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Electrical properties of glasses in the Na2O–MoO3–P2O5 system
September 2001
Materials Letters 50 Ž2001. 308–317 www.elsevier.comrlocatermatlet
Electrical properties of glasses in the Na 2 O–MoO 3 –P2 O5 system L. Bih a,b, M. El Omari c , J.M. Reau ´ d,) , A. Nadiri b, A. Yacoubi b, M. Haddad e a
Departement de Chimie, Faculte´ des Sciences et Techniques d’Errachidia, B.P. 509, Boutalamine, Errachidia, Morocco ´ Laboratoire de Chimie Minerale Appliquee, ´ ´ Faculte´ des Sciences de Meknes, ` B.P. 4010, Beni M’hamed, Meknes, ` Morocco c Departement de Chimie, Faculte´ des Sciences de Meknes, ´ ` B.P. 4010, Beni M’hamed, Meknes, ` Morocco d Institut de Chimie de la Matiere ` Condensee ´ de Bordeaux (ICMCB), UPR 9048 CNRAS, 87 AÕenue Dr. Albert Schweitzer, 33608 Pessac Cedex, France e Laboratoire de Spectronomie Physique, Faculte´ des Sciences de Meknes, ` B.P. 4010, Beni M’hamed, Meknes, ` Morocco
b
Received 26 July 2000; received in revised form 15 December 2000; accepted 20 December 2000
Abstract A range of coloured electronic or mixed ionic–electronic glasses has been evidenced in the Na 2 O–MoO 3 –P2 O5 system. The properties of these glasses have been studied along different composition lines corresponding either to a fixed Na 2 O content or a constant MorP ratio. An EPR spectroscopy investigation of these glasses has allowed to determine the Mo 5q ion percentages in these materials. The electrical properties of these glasses have been studied by impedance spectroscopy, and the electronic and ionic contributions have been evaluated. The properties of these sodium glasses have been compared with those of lithium glasses with the same compositions. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Ionic conductivity; Electronic conductivity; EPR spectroscopy; Impedance spectroscopy; Phosphate glasses
1. Introduction Glasses containing transition metal ions offer much interest because of their potential applications in electrochemical, electronic and electro-optical devices w1–4x. Selecting molybdenum oxide as transition metal oxide, binary MoO 3 –P2 O5 glasses were proved stable over a wide range of compositions w5x, and consequently, the addition of alkaline oxides has given rise to the existence of very wide glassy ranges in the A 2 O–MoO 3 –P2 O5 ŽA s Li, Na. systems w6,7x.
) Corresponding author. Tel.: q33-556-846-332; fax: q33-556846-634. E-mail address: [email protected] ŽJ.M. Reau ´ ..
Glasses with a high concentration of alkaline oxide are superionic conductors w8,9x, whereas those issued from mixing of former oxides only are purely electronic, with a polaronic conduction mechanism due to an exchange process among the transition metal ions of different valence states w10–13x. A recent investigation of the electrical properties of glasses in the Li 2 O–MoO 3 –P2 O5 system has allowed to show a Across overB from a predominantly electronic conductivity in the alkaline oxide poor region to a mixed ionic–electronic conductivity in the alkaline oxide-richer region w14x. The Mo 5q and Mo 6q cation percentages in the studied glasses have been evaluated, and the electronic and ionic contributions to the total conductivity have been determined. The existence of a polaronic hopping conduction
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 2 4 5 - 2
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mechanism between Mo 5q and Mo 6q cations has been suggested in the purely electronic conductor glasses, and an ion–polaron soft coupling has been proposed in the mixed ionic–electronic conductor glasses w15x. The purpose of the present paper is the investigation of electrical properties of various glassy compositions in the Na 2 O–MoO 3 –P2 O5 system and the establishment of correlations between the glassy structure and the electrical properties. A comparative study of different properties of sodium glasses with those of lithium glasses with the same compositions is reported.
2. Experimental The glasses of the Na 2 O–MoO 3 –P2 O5 ternary system were prepared, as usual w7x, by mixing and finely grinding appropriate amounts of Na 2 CO 3 ŽMerck, 99.5%., NH 4 H 2 PO4 ŽMerck, 99.5%. and MoO 3 ŽAldrich, 99.9%., then heating mixtures in a platinum crucible at T f 650 K to remove volatile products, and melting at T f 1100–1200 K for a few hours. The melts were then poured on a stainless steel plate preheated at T f 450 K, and the vitreous samples obtained were annealed at T f 650 K to relieve residual internal stress, and slowly cooled at room temperature w7x. The amorphous nature of samples was checked by XRD analysis, using a Seifert XRD 3000 diffractometer. The glass transition temperatures ŽTg. were determined by DTA analysis, using a Seiko DTA thermal analyser. The EPR experiments were made at room temperature on coloured glassy samples using a Brucker EMX spectrometer operating at X-band frequencies Žf 9.5 GHz.. The Mo 5q cation percentages were determined by the dual sample cavity method w10,15x, and the computer simulations of the EPR spectra were performed on a computer using a laboratorydeveloped software. The a.c. and d.c. electrical measurements were carried out in order to define the electronic and ionic contributions to the total conductivity. The total conductivity was determined by the complex impedance method w16x. The a.c. electrical measurements, using a frequency response analyser ŽSolartron 1260., were
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done in vacuum in the 10y1 –10 6 frequency range, and over a thermal interval of 293–520 K in several temperature cycles. The electronic contribution to the total conductivity was evaluated from d.c. electrical measurements by means of the four point method w17x, using a Keithley 487 picoamperemeter.
3. Results and discussion 3.1. Study of physical properties A wide glassy range has been shown in the Na 2 O–MoO 3 –P2 O5 system represented in form of an equilateral triangle where the apexes are Na 2 O, ŽMoO 3 . 2 and P2 O5 ŽFig. 1.. The composition dependencies of physical and electrical properties of the glasses have been determined along three series of materials with the following compositions: v
v
v
0.25Na 2 O–0.75w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x for series 1; 0.45Na 2 O–0.55w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x for series 2; yNa 2 O– Ž1 y y .w0.40ŽMoO 3 . 2 –0.60ŽP2 O5 .x for series 3.
