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Crystal growth of praseodymium molybdate by gel method
International Journal of Inorganic Materials 3 (2001) 675–680
Crystal growth of praseodymium molybdate by gel method Sanjay Pandita a , Vinay Hangloo a , K.K. Bamzai a , P.N. Kotru a , *, Neera Sahni b b
a Department of Physics, University of Jammu, Jammu 180 006, India Department of Geology, Panjab University, Chandigarh 160 014, India
Received 1 June 2000; received in revised form 8 May 2001; accepted 11 August 2001
Abstract Experiments for the growth of praseodymium molybdate crystals using a silica gel method of crystal growth in the system PrCl 3 –(NH 4 ) 6 Mo 7 O 24 –NH 4 NO 3 –HNO 3 –Na 2 SiO 3 are reported. The Energy Dispersive Analysis of X-rays (EDAX) results suggest the crystals to be praseodymium heptamolybdate of the type Pr 2 Mo 7 O 24 . The crystals assumed varied morphologies, including spherulites, single crystal platelets, multifaceted and coalesced crystals. Some crystals exhibited a twinned structure. The spherulitic morphologies resulted from either the aggregation of crystals joining in a spherical envelope or the formation of an interpenetrating type with complex twinning. The layered structure on the crystal surfaces suggest that the crystals grew by two-dimensional spreading and a pile-up of layers. 2001 Published by Elsevier Science Ltd. Keywords: Crystal growth; Praseodymium molybdate; Gel method; Morphology; EDAX
1. Introduction Rare earth molybdates, in general, potentially have many applications in science and technology due to their fluorescent, laser, piezoelectric, ferroelastic and ferroelectric properties. The rare earth molybdates bearing the general formula R 2 (MoO 4 ) 3 have been acknowledged as ferroelectric materials [1]. Nassau and co-workers [2] reported a comprehensive study of rare earth molybdates, noting the profusion of different structural types, further complicated by temperature dependent polymorphism. Wide ranging phase transition studies of rare earth molybdates involving various structural modifications have also been reported [3]. A comprehensive study on the growth of rare earth molybdates, using Czochralski crystal growth procedures, and keeping the temperature dependent polymorphism and thermodynamic metastability of the individual components in consideration, has been dealt with by Brixner [4]. While considering the varying valency character of the rare earth ions when combining with molybdate ions, it is interesting to conduct studies on the crystal growth of the rare earth molybdate family at ambient temperature involving the controlled combination
of ions through a gel medium. Bhat et al. [5–7] reported the growth of La, Nd and mixed La–Nd heptamolybdate crystals as having the general formula R 2 Mo 7 O 24 . The technique, popularly known as the gel method has been used by researchers and substantial information has been obtained regarding the scientific data of several types of crystals [8–32]. So far not much attention has been given to the growth of rare earth molybdates using the gel technique. In this paper, the growth of single rare earth praseodymium heptamolybdate crystals in silica gels is reported for the first time.
2. Experimental Growth of praseodymium molybdate crystals is achieved by employing a single gel single tube technique in the system PrCl 3 –(NH 4 ) 6 Mo 7 O 24 –NH 4 NO 3 –HNO 3 – Na 2 SiO 3 . 284.20-g of Na 2 SiO 3 ?9H 2 O was dissolved in 1000 ml of distilled water so as to have a gel concentration of 1 M. The silica gel solution was left undisturbed for a few days and a clear solution of silica gel was obtained on decantation. The silica gel was then mixed with the lower reactant by adding ammonium molybdate and ammonium nitrate, each weighing 10 g, to 65 ml of distilled water. The solution was thoroughly mixed with the help of a
*Corresponding author. Tel.: 191-191-453-079; fax: 191-191-453079. E-mail address: pn [email protected] (P.N. Kotru). ] 1466-6049 / 01 / $ – see front matter 2001 Published by Elsevier Science Ltd. PII: S1466-6049( 01 )00192-1
676
S. Pandita et al. / International Journal of Inorganic Materials 3 (2001) 675 – 680
Fig. 1. Schematic diagram of a crystallizer.
