www.elsevier.com/locate/micromeso

Microporous and Mesoporous Materials 71 (2004) 57–63

Phase transformations in mesostructured VPO/surfactant composites

Moises A. Carreon a, Vadim V. Guliants a,*, M. Olga Guerrero-P�erez b,Miguel A. Ba~nares b

a Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USAb Instituto de Cat�alisis y Petroleoqu�ımica, CSIC, Marie Curie, 2, E-28049 Madrid, Spain

Received 30 December 2003; received in revised form 8 March 2004; accepted 11 March 2004

Available online 22 April 2004

Abstract

Mesostructured VPO phases were obtained by soft-chemistry approaches in the presence of charged surfactants. These phases

underwent phase transformations to various mesostructured and amorphous VPO phases during thermal treatment in N2 at 400 �C.Lamellar-to-hexagonal, hexagonal-to-disordered mesostructure and cubic-to-amorphous transformations were followed by XRD

and in situ Raman spectroscopy. XRD detected the long-range structural changes in the mesostructured VPO phases during these

phase transformations. Raman spectroscopy is sensitive to the short-range order in mixed metal oxides and provided comple-

mentary information on the nature of the local structural changes in the V–O–P framework and surfactant molecules during these

transformations. The onset of these transformations below template decomposition temperatures is discussed in terms of the

changes in the surfactant packing parameter, g.� 2004 Elsevier Inc. All rights reserved.

Keywords: Mesostructured VPO; Phase transformations; Nitrogen heat treatment; Raman spectroscopy; Surfactant packing parameter

1. Introduction

Since the discovery of M41S family of silicate meso-

porous molecular sieves [1,2] there has been great

interest in the synthesis of structurally similar non-sili-

cate phases. In particular, transition metal oxides are

very attractive because of their redox properties and

potential as catalysts and catalytic supports. Several

transition metal oxides have been reported in the liter-

ature and recently reviewed by Sch€uth [3]. Vanadium-containing metal oxides possess highly promising

catalytic properties for the selective oxidation of lower

alkanes [4]. In particular, vanadium–phosphorus–oxide

is the only commercial catalytic system for the selective

vapor-phase oxidation of a lower alkane, i.e. n-butane to

maleic anhydride [5].

Several mesostructured VPO phases have been re-

cently reported [6–11]. Iwamoto and coworkers [6]synthesized mesostructured hexagonal vanadium–phos-

phorus–oxide materials using alkyltrimethyl ammonium

*Corresponding author.

E-mail address: vadim.guliants@uc.edu (V.V. Guliants).

1387-1811/$ - see front matter � 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2004.03.019

surfactants (C12–C16). Doi and Miyake [7] reported a

hexagonal mesostructured VPO phase obtained fromthe VPO catalyst precursor, VOHPO4 Æ 0.5H2O, by sur-

factant intercalation and a subsequent hydrothermal

treatment. Amoros and coworkers [8] described the

synthesis of novel hexagonal-type mesostructured ox-

ovanadium phosphates (ICMUV-2). Mizuno et al. [9]

obtained hexagonal, cubic and lamellar mesostructured

vanadium–phosphorus–oxides. Carreon and Guliants

[10] studied the effect of the surfactant nature, chainlength and concentration in the formation of hexagonal,

cubic and lamellar mesostructured VPO phases.

The vanadium–phosphorus–oxide (VPO) system is

characterized by facile formation and interconversion of

a number of crystalline and amorphous VIII, VIV and VV

phosphates [11]. Several crystallographic phases were

previously reported in the bulk catalytic vanadium–

phosphorus–oxide system: a-, b- and c-VOPO4, VO(HPO4) Æ 0.5H2O, VO(HPO4) Æ 4H2O, VO(H2PO4)2,

VO(PO3)2 and b- and c-(VO)2P2O7 [12,13]. Phase

transformations in these dense phases have been studied

extensively and recently reviewed by Xue and Schrader

[14]. These phase transformations proceed via (i) ther-

mal treatment in N2 or air; (ii) reduction in n-butane and

58 M.A. Carreon et al. / Microporous and Mesoporous Materials 71 (2004) 57–63

(iii) thermal transformation in the H2O/air or H2O/N2

environment.

