Glass formation in alkali molybdate systems

JOURNALOF NON~CRYSTALLINESOLIDS 1 (1968) 18--28 © North-Holland Publishing Co., Amsterdam

GLASS FORMATION

IN ALKALI MOLYBDATE

SYSTEMS

J. C. Th. G. M. VAN DER WIELEN, H. N. STEIN and J, M. STEVELS Laboratory of lnorganic Chemistry, Technological University, Eindhoven, The Netherlands Received 18 March 1968 Glass formation in alkali molybdate systems is investigated through measurements of the critical cooling rate, i.e. the cooling rate necessary to prevent crystallization entirely. Similarities and differences with alkali tungstate systems are noted, and the latter are explained by postulating a more continuous transition of the molybdenum ion from tetrahedral to octahedral coordination with decreasing alkali oxide content of the systems as compared with the behavior of tungstates, where tetrahedral coordination is found in all glasses. In addition, disproportionation of dimolybdate ions into monomers and higher polymeric units is assumed to be less pronounced in molybdate systems than the disproportionation of ditungstate ions in the corresponding tungstate systems. Density and thermal expansion data on molten alkali molybdate systems indicate that glass forming melts have a relatively spacious structure, in accordance with the idea that glass formation is facilitated by the formation of chains of (distorted) tetrahedra.

1. Introduction Glass f o r m a t i o n in alkali m o l y b d a t e systems was m e n t i o n e d first by B a y n t o n et a l l ) . It was considered o f interest to devote to these systems a m o r e t h o r o u g h investigation, especially since a c o m p a r i s o n with glass f o r m i n g tendencies in alkali tungstate systems z) might reveal influences o f substitution of one central ion b y one t h a t is chemically similar. O b s e r v a t i o n s in alkali tungstate systems were consistent with the h y p o thesis, t h a t on a d d i t i o n o f WO3 to a M z W O 4 melt ( M = alkali ion) initially glass f o r m a t i o n is p r o m o t e d t h r o u g h f o r m a t i o n o f chains o f W O 2 2 tetrahedra, b u t ultimately is c o u n t e r a c t e d by transition o f tungsten ions into o c t a h e d r a l c o o r d i n a t i o n , necessitating a higher degree o f t h r e e - d i m e n s i o n a l c o n d e n s a t i o n o f the anionic structural units. This m o d e l has been tested in the present investigation for the case o f m o l y b d a t e systems, using c o m p l e m e n t a r y techniques.

2. Experimental 2.1. MATERIALS R e a g e n t grade chemicals ( p r o analysi, M e r c k ) were used t h r o u g h o u t . A l k a l i c a r b o n a t e s , used as sources for alkali oxides, were dried at a temperature of 100°C before being weighed. 18

GLASS FORMATION IN ALKALI MOLYBDATE SYSTEMS

]9

Samples for measurements of critical cooling rates and for phase diagram studies were prepared as follows: Weighed portions of alkali carbonate and MoO 3 were mixed with dried benzene into a paste and milled in an agate ball mill. The mixture was dried afterwards in a platinum crucible at 100 °C in order to remove the benzene, and then, before further measurements were made, sintered at 400 °C for at least 14 days. Samples for density measurements in the melt were prepared by dry mixing M2CO3 and MoO3 in the appropriate ratio, then melted before being introduced into the apparatus, and melted again when being measured. 2.2. METHODS The methods for measuring critical cooling rates and infrared spectra have been described previously~). Density and thermal expansion in the melt were measured by Mackenzie's method 3), involving the measurement of the weight change of a platinum bob on submersion in the melt. Although the apparatus enabled the investigator to degass the sample before measurement in order to remove air bubbles adhering to the bob, in practice this was not necessary: the melts had very 800 Temperafure

/

foc

t,.+

?00 606 500400300206I0C 0 0

(t i2 MoO~)

.90

=

._~.lx lO0

lO0 (~03)

Fig. 1. Phase diagram Li2MoO4-MoOs. y ~ "molar ratio" MoO3/Li20. (*) congruent melting points, (+) liquidus points, (O) solidus points.