The glassy compositions studied in that way are similar to those of lithium glasses in the Li 2 O– MoO 3 –P2 O5 system w14x. Series 1 and 2 offer a fixed alkaline oxide content and series 3 is characterized by a constant MorP ratio. The physical properties of the sodium glasses studied are gathered in Table 1. The sodium glasses are coloured as those of lithium, but they are slightly paler. Considering the values of Tg relative to series 1 and 2, a flat Tg maximum appears for x f 0.40, a result close to that observed for the analogous lithium glasses. These Tg maxima result from two opposite influences: a Tg increase due to a larger cross-link density with increasing x w5x and a Tg decrease due to a decreasing mean bond strength as the covalence of the M`O bonds decreases from M s P to M s Mo w18x. On the other hand, the introduction of the Na 2 O basic oxide into the binary phosphomolybdate glass, in series 3, results in a decrease of the mean bond strength and a Tg decrease, as in the lithium glasses of the same series w14x.
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Fig. 1. Glassy domain of the Na 2 O– ŽMoO 3 . 2 –P2 O5 system Žshaded area.: the composition of the samples of the three studied series are indicated by dots.
The composition dependence of Tg is confirmed by that of the optical basicity Ž L., which rises very weakly with increasing x in series 1 and 2, but on the contrary, increases significantly with increasing y in series 3 ŽTable 1.. These results are consequently very close to those obtained with the corresponding lithium glasses w14x.The physical properties of glasses of the A 2 O–MoO 3 –P2 O5 ŽA s Li, Na. systems do not depend practically on the nature of the alkali oxide. 3.2. EPR inÕestigation Some EPR spectra, shown in Fig. 2a, have been assigned to Mo 5q cations in their oxygen environment. They consist of a main central line surrounded by less intense satellite lines. The central line arises from even molybdenum isotopes Ž I s 0; natural abundance: 74.62%., the satellite lines correspond to the hyperfine structure from odd 95 Mo and 97 Mo isotopes Ž I s 5r2; natural abundance: 15.78% and 9.69%, respectively.. The spectra obtained are simi-
lar to those of lithium glasses, but the intensity of RPE signal is weaker for the sodium sample than for the corresponding lithium sample ŽFig. 2b., resulting in a smaller Mo 5q percentage for the sodium glass, in agreement with its less pronounced coloration. It can be assumed that Mo 5q cation in the sodium glasses studied has an octahedral coordination with a weak axial distortion, as in the lithium ones w6,14, 19,20x. The Mo 5q cation percentages in the sodium glasses have been evaluated by comparison with a AVarian standardB reference sample, as in the lithium glasses w6x, and the Mo 5qrMo tot and Mo 5qrMo 6q ratios have been calculated for each glass studied ŽTable 1.. Fig. 2c and d give, respectively, the variations of these ratios as a function of x for series 1 and of y for series 3; the curves relative to the homologous lithium series are reported, by way of comparison, in these figures. Whatever x is, the sodium glasses are characterized by smaller values of these ratios. Considering series 1, both ratios offer, whatever the alkaline oxide, a maximum for x f 0.30 ŽFig.
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Table 1 Physical data of studied glasses Glasses
xry
MorP
d Žgrcm3 .
L
Tg ŽK.
Colour
Mo 5qr Mo tot
%Mo 5q
%Mo 6q
Mo 5qr Mo 6q
Series 1: 0.25Na 2 O–0.75 w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x
0.13 0.20 0.27 0.33 0.40 0.47 0.53 0.60 0.09 0.18 0.27 0.36 0.45 0.55 0.10 0.20 0.25 0.30 0.40 0.50 0.60
0.15 0.25 0.37 0.50 0.67 0.89 1.13 1.50 0.10 0.22 0.37 0.56 0.82 1.22 0.67 0.67 0.67 0.67 0.67 0.67 0.67
2.62 2.74 2.87 2.98 3.13 3.25 3.36 3.46 2.59 2.76 2.91 3.06 3.19 3.32 3.19 3.17 3.13 3.12 3.12 3.08 3.05
0.431 0.432 0.433 0.434 0.435 0.436 0.437 0.438 0.480 0.481 0.482 0.482 0.483 0.484 0.410 0.426 0.435 0.445 0.469 0.499 0.538
593 608 613 624 658 649 640 634 580 587 610 632 627 610 694 670 658 656 641 625 590
blue-green blue-green blue-green dark green green green green green pale green pale green brown-yellow brown-yellow yellow yellow blue-green blue-green green brown brown yellow yellow
0.31 0.38 0.41 0.42 0.14 0.04 0.02 0.01 0.08 0.04 0.02 0.01 0.01 0.01 0.30 0.32 0.14 0.09 0.03 0.01 0.01
5.60 10.04 13.97 16.74 6.41 2.21 1.29 0.89 0.78 0.72 0.52 0.41 0.38 0.37 15.95 15.49 6.40 3.78 1.34 0.21 0.14
12.17 16.05 19.71 23.26 39.74 49.92 55.60 61.18 8.65 17.30 25.34 32.65 39.30 46.08 36.99 32.99 39.74 39.97 37.37 33.12 27.45
0.46 0.63 0.71 0.72 0.16 0.04 0.02 0.01 0.09 0.04 0.02 0.01 0.01 0.01 0.43 0.47 0.16 0.09 0.04 0.01 0.01
Series 2: 0.45Na 2 O–0.55 w x ŽMoO 3 . 2 – Ž1 y x . ŽP2 O5 .x
Series 3: yNa 2 O– Ž1 y y . w0.40ŽMoO 3 . 2 –0.60ŽP2 O5 .x
2c., i.e. for a value of the MorP ratio close to 0.43; the origin of such maxima, which are observed also in the binary ŽMoO 3 –P2 O5 . glasses w10x, is probably structural w5,11x. It can be attributed in the sodium glasses as in those of lithium w14x to the presence of a maximum of groupings of MoOPO4 type which involve mainly P`O`Mo linkages w5x. As a matter of fact, according to Bridge and Patel w11x, the percentages of MoOPO4 and MoO 2 ŽPO 3 . 2 groupings in the MoO 3 –P2 O5 glasses are maxima for MorPf 0.50. Considering series 3, the maxima observed in the lithium case can be also detected in the sodium case ŽFig. 2d., but they are, of course, much more flat and inaccurate. It has been shown that the presence of these maxima in the lithium series results from two opposite influences; on the one hand, the addition of Li 2 O to the ŽMoO 3 –P2 O5 . glasses breaks preferentially the P`O`P units with creation of groupings of orthophosphate type, which stabilize the valence q5 of molybdenum and, on the other hand, involves simultaneously an increase of molybdenum ions in the highest oxidation state w14x. This explanation is
also valid in the case of the sodium series, where the larger aptitude of Naq ions to stabilize the molybdenum in the 6 q oxidation state results in the existence of very flat and little intense maxima. This larger aptitude of Naq ions to stabilize the valence q6 of molybdenum justifies the weak values of Mo 5q percentages determined in the sodium glasses of series 2, which have a high Na 2 O content ŽTable 1.; they are, as a matter of fact, clearly smaller than those of homologous lithium glasses w14x. 3.3. Study of electrical properties Complex impedance diagrams, i.e. ZY Ž V . as a function of ZX Ž V ., have been plotted at various temperatures for the glasses studied. The experimental spectra of sodium glasses can be classified, as those of lithium ones w14x, into two groups. The spectra of the first group, which consist of a single semicircular arc, whatever the temperature, correspond to materials which would be predominantly electronic conductors. Those of the second group
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Fig. 2. Ža. Experimental EPR spectra of sodium glasses of series 1. Žb. Comparison of EPR spectra of sodium and lithium glasses of series 1 and corresponding to x s 0.27. Žc. Comparison of Mo 5qrMo tot and Mo 5qrMo 6q ratios for sodium and lithium glasses of series 1. Žd. Comparison of Mo 5qrMo tot and Mo 5qrMo 6q ratios for sodium and lithium glasses of series 3.
