n 57 (2006) 30–35

Materials Characterizatio

Pre-treated and treated hydrothermal preparation of NaVP2O7·H2Ocrystalline material and its characterization

M.J. Mahesh ⁎, G.S. Gopalakrishna, Ashamanjari

Department of Studies in Geology, University of Mysore, Manasagangotri, Mysore 570 006, India

Received 6 October 2005; accepted 5 December 2005

Abstract

NaVP2O7·H2O crystalline material was synthesized by a hydrothermal technique at moderate P–T condition. The resultantmaterial was characterized by SEM, powder XRD, DTA, FTIR and magnetic susceptibility studies. Powder X-ray studies revealedthat the NaVP2O7·H2O compound has crystallized in monoclinic system with cell parameters a=22.1737, b=8.0520, c=11.4670Å, β=103.751, V=1988.66 Å3. Repetitive hydrothermal treatment of the material yielded improved size and quality of crystals.Magnetically this material is paramagnetic; however the μeff decreased as the size of the crystals increased.© 2005 Elsevier B.V. All rights reserved.

Keywords: Hydrothermal synthesis; Vanadium phosphate; XRD; DTA; FT-IR; Paramagnetic

1. Introduction

The solid phase inorganic phosphates have captivat-ed the attention of materials scientists due to their wideapplications such as fertilizers, phosphors, ion con-ductors, dielectric, pyroelectric, ferroelectric, piezoelec-tric property, conventional solid state laser materials,polarizer and good catalytic performance, biotechno-logical materials, sorbents and magnetic materials [1–7]and considerable technological importance like synthet-ic products, acid salts, hydrates and polymorphicverities [8,9]. Vanadium phosphates are interesting asa low dimensional spin system [10,11] and very suitablehost for the rare earth ions because of their luminescentbehavior, typical magnetic properties and relativelysimple crystal field splitting. Some rare earth vanadates

⁎ Corresponding author.E-mail address: mjmahe@yahoo.com (M.J. Mahesh).

1044-5803/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.matchar.2005.12.002

are commonly used as laser materials [12,13]. Theoxovanadium phosphate system represents an importantsubclass of the family of inorganic phosphates. Thedramatic extensive chemistry of the V–O–P systemderives its impetus from observations related to vanadylpyrophosphate VO2P2O7 [14,15]. Here, we are report-ing the hydrothermal synthesis and characterization of anew group of hydrate sodium vanadium phosphatecrystalline material; NaVP2O7·H2O and also discuss theeffect that hydrothermal pre-treatment and treatment hason the size and morphology of crystals and their thermaland magnetic properties.

2. Experimental procedure

All chemicals were used as obtained without furtherpurification. Vanadium penta-oxide (V2O5, 99%),sodium hydroxide (NaOH, 99.99%), ortho-phosphoricacid (H3PO4, 85%) and nitric acid (HNO3, 69–72%),

31M.J. Mahesh et al. / Materials Characterization 57 (2006) 30–35

were purchased from Glaxo, India. NaVP2O7·H2Ocrystals were synthesized by a hydrothermal techniqueat moderate temperature and pressure conditions byfollowing an earlier reported procedure [19,20], whichis shown as a flow chart (Fig. 1). Synthesis of phosphatematerials by hydrothermal technique is one the bettermethods compared to other methods, owing to itsadvantages such as the fact synthesis takes place atrelatively low temperature and it is a closed system,hence gaseous fugacity plays an important role in thesynthesis process. The resultant products of the experi-ments are in purer form [21]. The experiments were

Fig. 1. Hydrothermal synthesis flow chart for NaVP2O7·H2Ocrystalline materials.