However, little information exists in the literature

concerning phase transformations in mesostructured

transition metal oxides. Recently, we reported the firstexample of mesostructured VPO phases that underwent

phase transformations to various mesostructured and

dense VPO phases depending on the post-synthesis

treatment [15]. Phase transformations of mesostructured

VPO were observed during ethanol Soxhlet extraction

and thermal treatment in N2. In this communication we

follow these mesophase transformations occurring dur-

ing thermal treatment in nitrogen by XRD and in situRaman spectroscopy.

1 2 3 4 5 6 7 8 9 102 theta

Inte

nsity

(a.u

.)

001

001

003002

100

100002

200110

100

x 10

a

c

b

d

Fig. 1. XRD patterns of mesostructured VPO during the lamellar-to-

hexagonal phase transformation: (a) as-synthesized lamellar, and

thermally treated in N2 at (b) 200 �C (lamellar + hexagonal), (c) 300 �C(disordered hexagonal) and (d) 400 �C (hexagonal).

2. Experimental

2.1. Reagents

VOSO4 (Aldrich), H3PO3 and H3PO4 (85%) (FisherChemicals) were used as the VPO sources. The pH of the

aqueous solutions was adjusted with NH4OH and HCl

(Fisher Chemicals). Cationic, anionic and alkylamine

surfactants were employed in the synthesis: alkyltrim-

ethyl ammonium bromides, CH3(CH2)nN(CH3)3Br,

n ¼ 11; 13; 15; 17 (Aldrich); monododecyl phosphate,

CH3(CH2)11OPO(OH)2 (Lancaster), sodium hexade-

cane sulphonate, CH3(CH2)15SO3Na (Lancaster),dodecyl sodium sulfate salt, CH3(CH2)11OSO3Na (Al-

drich) and primary alkylamines, CH3(CH2)nNH2,

n ¼ 11,15,17 (Lancaster).

2.2. Synthesis

Mesostructured VPO phases were prepared under

mild conditions by reacting aqueous solutions contain-ing vanadium (VOSO4) and phosphorus (H3PO3 or

H3PO4) sources in the presence of surfactants as de-

scribed elsewhere [10,15]. Typical synthesis composi-

tions were (on molar basis): H2O/surfactant¼ 300–525,surfactant/V¼ 0.6–1.1. The P/V molar ratio was kept

constant at �1.1. The as-prepared mesophases were

thermally treated in nitrogen at 400 �C for 6 h (3 �C/min). Inert nitrogen environment was chosen in order toavoid overoxidation of the mesostructured and bulk

V(IV) phosphorus oxide phases [10,15].

2.3. Characterization

Powder X-ray diffraction (XRD) patterns were re-

corded on a Siemens D-500 spectrometer using Cu Karadiation with a step size of 0.02�/s. Raman spectra werecollected on a Renishaw Micro-Raman System 1000.

The samples were excited with the 514 nm Ar line in an

in situ treatment cell (Linkam TS-1500), which allows

temperature treatments under flowing gases. The CHN

elemental analyses were carried out by Robertson Mi-

crolit Laboratories, Inc., Madison, NJ. The TGA data

were collected on a Perkin–Elmer TGA7 thermogravi-

metric analyzer under nitrogen flow at 400 �C at 5 �C/min temperature ramp. The average oxidation state of

vanadium was determined by the double titration

method [10].

3. Results and discussion

3.1. Lamellar-to-hexagonal phase transformation

A pure lamellar VPO mesophase was prepared as

described elsewhere [10]. CH3(CH2)15N(CH3)3Br was

used as the cationic surfactant. The XRD patterns for

the as-synthesized sample and samples thermally treated

under nitrogen atmosphere at different temperatures are

shown in Fig. 1. The XRD pattern of the as-synthesized

sample shows the characteristic (0 0 1), (0 0 2) and (0 0 3)reflections of the lamellar phase, with average unit cell

parameter c ¼ 24:7� 0:08 �A. After treating the as-syn-thesized sample at 200 �C in nitrogen for 6 h, XRD

shows the presence of an extra reflection at �44 �Að2h � 2Þ which was tentatively assigned to the (1 0 0)

reflection of the hexagonal phase. Two strong reflections

corresponding to the (0 0 1) and (0 0 2) planes of the

lamellar phase were also present, indicating that afterthermal treatment at 200 �C for 6 h a mixture of lamellarand hexagonal phases was obtained. The (0 0 1) and

(0 0 2) reflections of the lamellar phase shifted to smaller

M.A. Carreon et al. / Microporous and Mesoporous Materials 71 (2004) 57–63 59

d-spacings with increasing temperature due to removalof water prior to formation of the hexagonal phase.