20

J. C. TH. G. M. VAN DER WIELEN, H. N. STEIN AND J. M. STEVELS

low viscosities ( ~ 10- 2 p), resulting in easy removal of air bubbles. The same circumstance, however, prevented the authors to measure viscosity isotherms of the melts exactly, since viscosities were beyond the range of the method employed. The size of the platinum bob used did not have any detectable influence on measured density values, indicationg the absence of appreciable surface energy effects. 3.1. Results and discussion 3.1. PHASE DIAGRAM AND CRITICALCOOLINGRATES Phase diagram studies confirmed the K 2 M o O 4 - M o O 3 diagram communicated by Spitzyn and Kuleshov 5) and by Mokhosoev, Kuleshov and Fedorov 6) [as opposed to that published by Hoermann4)]. The Na2MoO4 M o O 3 diagram found was similar to that communicated by Hoermann a) and by Spitzyn and KuleshovS), while the Li2MoO4-MoO 3 diagram found in the present investigation (fig. l) differs from that communicated by Hoermann4). Some data for this system are shown in table 1. Since Hoermann used a DTA technique for his phase diagram studies, one may expect to find equilibrium values among his data only if equilibrium is attained rapidly. The critical cooling rates of alkali molybdate systems are represented in fig. 2. Although the shapes of most curves are similar to those found for alkali tungstate systems2), showing a minimum near the composition MEMO207, there are some remarkable differences: TABLE 1 Compounds and their melting points in the system Li~MoO4-MoOa Compound

Melting point (°C)

Li2MoO4 (congruent) Li2Mo204 (congruent) LizMoaOlo (congruent)

706 593 607

Compound LizMo4013 (incongruent) MoOz (congruent)

Melting point (°C) 591 795

a) In alkali molybdate systems, Li + is the cation promoting glass formation to the highest degree: glass formation tendencies decrease with increasing ionic radius ( L i + > N a + > K + > R b + > C s + ; cf. for tungstate systems: Na + > K + > Rb + > Li + > Cs+). b) The lithium system is the only one among the alkali molybdates showing a greater tendency for glass formation than the corresponding tungstate systems: Critical cooling rate curves for Na, K and Rb molybdates lie at

GLASS FORMATION IN ALKALI MOLYBDATE SYSTEMS

21

I0 000 Crii:io21

cooli~ rc#e

(°c ~ec-I)

1000

iO0

\J ¢0 I 0

Fig. 2.

I

I

I

i

I

t

50

I

IO0

Critical cooling rate as a function of composition in the systems M 2 0 . y MOO3.

wave numtx~ fcm-~) f600 f400

f200

fO00 900

800

700

.s o

o

Fig. 3.

7

~

9

10

11

12 13 14 15 Wave lengfh (fl)

Infrared spectra of the crystalline monomolybdates M~MoO4.

22

J . c . TH. G. M. VAN DER W1ELEN, H . N . STEIN AND J. M. STEVELS

substantial higher cooling rate values than the corresponding tungstate systems. c) The Li molybdate critical cooling rate vs. composition curve shows a distinct "shoulder" at L i 2 0 . 2 MOO3, presumably connected with relatively rapid crystallization of Li2Mo207 (cf. Dietzel's observations on the "glassiness" of alkali silicate melts 7). For the comparison of data obtained by critical cooling rate measurements with data obtained by a measurement of crystal growth, see ref. 8. 3.2. COORDINATIONOF MOLYBDENUMIONS IN SOLID PHASES In order to discuss these differences, some data on the coordination of molybdenum are presented. Among the crystalline monomolybdate compounds, the structures of LiEMO049), NaEMO041°) and K2MoO4 zl) are known; though the structures are different (Li2MoO 4 having a phenakite structure, NaeMo04 a spinel structure, the K2Mo04 structure being of a type hitherto unknown), they resemble each other in the coordination of the Wavenumber (¢m-1) 1000 900 800 700

1600 1400 1200

/

\

t I

o

07 _

..~

S

.