carried out in Morey-type autoclaves provided withTeflon liners at 260 °C and pressure ranges of 60–80bars. Initially, a known quantity of NaOH and H3PO4

were placed in a Teflon-lined stainless steel autoclavewith a capacity of 50 ml. Later a known quantity ofV2O5 and mineralizer HNO3 were added to it and thereactants were briefly stirred till homogeneity attainedand initial pH measured was 4.12. The crystallizationwas carried out by spontaneous nucleation and the rateof nucleation was controlled through programmed slowrate of heating. The experiments were run for 8 daysfollowed by instant quenching to ambient conditionsand then washed thoroughly with distilled water usingultra sonic cleaner; the final pH measured for theresultant product was 3.71·NaVP2O7·H2O crystallinematerial was obtained under the following molar ratios(in grams):

NaOH : V2O5 : H3PO4 : HNO3 :: 1:5–2:5 : 1:0–2:0: 10–12 : 0:5–0:8:

3. Characterization

SEM study has made by using JOEL, JSM-840AScanning Microscope. Powder XRD spectra haverecorded for uniform phase materials by RigakuMiniflex diffractometer with CuKα radiation (λ=1.5405 Å) at room temperature. The peaks have beenindexed and cell parameters were calculated by reciprocallattice method using the spatial indexing computerprogram (Crysfire-2002). Hydrothermally treated NaVP2O7·H2O crystals have crystallized in the monoclinicsystem with cell parameters; a=22.1737, b=8.0520,c=11.4670 Å, β=103.751 and V=1988.66 Å3. The DTAcurve was recorded using DTA/ETA instrument (Model021 NAL India) from room temperature to 600 °C.The FTIR-Spectrum was recorded in the range of4000–400 cm−1 by high resolution JASCO FT/IR-460-Plus Fourier Transformation Infrared Spectrophotome-ter. Magnetic susceptibility measurements were madefor both pretreated and treated materials using Gouybalance method by applying field strength of 0.5–3.0 kGat 300 K.

4. Results and discussion

The crystalline materials obtained by hydrothermalsynthesis are relatively tiny, exhibiting needle andirregular shapes having size of 0.25–2.5 μm andtwinning is common (Fig. 2). It has been reported thatthe repetitive hydrothermal synthesis process yieldedbetter crystallinity and increased the size of the crystals

Fig. 2. SEM photograph of hydrothermally pre-treated NaVP2O7·H2O crystalline materials.

32 M.J. Mahesh et al. / Materials Characterization 57 (2006) 30–35

[19]. Keeping this in mind, the synthesis process wasfurther continued. That is, the hydrothermally synthe-sized material was treated further with a known quantityof 6 N H3PO4 solutions (material :6N H3PO4 solu-tions :: 1 :10) under similar experimental conditions for aperiod of 1–5 h. It was observed that the duration of thehydrothermal treatment of the material has no signifi-cant influence on the size and shape of the crystals andhence the duration of the treatment process was fixed toa minimum duration (one hour). Again, the resultantproduct was thoroughly washed with distilled water.This repetitive hydrothermal treatment enhanced thedegree of crystallinity and size to manifolds (0.3–0.8μm) especially along c-axis shown in Fig. 3. The shapesalso have changed to a tabular habit.

The XRD spectra of the pre-treated and treatedmaterials exhibited sharp intensive peaks. No impuritieswere observed and the considerable fine and sharpness

Fig. 3. SEM photograph of hydrothermally trea

of the peaks indicates high crystallinity of the materials(Fig. 4a–b). The peaks have been indexed and cellparameters were calculated by reciprocal lattice methodusing special indexing computer program (Crysfire-2002) [22]. It is crystallized in the monoclinic systemhaving cell parameters; a= 22.1737, b= 8.0520,c=11.4670 Å, β=103.751 and V=1988.66 Å3. TheXRD spectra of the hydrothermally treated materialsexhibit sharp intensive peaks compared to the hydro-thermally pretreated materials, clearly indicating a betterdegree of crystallinity without change in the crystallo-graphic systems. Standard deviation and figures of meritare meager and are well within the permissible limits.The fineness and sharpness of the peaks clearly indicatea purer form of NaVP2O7·H2O crystals for thehydrothermally treated material.

To provide more information on the NaVP2O7·H2Ocrystalline materials, it is judged important to understand

ted NaVP2O7·H2O crystalline materials.