also contains a semicircular arc at higher frequencies with, in addition, an inclined spike at lower frequen-
cies, which together are characteristic of the behavior of ion-conducting solids with blocking electrodes.
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The sodium glasses of series 3 are, in that way, characterized, as the lithium homologous series, by a crossover from electronic to mixed ionic–electronic conductivity when y increases. The temperature dependence of the bulk conductivity is of Arrhenius-type for each studied sample; a linear fit to stot T s s 0 expŽyD EsrkT ., with a correlation coefficient r s 0.98 has been shown. The electrical data relative to the glasses of the three series are listed in Table 2. The composition dependence of log stot at 473 K is given in Fig. 3a for series 1 and 2, and in Fig. 3b for series 3. The curves relatives to lithium series are reported, by comparison, in Fig. 3a and b. In each series, the composition dependence of stot for the sodium glasses is analogous to that observed for the lithium glasses and the conductivity of the sodium glass is generally weaker than that of the corresponding lithium glass. As a matter of fact, stot is always higher for the lithium glasses than for the sodium ones, except for x F 0.20 in series 1. The sodium first series offers, as that of lithium, a stot minimum, which means that there is a competition between different types of conductivity
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in this composition range. The minimum appears, in the sodium case, for x f 0.33, i.e. for an x value slightly higher than in the lithium case Ž x f 0.27.; on the other hand, the lithium and sodium glasses of series 2 offer a weak increase of stot with increasing x, and those of series 3, a fast increase of stot when y increases, at least for y F 0.40. The temperature dependence of electronic conductivity se , issued from d.c. measurements, is of Arrhenius-type. The samples characterized by se f stot can be considered as purely electronic conductors; when se - stot , the temperature dependence of ionic contribution is deduced from those of stot and se for the samples concerned and considered as mixed ionic–electronic conductors. The composition dependence of se and s i at 473 K is given in Fig. 4a–c for the three series of sodium glasses and, by comparison, for lithium homologous series. Whatever the alkaline element, the glasses of series 1 corresponding to x ) 0.27 are mainly electronic conductors: se increases quickly with increasing x in the Ž0.27 - x - 0.45. range, then more slowly for x ) 0.45; on the other hand, for a given x
Table 2 Electrical parameters of studied glasses Glasses
xry
Log stot w Vy1 cmy1 x, T s 473 K
Log s 0 w Vy1 cmy1 x
Ž D Es . tot weVx
Ž D Es . i weVx
Ž D Es .e weVx
Log se w Vy1 cmy1 x, T s 473 K
Log s i w Vy1 cmy1 x, T s 473 K
Series 1: 0.25Na 2 O–0.75 w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x
0.13 0.20 0.27 0.33 0.40 0.47 0.53 0.60 0.09 0.18 0.27 0.36 0.45 0.55 0.10 0.20 0.25 0.30 0.40 0.50 0.60
y4.55 y7.62 y7.78 y8.36 y7.59 y7.54 y7.28 y6.84 y6.19 y5.96 y6.33 y5.67 y5.72 y5.50 y8.80 y7.91 y7.59 y6.50 y5.84 y5.28 y5.13
10.50 9.10 1.61 1.99 2.97 2.92 3.07 3.51 6.07 6.20 4.44 5.10 4.94 5.16 3.04 2.86 2.97 3.20 3.65 4.53 5.00
1.16 1.32 0.63 0.72 0.74 0.73 0.72 0.72 0.90 0.89 0.76 0.76 0.75 0.75 0.86 0.76 0.74 0.66 0.64 0.67 0.70
1.20 1.40 – – – – – – 0.89 0.91 – 0.77 0.87 0.90 – – – 0.69 0.63 0.67 0.70
0.87 1.04 – 0.72 0.74 0.73 0.72 0.72 0.95 0.88 – 0.74 0.74 0.73 0.86 0.76 0.74 0.66 0.70 0.78 0.63
y8.63 y8.57 – y8.36 y7.59 y7.54 y7.28 y6.84 y6.72 y6.41 – y6.26 y5.95 y5.63 y8.80 y7.91 y7.59 y6.60 y6.60 y6.74 y7.41
y4.55 y7.68 – – – – – – y6.36 y6.16 – y5.80 y5.78 y6.09 – – – y7.18 y5.93 y5.30 y5.14
Series 2: 0.45Na 2 O–0.55 w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x
Series 3: yNa 2 O– Ž1 y y . w0.40ŽMoO 3 . 2 –0.60ŽP2 O5 .x
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Fig. 3. Variation of log stot at 473 K as a function of Ža. x for series 1 and 2, Žb. y for series 3 of sodium and lithium glasses.
