Davoli, Ivan

Dingwell, Donald B.

Mottana, Annibale

Paris, Eleonora

Romano, Claudia

Seifert, Friedrich

English

Open Access LMU

in-situ h i g h tempera tu re s tudy of t i t a n i u m c o o r d i n a t i o n in Rb-s i l i ca te g lass and l iqu id to 1400 ° C F. Se i fer t * ; Ε. Par is** ; D.B. Dingwall*; A. Mot tana** * ; C. Romano* * * * ; I. Davo l i * * * * * 'Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany "Dipartimento di Scienze delta Terra, Universitä di Camerino, I-62032 Camerino, Italy ' "Dipart imento di Scienze Geologiche, Terza Universitä di Roma, 1-00146 Roma, Italy " "D ipar t imento di Scienze della Terra, Universitä di Roma "La Sapienza", 1-00185 Roma, Italy " " •D ipar t imento di Matematica e Fisica, Universitä di Camerino, I-62032 Camerino, Italy Abstract In-situ high-temperature XANES spectra at the Ti-K edge demonstrate that the coordination number of Ti in Rb2~nSi40n in the glassy, supercooled and stable liquid regimes remains essentially unchanged at a mean value close to 5 up to a temperature of 1400 ° C . A slight increase in the intensity of the pre-edge peak with temperature can be taken as evidence for a minor decrease in the average coordination number of Ti (from ca. 5.0 in the glassy state to ca. 4.75 at 1400 °C). Introduction Most studies of melt structures have been performed on their quenched equivalents, i. e. glasses and assumed that these structures represent that of the melt near the glass-transition temperature (cf. Mysen, 1988). A limited temperature range (on the order of several tens of degrees) is accessible for such studies by changing the quench-rate (Dingwell and Webb, 1990). For covering a wider temperature range, in particular into the region of stable liquids, in-situ techniques have to be used instead. Such studies are rare, mostly because of methodological difficulties. It has been shown, however, that the main structural features such as the topology of the network remains unchanged with temperature in fully polymerized melts such as GeÜ2 (Seifert et al., 1981) and that, in •depolymerized melts, only rather subtle changes occur, such as in the connectivity of S 1 O 4 tetrahedra (i.e. the distribution of Q-species) via disproportionation reactions of the type Q 3 = Q 4 + Q 2 (Brandriss and Stebbins, 1988; Mysen and Frantz. 1992; Seifert et al., 221 1981). Furthermore, minor coordination changes of Si and Al have been found (e.g. Poe et al.. 1992 a, b). It is the purpose of the present note to describe in-situ measurements on a Rb2TiSi40-j 1 glass and melt via Ti-K-edge XANES in the temperature range of 25 to 1~400 °C . This composition has been chosen as a result of a general comparison of X n + 2 / n T i S i 4 0 n glasses (where X is Li, Na, K, Rb, Cs, Ca, Sr, Ba, and AI) (Dingweü et al., 1993). This melt is expected, for reasons to be discussed below, to have an intermediate coordination number of Ti between 4 and 6 and therefore to be susceptible to variation with temperature. The need of in-situ spectroscopic data for explaining anomalous macroscopic properties in Ti-bearing melts has been emphasized recently by Lange and Navrotsky (1993). Experimental Starting materials for the preparation of a glass of Rb2TiSi40i 1 composition were powder mixtures of Si02 (99.95), T1O2 (99.8) and Rb2C03 (99) which were fused and homogenized at 1400°C in air in a M0S12 box furnace in a R crucible and then poured onto a steel plate for cooling. An analysis is provided by Dingwell et al. (1993) The X-ray absorption of the glass and liquid sample has been measured at temperatures between 25 and 1400 °C in a high-temperature cell (Seifert et al., 1993) at the PWA line of the National Synchrotron Radiation Facility, Frascati, Italy, at the Ti K-edge (4966 eV) using a (111) channel-cut Si