Immobilization of mo6+ in monazite lattice: Synthesis and characterization of new phosphomolybdates, La1-xCa xP1-yMoyO4, Where x=y=0.1-0.9

Article abstract of DOI:10.1111/j.1551-2916.2010.04183.x

In the process of high-level nuclear waste (HLW) disposal, monazite, LaPO₄ is considered as one of the potential phosphate ceramics to immobilize lanthanides and actinides. HLW along with hazardous long-lived nuclides contains elements such as Zr, Mo, etc. in higher wt%. In the current investigation, an attempt was made to introduce Mo into the monazite lattice. The phosphomolybdates of the formula La_1-xCa _xP_1-yMo_yO₄, x, y=0.1-0.9 were synthesized by a solution route and characterized by powder X-ray diffraction, FT-IR, TGA/DTA, and scanning electron microscopic techniques. The phases with compositions La_0.9Ca_0.1P_0.9Mo _0.1O₄, La_0.8Ca_0.2P _0.8Mo_0.2O₄, and La_0.7Ca _0.3P_0.7Mo_0.3O₄ were found to be crystallized in monazite structure. The composition La_0.1Ca _0.9P_0.1Mo_0.9O₄ with X=Y=0.9 crystallized in the scheelite structure. The compositions indicate the immobilization of a4.1, 8.5, 12.9, and 42.5 at. wt% of Mo. With x, y=0.6, 0.5, 0.4, 0.3, and 0.2, along with the monazite phase, formation of a secondary phase LaP₃O₉ was observed.

Full text of DOI:10.1111/j.1551-2916.2010.04183.x

J. Am. Ceram. Soc., 94 [4] 1008–1013 (2011)
DOI: 10.1111/j.1551-2916.2010.04183.x
r 2011 The American Ceramic Society
ournal
J
Immobilization of ‘‘Mo61’’ in Monazite Lattice: Synthesis and
Characterization of New Phosphomolybdates, La1ÀxCaxP1ÀyMoyO4,
Where x 5 y 5 0.1–0.9
Sathasivam Pratheep Kumar and Buvaneswari Gopalw
Materials Division—Chemistry, School of Advanced Sciences, VIT University, Vellore 632 014, Tamil Nadu, India
In the process of high-level nuclear waste (HLW) disposal, mo-
nazite, LaPO4 is considered as one of the potential phosphate
ceramics to immobilize lanthanides and actinides. HLW along
with hazardous long-lived nuclides contains elements such as Zr,
Mo, etc. in higher wt%. In the current investigation, an attempt
was made to introduce ‘‘Mo’’ into the monazite lattice. The
phosphomolybdates of the formula La1ÀxCaxP1ÀyMoyO4, x,
y 5 0.1–0.9 were synthesized by a solution route and character-
ized by powder X-ray diffraction, FT-IR, TGA/DTA, and scan-
ning electron microscopic techniques. The phases with
compositions La0.9Ca0.1P0.9Mo0.1O4, La0.8Ca0.2P0.8Mo0.2O4,
and La0.7Ca0.3P0.7Mo0.3O4 were found to be crystallized in mo-
nazite structure. The composition La0.1Ca0.9P0.1Mo0.9O4 with
X 5 Y 5 0.9 crystallized in the scheelite structure. The compo-
sitions indicate the immobilization of B4.1, 8.5, 12.9, and 42.5
at. wt% of ‘‘Mo.’’ With x, y 5 0.6, 0.5, 0.4, 0.3, and 0.2, along
with the monazite phase, formation of a secondary phase
LaP3O9 was observed.
are reported. Examples include: NaMo2(PO4)3, AMo2P3O12
(A5 K, Rb, Ti), and AMo2(PO4)3 (A5 Ba, Sr, Ca).15–17 Sim-
ilarly, ‘‘Mo41’’ substitution in pyrochlore structure is also stud-
ied.18 Even though these phases can accommodate higher
percentage of molybdenum, they are not stable under an ambi-
ent condition in air over a period of time.
Petkov et al.13 reported the incorporation of Mo61 in NZP
lattice. Stewart and Vance explored various systems of
multiphasic ceramic wasteforms such as NZP–powellite,
pyrochlore–sodium titanium phosphate (NTP)–perovskite, and
NTP–perovskite–powellite as potential hosts for the incorpora-
tion of ‘‘Mo41’’ and ‘‘Mo61.’’18 Chourasia et al.19 have studied
the structural details of the ‘‘Mo61’’-inserted NZP phase of the
formula Na0.9Zr2Mo0.1P2.9O12.