value, se is higher in the lithium glass than in the sodium one. For x - 0.27, the glasses are, on the contrary, characterized by an ionic conductivity which decreases very quickly when x increases. An 1 H NMR investigation of the x s 0.13 and x s 0.20 sodium glasses, carried out at room temperature, shows that when x decreases, the line becomes narrower and, consequently, the proton mobility is larger, a result in agreement with the variation of s i with x ŽFig. 5.. The protonic nature of the conductivity suggested previously in those lithium glasses w14x is in this way, confirmed in the sodium glasses; it is due to the hygroscopicity of the glasses in this composition range. The variations of se and s i with x for the sodium glasses of series 2 are analogous to those observed for the lithium glasses and, whatever x, Ž se . Na and Ž s i . Na are respectively less than Ž se . Li and Ž s i . Li ŽFig. 4b.. Ž se . Na and Ž se . Li increase softly when x increases. A maximum of s i appears in the sodium and lithium series, approximately for x f 0.35, i.e. for an MorP ratio close to 0.50. The s i maximum appears consequently, whatever the alkaline element, for the composition having a disorder maximum between both glass-formers. An analogous result has been observed for ionic glasses in the Li 2 O–P2 O5 –Nb 2 O5 system w21x. The variation of s i with y in series 3 of sodium glasses is similar to that observed for the lithium homologous series: s i appears above y f 0.30, and is growing with y more quickly in the lithium case than in the sodium case ŽFig. 4c.. On the contrary, the variations of se with y differ: in the case of lithium, se increases fast for y - 0.30 and more weakly for higher y values, and it results from the relative variations of se and s i that the lithium glasses of series 3 are electronic conductors for y - 0.30 and mixed ionic–electronic conductors for y ) 0.40 w14x. The variation of se with y, in the case of sodium, is characterized by the presence of a maximum for y f 0.40; consequently, the sodium glasses of series 3 are electronic conductors for y - 0.30, then mixed in the Ž0.30 - y - 0.50. range, as the lithium homologous glasses. However, above y s 0.50, the sodium glasses become progressively ionic conductors: in fact, the electronic transport numbers calculated for y s 0.50 and 0.60 are, respectively, equal to f 4 P 10y2 and f 5 P 10y3 .
L. Bih et al.r Materials Letters 50 (2001) 308–317
315
Fig. 4. Variation of log se and log s i at 473 K as a function of Ža. x for series 1, Žb. x for series 2, and Žc. y for series 3 of sodium and lithium glasses.
L. Bih et al.r Materials Letters 50 (2001) 308–317
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and it results in a se drop. The Ž y s 0.60. glass can be considered as a purely ionic conductor.
4. Conclusions A wide-coloured glassy range has been shown and characterized in the Na 2 O–MoO 3 –P2 O5 system. Various properties have been studied along particular composition lines corresponding either to a fixed Na 2 O content or a constant MorP ratio, and compared with those of lithium glasses with the same compositions: v
1
Fig. 5. H NMR spectra ŽkHz. at room temperature of sodium glasses of series 1 and corresponding to x s 0.13 and 0.20.
v
v
The presence of maxima of Mo 5qrMo tot and Mo 5qrMo 6q ratios, observed as functions of x and y in the sodium glasses as in those of lithium ŽFig. 2c,d., has no effect on the composition dependence of electronic and ionic conductivities. The structural origin of these maxima is in that way confirmed. It results that these sodium glasses probably have a structure close to that of lithium homologous glasses. Independent of the nature of the alkaline ion, the se conductivity in the glasses studied can be assumed to be due to electronic hopping from the lower valence state Mo 5q Ždonor level. to the higher valence state Mo 6q Žacceptor level.. This conduction mechanism is similar to that proposed in other transition metal oxides w13,22,23x. The comparison of electrical properties of glasses belonging to series 3 shows a similar behavior in the 0 - y - 0.40 and 0 - y - 0.50 ranges, respectively, for the sodium and lithium glasses: a simultaneous increase of se and s i is observed when y increases. Independent of the nature of the alkaline ion, the ionic and electronic motions appear as closely correlated, and the hypothesis of an ion–polaron soft coupling, suggested in the case of lithium glasses w14x, can be extended to homologous sodium glasses. When y is higher than 0.40 in the sodium series, the proportion of Mo 5q becomes very small ŽTable 1.
series 1: 0.25Na 2 O–0.75w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x; series 2: 0.45Na 2 O–0.55w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x; series 3: y Na 2 O– Ž1 y y .w0.40 ŽMoO 3 . 2 – 0.60ŽP2 O5 .x.
The percentages of Mo 5q ion in these glasses have been determined by EPR spectroscopy. For a given composition, the Mo 5q ion percentage in the sodium glass is lower than that in the lithium glass, and the colour of which is deeper. Whatever the alkaline element, maxima of Mo 5qrMo tot and Mo 5qrMo 6q ratios have been evidenced in the glasses of series 1 for an MorP ratio close to 0.50, and attributed to a maximum of units of MoOPO4 type in the glass. The glasses of series 3 offer, for y f 0.10, maxima of Mo 5qrMo tot and Mo 5qrMo 6q ratios, the origin of which is also structural: the addition of A 2 O ŽA s Li, Na. into the binary ŽMoO 3 –P2 O5 . glasses results first in a supplementary formation of MoOPO4 groupings due to the preferential breaking of P`O bonds instead of Mo`O bonds; this influence is favourable to an increase of Mo 5q ion percentage, but is counterbalanced by the increase of Mo 6q ion percentage with increasing A 2 O content. The Mo 5q ion percentages, smaller in the sodium glasses than in the homologous lithium ones, are justified by the larger aptitude of Naq ions to stabilize the valence q6 of molybdenum. Whatever the alkaline element, lithium or sodium, the glasses of series 1 are mainly electronic conductors, those of series 2 are mixed conductors, those of
L. Bih et al.r Materials Letters 50 (2001) 308–317
series 3 are mainly electronic for y - 0.30, and mixed above that value. Purely ionic glasses are even obtained for y s 0.60 in series 3 of sodium. The conductivity of the sodium glass, either electronic or ionic, is always weaker than that of lithium glass of the same composition. No correlation is evidenced between the variation of the electronic or ionic conductivities, as a function of x or y, and the presence of maxima of the Mo 5qrMo tot and Mo 5qrMo 6q ratios in the lithium and sodium glasses studied.