monochromator with an energy resolution of 1 eV, at steps of 0.2 eV. The powdered glassy starting material was pressed into a Pt mesh (mesh size ca. 0.3 mm) and this mesh was then mounted between two MgO blocks held together by Pt wires. The MgO blocks were drilled in such away as to ensure unattenuated passage of photons. This assembly was then inserted into a horizontal Pt-wound tube furnace. The surface tension of the melt was sufficient to keep a film of appropriate thickness suspended vertically for the period of the measurements (several hours). Temperature was measured and controlled by a Pt-Pt90RhlO thermocouple and held constant to ±1°C. Between individual scans, temperature was allowed to equilibrate for ca. 30 minutes. A Ti metal foil (EXAFS Materials, 4966.0 eV) was used for energy calibration at room 222 temperature. Experimental XANES data have been reduced by subtraction of background and normalisation of the spectra at the absorption edge. Results and Discussion Rg . 1 shows a (normalized) comparison of the spectra obtained. It should be noted that all spectra except the room temperature spectrum are in the region of the relaxed (stable or metastable) liquid, i.e. above the glass transition temperature Tg which, for this composition, lies between 550 and 600 ° C (Takahashi et a!., 1977). At these elevated temperatures rapid structural relaxation and consequently an equilibrium structural state are assumed. The overall shape of the absorption spectra is nearly independent of temperature, although with increasing temperature a general broadening of the features just above the edge can be observed, as well as increasing noise. The most important result is, however, that the relative height of the pre-edge peak only varies marginal ly with temperature, f rom 4 5 % (normalized) height at room temperature (corresponding to a structural state near T g at 550 - 600 ° C ) to 52 - 53 % at the highest temperature employed. We therefore conclude that the overall structure of the melt shows little change with temperature. In detail, however, we observe a systematic temperature dependence: although the variation in pre-edge intensity with temperature is not much larger than the mean standard deviation of the individual determinations, a trend of increasing pre-edge height with temperature is apparent from the data (Fig. 2). It has been shown (Waychunas. 1987; Paris et a l . 1993; Dingwell et al. , 1993) that the height of the T i 4 + pre-edge is inversely correlated with coordination number (CN) of t i tanium. It is also clear however, that appreciable ranges are observed at a given coordination number, most likely related to varying degrees of distortion of the polyhedra. The intensity of the pre-edge peak changes from 1 - 29 % for CN = 6 to 40 - 45 % for CN = 5 to 57 - 84 for CN = 4 in the crystalline state. A least-squares fit on the variation of pre-edge intensity as a function of coordination number has been obtained (Paris et at., 1993). Applying this relationship for crystalline standards to the present liquid data, the average coordination number of Ti in the R b 2 T i S i 4 0 i 1 glass is close to 5.0, and the change in pre-edge height reported in R g . 2 then corresponds to a decrease in average coordination number on the order of 0.25 in the temperature range between T g and 223 1400 °C . Such a decrease of CN with temperature may be expected on very general crystal chemical grounds; what is surprising, however, is that the magnitude of this effect is so small. It should be emphasized that, given the lack of energy resolution in the XANES absorption spectra, we would not be able to distinguish between a fivefold and a mixed 4+6-fold coordination of titanium, i.e. we cannot determine the width of the distribution curve of coordination