Monazite (LaPO4) is a natural orthophosphate considered as
one of the most water-resistant minerals. Monazite crystallizes
in a monoclinic structure with the space group of P21/n. A three-
dimensional network of the structure is formed by intercon-
nected REO9 polyhedra and PO4 tetrahedra. The crystal
chemistry of monazite structure exhibits considerable flexibility
to incorporate ions of different sizes and charges.20–22 Studies on
the incorporation of lanthanides,23 actinides,24–26 and a combi-
nation of trivalent and divalent27 ions have been reported. The
phase stability of monazite compound under radiation damage28
is found to be excellent. Terra et al.23 have studied lanthanum–
gadolinium monazite, La1ÀxGdxPO4 2a4s wasteform for radioac-
I. Introduction
IGH-LEVEL nuclear waste (HLW) contains a mixture of ra-
H
dioactive metal ions of different sizes and charges.1 Boro-
silicate glass, iron phosphate glass, glass–ceramics, synroc, and
phosphate-based ceramics have been studied as host lattices for
the immobilization of such radioactive waste elements.2–6 For
long-term storage, the waste immobilized lattice (wasteform)
should possess phase stability and high leach resistance. Gener-
ally, phosphate ceramics are considered as highly stable and
low-soluble minerals. Phosphate-based ceramics such as apa-
tites,7 britholites,8 monazites,9 sodium zirconium phosphate
(NZP),10 and zirconium phosphates11 are considered as poten-
tial single phasic crystalline host lattices for the immobilization
of radioactive elements. Based on their structural flexibility, ions
of different sizes and charges are accommodated into their crys-
tal lattice. Molybdenum is one of the major waste elements
present in HLW and its immobilization in an inert matrix is es-
sential from the nuclear waste disposal point of view.12 At pres-
ent, ‘‘Mo’’ and other nuclides are solidified by incorporation in
borosilicate glass.13 It is suggested that incorporation of ‘‘Mo’’
into monophasic phosphate ceramics may result in low leach
rate of molybdenum as compared with other matrices.13
tive waste storage. Bregiroux et al.
have studied Pu31 and
Am31 containing monazite phases. The incorporation of tho-
rium and uranium in the monazite structure is reported by Mc-
Carthy et al.25 Double orthophosphates of tetravalent actinides
such as Ce, Th, U, Np, and Pu crystallizing in monazite structure
have been investigated by Kitaev et al.26 Monazite-based waste-
forms are found to be highly leach resistant even at elevated
temperatures. Nevertheless, studies on molybdenum (Mo61
)
containing monazite wasteform have not been reported. Hence,
in the current study the feasibility of substitution of P51 by Mo61
in the monazite lattice is explored. The effect of an increase in
‘‘Mo’’ content on the monazite phase formation is investigated.
II. Experimental Procedure
(1) Materials
Molybdenum is a multivalent element; it exists in compounds
with 14 or 16 oxidation states. If the wasteforms are fabricated
in air, ‘‘Mo’’ will occur in hexavalent form and in an inert
atmosphere it could exist in tetravalent form. The high-level
radioactive waste composition contains B13 wt% of MoO3 in
which molybdenum is in ‘‘16’’ oxidation state.14 NZP structure
compounds containing ‘‘Mo41’’ with octahedral coordination
La2O3 (99.9%, SRL Pvt. Ltd., Mumbai, India), CaCO3 (991%,
Sigma Aldrich, St. Louis, MO), SrCO3 (991%, Sigma Aldrich),
NH4H2PO4 (99%, S. D. Fine chemicals, Mumbai, India), and
(NH4)6Mo7 Á 4H2O (98.5%, Titan Biotech Ltd., Delhi, India).