Further thermal treatment in nitrogen at 300 �C leads tothe disappearance of the lamellar phase and the

appearance of a broad reflection at �40 �A (2h � 2:2)which probably corresponds to the (1 0 0) plane of a

disordered hexagonal phase with unit cell parameter

a � 46 �A. There is a decrease of �4 �A in the d-spacing ofthe hexagonal (1 0 0) reflection of this sample as com-

pared to the sample treated at 200 �C, indicating pro-gressive surfactant removal and shrinkage of the VPO

framework at higher temperature. Finally, thermal

treatment in nitrogen at 400 �C leads to a completetransformation to the hexagonal mesostructure. This

sample shows the presence of reflections corresponding

to the (1 0 0), (1 1 0) and (2 0 0) planes of the hexagonal

phase with an average unit cell parameter a ¼ 38:3�0:33 �A.The Raman spectra of various crystalline VPO phases

have been reported and discussed by Volta and

coworkers [12] and Guliants et al. [16]. The Ramanspectrum of the lamellar mesophase at 50 �C (Fig. 2)

showed the bands at 790 cm�1 corresponding to the

symmetric P–O–P stretch, at 1070 and 1130 cm�1 cor-responding to the V–O–P stretches and 1300 cm�1 cor-responding to the P–O stretch of the (PO2)n chains. The

same bands were present at 100 �C. However, the 1130cm�1 band corresponding to the V–O–P stretch band

was more intense. These Raman bands underwent sig-nificant changes at 150–200 �C and were no longer vis-

ible above 200 �C, which coincided with an onset of themesophase transformation detected by XRD. The

characteristic C–H and C–C bands of the surfactant

10050090013001700210025002900

Raman shift (cm-1)

FRESH

100ºC

150ºC

300ºC

400ºC

C-C

P-O-P

C-H

P-O

V-O-P

V-O-P

200ºC

350ºC

250ºC

Fig. 2. In situ Raman spectra of mesostructured VPO during the

lamellar-to-hexagonal phase transformation in nitrogen atmosphere.

were present at �2900 and �1450 cm�1, respectively.The Raman bands at �2900 cm�1 were not observed fortemperatures above 150 �C, indicative of progressivesurfactant decomposition. This observation is in agree-

ment with the appearance of a broad Raman band near1600 cm�1, characteristic of the sp2 carbon species [17–21]. Two new Raman bands near 1590 and 1350 cm�1

became evident as the treatment temperature increased.

These bands most likely correspond to the sp2 carbon in

graphitic materials [22–24]. The Raman band at 1590

cm�1 corresponds to the E2g C–C stretching mode,

which is the only possible band in single crystal graphite

[22]. The Raman band at 1355 cm�1 assigned to the A1g

mode [22] is indicative of defects in the graphite struc-

ture. The position of this band depends on the nature of

the carbon material [21]. Tuinstra and Koenig showed a

linear correlation between 1000/La (where La is the

crystal size in �A) and the relative Raman intensity of thegraphitic bands at 1350 and 1590 cm�1 [22]. It is

therefore possible to estimate the size of graphitic

deposits (�58 �A) formed at 400 �C. Apparently, anenhancement of the Raman band intensity at �1350cm�1 indicates increased structural disorder of graphiticmaterials [25]. The second-order Raman spectra of

carbonaceous solids may display various bands

depending on their long-range order. The highly ordered

solids, such as graphite, exhibit two Raman bands at

2700–3200 cm�1 [18,21,26]. The so-called G band ob-

served at �2660 cm�1 has two components [21,26],whose relative positions are structure-sensitive. How-

ever, disordered materials either exhibit poorly defined

second-order Raman bands or lack them altogether

[21,26]. The carbon deposits generated from occluded

surfactant molecules in the present study are structur-

ally disordered, which is confirmed by the absence of

the second-order Raman bands in the corresponding

spectra.The CHN analysis results and the VPO mesophases

observed for the lamellar-to-hexagonal transformation

are shown in Table 1. The carbon content decreases

significantly at 300 and 400 �C leading to structural

rearrangement at the inorganic–organic interface and

the observed lamellar-to-hexagonal transformation. The

TGA analysis in nitrogen for the as-synthesized lamellar

phase is shown in Fig. 3. In the 25–200 �C region, thereis a �4 wt.% loss due to desorption of water, followed