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Materials Letters 50 Ž2001. 308–317 www.elsevier.comrlocatermatlet
Electrical properties of glasses in the Na 2 O–MoO 3 –P2 O5 system L. Bih a,b, M. El Omari c , J.M. Reau ´ d,) , A. Nadiri b, A. Yacoubi b, M. Haddad e a
Departement de Chimie, Faculte´ des Sciences et Techniques d’Errachidia, B.P. 509, Boutalamine, Errachidia, Morocco ´ Laboratoire de Chimie Minerale Appliquee, ´ ´ Faculte´ des Sciences de Meknes, ` B.P. 4010, Beni M’hamed, Meknes, ` Morocco c Departement de Chimie, Faculte´ des Sciences de Meknes, ´ ` B.P. 4010, Beni M’hamed, Meknes, ` Morocco d Institut de Chimie de la Matiere ` Condensee ´ de Bordeaux (ICMCB), UPR 9048 CNRAS, 87 AÕenue Dr. Albert Schweitzer, 33608 Pessac Cedex, France e Laboratoire de Spectronomie Physique, Faculte´ des Sciences de Meknes, ` B.P. 4010, Beni M’hamed, Meknes, ` Morocco
b
Received 26 July 2000; received in revised form 15 December 2000; accepted 20 December 2000
Abstract A range of coloured electronic or mixed ionic–electronic glasses has been evidenced in the Na 2 O–MoO 3 –P2 O5 system. The properties of these glasses have been studied along different composition lines corresponding either to a fixed Na 2 O content or a constant MorP ratio. An EPR spectroscopy investigation of these glasses has allowed to determine the Mo 5q ion percentages in these materials. The electrical properties of these glasses have been studied by impedance spectroscopy, and the electronic and ionic contributions have been evaluated. The properties of these sodium glasses have been compared with those of lithium glasses with the same compositions. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Ionic conductivity; Electronic conductivity; EPR spectroscopy; Impedance spectroscopy; Phosphate glasses
1. Introduction Glasses containing transition metal ions offer much interest because of their potential applications in electrochemical, electronic and electro-optical devices w1–4x. Selecting molybdenum oxide as transition metal oxide, binary MoO 3 –P2 O5 glasses were proved stable over a wide range of compositions w5x, and consequently, the addition of alkaline oxides has given rise to the existence of very wide glassy ranges in the A 2 O–MoO 3 –P2 O5 ŽA s Li, Na. systems w6,7x.
) Corresponding author. Tel.: q33-556-846-332; fax: q33-556846-634. E-mail address: [email protected] ŽJ.M. Reau ´ ..
Glasses with a high concentration of alkaline oxide are superionic conductors w8,9x, whereas those issued from mixing of former oxides only are purely electronic, with a polaronic conduction mechanism due to an exchange process among the transition metal ions of different valence states w10–13x. A recent investigation of the electrical properties of glasses in the Li 2 O–MoO 3 –P2 O5 system has allowed to show a Across overB from a predominantly electronic conductivity in the alkaline oxide poor region to a mixed ionic–electronic conductivity in the alkaline oxide-richer region w14x. The Mo 5q and Mo 6q cation percentages in the studied glasses have been evaluated, and the electronic and ionic contributions to the total conductivity have been determined. The existence of a polaronic hopping conduction
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 2 4 5 - 2
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mechanism between Mo 5q and Mo 6q cations has been suggested in the purely electronic conductor glasses, and an ion–polaron soft coupling has been proposed in the mixed ionic–electronic conductor glasses w15x. The purpose of the present paper is the investigation of electrical properties of various glassy compositions in the Na 2 O–MoO 3 –P2 O5 system and the establishment of correlations between the glassy structure and the electrical properties. A comparative study of different properties of sodium glasses with those of lithium glasses with the same compositions is reported.
2. Experimental The glasses of the Na 2 O–MoO 3 –P2 O5 ternary system were prepared, as usual w7x, by mixing and finely grinding appropriate amounts of Na 2 CO 3 ŽMerck, 99.5%., NH 4 H 2 PO4 ŽMerck, 99.5%. and MoO 3 ŽAldrich, 99.9%., then heating mixtures in a platinum crucible at T f 650 K to remove volatile products, and melting at T f 1100–1200 K for a few hours. The melts were then poured on a stainless steel plate preheated at T f 450 K, and the vitreous samples obtained were annealed at T f 650 K to relieve residual internal stress, and slowly cooled at room temperature w7x. The amorphous nature of samples was checked by XRD analysis, using a Seifert XRD 3000 diffractometer. The glass transition temperatures ŽTg. were determined by DTA analysis, using a Seiko DTA thermal analyser. The EPR experiments were made at room temperature on coloured glassy samples using a Brucker EMX spectrometer operating at X-band frequencies Žf 9.5 GHz.. The Mo 5q cation percentages were determined by the dual sample cavity method w10,15x, and the computer simulations of the EPR spectra were performed on a computer using a laboratorydeveloped software. The a.c. and d.c. electrical measurements were carried out in order to define the electronic and ionic contributions to the total conductivity. The total conductivity was determined by the complex impedance method w16x. The a.c. electrical measurements, using a frequency response analyser ŽSolartron 1260., were
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done in vacuum in the 10y1 –10 6 frequency range, and over a thermal interval of 293–520 K in several temperature cycles. The electronic contribution to the total conductivity was evaluated from d.c. electrical measurements by means of the four point method w17x, using a Keithley 487 picoamperemeter.
3. Results and discussion 3.1. Study of physical properties A wide glassy range has been shown in the Na 2 O–MoO 3 –P2 O5 system represented in form of an equilateral triangle where the apexes are Na 2 O, ŽMoO 3 . 2 and P2 O5 ŽFig. 1.. The composition dependencies of physical and electrical properties of the glasses have been determined along three series of materials with the following compositions: v
v
v
0.25Na 2 O–0.75w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x for series 1; 0.45Na 2 O–0.55w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x for series 2; yNa 2 O– Ž1 y y .w0.40ŽMoO 3 . 2 –0.60ŽP2 O5 .x for series 3.