numbers which may also change with temperature. Given the ranges of pre-edge intensity at constant coordination numbers, the subtle variation with temperature could, in principle, also reflect a change of site distortion at constant coordination number. We favour, however, for the large variations resulting from chemical substitution a variation in average coordination number. This is because of the large variations in the volume per oxygen attributed to titanium in these melts which is evident from the density data for these melts (Dingwell, 1992). Acknowledgements: We thank Prof. Emilio Burattini and his PWA staff for assistance in the operation. Financial support from Fonds der Chemischen Industrie, Frankfurt; CNR, Rome (CT92.00799.CT05) and Vigoni exchange programme of CRUI and DAAD is gratefully acknowledged. R e f e r e n c e s : ΙΛΙ Brandriss, M.E.; Stebbins, J.F.: Effects of temperature on the structure of silicate liquids: 2 9 S i NMR results. Geochimica et Cosmochimica Acta 52 (1988) p. 2659-2669. 121 Dingwell, D.B.. Density of some titanium-bearing silicate liquids and the compositional dependence of the partial molar volume of T1O2- Geochimica et Cosmochimica Acta 56(1992) p. 3403-3407. 12,1 Dingwell, D.B.; Paris, E.; Seifert, F.; Mottana, Α.; Romano, C : An X-ray absorption study of Ti-bearing silicate glasses, (submitted to Physics and Chemistry of Minerals) (1993) 224 IAI Dingwelt. D.B.; Webb, S. L : Relaxation in silicate melts. European Journal of Mineralogy 2 (1990) ρ 427-449. fSI Lange, R A ; Navrotsky, Α.: Heat capacities of TiOa-bearing silicate liquids: Evidence for anomalous changes in configurational entropy with temperature. Geochimica et Cosmochimica Acta 57 (1993) p. 3 0 0 1 - 3 0 1 1 . m Mysen, B.O.: Structure and Properties of Silicate Melts. Elsevier (1988) 354 pp. !7f Mysen, B.O.; Frantz. J.D.: Raman spectroscopy of silicate melts at magmatic temperatures: Na20 -S i02 , K20 -S i02 and Ü 2 0 - S i 0 2 binary compositions in the temperature range 25-1475°C. Chemical Geology 96 (1992) p. 321-332. 78 / Paris, E.; Romano, C ; Mottana, Α.; Seifert. F.; Dingwell, D.B.: Ti K-edge XANES study of crystalline Na2"nSi05 and the systematics of pre-edge intensity versus coordination number.(submitted to Physics and Chemistry of Minerals) (1993) /9/ Poe, B.T.; McMillan, P.F.; Angell, Α.; and Sato, R.K.: Al and Si coordination in S i02-A l203 glasses and liquids. A study by NMR and IR spectroscopy and MD simulations. Chemical Geology 96 (1992a) p. 333-349 /10 / Poe. B.T.; McMillan, P.F.; Cote. B.; Massiot, D.; Coutures, J.P.: S1O2-AI2O3 liquids: in-situ study by high-temperature 2 7 A t NMR spectroscopy and molecular dynamics simulation. Journal of Physical Chemistry 96 (1992b) p. 8220-8224. / 1 1 / Seifert; F., Mysen; B.O.; Virgo, D.: Structural similarity between glasses and melts relevant to penological processes. Geochimica et Cosmochimica Acta 45 (1981) 1879-1884. / 12 / Seifert, F.; Paris, Ε.; Dingwell, D. B.; Davoli. I.; Mottana, Α.: A high-temperature device for in situ measurement of X-ray absorption spectra. Condensed Matterand Materials Comm. 1 (1993) (in press) 225 713/ Takahashi, Κ.; Mochida, Ν1.; Yoshida, Υ.: Properties and structure of silicate glasses containing tetravalent cations. Journal of the Ceramic Society of Japan 85 (1977} :p. 330-340. /14 / Waychunas, G.A.: Synchrotron radiation XANES spectroscopy of Tl in minerals: Effects of Ti bonding distances, Ti valence, and site geometry on absorption edge structure. American Mineralogisl 72 (1987) p. 89-101. F igu re c a p t i o n s Fig. 1 XANES spectra of Rb2TiSi40-j 1 melt and glass, normalized to the maximum height of the absorption edge. Note the general similarity of the topology of the spectra. 226 4 9 6 0 4 9 8 0 5 0 0 0 5 0 2 0 5 0 4 0 ENERGY (eV) 2 2 7 