by the decomposition of surfactant (�32 wt.%) at 200–400 �C. Therefore, the cationic surfactant remains

essentially intact at temperatures below 200 �C. This issupported by the results of the CHN analysis which

indicated only a slight decrease in carbon content from

44.2 at room temperature to 43.9 wt.% at 200 �C. Somechanges in the Raman bands of the hydrocarbon speciesare evident at 150 �C indicative of the early stages of

surfactant decomposition, which starts via hydrogen

loss from the surfactant and formation of unsaturated

Table 1

CHN analysis of mesostructured VPO for lamellar-to-hexagonal phase transformation. (weight percent)

Sample ID C H N C/Na C/Nb H/Cb Mesophase

As-synthesized 44.2 8.8 2.7 16.4 19.4 2.4 Lamellar

Activated N2/200 �C 43.9 8.9 2.6 16.9 19.4 2.4 Lamellar +hexagonal

Activated N2/300 �C 19.4 2.8 1.5 12.9 15.5 1.7 Disordered hexagonal

Activated N2/400 �C 7.6 1.6 0.9 8.4 10.3 2.5 Hexagonal

a From elemental analysis.b From TGA results.

60

65

70

75

80

85

90

95

100

0 100 200 300 400Temperature (°C)

Wei

ght l

oss

(%)

Fig. 3. TGA curve of the as-synthesized lamellar mesostructured VPO

phase under nitrogen flow at 400 �C at 5 �C/min.

60 M.A. Carreon et al. / Microporous and Mesoporous Materials 71 (2004) 57–63

hydrocarbon species. However, the surfactant decom-

position is very minor below 200 �C because the Raman

technique is �50 times more sensitive to the presence ofunsaturated sp2 carbons than that of sp3 carbons when

excited with the 514 nm radiation [27]. By considering

the weight losses from TGA (Fig. 3), we estimated thenumber of moles of each species (C,N,H) to evaluate C/

N and H/C ratios for the as-synthesized, 200, 300 and

400 �C nitrogen heat treated samples as shown in Table

1. The observed numbers for the as-synthesized sample

(C=N ¼ 19:4 and H=C ¼ 2:4) are very close to the

theoretical value of C=N ¼ 19:0 and H=C ¼ 2:2,respectively. The average vanadium oxidation state for

all as-synthesized and nitrogen thermal treated samplesdetermined by a double titration technique [10] was in

the 3.8–4.0 range.

The phase transformations in mesostructured systems

have been explained by changes in the surfactant pack-

ing parameter g used as a structural index to predict theformation of different mesophases [10,15,28–33]. It is

defined as g ¼ V =aol, where the term V is composed of

two contributions (i.e. combined volume of surfactantchains and cosolvent organic molecules located between

the chains), ao is the surfactant headgroup area at the

micelle surface and l is the surfactant tail length. Otherfactors that may be responsible for the phase transfor-

mations are structural rearrangements of the inorganic-

organic phases, changes in hydration behavior and themigration of small organic molecules [29]. Prior to the

template decomposition the surfactant packing param-

eter may be used as a valid structural index to explain

the occurrence of these mesophase transformations. Vand l change proportionally and their ratio has been

shown to be constant V =l ¼ 21 �A2 for a single surfac-

tant headgroup [33]. Therefore, the surfactant head-

group area at the micelle surface, ao is expected to definethe surfactant packing parameter values. In order to

elucidate the effect of surfactant packing parameter in

the lamellar-to-hexagonal phase transformation, the as-

synthesized lamellar sample was thermally treated in

nitrogen at 180 �C, which is slightly below the onset of

surfactant decomposition at 200 �C. The XRD pattern

(not shown here) is similar to that at 200 �C (Fig. 1b).