(2) Synthesis
The compounds based on the general formula La1ÀxCaxP1Ày
MoyO4, where x 5 y 5 0.0–1.0 were synthesized. Lanthanum
oxide was preheated at 10001C for 12 h before use. Stoichio-
metric amounts of lanthanum oxide and calcium carbonate were
dissolved in 1:1 nitric acid separately (solution I and solution II,
respectively). Similarly, stoichiometric amounts of ammonium
molybdate and ammonium dihydrogen phosphate were dis-
W. Lee—contributing editor
Manuscript No. 27787. Received April 3, 2010; approved September 17, 2010.
wAuthor to whom correspondence should be addressed. e-mail: gopalbhu@yahoo.com
1008

April 2011
Immobilization of ‘‘Mo61’’ in Monazite Lattice
1009
solved in distilled water separately (solution III and solution IV,
respectively). The solutions were mixed in the following order
under constant stirring. First, solution I was mixed with solution
II. To the homogenized mixture, solution III was added. Finally,
to the above mixture, solution IV was added drop wise. The
mixture was continuously stirred to maintain the homogeneity.
The pH of the mixed solution (o1.0) was slightly increased and
maintained at 2.5–3.0 using ammonia solution. The precipitate
formed was filtered and dried at 1501C for 24 h. The dried pre-
cursor was ground well using a mortar and pestle and the pow-
der was heat treated in air at 6001C for 5 h. The cooled product
was ground well and heated at 8001C for 24 h. Strontium an-
alogues of the formula La1ÀxSrxP1ÀyMoyO4 where x, y 5 0.0–
1.0 were synthesized by a similar procedure.
Table I. Crystallization of La1ÀxCaxP1ÀyMoyO4
Compositions, Where x 5 y 5 0–1.0
At.%
Crystallization of the phase
La1ÀxCaxP1ÀyMoyO4
of Mo
after 6001C calcination
x 5 0
0
4.06
8.45
Monazite
Monazite
Monazite
Monazite
Monazite1LaP3O9
Monazite1LaP3O9
Monazite1LaP3O9
Monazite1LaP3O9
Monazite1LaP3O9
Scheelite
x 5 0.1
x 5 0.2
x 5 0.3
x 5 0.4
x 5 0.5
x 5 0.6
x 5 0.7
x 5 0.8
x 5 0.9
x 5 1
12.87
17.42
22.11
26.95
31.95
37.12
42.45
47.97
(3) Characterization
Scheelite
The synthesized products were characterized using powder
X-ray diffraction (XRD) method (CuKa, PNAnalytical, Al-
melo, the Netherlands) at room temperature. The lattice param-
eters were obtained by LSQ fitting of high-angle reflections. The
infrared spectra (FT-IR spectrometer—ThermoNicolate Com-
pany Avatar 330, Madison, WI) were recorded using the KBr
disk technique. The chemical analysis of selected compositions,
viz., La0.9Ca0.1P0.9Mo0.1O4, La0.8Ca0.2P0.8Mo0.2O4, La0.7Ca0.3
P0.7Mo0.3O4, and La0.1Ca0.9P0.1Mo0.9O4 obtained after the
heat treatment at 6001C was performed by electron probe
microanalysis (EPMA) using a Jeol 8600M Electron Probe
Microanalyzer (JEOL Surface Analysis Instruments, Tokyo,
Japan). The powder samples were pressed into pellets and the
pellets were analyzed with a beam of 10 mm. For each sample,
four points were analyzed. Thermogravimetric analysis was
performed in air using TA Instruments Model SDT Q600
(TA Instruments, New Castle, DE) in the temperature range
of RT–12001C. The heating rate was 201C/min. Scanning elec-
tron microscopic (SEM) analysis of the selected samples was
carried out using a Cambridge S360 SEM (Class One Equip-
ment, Decatur, GA).
The sharpness of the peaks increases with the increasing ‘‘Mo’’
content. Analysis of the XRD data indicates that the composi-
tions La0.9Ca0.1P0.9Mo0.1O4, La0.8Ca0.2P0.8Mo0.2O4, and La0.7
Ca0.3P0.7Mo0.3O4 crystallize in monoclinic structure and are
isostructural with monazite. The diffraction patterns are indexed
based on the reported data of LaPO4 [JCPDS file 32-0493]. With
a further increase in ‘‘Mo’’ content (y), formation of monazite
phase along with a secondary phase LaP3O9 is observed in the
powder XRD patterns of the compositions La0.6Ca0.4P0.6
Mo0.4O4, La0.5Ca0.5P0.5Mo0.5O4, La0.4Ca0.6P0.4Mo0.6O4, La0.3
Ca0.7P0.3Mo0.7O4, and La0.2Ca0.8P0.2Mo0.8O4. Such an observa-
tion of the formation of LaP3O9 is reported by Damien et al.30
Powder XRD analysis shows the composition La0.1Ca0.9P0.1
Mo0.9O4 crystallizes in the scheelite structure. The pattern is in-
dexed based on the reported data of CaMoO4 (JCPDS file 85-
1267). Thus, it is concluded that up to a maximum of 12.9 at.