C: An X-ray absorption study of Ti-bearing silicate glasses, (submitted to Physics and Chemistry of Minerals)

D.B.: Ti K-edge XANES study of crystalline Na2&quot;nSi05 and the systematics of pre-edge intensity versus coordination number.(submitted to Physics and Chemistry of Minerals)

Density of some titanium-bearing silicate liquids and the compositional dependence of the partial molar volume of T1O2-

Effects of temperature on the structure of silicate liquids: 29Si NMR results.

G.A.: Synchrotron radiation XANES spectroscopy of Tl in minerals: Effects of Ti bonding distances, Ti valence, and site geometry on absorption edge structure.

Heat capacities of TiOa-bearing silicate liquids: Evidence for anomalous changes in configurational entropy with temperature.

Raman spectroscopy of silicate melts at magmatic temperatures:

Structure and Properties of Silicate Melts.

Α.: A high-temperature device for in situ measurement of X-ray absorption spectra.

In Situ high temperature study of titanium coordination in a Rb-silicate glass and liquid to 1400°C

Universität München: Elektronischen Publikationen

in-situ h i g h tempera tu re s tudy of t i t a n i u m c o o r d i n a t i o n 
in Rb-s i l i ca te g lass and l iqu id to 1400 ° C 
F. Se i fer t * ; Ε. Par is** ; D.B. Dingwall*; A. Mot tana** * ; 
C. Romano* * * * ; I. Davo l i * * * * * 
'Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany 
"Dipartimento di Scienze delta Terra, Universitä di Camerino, I-62032 Camerino, Italy 
' "Dipart imento di Scienze Geologiche, Terza Universitä di Roma, 1-00146 Roma, Italy 
" "D ipar t imento di Scienze della Terra, Universitä di Roma "La Sapienza", 1-00185 Roma, Italy 
" " •D ipar t imento di Matematica e Fisica, Universitä di Camerino, I-62032 Camerino, Italy 
Abstract 
In-situ high-temperature XANES spectra at the Ti-K edge demonstrate that the 
coordination number of Ti in Rb2~nSi40n in the glassy, supercooled and stable liquid regimes 
remains essentially unchanged at a mean value close to 5 up to a temperature of 1400 ° C . A slight 
increase in the intensity of the pre-edge peak with temperature can be taken as evidence for a 
minor decrease in the average coordination number of Ti (from ca. 5.0 in the glassy state to ca. 
4.75 at 1400 °C). 
Introduction 
Most studies of melt structures have been performed on their quenched equivalents, i. e. 
glasses and assumed that these structures represent that of the melt near the glass-transition 
temperature (cf. Mysen, 1988). A limited temperature range (on the order of several tens of 
degrees) is accessible for such studies by changing the quench-rate (Dingwell and Webb, 1990). 
For covering a wider temperature range, in particular into the region of stable liquids, in-situ 
techniques have to be used instead. Such studies are rare, mostly because of methodological 
difficulties. It has been shown, however, that the main structural features such as the topology of 
the network remains unchanged with temperature in fully polymerized melts such as GeÜ2 (Seifert 
et al., 1981) and that, in •depolymerized melts, only rather subtle changes occur, such as in the 
connectivity of S 1 O 4 tetrahedra (i.e. the distribution of Q-species) via disproportionation reactions 
of the type Q 3 = Q 4 + Q 2 (Brandriss and Stebbins, 1988; Mysen and Frantz. 1992; Seifert et al., 
221 
1981). Furthermore, minor coordination changes of Si and Al have been found (e.g. Poe et al.. 
1992 a, b). 
It is the purpose of the present note to describe in-situ measurements on a Rb2TiSi40-j 1 
glass and melt via Ti-K-edge XANES in the temperature range of 25 to 1~400 °C . This composition 
has been chosen as a result of a general comparison of X n + 2 / n T i S i 4 0 n glasses (where X is Li, 
Na, K, Rb, Cs, Ca, Sr, Ba, and AI) (Dingweü et al., 1993). This melt is expected, for reasons to be 
discussed below, to have an intermediate coordination number of Ti between 4 and 6 and 
therefore to be susceptible to variation with temperature. The need of in-situ spectroscopic data 
for explaining anomalous macroscopic properties in Ti-bearing melts has been emphasized 
recently by Lange and Navrotsky (1993). 
Experimental 
Starting materials for the preparation of a glass of Rb2TiSi40i 1 composition were powder 