There is an increase in the d-spacing from �30 to 44 �Aafter the nitrogen heat treatment. Within liquid crys-

talline mesophases, the d-spacing provides a way to

measure indirectly the area per head group ao [34]. Astemperature increases and hydroxyl groups at the inor-

ganic–organic interface are gradually removed, deinter-

calation of the layered mesostructure most likely takes

place. At this point, the hydrophobicity of the charged

headgroups increases and therefore the repulsion be-tween them, pushing the headgroups further apart and

leading to an increase in the surfactant headgroup area

at the interface ao and to a decrease in the surfactantpacking parameter g. This will favor the formation ofmore surface curvature mesostructures (i.e. rod-like

hexagonal mesophase). Also, as temperature increases,

the extent of the inorganic framework condensation

increases. Therefore, the charge density of the VPOframework will change, and in order to maintain charge

balance at the interface, the surfactant will pack into a

mesophase with a higher curvature surface such that

observed in the hexagonal mesophase. The coexistence

of the lamellar and hexagonal phases during the heat

treatment suggests that the transformation takes place

in the solid phase as proposed previously for the silicate

system [29].The proposed mechanism for the lamellar-to-hexag-

onal transformation is shown in Fig. 4. When the

H2O

H2O 200˚C

400˚C

300˚C LAMELLAR

g=1

HEXAGONAL g=1/2

L+H g<1

DISORDERED HEX g<1

Fig. 4. Proposed mechanism for the lamellar-to-hexagonal phase

transformation.

02 4 6 8 10

2 theta

Inte

nsity

(a.u

.)100

110 200

100

a

b

Fig. 5. XRD patterns of mesostructured VPO during the hexagonal-to-

disordered phase transformation: (a) as-synthesized hexagonal, (b)

disordered mesostructure after thermal treatment in N2 at 400 �C.

10050090013001700210025002900

Raman shift (cm-1)

FRESH

200ºC

400ºC

300ºC

C-H

C-C

100ºC

150ºC

250ºC

350ºC

Fig. 6. In situ Raman spectra of mesostructured VPO during the

hexagonal-to-disordered phase transformation in nitrogen atmosphere.

M.A. Carreon et al. / Microporous and Mesoporous Materials 71 (2004) 57–63 61

lamellar mesophase is heated to 200 �C in N2, the

intercalated water is removed gradually leading to in-

creased repulsion of surfactant headgroups at the sur-

face of vanadium phosphate layers and a partial

surfactant deintercalation. The interlayer spacing starts

to shrink as supported by the changes in the lowest d-spacing observed for the sample at 200 �C in N2 ascompared to the as-synthesized sample (Fig. 1). Sur-

factant deintercalation, in turn, effectively increases the

surfactant headgroup area at the layer surface, ao, andreduces the surfactant packing parameter, g, whicheventually triggers the phase transformation to a lower ghexagonal phase. Further thermal treatment at 300 �C inN2 leads to the template decomposition and a more

extensive structural rearrangement of the inorganic VPOframework into a disordered hexagonal mesostructure.

Finally, at 400 �C in N2 a hexagonal phase with im-

proved structural order is observed which may be

attributed to the enhanced condensation and cross-

linking of the VPO framework. Some residual template

corresponding to �17% of the original carbon present in

the as-synthesized sample remains occluded in the mes-

opores as suggested by the CHN analysis, probably in aform of graphitic deposits observed by Raman spec-

troscopy.

3.2. Hexagonal-to-disordered phase transformation

A hexagonal mesophase was prepared using CH3-

(CH2)11NH2 as the surfactant [10]. This phase showed

three strong reflections at 34 �A (2h ¼ 2:6), 21.5 �A(2h ¼ 4:1) and 16.6 �A (2h ¼ 5:3) corresponding to the(1 0 0), (1 1 0) and (2 0 0) planes of the 2D hexagonal

structure with an average unit cell parameter

a ¼ 40:2� 2:5 �A. After the thermal treatment in nitro-gen at 200–400 �C the samples showed only one intense