Fig. 4. Powder X-ray patterns of: a) pre-treated and b) treated of NaVP2O7·H2O crystalline materials.

33M.J. Mahesh et al. / Materials Characterization 57 (2006) 30–35

the thermal properties by the DTA results shown in Fig. 5(a–b). The results yielded endothermic peaks at 281 and362 °C due to the irreversible phase thermal transition ofthe crystalline material. Indicating this result, hydrother-mal treated material more stable and crystalline compareto pre-treated crystalline material.

FTIR-Spectra of the materials exhibited prominentmultiple vibrational bands, especially in four frequencyregions, i.e., at ν1=3654.09–3535.64 cm− 1, ν2 =2667.66–2337.9 cm−1, ν3=1811.79–1432.65 cm−1 andν4=1249.65–412.58 cm−1 as shown in Fig. 6(a–b). Thevibrations at ν1 are due to the presence of H–O–Hmolecule. ν2 and ν3 are due to the Na–O–Na and V–O–Vbonding, and ν4 is due to the presence of P–O–Pbonding. The degree of multiplication and fineness in

Fig. 5. DTA curves of: a) pre-treated and b) treated NaVP2O7·H2Ocrystalline materials.

the spectrum of the phosphates increases as the degreeof polymerization increases in the [PO4] tetrahedral[19,20]. Both the pretreated and treated materials of thepresent investigations exhibit a number of multiplesplitting and sharp peaks in the middle to lowerfrequency regions. However, the multiplication andsharpness is relatively more prominent and intensive inthe treated materials, especially at 1256.33, 1170.56,1110.21 and 960.64 cm−1, clearly indicating a highdegree of polymerization of [PO4]

3− to [P2O5] com-pared to the pretreated materials.

The magnetic susceptibility measurements weremade on hydrothermally pre-treated and treated materi-als. Hydrothermally pre-treated NaVP2O7·H2O materi-als show a magnetic moment (μeff) range of 1.92–2.51BM and hydrothermally treated materials have a μeffrange of 1.56–2.17 BM, respectively; correspondingplots are shown in Fig. 7(a–b). In general, the magneticmoment of vanadium derivatives falls in two broadclasses in the range of 2.8–3.1 BM, and the calculatedmagnetic moment spin-only value is 2.83 BM [23,24].The vanadium derivative compounds with one unpairedelectron is either low spin octahedral or planar; whereasthe compounds with three unpaired electrons are eitherhigh spin octahedral or spin-only formula. The high spinoctahedral derivatives of vanadium have μeff valuesranging from 2.8–3.1 BM and the spin-only formula forthree unpaired electrons is only 2.83 BM, whereas thepure phosphate derivatives are diamagnetic [23]. Thepresent investigated compounds, both hydrothermallypretreated and treated materials, are paramagnetic andexhibit relatively lower μeff values than the μeff ofvanadium derivatives. These phenomena could be

Fig. 6. FT-IR spectra of: a) pre-treated and b) treated NaVP2O7·H2O crystalline materials.

34 M.J. Mahesh et al. / Materials Characterization 57 (2006) 30–35

attributed due to the diamagnetic phosphorus ionsbonded with the vanadium ions through commonoxygen ions in the structure. It has been reported thatthe magnetic properties of nanoparticles show a highpresence of net magnetic moment at the surface due tothe large surface /volume ratio [19,25]. In the presentinvestigated materials, it also has been observed that thehydrothermally pretreated materials have relativelyhigher values of μeff than the treated materials. This

Fig. 7. Magnetic moment of: a) hydrothermally pre-treated and b)treated NaVP2O7·H2O materials.

may be related to the 3–4-fold increase in the size of thehydrothermally treated materials relative to the pre-treated materials. The μeff values of both compoundshave shown positive correlation with the magnetic fieldstrength, indicating these are frequency-dependentparamagnetic materials.