.~

~

"

I

~Mo2oz

=II,I J,l 6

8

9

.

I~

I

12 13 14. Wavelength(fl)

I

15

Fig. 4. Infrared spectra of the crystalline dimolybdates M~Mo207.

GLASS FORMATION IN ALKALI MOLYBDATE SYSTEMS

23

Mo 6 + ion, this being in all cases a tetrahedral one. The site symmetry of the MoO 2- tetrahedra is highest in the spinel structure (Na2MoO4), much lower in the phenakite structure (Li2MoO¢) and very low in K2MoO~. Among the crystalline dimolybdates, Na2M0207 is the only one whose structure has been determined 10,12); it consists of infinite chains of MoO66octahedra connected by 1) bridging oxygen ions, and 2) MoO 2- tetrahedra sharing two oxygen ions with different octahedra. The structure of K2Mo3Olo 13'14) consists of infinite chains of deformed octahedra and tetragonal pyramids, the Mo 6+ ion being in 6- and 5-coordination respectively. In MoO 3 the molybdenum ion has octahedral coordination; the octahedra are mutually connected by bridging oxygen ions 10). Turning to infrared spectra, it will be seen that those of the monomolybdates (fig. 3.) all show a strong absorption peak at 835 cm -1 (12.2/~m), consistent with the same coordination of molybdenum in those structures (isolated tetrahedra). The lowering of the site symmetry of the MoO ] - group when going from Na2MoO4 to Li2MoO4 and to KzMoO4 appears to in-

-,,--- Wavenumber(cm-I) iO00 9OO

(~00

7OO

/

f

/

6

?

8

,9

10

"/1

f2

13

1~

Wave/ength(/z)

15

Fig. 5. Infrared spectra of lithium molybdate glassesLi~O.y MOO3.

24

J. C. TH. G. M. VAN DER WIELEN, H. N. STEIN AND J. M. STEVELS

fluence appreciably neither the wave number of the principal band nor the degeneracy of the main energy levels involved. Dimolybdate spectra (fig. 4) are much more complicated and show pronounced differences both mutually and when compared with the monomolybdate spectra. The infrared spectra of alkali molybdate glasses (figs. 5 and 6) though resembling superficially those of the crystalline monomolybdates, differ from the latter on closer inspection. The principal absorption peak of the Wave number (cm-1) 1000 900 800 TOO

1600 1400 1200

6

7

8

g

10

11

12

13

I/-/-

15

Wavelengthier) Fig. 6.

Infrared spectra of sodium molybdate glassesNazO .y MOO3.

monomolybdate spectra (835 cm- 1) is still found, but in addition there are other strong bands (880-890 cm- 1, 920-950 cm- 1, 1600-1610 cm- 1) that are found weakly, if at all, in the spectra of crystalline monomolybdates. Especially the 880-890 cm- 1 peak is very strong. The same phenomenon is found in tungstate glasses, though here the difference in wave number between the monotungstate peak (830 cm-l) and the strongest tungstate glass peak (860-870 cm- 1) is less pronounced.

GLASS FORMATIONIN ALKALIMOLYBDATESYSTEMS

25

An exact band assignment for the molybdates is not possible at present. However, the data indicate substantial differences between the molybdenum and the tungsten ions as regards their coordination, the situation in molten molybdates being probably more complicated than in molten tungstates. 3.3. MEASUREMENTSON MOLTENSYSTEMS Complementary information about the coordination of the molybdenum ion is found in the density and thermal expansion isotherms of molten alkali molybdate systems. Densities of molten alkali molybdate systems have been reported previously by Morris and Robinson 15); their data, however, do not permit of comparing all systems at one temperature. In fig. 7. density data obtained at 920 °C (by interpolation of density temperature curves) are summarized as relative deviation from "ideal" density, the latter being that found by linear interpolation between the end