wt% of ‘‘Mo’’ could be accommodated in its 16 oxidation state
in the monazite lattice without any structural changes. In the
case of the scheelite phase of the composition La0.1Ca0.9P0.1
III. Results and Discussion
(1) Phase Formation, Structural Analysis, and Thermal
Stability
(A) Powder XRD : In order to understand the extent of
accommodation of ‘‘Mo61’’ into the tetrahedral site of monazite
lattice, compositions of the general formula La1ÀxCax
P1ÀyMoyO4, where x 5 y 5 0.1–0.9 have been synthesized. Be-
cause of the replacement of trivalent ‘‘La’’ by divalent ‘‘Ca’’ for
charge compensation, it is expected that with lower ‘‘Mo’’ con-
tent the composition may prefer to crystallize in monazite struc-
ture while the higher ‘‘Mo’’ concentration may lead to the
crystallization in scheelite structure. The radii of the ions under
51
consideration are: Mo61 (tetrahedral)5 0.41A, P
(tetra-
hedral) 5 0.17 A, La (ninefold)5 1.21 A, and Ca (nine-
˚
31
21
˚
˚
fold) 5 1.18 A. Stabilization of ‘‘Mo61’’ in monazite lattice
29
˚
and the pure phase formation were studied by characterizing all
the products of the composition La1ÀxCaxP1ÀyMoyO4 obtained
by heating the precursor at 6001–8001C. Powder XRD of the
products obtained by heating the precursors at 6001C designates
the effect of the concentration of ‘‘Mo’’ in the formation of sin-
gle-phase ‘‘Mo’’-incorporated monazite. The details of the crys-
tallization of the compositions into monazite and scheelite
structure are given in Table I. However, heating the precursors
of the compositions at 8001C yields the monazite phase, LaPO4
as the major product. This could probably be due to the loss of
molybdenum that occurs on heating the precursors above
6001C. A similar observation has been reported by Petkov and
Sukhanov12 in their study on the immobilization of molybde-
num into NZP ceramics.
The powder XRD patterns of the nine compositions prepared
at 6001C are compiled in Figs. 1 and 2. The patterns of the end
products LaPO4 and CaMoO4 are provided for comparison.
Fig. 1. Powder X-ray diffraction patterns of La1ÀxCaxP1ÀyMoyO4,
where x 5 y 5 0, 0.1, 0.2, 0.3, and 0.5.

1010
Journal of the American Ceramic Society—Pratheep Kumar and Gopal
Vol. 94, No. 4
stallinity of the scheelite structured phase La0.1Ca0.9P0.1Mo0.9O4
indicates an ordered arrangement of the ions in the lattice.
