Journal of Non-Crystalline Solids 356 (2010) 87–92

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Journal of Non-Crystalline Solids

journal homepage: www.elsevier .com/ locate/ jnoncrysol

Synthesis and characterization of tin containing molybdophosphate andtungstophosphate glasses

S. Chenu a, J. Rocherullé a,*, R. Lebullenger a, O. Merdrignac a, F. Cheviré a, F. Tessier a, H. Oudadesse b

a UMR 6226 Sciences Chimiques, Equipe Verres et Céramiques, Campus de Beaulieu, 35042 Rennes, Franceb UMR 6226 Sciences Chimiques, Equipe Chimie du Solide et des Matériaux, Campus de Beaulieu, 35042 Rennes, France

a r t i c l e i n f o

Article history:Received 6 November 2008Received in revised form 24 September2009

Keywords:CrystallizationGlass–ceramicsX-ray diffraction

0022-3093/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.jnoncrysol.2009.11.001

* Corresponding author.E-mail address: Jean.rocherulle@univ-rennes1.fr (J

a b s t r a c t

Two series of tin containing molybdophosphate and tungstophosphate glasses and glass–ceramics wereachieved by means of a domestic microwave oven under air. During melting, a redox reaction takes placebetween Sn2+ and Mo6+ generating the crystallization of a Na(SnIV,MoIV)(PO4)3 solid solution with a NZP(NaZr2P3O12) type structure. Such reactivity was not underlined in the case of the W-series. All the sam-ples were characterized from a thermal and mechanical point of view as a function of the RO3 (R = Mo, W)for SnO substitution. Two types of behaviors were identified. For the Mo-series, all the characteristics,except density, present an extremum value for the chemical composition with a 1:1 SnO–MoO3 molarratio. This is strongly correlated to the amount of NZP crystals present in the glass ceramic, the differentbehavior observed for the density being due to the low compactness of the NZP phase. For the W-series,all these different characteristics varies monotonously according to a progressive strengthening of thenetwork by replacement of a low field strength ion (Sn2+) by a higher field strength ion (W6+). In addition,the solid state reactivity of a 1:1 SnO–RO3 mixture was examined confirming the absence of any redoxprocess between SnO and WO3 during the glass synthesis.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction quence of a small degree oxygen loss during the preparation of

Transition metal oxides like MoO3 and WO3 are known to formglasses with P2O5 in a wide range of composition. However theaddition of alkali oxides considerably enhances the glass formingability and decreases the glass transition temperature. Based onseveral studies conducted on the ternary composition diagramsRO3—R02O—P2O5 (R = Mo, W and R0 = Li, Na, K), a structural modelhas been proposed and the chemistry of these glasses have beeninvestigated in detail and discussed in terms of structural entities[1,2]. Furthermore, evidence was shown of the coexistence of dif-ferent oxidation states for the transition metal (i.e. R5+, R6+). As aconsequence, a cross-over from a predominantly electronic con-ductivity to a mixed ionic-electronic conductivity in the alkalineoxide richer-region has been mentioned [3,4].

If PbO is added to the molybdophosphate glasses instead of analkali oxide, it plays a dual structural role, both as a network for-mer up to a certain level and as a network modifier above this le-vel. This aspect has been discussed in the light of variousexperimental results [5,6]. A concomitant effect of the changed roleof Pb is on the concentration of Mo5+ in the glass and on the chan-ged predominant mechanism of conductivity. This is a conse-

ll rights reserved.

. Rocherullé).

these glasses [5]. In the presence of modifier oxides, oxygen lossis suppressed and the concentration of Mo5+ is lowered due tothe increased O2� ion activity in the melt [5].

SnO can also be added to sodium metaphosphate (NaPO3) andin the pseudo-ternary system SnO–NaPO3, glass formation occursover a wide range of composition [7]. An interesting feature of thisglass system is the existence of a redox equilibrium between Sn2+

and Sn4+, the ratio Sn4+/Sn2+ being essentially independent of thechemical composition [7]. The Na+ ion conductivity has also beenstudied as a function of the tin content [8]. It is interesting to notethat the network former – network modifier roles of Pb2+ and Sn2+

are opposite to each other. At lower concentrations, SnO acts as amodifier while PbO acts as a network former and at higher concen-trations, the roles are opposite.