Preliminary impedance spectroscopy results ofhydrothermally pre-treated and treated materials areencouraging. Ionic conductivity values of these materi-als are in the range of 10−5 to 10−3 and 10−6 to 10−3

cm−1, respectively. Detailed impedance spectroscopystudies are underway.

5. Conclusions

NaVP2O7·H2O crystalline materials were synthesizedby a hydrothermal method. The hydrothermal treatmentof the synthesized materials yielded better crystallinitywith well-developed morphology. Powder XRD resultshave revealed that the degree of crystallinity and size ofthe crystallites increased manifold due to the hydrother-mal treatment. DTA results indicate relatively moderatethermal stability. FT-IR clearly demonstrated that thesubject compound has exhibited more splitting andsharpness, especially in the low frequency regions,indicating condensation of [PO4]

3− to [P2O7]. Thesematerials are frequency-dependent paramagnetic;

35M.J. Mahesh et al. / Materials Characterization 57 (2006) 30–35

however, the μeff values decreased as the crystallinityand size of the materials increased.

Acknowledgement

The authors would like to express thanks to DST,Government of India for financial assistance under theproject SP/S2/M-44/96-97.

References

[1] Aurivillius B, Lindblom CI, Stenson P. Acta Chem Scand1964;16(6):1555.

[2] Firsov AV, Skorokhodov NE, Astafiev AV, Bush AA, Stefano-vich SYu, Venetsev YuN. Sov Phys Crystallogr 1984;29(3):304.

[3] Prasad KVR, Varma KBR. Ferroelectrics 1994;158:205.[4] Bush AA, Venevstev YuN. Russ J Inorg Chem 1986;31(5):759.[5] Shonai Tomohiro, Higuchi Mikio, Kodaira Kohei. Mater Res

Bull 2000;35:225–32.[6] Shonai Tomohiro, Higuchi Mikio, Kodaira Kohei, Ogawa

Takayo, Wada S, Machida Hiroshi. J Cryst Growth 2002;24:159–64.

[7] Kanazawa T. Inorganic phosphate materials. Materials sciencemonographs, vol. 52. Amsterdam: Elsevier; 1989.

[8] Fim RC, Zubieta J, Haushalter RC. Prog Inorg Chem2002;51:451.

[9] Corbridge DEC. Phosphorus. Amsterdam: Elsevier; 1978.[10] Prokofiev AV, Bullesfeld F, Assmus W. Mater Res Bull

2000;35:1859–68.[11] Alarcon J. J Mater Sci 2001;36:1189–95.[12] Nag A, Ghosh D, Wanklyn BM. Solid State Commun

1998;108:265–70.[13] Bullesfeld F, Prokofiev AV, Assmus W, HiSchwenk, Wichert D,

Low U, Luthi B. Physica B 1999;259–261:969–70.[14] Centi G, Trifiro F, Ebner JR, Franchetti VM. Chem Res

1988;88:55.[15] Centi G, editor. Vanadyl pyrophosphate catalysis. Catalysis

today, vol. 16. Amsterdam: Elsevier; 1993.[19] Gopalakrishna GS, Mahesh MJ, Ashamanjari KG, Shashidhar-

aprasad J. J Cryst Growth 2005;284:495–501.[20] Mahesh MJ, Gopalakrishna GS, Ashamanjari KG, Shashidhara

Prasad J. Indian J Phys 2005;79:37.[21] Rabenau A. Angew Chem Int Ed Engl 1985;24:1026.[22] Shirley R. The Crysfire 2002 System for automatic powder

indexing: user's manual. 41 Guildford Park Avenue, Guildford,Surrey GU2 7NL, England: The Lattice Press; 2002.

[23] Lee JD. Text book of concise inorganic chemistry. 5th edition.1996. p. 664–5.

[24] Raj Ranian JHa, JHa DK, Mishra JP. Asian J Chem 1999;11(3):694–8.

[25] Baiiobre-Lopez Manuel, Vázquez-Vázquez Carlos, RivasJosé, Arturo López-Quintela M. Nanotechnology 2003;14:318–22.