The lattice parameters of the monoclinic unit cells of the com-
positions La0.8Ca0.2P0.8Mo0.2O4 and La0.7Ca0.3P0.7Mo0.3O4 have
been evaluated. Similarly, the dimensions of the tetragonal unit
cell of La0.1Ca0.9P0.1Mo0.9O4 have been calculated. The lattice pa-
rameters are given in Table III. The values of the parent phases
LaPO4 and CaMoO4 are given for comparison. With ‘‘Mo’’ sub-
stitution at ‘‘P’’ site and ‘‘Ca’’ substitution at ‘‘La’’ site, an increase
in ‘‘a’’ lattice parameter and a decrease in ‘‘b’’ and ‘‘c’’ parameters
are observed. An increase in ‘‘b’’ value is observed in both the
compositions. Such changes in the unit cell dimensions could be
correlated to the size of the ions occupying the ‘‘La’’ site and ‘‘P’’
site in the monazite structure. Substitution of P51 (r50.17A) by
˚
Mo61 (0.41 A) in LaPO4 enlarges the size of a fraction of inter-
˚
connecting tetrahedra made of MoO4. The substitution of La31
21
˚
˚
(rc 51.21 A) by Ca (rc 51.18 A) replaces a fraction of LaO9
polyhedra by slightly smaller CaO9 polyhedra. Thus, the com-
bined effect of the substitution of ‘‘La31’’ by slightly smaller
‘‘Ca21’’ and ‘‘P51’’ by larger ‘‘Mo61’’ results in the corresponding
change in the unit cell parameters and unit cell volume. An overall
increase in the unit cell volume has been observed. Comparison of
the lattice parameters of La0.1Ca0.9P0.1Mo0.9O4 with that of the
parent phase CaMoO4 indicates the feeble influence of the intro-
duction of ‘‘La’’ and ‘‘P’’ (x50.1) on the scheelite lattice. The
change in ‘‘a’’ lattice parameter is found to be negligible. A slight
increase in the ‘‘c’’ lattice parameter is observed. However, the
crystal chemical substitution of ‘‘Mo61’’ by ‘‘P51’’ is confirmed by
FT-IR spectroscopy and elemental analysis (Table II).
In order to study the effect of replacement of ‘‘Ca21’’ by bigger
‘‘Sr21,’’ compositions of the general formula La1ÀxSrxP1ÀyMoyO4
have been synthesized under similar experimental conditions.
Structural analysis by powder XRD indicates that only the com-
position of the formula La0.9Sr0.1P0.9Mo0.1O4 crystallizes in the
pure monazite structure. From x5y50.2 onward appearance of
LaP3O9 along with the monazite phase has been observed up to
x5y50.4. With a further increase in ‘‘Sr21’’ and ‘‘Mo61’’ con-
tent, LaP3O9 is found to be the major phase. Compared with its
calcium analogue crystallization in scheelite structure is not ob-
served in the case of the composition La0.1Sr0.9P0.1Mo0.9O4. Pow-
der XRD patterns of selected compositions of the formula
La1ÀxSrxP1ÀyMoyO4 are shown in Fig. 3
(B) Infrared Spectroscopic Analysis: The replacement of
‘‘PO4’’ moiety by ‘‘MoO4’’ tetrahedron in monazite structure is
also confirmed by the presence of their characteristic IR bands
associated with stretching and bending vibrational modes. The
infrared spectra of the selected compositions based on XRD
such as La0.9Ca0.1P0.9Mo0.1O4, La0.8Ca0.2P0.8Mo0.2O4 La0.7
Ca0.3P0.7Mo0.3O4, and La0.1Ca0.9P0.1Mo0.9O4 are compiled in
Fig. 4. The IR spectra of pure LaPO4 and CaMoO4 are also
given for comparison. The transition of the structure of the
composition from monazite to scheelite with the increasing con-
tent of ‘‘Mo’’ is visible. The characteristic bands appearing in
the spectra of these compositions confirm the simultaneous
presence of the phosphate and molybdate tetrahedral groups.
It is noted that the bands appear at B615, 578, 562, and
540 cmÀ1 in the n4 region, and the n3 bands found in the range
of 990–1100 cmÀ1 are attributed to the asymmetric bending and
Fig. 2. Powder X-ray diffraction patterns of La1ÀxCaxP1ÀyMoyO4,
where x 5 y 5 0.6, 0.7, 0.8, 0.9, and 1.0.
Mo0.9O4, the molybdenum content is 42.4 at. wt% and lantha-
num content is 6.8 at. wt%.
The results of the chemical analysis of the compositions
La0.9Ca0.1P0.9Mo0.1O4, La0.8Ca0.2P0.8Mo0.2O4, La0.7Ca0.3P0.7
Mo0.3O4, and La0.1Ca0.9P0.1Mo0.9O4 by ‘‘EMPA’’ in terms of
the wt% of oxides are given in Table II. The relatively low stan-
dard deviations show that the samples are homogeneous. The
experimentally determined values of at. wt% of the elements are
found to be in good agreement with the calculated values of the
nominal compositions.