In this paper, we present our investigations conducted on a glassseries synthesized by means of a microwave heating furnace. Thechemical compositions can be described by the following formula:

80NaPO3—ð20� xÞSnO—xRO3ðR ¼Mo;WÞ:

Using different experimental techniques, we report severalthermal and physical characteristics such as glass transitiontemperature (Tg), coefficient of thermal expansion (CTE), density,Vickers microhardness (Hv) and the different elastic moduli. Thechemical durability of these glasses has been measured at 50 �C

88 S. Chenu et al. / Journal of Non-Crystalline Solids 356 (2010) 87–92

in a water medium at pH 7. We also have examined the reactivitybetween SnO and RO3 by means of X-ray powder diffraction (XRD)and simultaneous thermogravimetry and differential thermal anal-ysis (TGA–DTA).

350

450

550

650

750

850

950

1050

1150

0 60 120 180 240 300 360Time (s)

Tem

pera

ture

(°C

)

80NaPO3 - 10SnO - 10RO3

100NaPO3

Fig. 1. Melting temperature profile as a function of heating time for typical batches.

2. Experimental

Glasses have been elaborated by a simple microwave meltingmethod [9–11]. The microwave route to synthesis offers severaladvantages over conventional methods, the foremost of whichare the very short time scales required for the preparation andthe homogeneity of products. A critical requirement is the couplingof at least one of the reactants to the microwave field to initiateand drive the melting reaction. It has been reported that NaH2-

PO4�2H2O has a great microwave-absorbing ability [12]. In a typicalexperiment, required quantities of starting materials, NaH2-

PO4�2H2O (Acros Organics), SnO (99.9% Alfa Aesar), WO3 (99+% Al-drich) and MoO3 (99+% Acros Organics), are placed in a silicacrucible and exposed to microwave irradiation up to 10 min in adomestic microwave oven (Kerwave�, 2.45 GHz, 750 W nominalpower). Within 3 min of exposure, the hydrated phosphate decom-poses completely to NaPO3 increasing the temperature of the batch[13]. A homogeneous melt is obtained after 7 min [14], then themelt is poured between two stainless steel plates and the glass isannealed for one hour at Tg to relieve residual stress before coolingdown slowly to room temperature. During the glass synthesis, anoptical pyrometer (IGA 140 Impac�) is used to monitor the temper-ature of the melt through a hole drilled on the top of the micro-wave oven cavity. The samples were stored in a desiccator andtaken out only during the time of experimental studies.

The amorphous or crystalline character of the sample waschecked by X-ray powder diffraction (XRD). Data were collectedat room temperature with a PHILIPS diffractometer (Model PW1830) using CuKa radiation (Ka1 = 1.5406 Å, Ka2 = 1.5444 Å) se-lected with a diffracted-beam graphite monochromator and theBragg–Brentano optics. The 10�–70� (2h) angular range wasscanned in 0.02� (2h) steps with a counting time set at 1 s step�1.The ICDD PDF database, available in the program search/matchfrom the PC software package X’PERT supplied by PHILIPS, wasinterrogated in order to reveal possible crystallization.

The glass transition temperature was determined from DTAexperiments conducted at 10 �C min�1 and performed with a TAinstruments SDT 2960. The overall accuracy of this instrument isexpected to be within ±2 �C. The coefficient of thermal expansion(CTE) was calculated from a thermomechanical experiment (TAInstruments� Model TMA 2940) performed with a 5 �C min�1 heat-ing rate and a preload force of 0.1 N. The precision of the CTE deter-mination is ±2 � 10�7 K�1 in the 50–250 �C range.

The density of the samples was measured at room temperatureusing Archimedes method with absolute alcohol as immersionfluid. The error in the determination of the density is ±0.02 g cm�3.

Young’s modulus (E), bulk modulus (G) and shear modulus (K)were calculated from experiments using the ultrasonic velocitytechnique. This technique is based on time-of-flight measurementsusing the pulse-echo technique [15]. Vickers microhardness mea-surements have been carried out on Matsuzama� MXT 70 equip-ment by the indentation technique using a 50 g load applied for 5 s.