mixtures of Si02 (99.95), T1O2 (99.8) and Rb2C03 (99) which were fused and homogenized at 
1400°C in air in a M0S12 box furnace in a R crucible and then poured onto a steel plate for cooling. 
An analysis is provided by Dingwell et al. (1993) 
The X-ray absorption of the glass and liquid sample has been measured at temperatures 
between 25 and 1400 °C in a high-temperature cell (Seifert et al., 1993) at the PWA line of the 
National Synchrotron Radiation Facility, Frascati, Italy, at the Ti K-edge (4966 eV) using a (111) 
channel-cut Si monochromator with an energy resolution of 1 eV, at steps of 0.2 eV. The 
powdered glassy starting material was pressed into a Pt mesh (mesh size ca. 0.3 mm) and this 
mesh was then mounted between two MgO blocks held together by Pt wires. The MgO blocks 
were drilled in such away as to ensure unattenuated passage of photons. This assembly was then 
inserted into a horizontal Pt-wound tube furnace. The surface tension of the melt was sufficient to 
keep a film of appropriate thickness suspended vertically for the period of the measurements 
(several hours). Temperature was measured and controlled by a Pt-Pt90RhlO thermocouple and 
held constant to ±1°C. Between individual scans, temperature was allowed to equilibrate for ca. 30 
minutes. A Ti metal foil (EXAFS Materials, 4966.0 eV) was used for energy calibration at room 
222 
temperature. Experimental XANES data have been reduced by subtraction of background and 
normalisation of the spectra at the absorption edge. 
Results and Discussion 
Rg . 1 shows a (normalized) comparison of the spectra obtained. It should be noted that all 
spectra except the room temperature spectrum are in the region of the relaxed (stable or 
metastable) liquid, i.e. above the glass transition temperature Tg which, for this composition, lies 
between 550 and 600 ° C (Takahashi et a!., 1977). At these elevated temperatures rapid structural 
relaxation and consequently an equilibrium structural state are assumed. The overall shape of the 
absorption spectra is nearly independent of temperature, although with increasing temperature a 
general broadening of the features just above the edge can be observed, as well as increasing 
noise. The most important result is, however, that the relative height of the pre-edge peak only 
varies marginal ly with temperature, f rom 4 5 % (normalized) height at room temperature 
(corresponding to a structural state near T g at 550 - 600 ° C ) to 52 - 53 % at the highest 
temperature employed. We therefore conclude that the overall structure of the melt shows little 
change with temperature. In detail, however, we observe a systematic temperature dependence: 
although the variation in pre-edge intensity with temperature is not much larger than the mean 
standard deviation of the individual determinations, a trend of increasing pre-edge height with 
temperature is apparent from the data (Fig. 2). 
It has been shown (Waychunas. 1987; Paris et a l . 1993; Dingwell et al. , 1993) that the 
height of the T i 4 + pre-edge is inversely correlated with coordination number (CN) of t i tanium. It is 
also clear however, that appreciable ranges are observed at a given coordination number, most 
likely related to varying degrees of distortion of the polyhedra. The intensity of the pre-edge peak 
changes from 1 - 29 % for CN = 6 to 40 - 45 % for CN = 5 to 57 - 84 for CN = 4 in the crystalline 
state. A least-squares fit on the variation of pre-edge intensity as a function of coordination number 
has been obtained (Paris et at., 1993). Applying this relationship for crystalline standards to the 
present liquid data, the average coordination number of Ti in the R b 2 T i S i 4 0 i 1 glass is close to 
5.0, and the change in pre-edge height reported in R g . 2 then corresponds to a decrease in 
average coordination number on the order of 0.25 in the temperature range between T g and 
223 
1400 °C . Such a decrease of CN with temperature may be expected on very general crystal 
chemical grounds; what is surprising, however, is that the magnitude of this effect is so small. It 
should be emphasized that, given the lack of energy resolution in the XANES absorption spectra, 