XRD peak (Fig. 5) with a unit cell parameter a � 38 �A.The disappearance of the (1 1 0) and (2 0 0) reflections

suggested the increase of structural disorder. The d-spacings decreased as the temperature increased due to

the shrinkage of the inorganic framework. In this case,

the heat treatment induced changes in the hydration

behavior in the inorganic–organic interface distorting

the hexagonal mesophase and leading to a more disor-

dered mesostructure.The Raman bands corresponding to the VPO bonds

(the symmetric P–O–P stretching band and P–O stretch

of the (PO2)n chains) observed in the Raman spectra of

the lamellar phase (Fig. 2) were not observed in the

spectra of the hexagonal phase (Fig. 6). Only the char-

acteristic C–H and C–C vibrations corresponding to the

occluded surfactant were present at �2900 and �1450

400°C

350°C

300°C

C-H

62 M.A. Carreon et al. / Microporous and Mesoporous Materials 71 (2004) 57–63

cm�1, respectively. The C–H Raman bands at �2900cm�1 were not observed at temperatures above 150 �Cdue to surfactant decomposition and formation of oc-

cluded graphitic species. The changes in the Raman

spectra reflected a transformation of the sp3 to unsatu-rated sp2 carbon species, also observed during the phase

transformation of the lamellar phase (Fig. 2).

100

150°C

FRESH

50090013001700210025002900

Raman shift (cm-1)

C-C

P-O V-O-P

V-O

P-O-P

Fig. 8. In situ Raman spectra of mesostructured VPO during the cubic-

to-amorphous phase transformation in nitrogen atmosphere.

3.3. Cubic-to-amorphous phase transformation

A pure cubic mesophase was prepared employing the

CH3(CH2)15SO3Na surfactant as described elsewhere

[10]. The sample shows the presence of four strongreflections at 40.6 �A (2h ¼ 2:2), 35.3 �A (2h ¼ 2:5), 20.6�A (2h ¼ 4:3) and 13.7 �A (2h ¼ 6:4), corresponding to the(2 1 1), (2 2 0), (4 0 0) and (6 2 0) planes of the cubic

mesophase with an average unit cell parameter a ¼88:9� 7:3 �A (Fig. 7). After thermal treatment in nitro-

gen at 200–400 �C the sample was XRD-amorphous

indicating the collapse of the mesostructure and the

limited thermal stability of the cubic mesophase.The Raman spectrum of the cubic phase shown in

Fig. 8 exhibited all the Raman bands previously ob-

served in the Raman spectra of the lamellar phase (Fig.

2): at 790 cm�1 (the symmetric P–O–P stretching band);1070 and 1130 cm�1 (the V–O–P stretch); and at 1300

cm�1 (the P–O stretch of the (PO2)n chains). One addi-

tional band appeared in the spectrum of the cubic phase

at 1000 cm�1, and has been assigned to the V–O stretchof the V@O bond [16]. As temperature increases, the

structural order was progressively lost leading to an

amorphous material as indicated by the XRD and

Raman. This thermal treatment also led to a similar

1 2 3 4 5 6 7 8 9 102 theta

Inte

nsity

(a.u

.)

211

400220 620

a

b

Fig. 7. XRD patterns of mesostructured VPO during the cubic-to-

amorphous phase transformation: (a) as-synthesized hexagonal, (b)

amorphous after thermal treatment in N2 at 400 �C.

transformation of the surfactant carbon sp3 into the sp2

graphitic species.

4. Conclusions

The phase transformations observed for mesostruc-

tured VPO phases in the present study complement

those previously observed for the dense VPO phases.

Mesostructured vanadium–phosphorus–oxides (VPO)

underwent phase transformations to various meso-

structured and amorphous VPO phases during thermal

treatment in N2 at 400 �C. These mesophase transfor-mations were followed by XRD, CHN, TGA and in situ

Raman spectroscopy. XRD detected the long-range

structural changes in the mesostructured VPO phases

during these phase transformations. Raman spectros-

copy is sensitive to the short-range order in mixed metal

oxides and provided complementary information on the

nature of the local structural changes in the V–O–P

framework and surfactant molecules during thesetransformations. The observed phase transformations

were discussed in terms of changes in the surfactant

packing parameter, hydration behavior and structural

rearrangements of the organic–inorganic mesophases

during nitrogen heat treatment.

Acknowledgements

V.V. Guliants acknowledges the National Science

Foundation for the NSF Career Award (CTS-0238962).

M.A. Carreon thanks CONACYT (Mexico) for the

M.A. Carreon et al. / Microporous and Mesoporous Materials 71 (2004) 57–63 63

scholarship award. M.A. Ba~nares acknowledges fundingfrom CICYT, Spain (IN96-0053) for the acquisition of

the Raman system.

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