magnetic stirrer and then after about 0.5 h, 30 ml of conc. HNO 3 was added to it drop by drop till a white precipitate was formed. To this was added 30 ml of distilled water so
as to make a total volume of 125 ml of lower reactant having a molarity of 0.06. The silica gel charged with the lower reactant (providing molybdenum ions) of a desired pH was then transferred to a crystallizer in the form of a single test tube (diameter52.5 cm and length520 cm), as shown schematically in Fig. 1. The gel of the desired pH was then allowed to set and age for a specific time. In our experimentation, gel ageing of 24, 48, 72 h or even longer was tried. This was followed by pouring the upper reactant (PrCl 3 ) of a desired molarity along the sides of the tube. This ensured that the gel did not break. All experiments were conducted at room temperature (|258C). Fig. 2 shows the growth of praseodymium molybdate crystals in a crystallizer. The grown crystals were examined under a metallurgical microscope (Epignost / Neophot-2; Carl Zeiss, Germany) and a scanning electron microscope (model JEOL JSM-25 S) coupled with the EDAX, KeVex Delta Class system. Before examination under the SEM, the crystals were coated with gold by an ion-sputter coater model JFC-1100 (JEOL).
3. Results and discussion
Fig. 2. Growth of praseodymium heptamolybdate in a crystallizer.
In order to establish the optimum conditions conducive for the growth of praseodymium heptamolybdate in the form of single crystals in a size suitable for scientific investigations, several experiments were performed under varying conditions of different growth parameters, viz. gel pH, concentration of upper and lower reactants and gel
S. Pandita et al. / International Journal of Inorganic Materials 3 (2001) 675 – 680
ageing. Table 1 gives a summary of the details of these experiments and the results obtained. The net outcome of these experiments is that the optimum conditions for the growth of crystals of a good size and quality are: pH 4.5; concentration of upper reactant50.25 M; concentration of lower reactant50.06 M; gel ageing572 h; gel concentration50.5 M. Bigger crystals were obtained by following a procedure of concentration programming. In these experiments, 0.5 M of upper reactant was poured initially over the gel (gel concentration50.5 M and gel age572 h) charged with the lower reactant (ammonium molybdate and ammonium nitrate) of concentration 0.6 M. After every 144 h, the upper reactant was drained off by a dropper and replaced by the upper reactant of a higher concentration of molarities viz., 1, 1.5, 2, 2.5, 3, 3.5 and 4 M, each time keeping all other growth parameters constant. The procedure increases the final size to 1 mm 3 with near cubic morphology, which is more than double the maximum size
677
obtained by the normal methods of growth, but does not affect the morphology of crystals. It takes about 1 month for the crystals to grow to the maximum possible size whether the experiments are performed under normal process of growth or concentration programming. In order to verify if the starting compositions and the consequent reaction through diffusion, as controlled by the silica gel, yields crystals of the expected composition, the crystals were examined by EDAX. Fig. 3 shows EDAX peaks corresponding to single rare earth praseodymium molybdate; the EDAX of praseodymium molybdate exhibits both praseodymium and molybdenum as expected. Table 2 shows the wt.% and at.% for praseodymium heptamolybdate, which suggests that the composition of the grown crystals may be Pr 2 Mo 7 O 24 . It is the first time that praseodymium heptamolybdate has been crystallized at ambient temperatures using the gel encapsulation technique. The crystals of praseodymium heptamolybdate assume
Table 1 Details of experiments used for the growth of praseodymium molybdate crystals Experiment
Constant parameters
Changing parameters
Results
Variation of gel ageing
UR conc. (0.25 M) LR conc. (0.06 M) Gel pH (5) Gel conc. (0.5 M)
Gel ageing: 24, 48, 72, 96, 144 h
(i) Morphology: Single crystal platelets, spherulites, cuboids, octagonals, hexagonals and crystal aggregates. (ii) Nucleation density: min. and max. at 24 and 72 h of gel age respectively. (iii) Crystallization at all ages.
Variation of pH
UR conc. (0.25 M) LR conc. (0.06 M) Gel age (72 h) Gel conc. (0.5 M)
Gel pH: 4, 4.5, 5, 5.5, 6, 6.5
(i) Morphology: Single crystal platelets, spherulites, cuboids, octagonals, hexagonals and crystal aggregates (ii) Nucleation density: min. and max. at 4 and 5 gel pH, respectively. (iii) pH values ,4 and .6 not conducive for crystal growth. (iv) Liesegang ring formation at pH 5.5
Variation of LR
UR Gel Gel Gel
LR conc. 0.06, 0.03, 0.01 M
(i) Morphology: Single crystal platelets, spherulites, cuboids, octagonals, hexagonals and crystal aggregates. (ii) Nucleation density: min. and max. at 0.03 and 0.06 M of LR, respectively. (iii) Crystals nucleate at all LR conc.