The powder XRD patterns of the phases crystallizing in the
monazite structure display broader peaks. The appearance of
very sharp peaks is observed in the case the composition crys-
tallizes in scheelite structure. It is also observed that the peaks
representing the formation of the secondary phase LaP3O9 are
sharper. This observation implies that most probably the substi-
tution of Mo61 for P51 in monazite structure distorts the lattice
while retaining the structure up to x, y 5 0.1–0.3. At the same
time, an increase in ‘‘Mo’’ content leads to monazite phase for-
mation to a limited level and ejects LaP3O9 as a secondary phase.
This could be due to the incompatibility in accommodating the
increased concentration of ‘‘Mo61’’ ion. Whereas, the high cry-
Table II. Electron Microprobe Analysis (wt%, oxides) Results of the Selected Compositions
Wt. % of oxides
SrO
Composition
La2O3
CaO
P2O5
MoO
Total
La0.9Ca0.1P0.9Mo0.1O4
La0.8Ca0.2P0.8Mo0.2O4
La0.7Ca0.3P0.7Mo0.3O4
La0.1Ca0.9P0.1Mo0.9O4
La0.9Sr0.1P0.9Mo0.1O4
85.16 (1.04)
81.85 (0.44)
76.21 (1.24)
19.70 (1.92)
83.32 (0.47)
0.98 (0.20)
1.72 (0.04)
3.00 (0.42)
16.93 (1.78)
—
—
—
—
8.64 (0.54)
7.99 (0.84)
7.33 (0.82)
1.85 (0.66)
8.49 (0.76)
5.23 (0.76)
9.22 (0.67)
13.81 (1.27)
64.20 (1.71)
4.81 (0.47)
100.01
100.78
100.35
102.68
100.57
—
3.95 (0.51)
The number in the parentheses is the standard deviation compositions.

April 2011
Immobilization of ‘‘Mo61’’ in Monazite Lattice
1011
Table III. Lattice Parameter Variation of La1ÀxCaxP1ÀyMoyO4
3
Cell volume (A )
˚
a (A)
˚
b (A)
˚
c (A)
˚
La1ÀxCaxP1ÀyMoyO4
b (deg.)
LaPO4 (present work)
La0.8Ca0.2P0.8Mo0.2O4
La0.7Ca0.3P0.7Mo0.3O4
La0.1Ca0.9P0.1Mo0.9O4
CaMoO4 (present work)
6.83 (2)
6.92 (1)
7.12 (2)
5.217 (2)
5.214 (2)
7.074 (5)
7.047 (2)
7.026 (3)
—
6.50 (2)
6.50 (1)
6.49 (1)
11.434 (4)
11.425 (4)
103.3 (7)
103.7 (3)
104.0 (1)
—
306.30
308.10
314.60
311.20
310.60
—
—
stretching vibrational modes of PO4 group of monazite, respec-
tively.31 The symmetric stretching vibrational mode of the P–O
bond appears at around 950 cmÀ1. The amplitude of P–O vib-
rational modes decreases with the increasing molybdenum con-
tent. In the spectra of these selected phases, the bands appearing
in the region of B800–892 cmÀ1 are ascribed to the asymmetric
stretching vibrational modes of ‘‘MoO4’’ tetrahedral.13,32 The
IR spectrum of the phase La0.1Ca0.9P0.1Mo0.9O4 resembles that
of scheelite (CaMoO4).
Thus, the structural and spectroscopic data suggest that the
integrity of the monazite structure is retained in the range of x,
y 5 0.1–0.3. The monazite lattice allows the substitution of
‘‘P51’’ by ‘‘Mo61’’ up to the maximum of 12.9 at. wt% of
‘‘Mo’’(Table I).
(C) Thermal Analysis: The thermal stability of the
phases La0.9Ca0.1P0.9Mo0.1O4, La0.8Ca0.2P0.8Mo0.2O4, La0.7
Ca0.3P0.7Mo0.3O4, and La0.1Ca0.9P0.1Mo0.9O4 has been studied
in the temperature range of RT–12001C in air. The compositions
with higher ‘‘Mo’’ content show lesser thermal stability than
the compositions with lower ‘‘Mo’’ content. The phase La0.7
Ca0.3P0.7Mo0.3O4 crystallizing in monazite structure shows
B9% mass loss in the temperature range of 7401–9301C. This
may be attributed to the loss of molybdenum occurring above
7001C. This observation corroborates the formation of LaPO4
as the major product on heating the precursor at 8001C for 24 h
as evidenced from the powder XRD analysis. Similarly, the
composition La0.1Ca0.9P0.1Mo0.9O4 crystallizing in the scheelite
structure registers B15% mass loss in the temperature range of
7001–9001C. No such striking mass loss is noticed in the case
of the phases with ‘‘Mo’’ content of y 5 0.1 and 0.2. The phases
of the compositions La0.7Ca0.3P0.7Mo0.3O4 (12.8 at. wt% Mo)
and La0.1Ca0.9P0.1Mo0.9O4 (42.4 at. wt% Mo) are thermally
stable up to the maximum temperature range of 7001–7501C.
