants, orthophosphoric acid (1M)
was mixed with silica gel of density 1.06 gm/cm
so that the pH of the mixture could be set to
6.0. The mixture was then transferred into test tubes. After gelation, the supernatant
solution calcium chloride (1M) was slowly added along the walls of the test tubes. To study
the effect of strontium, SrCl
(0.1 & 0.2 M) was mixed with calcium chloride and the
experiments were repeated as described above. After three weeks the crystals were harvested
and characterized.
Powder X-ray diffraction pattern was recorded on Bruker advance diffractometer within the
2θ range of 10 to 70°. The elemental composition of the specimen was determined using an
elemental analyzer JEOL JSX 3222 equipped with energy dispersive X- ray fluorescence
Vol.10, No.7 Role of Strontium on the Crystallization 627
system (XRF). The surface morphology of the samples was evaluated by scanning electron
microscopy (SEM). Thermal analyses were performed using SDT Q600 V8.3 Build 101
instrument. FTIR spectra of the grown crystals were recorded using Perkin Elmer, Spectrum
Rx1 detector and KBr beam splitter.
3.1 Crystal growth
A systematic investigation has been carried out to understand the role of strontium on the
crystallization of calcium phosphates under physiological conditions. Two different amount
of ‘Sr’ was added in the form of SrCl
to the growth environment. In all the cases, a white
precipitate has formed at the gel- solution interface within 12 minutes of adding supernatant
solution. White colored rings known as Liesegang rings have appeared just below the
interface within 6 hours. The number of these white colored rings increased with time and a
total of 16 such rings were observed after five days. In the mean time, the first few Liesegang
rings started dissolving slowly and tiny white spherulites have grown in that place. The
colorless and transparent needle shaped crystals have grown in between the Liesegang rings.
Approximately 10-15 needle shaped crystals were harvested after three weeks. Figs.1a and 1b
show the crystals grown in test tubes without and with (0.1M) addition of Sr.
Fig. 1. Calcium phosphate crystals in the test tubes (a) pure (b) 0.2M SrCl
628 K. Suguna and C. Sekar Vol.10, No.7
In strontium (0.1M) added experiments, the growth pattern was completely different. After
adding the supernatant solutions white precipitate was observed just below the interface.
After three days, white spherulites have appeared from the middle towards the bottom of the
tube as shown in Fig.1b. The number and size of spherulitic crystals increased with time.
Simultaneously a few tiny crystallites have appeared in the lower end of the gel medium
which continued to grow and assumed the coral (octopus) like morphology
. Figs. 2a and
2b show the needle and octopus like crystal grown without and with strontium addition.
Contrary to the pure system, Liesegang rings did not appear and there was no detectable
amount of white precipitate at the interface between supernatant solution and gel. As the
concentration of Sr was increased to 0.2M, the size and number of crystals got decreased
further. Thus the presence of excessive amount of ‘Sr’ suppresses the formation of nuclei and
their further development into large crystals.
Fig. 2. As grown CHPD crystals (a) pure (b) 0.2 M Sr
3.2 Powder X-ray Diffraction Analyses
The powder X-ray diffraction patterns were recorded for all the three types of products grown
without the addition of strontium i.e. the thick white precipitate formed at the gel- solution
interface, spherulitic and needle shaped crystals grown beneath the interface. The white
precipitate was identified as hydroxyapatite (JCPDS data (09-0432) [Fig.3].
The spherulitic crystals were found to be a mixture of monetite (JCPDS data (70-1425)) and
brushite. Fig. 4a shows the powder XRD pattern of monetite crystals grown without Sr. The
minor peaks observed at 2θ values 11.76˚ and 21.05˚ could be indexed for (0 2 0)
1) 2 (1
planes of CHPD. The intensity of these CHPD peaks increased with the addition
of strontium as shown in Fig. 4b. It can also be noted that the peaks shift towards lower angle
side which indicates that the ‘Sr’ enters into CHPD crystal lattice.