The chemical durability was evaluated from the weight loss ofbulk samples immersed in water at 50 �C. A constant ratio samplesurface/solution volume of 0.1 cm�1 was maintained in order tocompare the results. The faces of the samples were polished with1200 grit SiC paper and cleaned in alcohol. After each corrosiontest, the samples were dried at 100 �C until the weight remainedconstant. The dissolution rate (DR) of samples has been calculated

from the measured weight loss (DW), sample surface area (SA) andtime of immersion (t), using the following equation:

DR ¼ DWðgÞSAðcmÞ2 � tðminÞ

: ð1Þ

The reactivity between SnO and the transition metal oxides hasbeen studied by monitoring the weight of the reactants in equimo-lecular proportion as a function of the temperature up to 850 �C.The experiments have been conducted by means of a simultaneousDTA–TGA device (TA instruments SDT 2960) under static air andunder a nitrogen flow (70 ml min�1). A heating rate of 10 �C min�1

has been employed. The accuracy of the balance is expected to bebetter than 0.5 lg. In order to avoid any surface or mass effect, thepowder size of the reactants and the weight sample of the batcheswere remained constant, 50–100 lm and 50 mg, respectively.

3. Results

3.1. Synthesis

Fig. 1 shows a typical heating temperature profile obtained dur-ing synthesis. For illustration, we added the temperature profileobtained for a pure sodium metaphosphate glass, the weight ofthe batch, the crucible and its position inside the oven remainingthe same.

The rate of heating of the material depends on the absorptionenergy, which depends on the dielectric constant of the material.The dependence of dielectric constant on temperature varies fordifferent materials; some materials display significant increasewith increasing temperature leading to a thermal runaway. Obvi-ously, the maximum temperature also varies with the chemicalcomposition of the batch. Nevertheless, SnO, MoO3 and WO3 aregood microwave absorbers and the melting temperatures are al-ways close to 1000 �C for all the glasses studied in this work.

3.2. Physical characterization

Fig. 2 depicts two typical XRD patterns which have been ob-tained after the synthesis. All the W containing samples are glassymaterials and strictly amorphous. On the contrary, all the Mo con-taining compositions crystallize during melting, giving two-phase(glass and crystals) materials. By interrogation of the ICDD–PDF2database, the search-match fits well with both NaSnIV

2 ðPO4Þ3 (ICDD49-1198) and NaMoIV

2 ðPO4Þ3 (ICCD 84-1836). These compoundshave many structural similarities and belong to the NZP type struc-ture family for which NaZr2(PO4)3 is the most representative mem-ber. This allows us to assume that a solid solution exits, where

10 20 30 40 50 60 702 theta/degree

Rel

ativ

e in

tens

ity(110)

(104)

Fig. 2. XRD patterns obtained for 80NaPO3–9SnO–11RO3 with R = W (bottom) andR = Mo (top).

S. Chenu et al. / Journal of Non-Crystalline Solids 356 (2010) 87–92 89

Mo4+ can substitute Sn4+ in the tin coordination polyhedra, accord-ing to the following formula:

NaðSnIV2�xMoIV

x ÞðPO4Þ3 with0 6 x 6 2:

From a chemical point of view, it is required a redox reaction be-tween Sn2+ and Mo6+ in the melt according to:

Sn2þ þMo6þ ! Sn4þ þMo4þ: ð2Þ

This implies a complete reaction for the same proportions of thereactants and as a consequence, a maximum of the NZP molar frac-tion in the melt. This assumption has been verified by XRD analysisperformed on samples with various compositions. For these exper-iments, the counting time was increased up to 10 s step�1 in orderto improve the counting statistics. Fig. 3 shows the intensities oftwo diffraction peaks corresponding to the (1 1 0) and (1 0 4)reflections associated to the highest values of the relative intensity,100 and 88, respectively as reported in 49-1198 ICDD card. There isa clear evidence for a maximum in the NZP molar fraction occur-ring for an equimolecular mixture of the reactants. In this particu-lar case, because of the important quantity of crystalline phase, itwas impossible to remove completely after melting the two-phasematerial from the silica crucible.

Fig. 4 shows the evolution of different physical characteristicsas a function of the RO3 content. There is a different behavior be-tween the two series. For the Mo containing samples, all the char-acteristics present an extremum value for a 1:1 SnO to MoO3 molarratio except for the density which slightly decreases. This behavioris confirmed by the values of the different elastic moduli and of thedissolution rates which are summarized in Table 1. For example,the increase of the Young’s modulus is of 25% and the chemicaldurability of the 80NaPO3–9SnO–11MoO3 composition is 6000

0

10000

20000

30000

40000

50000

0 2 4 6 8 10 12 14 16 18 20% MoO3

Peak

s in

tens

ity (c

ount

s)

(110)

(104)

Fig. 3. Intensity of the (1 1 0) and (1 0 4) diffraction peaks for 80NaPO3–(20�x)SnO–xMoO3 samples.

times more important than those of the 80NaPO3–20SnO basedglass.