Variation of UR
LR conc. (0.06 M) Gel pH (5) Gel conc. (0.5 M) Gel age (72 h)
UR conc.: 0.25, 0.5,0.75, 1 M
(i) Morphology: Single crystal platelets, spherulites, cuboids, octagonals, hexagonals and crystal aggregates. (ii) Nucleation density: min. and max. at 0.25 and 0.5 M of UR, respectively. (iii) Crystals nucleate at all UR conc.
Variation of gel conc.
UR conc. (0.5 M) Gel age (72 h) Gel pH (5) LR conc. (0.06 M)
Gel conc.: 0.125, 0.25, 0.5 M
(i) Morphology: Single crystal platelets, spherulites, cuboids, octagonals, hexagonal and crystal aggregates. (ii) Nucleation density: min. and max. at 0.25 and 0.5 M of gel conc., respectively. (iii) Bigger crystal size at 0.025 M. (iv) Crystals nucleate at all gel conc.
conc. (0.5 M) age (72 h) pH (5) conc. (0.5 M)
UR5Upper reactant (praseodymium chloride); LR5lower reactant (gel charged with ammonium molybdate and ammonium nitrate). Dimensions of crystallizer and gel column in all experiments were: diameter52.5 cm, height520 cm and 13 cm, respectively.
S. Pandita et al. / International Journal of Inorganic Materials 3 (2001) 675 – 680
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Fig. 3. EDAX of praseodymium heptamolybdate showing peaks corresponding to expected elements Mo and Pr. Table 2 Data from EDAX analysis of Pr 2 Mo 7 O 24 Atomic percentage (at.%)
Weight percentage (wt.%)
Experimental
Calculated
Experimental
Calculated
Pr
Mo
Pr
Mo
Pr
Mo
Pr
Mo
20.7
79.3
22.2
77.8
27.7
72.3
29.5
70.4
Fig. 5. Twinned crystal of praseodymium molybdate.
Fig. 4. Multifaceted crystal with plane faces.
spherulitic, platelet, cuboid and coalesced crystal morphologies. Fig. 4 is a SEM showing a facetted crystal, Fig. 5 shows twinned crystals of praseodymium molybdate and Fig. 6 is an example of interpenetrating crystals leading to a spherulitic appearance of praseodymium heptamolybdate crystals. A typical butterfly morphology is exhibited by Fig. 7.
S. Pandita et al. / International Journal of Inorganic Materials 3 (2001) 675 – 680
Fig. 6. Interpenetrating crystals leading to spherulitic formation.
679
The spherulitic formation of organic compounds [33– 35], inorganic compounds in minerals [36–38] and artificially synthesized inorganic compounds [39–44] has been reported in the literature. It is generally believed that a spherulite develops from a single crystal nucleus [45]. However, Kotru et al. and Jain et al. [46,47] reported radiating fibres from multiple nuclei rather than a single nuclei resulting into spherulitic growth. The present observations suggest that the spherulitic morphology may result from either the interpenetration type of twinning or bunching together of a large number of crystallites in a spherical envelope. Some crystals do exhibit microstructures on their habit faces. Fig. 8a shows such microstructures, which suggest a layered growth of crystals. Fig. 8b shows growth layers spreading from a nucleus. The microstructures are suggestive of growth by two-dimensional spreading and piling-up of layers, indicative of growth at high supersaturations.
4. Conclusions From the above described experiments and results, the following conclusions were drawn:
Fig. 7. A typical butterfly morphology.
1. Employing a single gel, single tube, crystallizer and using a silica gel method of crystal growth in the system PrCl 3 –(NH 4 ) 6 Mo 7 O 24 –NH 4 NO 3 –HNO 3 – Na 2 SiO 3 , crystals of praseodymium heptamolybdate can be grown. 2. The gel-grown praseodymium heptamolybdate assumes varied morphologies including spherulites, cuboid, platelets, multifaceted and coalesced crystals. 3. Some crystals of praseodymium heptamolybdate bear a twinned structure.
Fig. 8. Two dimensional growth of crystals. (a) Microstructures showing layered growth and (b) growth layers spreading from a nucleus.
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4. The spherulitic morphologies result from aggregates of crystals joining in a spherical envelope or from an interpenetration type of complex twinning. 5. The crystals of praseodymium heptamolybdates grow by two-dimensional spreading and piling-up of layers.