Vol.10, No.7 Role of Strontium on the Crystallization 629
Fig. 3. Powder XRD pattern of hydroxyapatite
Fig. 4. Powder XRD patterns of monetite crystals (a) 0 M Sr and (b) 0.2 M Sr
630 K. Suguna and C. Sekar Vol.10, No.7
Fig. 5 shows the powder XRD pattern of the CHPD crystals grown without strontium (a) and
with 0.1M Sr (b) and 0.2M Sr (c) respectively. The needle shaped crystals were identified as
pure CHPD by comparing the JCPDS data (72-0713). The diffraction peaks of the crystals
grown with Sr were strong and sharp when compared to that of pure system. This result
indicates that the presence of Sr enhances the crystallinity of CHPD. A slight shift in the peak
positions and change in peak intensity confirmed that the Sr replace a certain amount of Ca in
Fig. 5. Powder XRD patterns of CHPD crystals (a) 0 M, (b) 0.1 M, and (c) 0.2M Sr
As already reported by Bigi et al.
the transformation of CHPD to HA was not observed in
the present study. Instead Sr addition seems to suppress the crystallization of HA and the
monetite was found to be the secondary phase.
The lattice parameters determined from the single crystal X-ray diffraction data obtained
using four-circle Nonius CAD4 MACH3 diffractometer (MoKα, λ = 0.71073 Ǻ ) are shown
in Table 1. There was a small enhancement in the lattice parameters of Sr (0.2 M) grown
CHPD crystals. This increase in lattice parameters could be due to the larger ionic radius of
Sr (1.13Ǻ) when compared to that of Ca (1.00 Ǻ). It may be that the Sr
provokes lattice
distortions which expand the brushite crystal lattice
Vol.10, No.7 Role of Strontium on the Crystallization 631
Table 1: Lattice parameters of Pure and Sr (0.2 M)
added CHPD crystals
Lattice parameters (Å)
Volume (Å
a b c β
Pure CHPD 5.808 15.176 6.236 116.36 492.5
CHPD+0.2M Sr 5.817 15.186 6.242 115.28 498.6
3.3 X-ray Fluorescence Spectrum
X-ray fluorescence spectra of brushite crystals are shown in Figure 6 which reveals the
presence of different elements such as Sr (Kα = 14.150 KeV), P (2.046 KeV) and Ca (Kα=
3.691 KeV, K
= 4.012 KeV). In pure sample, the Ca/P value was found to be very close to
that of CHPD (1.00) according to the chemical formula. In doped crystals, (Ca + Sr)/P ratio
was found to be 1.18 and 1.54 for the crystals grown with 0.1 and 0.2 M Sr respectively. As a
consequence the stoichiometry is no longer maintained in case of Sr doping into CHPD.
Fig. 6. X-ray fluorescence spectra of CHPD crystals (a) 0 M (b) 0.1 M (c) 0.2 M Sr
632 K. Suguna and C. Sekar Vol.10, No.7
3.4 SEM analysis
Fig. 7 shows the SEM images of octopus-like CHPD crystals grown in presence of Sr (0.2
M). The crystals grown without ‘Sr’ have needle-like morphology. The surfaces were found
to be fairly clean and defect free. On the other hand, the octopus-like crystals were found to
be branched dendrites (Fig. 7a). The branches exhibited curved needle shapes which on
further magnification showed the presence of large number of crystals arranged in eagle’s
wing-like shape (Fig. 7b). It may be that the presence of Sr enhances the nucleation process
of CHPD in particular which results in innumerous crystals. Boanini
CHPD crystal and reported that a relatively low Sr replacement of Ca induces a decrease in
the coherent length of the perfect crystalline domains and disturbs the shape of the crystals.
Fig.7. SEM pictures of CHPD crystal (a) pure (b) 0.2M Sr
3.5 Thermal Analysis
Figs. 8 and 9 illustrate thermal behavior of pure and strontium doped CHPD samples
recorded in the temperature range between 30-1200ºC at the rate of 20ºC/min in nitrogen
atmosphere. In pure sample the weight loss occurs in two stages. The major weight loss of
about 21% occurs between 103ºC and 199ºC which indicates the loss of lattice water. The
endothermic peak in DTA around 128ºC with the associated shoulders indicates the stepwise
removal of water during this temperature range. In the region (199-479ºC), two molecules of
combine and result in the elimination of a water molecule leading to the formation
of calcium pyrophosphate and nearly 74% of the sample is stable. The following chemical
reactions are expected to occur during the dehydration and decomposition stages
+ 4 H
+ H
Vol.10, No.7 Role of Strontium on the Crystallization 633
Nearly similar thermal behavior occurred in Sr doped CHPD also. The major weight loss of
about 21% occurs between 110ºC and 200ºC in the crystals grown with Sr (0.2M) addition.