On the contrary, all these characteristics vary monotonously forthe W containing glasses. According to Fig. 4 (data in filled square)the SnO for WO3 substitution yields to an increase of the glass tran-sition temperature, the density and the Vickers microhardnesswhile the CTE decreases.

3.3. Redox behavior

In order to clarify the differences observed between the two ser-ies, the solid state reactivity between SnO and the metal oxides,both under static air and under a nitrogen flow, has been studiedfrom DTA–TGA experiments. Furthermore, phase identificationhas been conducted by X-ray powder diffraction analysis.

3.3.1. Reactivity of tin oxideThe reactivity of SnO is depicted in Fig. 5. Under air, the TGA sig-

nal increases from 500 �C and reveals a maximum experimentalweight change (+11%) corresponding to a complete oxidation oftin, according to the following reaction:

SnOþ 12

O2 ! SnO2; ð3Þ

with a theoretical weight change of +11.9%. The DTA signal showstwo maxima at 520 and 600 �C which correspond to the two turningpoints deduced from the shape of the TGA derivative signal.

Under a nitrogen flow, no weight change or thermal event aredetected up to 750 �C. Nevertheless, a slight increase in weight athigher temperature can be observed on the TGA signal. This isdue to the presence of some residual oxygen gas in the furnace. Ta-ble 2 summarizes the result of the phase identification. Under air,SnO2 (ICDD 21-1250) is observed only, confirming the tin oxida-tion, while SnO remains the main crystalline phase under thenitrogen flow. However, the presence of a small amount of SnO2

is also detected.

3.3.2. SnO and MoO3

In a second step, we have studied in the same conditions anequimolecular batch of SnO and MoO3. These two experimentsare presented in Fig. 6(a).

Under air, the DTA signal exhibits two exothermic peaks at520 �C and 640 �C and one endothermic at 800 �C. If the last oneis undoubtedly assigned to the melting of MoO3, the exothermicpeaks correspond to the oxidation of SnO by the atmospheric oxy-gen. However, the second peak, observed at 600 �C in Fig. 5, isshifted towards higher temperatures (i.e. 640 �C). Considering theTGA signal, from room temperature up to 720 �C, there is a slightincrease in the weight sample (�4%) with a temperature onsetclose to 500 �C. Furthermore, an important weight loss occurs from720 �C.

All these remarks are consistent with the oxidation of SnO byatmospheric oxygen. In this case, a theoretical weight change of+5.7% is expected for an equimolecular mixture of the oxides be-cause MoO3 is not involved in the reaction.

The weight loss occurring from 720 �C is due to the beginning ofthe MoO3 volatilization which takes place during melting(Tm = 800 �C) as established by the DTA signal. The XRD analysis(see Table 2) only reveals the presence of two crystalline phases:SnO2 and a solid solution, formed during the melting of molybde-num oxide, with the 2MoO3–1SnO2 formulation (ICDD 36-1240).

Under a nitrogen flow, no weight change or thermal event, suchas the melting of MoO3, has been detected up to 850 �C. Moreover,the XRD analysis only shows the presence of SnO2 and MoO2 (ICDD32-0671). These results are consistent with a redox reaction withno weight change according to:

250

270

290

310

330

350

370

390

0 5 10 15 20% RO3

Gla

ss tr

ansi

tion

tem

pera

ture

(°C

)

2.7

2.8

2.9

3

3.1

3.2

3.3

0 5 10 15 20% RO3

Den

sity

(g.c

m-3

)175

185

195

205

215

225

0 5 10 15 20

% RO3

CTE

(10-7

°C-1)

210

230

250

270

290

310

0 5 10 15 20% RO3

Vick

ers

mic

roha

rdne

ss (k

g.m

m-2

)

Fig. 4. Physical characteristics of 80NaPO3–(20�x)SnO–RO3 system (R = W data in filled square – Mo data in empty triangle).

300 400 500 600 700 800Temperature (°C)

Del

ta T

(a.u

.) (e

xo

)

-2

0

2

4

6

8

10

12

Wei

ght c

hang

e (%

)

airnitrogen

Fig. 5. DTA and TGA curves of SnO under air or nitrogen.