References [1] Borchardt HJ, Bierstedt PE. Appl Phys Lett 1966;8:50. [2] Nassau R, Levinstein HJ, Loiacona GM. J Phys Chem Solids 1965;26:1805. [3] Drobyshev LA, Tomashpolskii YuYa, Saponov AI, Antonov GN, Fedulov SA, Venevstev YuN. Kristallografiya 1968;13:100; Drobyshev LA, Tomashpolskii YuYa, Saponov AI, Antonov GN, Fedulov SA, Venevstev YuN. Sov Phys Crystallogr 1969;13:964. [4] Brixner LH. J Cryst Growth 1973;18:297. [5] Bhat S, Kotru PN. Cryst Res Technol 1994;29(3):325. [6] Bhat S, Kotru PN, Koul ML. Mat Sci Eng 1994;B23:73. [7] Bhat S, Kotru PN, Koul ML. Mat Sci Eng 1995;B34:138. [8] Bhat VP, Patel RM. J Cryst Growth 1981;53:633. [9] Devanarayanan S, Kalkura Narayana S. J Mater Sci Lett 1991;10:497. [10] Desai CC, Rai JL. J Cryst Growth 1981;53:432. [11] Desai CC, Ramana MSV. J Cryst Growth 1988;91:126. [12] Desai CC, Ramana MSV. J Cryst Growth 1990;102:191. [13] Henisch HK. Crystal growth in gels, USA: The Pennsylvania State University Press, 1970. [14] Jain A, Razdan AK, Kotru PN. Mater Sci Eng 1991;B8:129. [15] Joshi MS, Antony AV. J Mater Sci 1978;13:939. [16] Kotru PN, Gupta NK, Raina KK. Cryst Res Technol 1986;21:126. [17] Kotru PN, Gupta NK, Raina KK. J Mater Sci 1986;22:90.
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45]
[46] [47]
Kotru PN, Gupta NK, Raina KK. J Mater Sci 1987;22:177. Kotru PN, Bhat S, Raina KK. J Mater Sci Lett 1989;8:587. Kumar RR, Ramana G, Ganana FD. J Mater Sci 1988;24:4535. Kumar RR, Ramana G, Ganana FD. J Mater Sci 1989;24:4531. Kurien V, Ittyachen MA. J Cryst Growth 1979;47:743. Mansotra V, Raina KK, Kotru PN. J Mater Sci 1991;26:3780. Marcher WC, Rosmalin Van GM. J Cryst Growth 1977;39:358. Ohia M, Tsulsumi M. J Cryst Growth 1982;53:652. Patel AR, Arora SK. J Cryst Growth 1976;14:843. Patel AR, Venkatesware AR. J Cryst Growth 1979;47:213. Garcia Ruiz JM. Mater Res Soc Bull 1985;20:1157. Garcia Ruiz JM. J Cryst Growth 1986;75:441. Santos A, Garcia Ruiz JM. Cryst Res Technol 1987;22:645. Sengupta S, Kar T, Gupta SPS. J Mater Sci Lett 1990;9:334. Shripathi T, Bhat HL, Narayana PS. J Mater Sci 1980;15:3095. Jackson JF, Hsu TS, Brasch JW, Polymer J. Science 1972;B10:207. Macchi EM. J Polymer Sci 1972;A10:45. Samuels SL, Wikes GL. Polymer preprints. Am Chem Soc Div Polymer Chem 1971;12:694. Ewart A. Miner Mag 1971;38:424. Ewing RC. Science 1974;184:561. Blau PJ, Axon HJ, Goldstein JI. J Geophys Res 1973;78:363. Blank Z, Brenner W. J Crystal Growth 1971;11:255. Morse HW, Warren CH, Donnay JDH. Am J Sci 1932;23:421. Morse HW, Donnay JDH. Am Mineralogist 1932;21:391. Nassau K, Cooper AS, Schriever JW, Prescott BE. J Solid State Chem 1973;8:260. Patel AR, Arora SK. J Crystal Growth 1973;18:199. Sohnel O, Mullin JW. J Crystal Growth 1982;60:239. Boltotov IE, Mauravev EA. In: Ovsienko DE, editor, Growth and imperfections of metallic crystals, New York: Consultants Bureau, 1968, p. 76. Kotru PN, Raina KK. J Cryst Growth 1988;91:221. Jain A, Razdan AK, Kotru PN. Mat Chem Phys 1996;45:180.