The glassy compositions studied in that way are similar to those of lithium glasses in the Li 2 O– MoO 3 –P2 O5 system w14x. Series 1 and 2 offer a fixed alkaline oxide content and series 3 is characterized by a constant MorP ratio. The physical properties of the sodium glasses studied are gathered in Table 1. The sodium glasses are coloured as those of lithium, but they are slightly paler. Considering the values of Tg relative to series 1 and 2, a flat Tg maximum appears for x f 0.40, a result close to that observed for the analogous lithium glasses. These Tg maxima result from two opposite influences: a Tg increase due to a larger cross-link density with increasing x w5x and a Tg decrease due to a decreasing mean bond strength as the covalence of the M`O bonds decreases from M s P to M s Mo w18x. On the other hand, the introduction of the Na 2 O basic oxide into the binary phosphomolybdate glass, in series 3, results in a decrease of the mean bond strength and a Tg decrease, as in the lithium glasses of the same series w14x.
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Fig. 1. Glassy domain of the Na 2 O– ŽMoO 3 . 2 –P2 O5 system Žshaded area.: the composition of the samples of the three studied series are indicated by dots.
The composition dependence of Tg is confirmed by that of the optical basicity Ž L., which rises very weakly with increasing x in series 1 and 2, but on the contrary, increases significantly with increasing y in series 3 ŽTable 1.. These results are consequently very close to those obtained with the corresponding lithium glasses w14x.The physical properties of glasses of the A 2 O–MoO 3 –P2 O5 ŽA s Li, Na. systems do not depend practically on the nature of the alkali oxide. 3.2. EPR inÕestigation Some EPR spectra, shown in Fig. 2a, have been assigned to Mo 5q cations in their oxygen environment. They consist of a main central line surrounded by less intense satellite lines. The central line arises from even molybdenum isotopes Ž I s 0; natural abundance: 74.62%., the satellite lines correspond to the hyperfine structure from odd 95 Mo and 97 Mo isotopes Ž I s 5r2; natural abundance: 15.78% and 9.69%, respectively.. The spectra obtained are simi-
lar to those of lithium glasses, but the intensity of RPE signal is weaker for the sodium sample than for the corresponding lithium sample ŽFig. 2b., resulting in a smaller Mo 5q percentage for the sodium glass, in agreement with its less pronounced coloration. It can be assumed that Mo 5q cation in the sodium glasses studied has an octahedral coordination with a weak axial distortion, as in the lithium ones w6,14, 19,20x. The Mo 5q cation percentages in the sodium glasses have been evaluated by comparison with a AVarian standardB reference sample, as in the lithium glasses w6x, and the Mo 5qrMo tot and Mo 5qrMo 6q ratios have been calculated for each glass studied ŽTable 1.. Fig. 2c and d give, respectively, the variations of these ratios as a function of x for series 1 and of y for series 3; the curves relative to the homologous lithium series are reported, by way of comparison, in these figures. Whatever x is, the sodium glasses are characterized by smaller values of these ratios. Considering series 1, both ratios offer, whatever the alkaline oxide, a maximum for x f 0.30 ŽFig.
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Table 1 Physical data of studied glasses Glasses
xry
MorP
d Žgrcm3 .
L
Tg ŽK.
Colour
Mo 5qr Mo tot
%Mo 5q
%Mo 6q
Mo 5qr Mo 6q
Series 1: 0.25Na 2 O–0.75 w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x
0.13 0.20 0.27 0.33 0.40 0.47 0.53 0.60 0.09 0.18 0.27 0.36 0.45 0.55 0.10 0.20 0.25 0.30 0.40 0.50 0.60
0.15 0.25 0.37 0.50 0.67 0.89 1.13 1.50 0.10 0.22 0.37 0.56 0.82 1.22 0.67 0.67 0.67 0.67 0.67 0.67 0.67
2.62 2.74 2.87 2.98 3.13 3.25 3.36 3.46 2.59 2.76 2.91 3.06 3.19 3.32 3.19 3.17 3.13 3.12 3.12 3.08 3.05
0.431 0.432 0.433 0.434 0.435 0.436 0.437 0.438 0.480 0.481 0.482 0.482 0.483 0.484 0.410 0.426 0.435 0.445 0.469 0.499 0.538
593 608 613 624 658 649 640 634 580 587 610 632 627 610 694 670 658 656 641 625 590
blue-green blue-green blue-green dark green green green green green pale green pale green brown-yellow brown-yellow yellow yellow blue-green blue-green green brown brown yellow yellow
0.31 0.38 0.41 0.42 0.14 0.04 0.02 0.01 0.08 0.04 0.02 0.01 0.01 0.01 0.30 0.32 0.14 0.09 0.03 0.01 0.01
5.60 10.04 13.97 16.74 6.41 2.21 1.29 0.89 0.78 0.72 0.52 0.41 0.38 0.37 15.95 15.49 6.40 3.78 1.34 0.21 0.14
12.17 16.05 19.71 23.26 39.74 49.92 55.60 61.18 8.65 17.30 25.34 32.65 39.30 46.08 36.99 32.99 39.74 39.97 37.37 33.12 27.45
0.46 0.63 0.71 0.72 0.16 0.04 0.02 0.01 0.09 0.04 0.02 0.01 0.01 0.01 0.43 0.47 0.16 0.09 0.04 0.01 0.01
Series 2: 0.45Na 2 O–0.55 w x ŽMoO 3 . 2 – Ž1 y x . ŽP2 O5 .x
Series 3: yNa 2 O– Ž1 y y . w0.40ŽMoO 3 . 2 –0.60ŽP2 O5 .x
2c., i.e. for a value of the MorP ratio close to 0.43; the origin of such maxima, which are observed also in the binary ŽMoO 3 –P2 O5 . glasses w10x, is probably structural w5,11x. It can be attributed in the sodium glasses as in those of lithium w14x to the presence of a maximum of groupings of MoOPO4 type which involve mainly P`O`Mo linkages w5x. As a matter of fact, according to Bridge and Patel w11x, the percentages of MoOPO4 and MoO 2 ŽPO 3 . 2 groupings in the MoO 3 –P2 O5 glasses are maxima for MorPf 0.50. Considering series 3, the maxima observed in the lithium case can be also detected in the sodium case ŽFig. 2d., but they are, of course, much more flat and inaccurate. It has been shown that the presence of these maxima in the lithium series results from two opposite influences; on the one hand, the addition of Li 2 O to the ŽMoO 3 –P2 O5 . glasses breaks preferentially the P`O`P units with creation of groupings of orthophosphate type, which stabilize the valence q5 of molybdenum and, on the other hand, involves simultaneously an increase of molybdenum ions in the highest oxidation state w14x. This explanation is
also valid in the case of the sodium series, where the larger aptitude of Naq ions to stabilize the molybdenum in the 6 q oxidation state results in the existence of very flat and little intense maxima. This larger aptitude of Naq ions to stabilize the valence q6 of molybdenum justifies the weak values of Mo 5q percentages determined in the sodium glasses of series 2, which have a high Na 2 O content ŽTable 1.; they are, as a matter of fact, clearly smaller than those of homologous lithium glasses w14x. 3.3. Study of electrical properties Complex impedance diagrams, i.e. ZY Ž V . as a function of ZX Ž V ., have been plotted at various temperatures for the glasses studied. The experimental spectra of sodium glasses can be classified, as those of lithium ones w14x, into two groups. The spectra of the first group, which consist of a single semicircular arc, whatever the temperature, correspond to materials which would be predominantly electronic conductors. Those of the second group
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Fig. 2. Ža. Experimental EPR spectra of sodium glasses of series 1. Žb. Comparison of EPR spectra of sodium and lithium glasses of series 1 and corresponding to x s 0.27. Žc. Comparison of Mo 5qrMo tot and Mo 5qrMo 6q ratios for sodium and lithium glasses of series 1. Žd. Comparison of Mo 5qrMo tot and Mo 5qrMo 6q ratios for sodium and lithium glasses of series 3.