we would not be able to distinguish between a fivefold and a mixed 4+6-fold coordination of 
titanium, i.e. we cannot determine the width of the distribution curve of coordination numbers 
which may also change with temperature. 
Given the ranges of pre-edge intensity at constant coordination numbers, the subtle 
variation with temperature could, in principle, also reflect a change of site distortion at constant 
coordination number. We favour, however, for the large variations resulting from chemical 
substitution a variation in average coordination number. This is because of the large variations in 
the volume per oxygen attributed to titanium in these melts which is evident from the density data 
for these melts (Dingwell, 1992). 
Acknowledgements: We thank Prof. Emilio Burattini and his PWA staff for assistance in the 
operation. Financial support from Fonds der Chemischen Industrie, Frankfurt; CNR, Rome 
(CT92.00799.CT05) and Vigoni exchange programme of CRUI and DAAD is gratefully 
acknowledged. 
R e f e r e n c e s : 
ΙΛΙ Brandriss, M.E.; Stebbins, J.F.: 
Effects of temperature on the structure of silicate liquids: 2 9 S i NMR results. Geochimica et 
Cosmochimica Acta 52 (1988) p. 2659-2669. 
121 Dingwell, D.B.. 
Density of some titanium-bearing silicate liquids and the compositional dependence of the 
partial molar volume of T1O2- Geochimica et Cosmochimica Acta 56(1992) p. 3403-3407. 
12,1 Dingwell, D.B.; Paris, E.; Seifert, F.; Mottana, Α.; Romano, C : 
An X-ray absorption study of Ti-bearing silicate glasses, (submitted to Physics and Chemistry 
of Minerals) (1993) 
224 
IAI Dingwelt. D.B.; Webb, S. L : 
Relaxation in silicate melts. European Journal of Mineralogy 2 (1990) ρ 427-449. 
fSI Lange, R A ; Navrotsky, Α.: 
Heat capacities of TiOa-bearing silicate liquids: Evidence for anomalous changes in 
configurational entropy with temperature. Geochimica et Cosmochimica Acta 57 (1993) p. 
3 0 0 1 - 3 0 1 1 . 
m Mysen, B.O.: 
Structure and Properties of Silicate Melts. Elsevier (1988) 354 pp. 
!7f Mysen, B.O.; Frantz. J.D.: 
Raman spectroscopy of silicate melts at magmatic temperatures: Na20 -S i02 , K20 -S i02 and 
Ü 2 0 - S i 0 2 binary compositions in the temperature range 25-1475°C. Chemical Geology 96 
(1992) p. 321-332. 
78 / Paris, E.; Romano, C ; Mottana, Α.; Seifert. F.; Dingwell, D.B.: 
Ti K-edge XANES study of crystalline Na2"nSi05 and the systematics of pre-edge intensity 
versus coordination number.(submitted to Physics and Chemistry of Minerals) (1993) 
/9/ Poe, B.T.; McMillan, P.F.; Angell, Α.; and Sato, R.K.: 
Al and Si coordination in S i02-A l203 glasses and liquids. A study by NMR and IR 
spectroscopy and MD simulations. Chemical Geology 96 (1992a) p. 333-349 
/10 / Poe. B.T.; McMillan, P.F.; Cote. B.; Massiot, D.; Coutures, J.P.: 
S1O2-AI2O3 liquids: in-situ study by high-temperature 2 7 A t NMR spectroscopy and 
molecular dynamics simulation. Journal of Physical Chemistry 96 (1992b) p. 8220-8224. 
/ 1 1 / Seifert; F., Mysen; B.O.; Virgo, D.: 
Structural similarity between glasses and melts relevant to penological processes. 
Geochimica et Cosmochimica Acta 45 (1981) 1879-1884. 
/ 12 / Seifert, F.; Paris, Ε.; Dingwell, D. B.; Davoli. I.; Mottana, Α.: 
A high-temperature device for in situ measurement of X-ray absorption spectra. Condensed 
Matterand Materials Comm. 1 (1993) (in press) 
225 
713/ Takahashi, Κ.; Mochida, Ν1.; Yoshida, Υ.: 
Properties and structure of silicate glasses containing tetravalent cations. Journal of the 
Ceramic Society of Japan 85 (1977} :p. 330-340. 
/14 / Waychunas, G.A.: 
Synchrotron radiation XANES spectroscopy of Tl in minerals: Effects of Ti bonding 
distances, Ti valence, and site geometry on absorption edge structure. American 
Mineralogisl 72 (1987) p. 89-101. 
F igu re c a p t i o n s 
Fig. 1 
XANES spectra of Rb2TiSi40-j 1 melt and glass, normalized to the maximum height of the 
absorption edge. Note the general similarity of the topology of the spectra. 
226 
4 9 6 0 4 9 8 0 5 0 0 0 5 0 2 0 5 0 4 0 
ENERGY (eV) 
2 2 7 


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In Situ high temperature study of titanium coordination in a Rb-silicate glass and liquid to 1400°C

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