Crystal growth of praseodymium molybdate by gel method Sanjay Pandita a , Vinay Hangloo a , K.K. Bamzai a , P.N. Kotru a , *, Neera Sahni b b
a Department of Physics, University of Jammu, Jammu 180 006, India Department of Geology, Panjab University, Chandigarh 160 014, India
Received 1 June 2000; received in revised form 8 May 2001; accepted 11 August 2001
Abstract Experiments for the growth of praseodymium molybdate crystals using a silica gel method of crystal growth in the system PrCl 3 –(NH 4 ) 6 Mo 7 O 24 –NH 4 NO 3 –HNO 3 –Na 2 SiO 3 are reported. The Energy Dispersive Analysis of X-rays (EDAX) results suggest the crystals to be praseodymium heptamolybdate of the type Pr 2 Mo 7 O 24 . The crystals assumed varied morphologies, including spherulites, single crystal platelets, multifaceted and coalesced crystals. Some crystals exhibited a twinned structure. The spherulitic morphologies resulted from either the aggregation of crystals joining in a spherical envelope or the formation of an interpenetrating type with complex twinning. The layered structure on the crystal surfaces suggest that the crystals grew by two-dimensional spreading and a pile-up of layers. 2001 Published by Elsevier Science Ltd. Keywords: Crystal growth; Praseodymium molybdate; Gel method; Morphology; EDAX
1. Introduction Rare earth molybdates, in general, potentially have many applications in science and technology due to their fluorescent, laser, piezoelectric, ferroelastic and ferroelectric properties. The rare earth molybdates bearing the general formula R 2 (MoO 4 ) 3 have been acknowledged as ferroelectric materials [1]. Nassau and co-workers [2] reported a comprehensive study of rare earth molybdates, noting the profusion of different structural types, further complicated by temperature dependent polymorphism. Wide ranging phase transition studies of rare earth molybdates involving various structural modifications have also been reported [3]. A comprehensive study on the growth of rare earth molybdates, using Czochralski crystal growth procedures, and keeping the temperature dependent polymorphism and thermodynamic metastability of the individual components in consideration, has been dealt with by Brixner [4]. While considering the varying valency character of the rare earth ions when combining with molybdate ions, it is interesting to conduct studies on the crystal growth of the rare earth molybdate family at ambient temperature involving the controlled combination
of ions through a gel medium. Bhat et al. [5–7] reported the growth of La, Nd and mixed La–Nd heptamolybdate crystals as having the general formula R 2 Mo 7 O 24 . The technique, popularly known as the gel method has been used by researchers and substantial information has been obtained regarding the scientific data of several types of crystals [8–32]. So far not much attention has been given to the growth of rare earth molybdates using the gel technique. In this paper, the growth of single rare earth praseodymium heptamolybdate crystals in silica gels is reported for the first time.
2. Experimental Growth of praseodymium molybdate crystals is achieved by employing a single gel single tube technique in the system PrCl 3 –(NH 4 ) 6 Mo 7 O 24 –NH 4 NO 3 –HNO 3 – Na 2 SiO 3 . 284.20-g of Na 2 SiO 3 ?9H 2 O was dissolved in 1000 ml of distilled water so as to have a gel concentration of 1 M. The silica gel solution was left undisturbed for a few days and a clear solution of silica gel was obtained on decantation. The silica gel was then mixed with the lower reactant by adding ammonium molybdate and ammonium nitrate, each weighing 10 g, to 65 ml of distilled water. The solution was thoroughly mixed with the help of a
*Corresponding author. Tel.: 191-191-453-079; fax: 191-191-453079. E-mail address: pn [email protected] (P.N. Kotru). ] 1466-6049 / 01 / $ – see front matter 2001 Published by Elsevier Science Ltd. PII: S1466-6049( 01 )00192-1
676
S. Pandita et al. / International Journal of Inorganic Materials 3 (2001) 675 – 680
Fig. 1. Schematic diagram of a crystallizer.