Od

-0.¢0

-0.15 i i J t~Oi 0

J ' r I I 100

- ~ x100

Fig. 7. Relative deviation of density of molten alkali molybdate systems M 2 0 . y MoOa from "ideal" density at 920 °C. ( + ) Lithium molybdates, (©) sodium molybdates, ( × ) potassium molybdates.

members M o O 3 and M z M o O 4. Thus, for a melt that may be obtained by adding p moles of M o O 3 to q moles of M2MoO+, the "ideal" density is taken as did =

P -- dMoo3 +

P+q

q

P+q

dM2MoO+.

26

J.c. TH. G. M. VAN DER WIELEN~ H.N. STEIN AND J. M. STEVELS

The relative deviation, plotted in fig. 7, is then defined as

p=

dexp

-1.

did

The curves have been calculated by taking dMoO3 ~--- 3.083

g cm-3

dLi2MoO 4 ---- 2.669

g c m - 3,

dNa2MoO 4 ---~ 2.568

g c m - 3,

dK2MoO4

(ref. 15),

2.327 g cm -3 .

It is seen that systems obtained by adding M o O 3 to K 2 M o O 4 form melts which show a relative density decrease, most pronounced towards the composition K20"2.5 MoO3[(y-1)/y,~0.6]. The same effect though less pronounced is found for the systems Na2MoO4-MoO3; in lithium molybdate systems, however, the relative density decrease, if any, is very small and superimposed on a relative density increase. It appears then that there are two counteracting effects determining the density of a melt: one, resulting in a relatively spacious structure towards the composition ( y - 1)/y ~0.5, that may be ascribed to formation of "chains" of MoO ] - tetrahedra. On addition of MoO3 to Mo2MO 4 the decreasing O/Mo ratio forces the MoO ] - tetrahedra to combine into chains that are, however, rather low-membered: at the composition M 2 0 . 2 . 5 M o O 3 the average chain length is between two and three tetrahedra. Chain length is not supposed to be uniform since the existence of disproportionation equilibria such as 2 M 0 2 0 2 - ~ MoO ] - + M03020

(1)

is probable. Such a structure is energetically favorable by reason of the tetrahedral coordination of molybdenum, but energetically unfavorable because the chains do not fit readily close to each other causing the appearance of large interstices. Ultimately, at still higher MoO 3 contents of the system, threedimensional units composed of MoO 6- octahedra (mutually connected through bridging oxygen ions) are formed. This results in a relative density versus composition relationship showing a minimum near ( y - 1 ) / y ~ 0 . 5 that is very distinct in K z M o O 4 - M o O 3 and Na2MoO4-MoO 3 systems. In Li2MoO4-MoO3 systems our data do not permit a definite conclusion. This effect appears to be crossed by another, resulting in a relative density increase for systems containing smaller alkali ions. As far as we can see, there are three possible interpretations for it:

GLASS FORMATION IN ALKALI MOLYBDATE SYSTEMS

27

a) Packing difficulties encountered in potassium molybdate systems arise to a lesser extent in lithium molybdate systems, because of smaller alkali ion size; b) Smaller alkali ions cause a shift of the disproportionation equilibria (1) towards more pronounced disproportionation through higher field strength ~): c) Smaller alkali ions effect a more pronounced distortion of the O-Mo-O bond angles by their higher field strength, resulting in easier packing. It is not possible to decide a priori which of these interpretations is the most probable one, though it will be seen later that the present authors favor a combination of the effects mentioned sub b) and c). The picture of the melt thus obtained is only a simplified one. In the first place, every partition of the molybdenum and oxygen ions of a melt over separate chains of three-dimensional units contains an element of arbitrariness: when will, for example, a tetrahedron distorted by the presence of adjacent oxygen ions be considered better as a distorted octahedron? In the second place the foregoing distinguishes sharply between tetrahedral and octahedral coordination of molybdenum. In reality a more continuous distribution of molybdenum over tetrahedra, all kind of distorted polyhedra and octahedra is made likely by the X-ray data obtained for crystalline K2Mo3Olo. However, the admittedly schematical way of treating these systems appears to be useful. 3.4.