90 S. Chenu et al. / Journal of Non-Crystalline Solids 356 (2010) 87–92

SnOþMoO3 ! SnO2 þMoO2: ð4Þ

3.3.3. SnO and WO3

Fig. 6(b) depicts the different experiments conducted in thesame conditions described in the above section for an equimolarSnO–WO3 mixture.

Under air, oxidation occurs at 500 �C and is complete at 850 �Cwith a weight change of �4%. The DTA signal shows an exothermic

Table 1Physical characteristics (E, K, G: elastic moduli and DR: dissolution rate).

Metal oxide SnO–WO3

Composition (mol%) 20–0 10–10

E (±1 GPa) 40 46K (±1 GPa) 31 40G (±1 GPa) 15 18�log (DR) (±0.06) (DR in g cm�2 min�1) 1.89 3.82

peak centered at 520 �C with a shoulder at 550 �C approximately.The XRD analysis reveals the presence of both SnO2 and WO3.

The reactivity of the same batch is completely different under anitrogen flow as depicted by Fig. 6(b). The TGA signal reveals aslight oxidation with a weight gain inferior to 1%, while the DTAcurve presents two endothermic peaks at 670 �C and 790 �C. Table2 lists the crystalline phases observed after this experiment. SnIIW-VIO4 (ICDD 27-0903) is the major phase and SnO2 and WO3 are de-tected as very minor phases.

4. Discussion

4.1. Physical characterization

Concerning the physical characterization of the materials, the dif-ferent behavior between the two series is due to the redox reactionoccurring during the melting of the Mo containing samples. The for-mation of NZP crystals is responsible of a compositional change ofthe residual glassy phase, resulting in an increase of the viscosityand of the glass transition temperatures. In the same way, the de-crease of the coefficient of thermal expansion is clearly related tothe quantity of this crystalline phase and a minimum value isreached for the most important molar fraction of the NZP phase. Fur-thermore, the evolution of the Vickers microhardness also presentsan extremum thus confirming the general trend for which crystalli-zation enhances the mechanical properties of a glass matrix.

SnO–MoO3

0–20 20–0 9–11 0–20

44 40 51 3934 31 38 2917 15 20 154.12 1.89 5.71 2.90

Table 2Summary of XRD phase identification (with ICDD card number in parenthesis) performed after DTA–TGA analysis for different batches (M for the main crystalline phase, m for theminor phase).

SnO SnO–MoO3 SnO–WO3

Air N2 Air N2 Air N2

SnO (M) (24–1342) SnO2 (M) (21–1250) SnO2 (M) (21–1250) SnO2 (M) (21–1250) SnWO4 (M) (27–0903)

SnO2 (M) (21–1250) SnO2 (m) (21–1250)

SnO2 (m (21–1250) SnO2–2MoO3 (36–1240) (M) MoO2 (M) (32–0671) WO3 (m) (43–1035) WO3 (m) (43–1035)

S. Chenu et al. / Journal of Non-Crystalline Solids 356 (2010) 87–92 91

However, the evolution of the density is different, the valuesslightly decreasing with the increase of MoO3 content. Densitycan be considered, in first approximation, as an additive property,thus the values should decrease because of the MoO3 densitywhich is lower than of SnO, 4.70 and 6.45 g cm�3 respectively.For the same reason, the crystallization of the NZP phase doesnot increase the density because of its low structure compactness,which is in the range from 3.51 to 3.86 g cm�3. These values corre-spond to the densities of the end members of the solid solution,NaMo2(PO4)3 and NaSn2(PO4)3 respectively.

Considering the W containing samples, the monotonous varia-tions of the physical characteristics are the consequence of thestrengthening of the network by replacement of the low fieldstrength Sn2+ ion by the higher field strength W6+ ion. Moreover,Sn2+ is considered as a modifier cation up to 30 mol% in glassesfrom the NaPO3–SnO pseudo-binary system [7]. On the contrary,W6+ for which W–O bond is significantly covalent (nearly as Si–Obond) acts as an intermediate. In general, the effects of the glass

300 400 500 600 700 800Temperature (°C)

Del

ta T

(a.u

.) (e

xo

)

-30

-25

-20

-15

-10

-5

0

5

Wei

ght c

hang

e (%

)

airnitrogen

DTA

TGA

Fig. 6a. DTA and TGA curves obtained for an equimolar SnO–MoO3 mixture underair or nitrogen.