magnetic stirrer and then after about 0.5 h, 30 ml of conc. HNO 3 was added to it drop by drop till a white precipitate was formed. To this was added 30 ml of distilled water so
as to make a total volume of 125 ml of lower reactant having a molarity of 0.06. The silica gel charged with the lower reactant (providing molybdenum ions) of a desired pH was then transferred to a crystallizer in the form of a single test tube (diameter52.5 cm and length520 cm), as shown schematically in Fig. 1. The gel of the desired pH was then allowed to set and age for a specific time. In our experimentation, gel ageing of 24, 48, 72 h or even longer was tried. This was followed by pouring the upper reactant (PrCl 3 ) of a desired molarity along the sides of the tube. This ensured that the gel did not break. All experiments were conducted at room temperature (|258C). Fig. 2 shows the growth of praseodymium molybdate crystals in a crystallizer. The grown crystals were examined under a metallurgical microscope (Epignost / Neophot-2; Carl Zeiss, Germany) and a scanning electron microscope (model JEOL JSM-25 S) coupled with the EDAX, KeVex Delta Class system. Before examination under the SEM, the crystals were coated with gold by an ion-sputter coater model JFC-1100 (JEOL).
3. Results and discussion
Fig. 2. Growth of praseodymium heptamolybdate in a crystallizer.
In order to establish the optimum conditions conducive for the growth of praseodymium heptamolybdate in the form of single crystals in a size suitable for scientific investigations, several experiments were performed under varying conditions of different growth parameters, viz. gel pH, concentration of upper and lower reactants and gel
S. Pandita et al. / International Journal of Inorganic Materials 3 (2001) 675 – 680
ageing. Table 1 gives a summary of the details of these experiments and the results obtained. The net outcome of these experiments is that the optimum conditions for the growth of crystals of a good size and quality are: pH 4.5; concentration of upper reactant50.25 M; concentration of lower reactant50.06 M; gel ageing572 h; gel concentration50.5 M. Bigger crystals were obtained by following a procedure of concentration programming. In these experiments, 0.5 M of upper reactant was poured initially over the gel (gel concentration50.5 M and gel age572 h) charged with the lower reactant (ammonium molybdate and ammonium nitrate) of concentration 0.6 M. After every 144 h, the upper reactant was drained off by a dropper and replaced by the upper reactant of a higher concentration of molarities viz., 1, 1.5, 2, 2.5, 3, 3.5 and 4 M, each time keeping all other growth parameters constant. The procedure increases the final size to 1 mm 3 with near cubic morphology, which is more than double the maximum size
677
obtained by the normal methods of growth, but does not affect the morphology of crystals. It takes about 1 month for the crystals to grow to the maximum possible size whether the experiments are performed under normal process of growth or concentration programming. In order to verify if the starting compositions and the consequent reaction through diffusion, as controlled by the silica gel, yields crystals of the expected composition, the crystals were examined by EDAX. Fig. 3 shows EDAX peaks corresponding to single rare earth praseodymium molybdate; the EDAX of praseodymium molybdate exhibits both praseodymium and molybdenum as expected. Table 2 shows the wt.% and at.% for praseodymium heptamolybdate, which suggests that the composition of the grown crystals may be Pr 2 Mo 7 O 24 . It is the first time that praseodymium heptamolybdate has been crystallized at ambient temperatures using the gel encapsulation technique. The crystals of praseodymium heptamolybdate assume
Table 1 Details of experiments used for the growth of praseodymium molybdate crystals Experiment
Constant parameters
Changing parameters
Results
Variation of gel ageing
UR conc. (0.25 M) LR conc. (0.06 M) Gel pH (5) Gel conc. (0.5 M)
Gel ageing: 24, 48, 72, 96, 144 h
(i) Morphology: Single crystal platelets, spherulites, cuboids, octagonals, hexagonals and crystal aggregates. (ii) Nucleation density: min. and max. at 24 and 72 h of gel age respectively. (iii) Crystallization at all ages.
Variation of pH
UR conc. (0.25 M) LR conc. (0.06 M) Gel age (72 h) Gel conc. (0.5 M)
Gel pH: 4, 4.5, 5, 5.5, 6, 6.5
(i) Morphology: Single crystal platelets, spherulites, cuboids, octagonals, hexagonals and crystal aggregates (ii) Nucleation density: min. and max. at 4 and 5 gel pH, respectively. (iii) pH values ,4 and .6 not conducive for crystal growth. (iv) Liesegang ring formation at pH 5.5
Variation of LR
UR Gel Gel Gel
LR conc. 0.06, 0.03, 0.01 M
(i) Morphology: Single crystal platelets, spherulites, cuboids, octagonals, hexagonals and crystal aggregates. (ii) Nucleation density: min. and max. at 0.03 and 0.06 M of LR, respectively. (iii) Crystals nucleate at all LR conc.