also contains a semicircular arc at higher frequencies with, in addition, an inclined spike at lower frequen-
cies, which together are characteristic of the behavior of ion-conducting solids with blocking electrodes.
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The sodium glasses of series 3 are, in that way, characterized, as the lithium homologous series, by a crossover from electronic to mixed ionic–electronic conductivity when y increases. The temperature dependence of the bulk conductivity is of Arrhenius-type for each studied sample; a linear fit to stot T s s 0 expŽyD EsrkT ., with a correlation coefficient r s 0.98 has been shown. The electrical data relative to the glasses of the three series are listed in Table 2. The composition dependence of log stot at 473 K is given in Fig. 3a for series 1 and 2, and in Fig. 3b for series 3. The curves relatives to lithium series are reported, by comparison, in Fig. 3a and b. In each series, the composition dependence of stot for the sodium glasses is analogous to that observed for the lithium glasses and the conductivity of the sodium glass is generally weaker than that of the corresponding lithium glass. As a matter of fact, stot is always higher for the lithium glasses than for the sodium ones, except for x F 0.20 in series 1. The sodium first series offers, as that of lithium, a stot minimum, which means that there is a competition between different types of conductivity
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in this composition range. The minimum appears, in the sodium case, for x f 0.33, i.e. for an x value slightly higher than in the lithium case Ž x f 0.27.; on the other hand, the lithium and sodium glasses of series 2 offer a weak increase of stot with increasing x, and those of series 3, a fast increase of stot when y increases, at least for y F 0.40. The temperature dependence of electronic conductivity se , issued from d.c. measurements, is of Arrhenius-type. The samples characterized by se f stot can be considered as purely electronic conductors; when se - stot , the temperature dependence of ionic contribution is deduced from those of stot and se for the samples concerned and considered as mixed ionic–electronic conductors. The composition dependence of se and s i at 473 K is given in Fig. 4a–c for the three series of sodium glasses and, by comparison, for lithium homologous series. Whatever the alkaline element, the glasses of series 1 corresponding to x ) 0.27 are mainly electronic conductors: se increases quickly with increasing x in the Ž0.27 - x - 0.45. range, then more slowly for x ) 0.45; on the other hand, for a given x
Table 2 Electrical parameters of studied glasses Glasses
xry
Log stot w Vy1 cmy1 x, T s 473 K
Log s 0 w Vy1 cmy1 x
Ž D Es . tot weVx
Ž D Es . i weVx
Ž D Es .e weVx
Log se w Vy1 cmy1 x, T s 473 K
Log s i w Vy1 cmy1 x, T s 473 K
Series 1: 0.25Na 2 O–0.75 w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x
0.13 0.20 0.27 0.33 0.40 0.47 0.53 0.60 0.09 0.18 0.27 0.36 0.45 0.55 0.10 0.20 0.25 0.30 0.40 0.50 0.60
y4.55 y7.62 y7.78 y8.36 y7.59 y7.54 y7.28 y6.84 y6.19 y5.96 y6.33 y5.67 y5.72 y5.50 y8.80 y7.91 y7.59 y6.50 y5.84 y5.28 y5.13
10.50 9.10 1.61 1.99 2.97 2.92 3.07 3.51 6.07 6.20 4.44 5.10 4.94 5.16 3.04 2.86 2.97 3.20 3.65 4.53 5.00
1.16 1.32 0.63 0.72 0.74 0.73 0.72 0.72 0.90 0.89 0.76 0.76 0.75 0.75 0.86 0.76 0.74 0.66 0.64 0.67 0.70
1.20 1.40 – – – – – – 0.89 0.91 – 0.77 0.87 0.90 – – – 0.69 0.63 0.67 0.70
0.87 1.04 – 0.72 0.74 0.73 0.72 0.72 0.95 0.88 – 0.74 0.74 0.73 0.86 0.76 0.74 0.66 0.70 0.78 0.63
y8.63 y8.57 – y8.36 y7.59 y7.54 y7.28 y6.84 y6.72 y6.41 – y6.26 y5.95 y5.63 y8.80 y7.91 y7.59 y6.60 y6.60 y6.74 y7.41
y4.55 y7.68 – – – – – – y6.36 y6.16 – y5.80 y5.78 y6.09 – – – y7.18 y5.93 y5.30 y5.14
Series 2: 0.45Na 2 O–0.55 w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x
Series 3: yNa 2 O– Ž1 y y . w0.40ŽMoO 3 . 2 –0.60ŽP2 O5 .x
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Fig. 3. Variation of log stot at 473 K as a function of Ža. x for series 1 and 2, Žb. y for series 3 of sodium and lithium glasses.