300 400 500 600 700 800Temperature (°C)

Del

ta T

(a.u

.) (e

xo

)

0

2

4

6

8

10

12

Wei

ght c

hang

e (%

)

airnitrogen

DTA

TGA

Fig. 6b. DTA and TGA curves obtained for an equimolar SnO–WO3 mixture underair or nitrogen.

composition on elastic moduli are similar to those found formicrohardness. As expected, the experimental values show thesame behavior.

4.2. Redox behavior

As regards the reactivity of tin oxide with air, the presence ofthe two maxima observed in the DTA signal does not allow us toconclude in a double step process for the SnO oxidation. In fact, thisbehavior is typical for any solid–gas reaction [16]. Atmosphericoxygen reacts first with the shell and then a diffusion processinhibits the oxidation of the core of the particle which takes placeat higher temperatures. In addition, the TGA derivative signal, notpresent in Fig. 5, has revealed the presence of two turning points atthe same temperatures giving consistency to this assumption.

As mentioned in the Section 3.2, the TGA–DTA raw data of anequimolecular SnO:MoO3 mixture give rise to a direct oxidationof tin oxide by the atmospheric oxygen. Nevertheless, this assump-tion is not consistent with the synthesis of samples from the Mo-series, because the crystallization of a NZP type phase requiresthe presence of Mo4+ only. Furthermore, the close values of theMo4+ and Sn4+ ionic radii, 69 pm and 65 pm respectively, give evi-dence for the Mo4+ for Sn4+ substitution in the MO6 octahedral sites[17]. Thus, we have to re-examine the reactivity of such a batch un-der air by considering the following two-step process:

SnOþMoO3 ! SnO2 þMoO2 ðDm=m0 ¼ 0%Þ; ð5Þ

SnO2 þMoO2 þ12

O2 ! SnO2 þMoO3 ðDm=m0 ¼ þ5:7%Þ: ð6Þ

The first step corresponds to a redox reaction between the reactantsand gives the SnIV and MoIV oxides. Then MoO2 is oxidized in MoO3

which melts and partially decomposes at 800 �C providing the crys-tallization of the phases mentioned in Table 2. The correspondingweight change is also consistent with the experimental value.

Concerning the reactivity of the equimolecular SnO:WO3 mix-ture, the TGA results are consistent with a direct oxidation of thetin oxide by the atmospheric oxygen according to the followingreaction:

SnOþWO3 þ12

O2 ! SnO2 þWO3 ðDm=m0 ¼ þ4:3%Þ; ð7Þ

and the experimental weight gain (�4%) fits well with this value.Furthermore, the two endothermic peaks are assigned to a phasetransition of SnWO4 phase at 670 �C for the first one and to themelting of these crystals at 790 �C for the second. All these resultsare in a good agreement with the literature data [18] and also withthe observations which have been made during the glass synthesisperformed under air, i.e. the absence of any redox reaction betweenthe metallic cations.

5. Conclusion

In this work, we have established the possibility to prepare tincontaining tungstophosphate glasses and tin containing molybdo-

92 S. Chenu et al. / Journal of Non-Crystalline Solids 356 (2010) 87–92

phosphate glass–ceramics by means of a domestic microwaveoven. The melting temperature of the batch, which has been mon-itored by using an optical pyrometer, varies slightly with thechemical composition but is always close to 1000 �C. Many physi-cal characteristics have been determined as a function of the RO3

(R = Mo, W) for SnO substitution and a difference in behavior hasbeen highlighted between the two series.

Considering the Mo-series, a redox reaction between Sn2+ andMo6+ occurs during melting and leads to the crystallization of aSnIV and MoIV continuous solid solution with a NZP type structure.Such a crystalline network is usually known for the interesting io-nic conduction properties it provides and also for its low thermalexpansion. The Sn/Mo molar ratio of the Na(Sn,Mo)(PO4)3 solidsolution is probably equal to 1, but further X-ray powder diffrac-tion analysis should be performed to confirm this hypothesis. Nev-ertheless, the solid state reactivity of a 1:1 SnO–MoO3 mixture,studied by simultaneous TGA–DTA experiments under air andnitrogen, has confirmed the results obtained during the glass syn-thesis, i.e. a redox reaction between the two metallic cations.

On the contrary, glasses from the W-series do not present anycrystalline phase after melting and quenching. Thus, the differentphysical characteristics always vary monotonously leading to thestrengthening of the network by replacement of the low fieldstrength Sn2+ ion by the higher field strength W6+ ion. In the sameway, the TGA–DTA experiments performed under air or nitrogenhave not revealed any redox activity between these two cations.

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