The mass loss corresponds well with the DTA results by the appearance of an endothermic
peak at 178 ºC with the shoulders. Thus there is an increase in the peak temperature which
indicates the improved thermal stability of CHPD due to Sr doping. In the second stage,
nearly 8% weight loss occurs and the rest of the sample (68%) was stable. The excess weight
loss (~ 6%) may be due to Sr doping into CHPD.
Fig. 8. TG-DTA curves CHPD Fig. 9. TG-DTA curves of Sr doped CHPD
3.6 FTIR Studies
The recorded FTIR spectra for pure and Sr doped CHPD crystal were depicted in Fig. 10. The
observed wave numbers, relative intensities and the assignments proposed for the crystals
under investigation were found to be in good agreement with the reported literature
[19, 20]
In the spectrum of pure CHPD, we can find two intense doublets; one with components at
3544 and 3489 cm
and the other with components at 3284 and 3168 cm
. In addition a
weak band around 2371 cm
and a sharp strong band around 1652 cm
have been observed.
These two doublets have quite different shapes; the high-wave number doublet consists of
sharp bands whereas the low-wave number doublet is much broader. The appearance of these
two doublets is attributed to the existence of two different types of water molecules in the
unit cell of brushite
. Petro et al.
reported that the high-wave number lines are due to a
loosely bound water molecule and the low wave number doublet to vibrations of those water
molecules which, according to the crystallographic data of Beevers
, forms direct bonds to
calcium atoms. In the Sr doped brushite, the low wave number doublets (3284 and 3168 cm
are slightly broadened and the peaks are shifted to 3295 and 3171 cm
respectively. This may
be due to the presence of strontium. In case of high wave number doublet the intensity of
peaks got reversed in the doped samples when compared to that of pure sample.
634 K. Suguna and C. Sekar Vol.10, No.7
Fig. 10. FTIR spectra of CHPD crystals (a) 0 M (b) 0.1 M (c) 0.2 M.
Presence of sharp band around 872 cm
in all IR spectra confirms the brushite mineral phase
. The sharp and strong band around 1652 cm
is assigned to the in-plane bending of water
molecules. CHPD is characterized by the splitting of phosphate bands in the region below
1600 cm
with more doublets. At 989 cm
a strong P-O stretching mode (υ
) is observed. In
the present work, the two bands at 576 and 527 cm
were assigned to the υ
mode vibration.
Peak intensity of hydrogen bonded HPO
ions at 1387 cm
in the case of pure sample
increases significantly in the Sr doped samples. The peak around 1215 cm
in the spectrum is
due to O-H in plane bending of HPO
group. The peaks around 795 and 666 cm
are due to
librations of water molecules and peak 795cm
is assigned to a rocking and the 666 cm
peak to a wagging librational motion respectively. No major shifts or additional peaks are
found in the case of Sr doped samples.
Simultaneous crystallization of calcium phosphates (brushite, hydroxyapatite, and monetite)
has occurred in sodium metasilicate gel under physiological conditions. The presence of Sr
has suppressed the formation of HA and promoted the monetite and brushite formation. In
addition, a significant change in morphology of CHPD from needle shape to octopus-like
shape has been observed. The XRD and XRF analyses confirmed the incorporation of Sr into
brushite crystallites. TG-DTA studies confirmed the improvement in thermal stability of
brushite due to Sr doping.
Vol.10, No.7 Role of Strontium on the Crystallization 635
The authors wish to thank Dr. A. Tamizhavel, Tata Institute of Fundamental Research,
Mumbai, for SEM and XRF analyses. K. Suguna thanks Dr. E. K. Girija for her helps in data
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