However, annealing the compounds at 12001C leads to the for-
mation of LaPO4 as the major phase in the case of La0.9
Ca0.1P0.9Mo0.1O4, La0.8Ca0.2P0.8Mo0.2O4, and La0.7Ca0.3P0.7
Mo0.3O4. Whereas, powder XRD analysis of the residue
obtained by annealing the phase La0.1Ca0.9P0.1Mo0.9O4 shows
LaPO4 as a minor phase with trace amount of CaMoO4 and
possibly the formation of Mo9O26 as a major phase.
(5) Morphological Changes
(A) SEM Studies: The morphology and the elemental
distribution of the four phases have been studied by SEM. The
micrographs with x and y contents equal to 0.9, 0.3, 0.2, and 0.1
are shown in Fig. 5. The change in the particle morphology with
the increasing ‘‘Mo’’ content is visible. With lesser ‘‘Mo’’
content where x, y 5 0.1 and 0.2, no defined particle shape is
Fig. 4. FT-IR spectra of (a) LaPO4, (b) La0.9Ca0.1P0.9Mo0.1O4,
(c) La0.8Ca0.2P0.8Mo0.2O4, (d) La0.7Ca0.3P0.7Mo0.3O4, (e) La0.1Ca0.9P0.1
Mo0.9O4, and (f)CaMoO4.
Fig. 3. Powder X-ray diffraction patterns of selected compositions of
the formula La1ÀxSrxP1ÀyMoyO4.

1012
Journal of the American Ceramic Society—Pratheep Kumar and Gopal
Vol. 94, No. 4
Fig. 5. Scanning electron micrographs of (a) La0.9Ca0.1P0.9Mo0.1O4, (b) La0.8Ca0.2P0.8Mo0.2O4, (c) La0.7Ca0.3P0.7Mo0.3O4, and (d) La0.1Ca0.9P0.1
Mo0.9O4.
observed and the particles are agglomerated. With higher ‘‘Mo’’
content, the particles attain a defined shape. In the case of x,
y 5 0.3, the beginning of a gentle growth of rod-like morphology
is demonstrated and the formation of complete rod-like particles
is observed for the compound La0.1Ca0.9P0.1Mo0.9O4. Thus,
SEM analysis reveals a gradual change in the morphology of
the powder samples. Such changes in the morphology of the
powder samples indicate the influence of the replacement of
‘‘PO4’’ moiety of monazite lattice by ‘‘MoO4’’ tetrahedra and
‘‘LaO9’’ polyhedra by ‘‘CaO9’’ polyhedra. With the increasing
‘‘Mo’’ and ‘‘Ca’’ contents, the particles transform from nonuni-
form shape and attained a uniform rectangular rod shape with x
and y 5 0.9. It is envisaged that the contribution toward such
morphological changes could mainly be due to the replacement
of smaller PO4 tetrahedra by bigger ‘‘MoO4’’ tetrahedra in com-
parison with the simultaneous replacement of ‘‘La31’’ by
‘‘Ca21.’’ The effect of the latter may be negligible due to the
similar size of the two ions.
and microscopic evidences confirm the crystal chemical incor-
poration of Mo61 in the monazite ceramic matrix. All the
compositions are prepared at 6001C. Based on the formation
of pure single phase, it is concluded that the maximum of
12.9 wt% of ‘‘Mo’’ could be accommodated in its 16 oxida-
tion state in the monazite lattice (nominal composition
La0.7Ca0.3P0.7Mo0.3O4). Elemental analysis of the sample
of this composition by EPMA shows 11.6 wt% of ‘‘Mo’’ with
a standard deviation of 1.0%.
Acknowledgments
The authors thank VIT University for providing all required facilities to carry
out the experiments.
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