Variation of UR
LR conc. (0.06 M) Gel pH (5) Gel conc. (0.5 M) Gel age (72 h)
UR conc.: 0.25, 0.5,0.75, 1 M
(i) Morphology: Single crystal platelets, spherulites, cuboids, octagonals, hexagonals and crystal aggregates. (ii) Nucleation density: min. and max. at 0.25 and 0.5 M of UR, respectively. (iii) Crystals nucleate at all UR conc.
Variation of gel conc.
UR conc. (0.5 M) Gel age (72 h) Gel pH (5) LR conc. (0.06 M)
Gel conc.: 0.125, 0.25, 0.5 M
(i) Morphology: Single crystal platelets, spherulites, cuboids, octagonals, hexagonal and crystal aggregates. (ii) Nucleation density: min. and max. at 0.25 and 0.5 M of gel conc., respectively. (iii) Bigger crystal size at 0.025 M. (iv) Crystals nucleate at all gel conc.
conc. (0.5 M) age (72 h) pH (5) conc. (0.5 M)
UR5Upper reactant (praseodymium chloride); LR5lower reactant (gel charged with ammonium molybdate and ammonium nitrate). Dimensions of crystallizer and gel column in all experiments were: diameter52.5 cm, height520 cm and 13 cm, respectively.
S. Pandita et al. / International Journal of Inorganic Materials 3 (2001) 675 – 680
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Fig. 3. EDAX of praseodymium heptamolybdate showing peaks corresponding to expected elements Mo and Pr. Table 2 Data from EDAX analysis of Pr 2 Mo 7 O 24 Atomic percentage (at.%)
Weight percentage (wt.%)
Experimental
Calculated
Experimental
Calculated
Pr
Mo
Pr
Mo
Pr
Mo
Pr
Mo
20.7
79.3
22.2
77.8
27.7
72.3
29.5
70.4
Fig. 5. Twinned crystal of praseodymium molybdate.
Fig. 4. Multifaceted crystal with plane faces.
spherulitic, platelet, cuboid and coalesced crystal morphologies. Fig. 4 is a SEM showing a facetted crystal, Fig. 5 shows twinned crystals of praseodymium molybdate and Fig. 6 is an example of interpenetrating crystals leading to a spherulitic appearance of praseodymium heptamolybdate crystals. A typical butterfly morphology is exhibited by Fig. 7.
S. Pandita et al. / International Journal of Inorganic Materials 3 (2001) 675 – 680
Fig. 6. Interpenetrating crystals leading to spherulitic formation.
679
The spherulitic formation of organic compounds [33– 35], inorganic compounds in minerals [36–38] and artificially synthesized inorganic compounds [39–44] has been reported in the literature. It is generally believed that a spherulite develops from a single crystal nucleus [45]. However, Kotru et al. and Jain et al. [46,47] reported radiating fibres from multiple nuclei rather than a single nuclei resulting into spherulitic growth. The present observations suggest that the spherulitic morphology may result from either the interpenetration type of twinning or bunching together of a large number of crystallites in a spherical envelope. Some crystals do exhibit microstructures on their habit faces. Fig. 8a shows such microstructures, which suggest a layered growth of crystals. Fig. 8b shows growth layers spreading from a nucleus. The microstructures are suggestive of growth by two-dimensional spreading and piling-up of layers, indicative of growth at high supersaturations.
4. Conclusions From the above described experiments and results, the following conclusions were drawn:
Fig. 7. A typical butterfly morphology.
1. Employing a single gel, single tube, crystallizer and using a silica gel method of crystal growth in the system PrCl 3 –(NH 4 ) 6 Mo 7 O 24 –NH 4 NO 3 –HNO 3 – Na 2 SiO 3 , crystals of praseodymium heptamolybdate can be grown. 2. The gel-grown praseodymium heptamolybdate assumes varied morphologies including spherulites, cuboid, platelets, multifaceted and coalesced crystals. 3. Some crystals of praseodymium heptamolybdate bear a twinned structure.
Fig. 8. Two dimensional growth of crystals. (a) Microstructures showing layered growth and (b) growth layers spreading from a nucleus.
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4. The spherulitic morphologies result from aggregates of crystals joining in a spherical envelope or from an interpenetration type of complex twinning. 5. The crystals of praseodymium heptamolybdates grow by two-dimensional spreading and piling-up of layers.
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