value, se is higher in the lithium glass than in the sodium one. For x - 0.27, the glasses are, on the contrary, characterized by an ionic conductivity which decreases very quickly when x increases. An 1 H NMR investigation of the x s 0.13 and x s 0.20 sodium glasses, carried out at room temperature, shows that when x decreases, the line becomes narrower and, consequently, the proton mobility is larger, a result in agreement with the variation of s i with x ŽFig. 5.. The protonic nature of the conductivity suggested previously in those lithium glasses w14x is in this way, confirmed in the sodium glasses; it is due to the hygroscopicity of the glasses in this composition range. The variations of se and s i with x for the sodium glasses of series 2 are analogous to those observed for the lithium glasses and, whatever x, Ž se . Na and Ž s i . Na are respectively less than Ž se . Li and Ž s i . Li ŽFig. 4b.. Ž se . Na and Ž se . Li increase softly when x increases. A maximum of s i appears in the sodium and lithium series, approximately for x f 0.35, i.e. for an MorP ratio close to 0.50. The s i maximum appears consequently, whatever the alkaline element, for the composition having a disorder maximum between both glass-formers. An analogous result has been observed for ionic glasses in the Li 2 O–P2 O5 –Nb 2 O5 system w21x. The variation of s i with y in series 3 of sodium glasses is similar to that observed for the lithium homologous series: s i appears above y f 0.30, and is growing with y more quickly in the lithium case than in the sodium case ŽFig. 4c.. On the contrary, the variations of se with y differ: in the case of lithium, se increases fast for y - 0.30 and more weakly for higher y values, and it results from the relative variations of se and s i that the lithium glasses of series 3 are electronic conductors for y - 0.30 and mixed ionic–electronic conductors for y ) 0.40 w14x. The variation of se with y, in the case of sodium, is characterized by the presence of a maximum for y f 0.40; consequently, the sodium glasses of series 3 are electronic conductors for y - 0.30, then mixed in the Ž0.30 - y - 0.50. range, as the lithium homologous glasses. However, above y s 0.50, the sodium glasses become progressively ionic conductors: in fact, the electronic transport numbers calculated for y s 0.50 and 0.60 are, respectively, equal to f 4 P 10y2 and f 5 P 10y3 .
L. Bih et al.r Materials Letters 50 (2001) 308–317
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Fig. 4. Variation of log se and log s i at 473 K as a function of Ža. x for series 1, Žb. x for series 2, and Žc. y for series 3 of sodium and lithium glasses.
L. Bih et al.r Materials Letters 50 (2001) 308–317
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and it results in a se drop. The Ž y s 0.60. glass can be considered as a purely ionic conductor.
4. Conclusions A wide-coloured glassy range has been shown and characterized in the Na 2 O–MoO 3 –P2 O5 system. Various properties have been studied along particular composition lines corresponding either to a fixed Na 2 O content or a constant MorP ratio, and compared with those of lithium glasses with the same compositions: v
1
Fig. 5. H NMR spectra ŽkHz. at room temperature of sodium glasses of series 1 and corresponding to x s 0.13 and 0.20.
v
v
The presence of maxima of Mo 5qrMo tot and Mo 5qrMo 6q ratios, observed as functions of x and y in the sodium glasses as in those of lithium ŽFig. 2c,d., has no effect on the composition dependence of electronic and ionic conductivities. The structural origin of these maxima is in that way confirmed. It results that these sodium glasses probably have a structure close to that of lithium homologous glasses. Independent of the nature of the alkaline ion, the se conductivity in the glasses studied can be assumed to be due to electronic hopping from the lower valence state Mo 5q Ždonor level. to the higher valence state Mo 6q Žacceptor level.. This conduction mechanism is similar to that proposed in other transition metal oxides w13,22,23x. The comparison of electrical properties of glasses belonging to series 3 shows a similar behavior in the 0 - y - 0.40 and 0 - y - 0.50 ranges, respectively, for the sodium and lithium glasses: a simultaneous increase of se and s i is observed when y increases. Independent of the nature of the alkaline ion, the ionic and electronic motions appear as closely correlated, and the hypothesis of an ion–polaron soft coupling, suggested in the case of lithium glasses w14x, can be extended to homologous sodium glasses. When y is higher than 0.40 in the sodium series, the proportion of Mo 5q becomes very small ŽTable 1.
series 1: 0.25Na 2 O–0.75w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x; series 2: 0.45Na 2 O–0.55w x ŽMoO 3 . 2 – Ž1 y x .ŽP2 O5 .x; series 3: y Na 2 O– Ž1 y y .w0.40 ŽMoO 3 . 2 – 0.60ŽP2 O5 .x.
The percentages of Mo 5q ion in these glasses have been determined by EPR spectroscopy. For a given composition, the Mo 5q ion percentage in the sodium glass is lower than that in the lithium glass, and the colour of which is deeper. Whatever the alkaline element, maxima of Mo 5qrMo tot and Mo 5qrMo 6q ratios have been evidenced in the glasses of series 1 for an MorP ratio close to 0.50, and attributed to a maximum of units of MoOPO4 type in the glass. The glasses of series 3 offer, for y f 0.10, maxima of Mo 5qrMo tot and Mo 5qrMo 6q ratios, the origin of which is also structural: the addition of A 2 O ŽA s Li, Na. into the binary ŽMoO 3 –P2 O5 . glasses results first in a supplementary formation of MoOPO4 groupings due to the preferential breaking of P`O bonds instead of Mo`O bonds; this influence is favourable to an increase of Mo 5q ion percentage, but is counterbalanced by the increase of Mo 6q ion percentage with increasing A 2 O content. The Mo 5q ion percentages, smaller in the sodium glasses than in the homologous lithium ones, are justified by the larger aptitude of Naq ions to stabilize the valence q6 of molybdenum. Whatever the alkaline element, lithium or sodium, the glasses of series 1 are mainly electronic conductors, those of series 2 are mixed conductors, those of
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series 3 are mainly electronic for y - 0.30, and mixed above that value. Purely ionic glasses are even obtained for y s 0.60 in series 3 of sodium. The conductivity of the sodium glass, either electronic or ionic, is always weaker than that of lithium glass of the same composition. No correlation is evidenced between the variation of the electronic or ionic conductivities, as a function of x or y, and the presence of maxima of the Mo 5qrMo tot and Mo 5qrMo 6q ratios in the lithium and sodium glasses studied.
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