GLASS FORMATION

Glass formation may be supposed to be favored by a structure comprising many chains, because such chains fit less readily into a periodical lattice than more simple structural units; besides, X-ray evidence indicates a rather specific sequence of MoOn polyhedra in crystalline Na2Mo2OT, entailing a high entropy change upon crystallization. Thus, the approximate coincidence of the composition of highest relative density decrease with that of easiest glass formation supports the interpretation given of the former. It is emphasized that a high relative density decrease in itself needs not favor glass formation; for in that case glass formation should be easiest in potassium molybdate systems (contrary to experience). Glass formation is favored by chain formation of MoO~- tetrahedra, evidenced by relative density decrease in K2MoO~-MoO 3 systems and to a lesser extent in Na2MoO4-MoO 3 systems and presumed to be present also in a very weak form, if any, in Li2MoO4-MoO 3 systems. As a reason for the reverse sequence of glass formation and densities of systems with different alkali ions, but equal M o O 3 content, a combination of the shift of the disproportionation and distortion of O-Mo-O bond angles through high field strength of small alkali ions [effects mentioned sub b) and c), respectively] appears to be

28

J . c . TH. G. M. VAN DER WIELEN, H. N. STEIN AND J. M. STEVELS

plausible, since it explains both the high glass formation tendency and the absence of relative density decrease. Untill now, the picture presented for the molybdate systems is essentially the same as that presented earlier for the tungstate systems. However, differences between both groups systems are evidenced by: a) differences in glass formation tendencies, b) differences in IR spectra. Both can be explained by assuming that molybdenum ions tend more toward coordinations other than strictly tetrahedral or octahedral ones, than tungsten: the distortion of MoO 2- tetrahedra counteracts the ordening effect exerted by Li + on its surroundings and consequently annihilates the special position occupied by Li ÷ in tungstate systems. A direct comparison of glass formation tendencies in systems of sodium molybdates and sodium tungstates, potassium molybdates and potassium tungstates etc. is difficult since at least two effects counteract: differences in degree of deformation of the tetrahedra, and differences in degree of disproportionation according to (1). A comparison will be possible only if data on the density of alkali tungstate melts are available.

References 1) P. L. Baynton, H. Rawson and J E, Stanworth, Nature 178 (1956) 910. 2) R. J. H. Gelsing, H. N. Stein and J. M. Stevels, Phys. Chem. Glasses 7 (6) (1966) 185. 3) J. D. Mackenzie, Rev. Sci. Instr. 27 (1956) 297. 4) F. Hoermann, Z. Anorg. Allgem. Chem. 177 (1929) 145. 5) V. I. Spitzyn and I. M. Kuleshov, Zh. Obshch. Khim. 21 (1951) 401,408 6) M. V. Mokhosoev, I. M. Kuleshov and P. I. Fedorov, Zh. Neorg. Khim. 7 (1962) 841. 7) A. Dietzel and H. Wickert, Glastech. Ber. 29 (1956) 1. 8) A. C. J. Havermans, H. N. Stein and J. M. Stevels, to be published. 9) W. I-L Zachariasen and H. A. Plettinger, Acta Cryst. 14 (1961) 229. 10) I. Lindqvist, Acta Chem. Scand. 4 (1950) 1066. 11) A. S. Koster, personal communication. 12) M. Seleborg, Acta Chem. Scand. 21 (1967) 499. 13) M. Seleborg, Acta Chem. Scand. 20 (1966) 2195. 14) B. M. Gatehouse and P. Leverett, Chem. Commun. (1967) 374. 15) K.B. Morris and P. L. Robinson, J. Phys. Chem. 68 (1964) 1194.