Medium-range order around titanium in a silicate glass studied by neutron diffraction
with isotopic substitution
L. Cormier*
Laboratoire de Mine ´ralogie-Cristallographie, Universite ´s Paris 6 et 7 et IPGP et UMR CNRS 7590, 4 place Jussieu,
75252 Paris Cedex 05, France
P. H. Gaskell
Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, England
G. Calas
Laboratoire de Mine ´ralogie-Cristallographie, Universite ´s Paris 6 et 7 et IPGP et UMR CNRS 7590, 4 place Jussieu,
75252 Paris Cedex 05, France
A. K. Soper
Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, England
Received 3 March 1998
The structure of a silicate glass of composition K
2
O•TiO
2
•2SiO
2
has been reinvestigated by neutron dif-
fraction with isotopic substitution of Ti. These data confirm Ti to be 5-coordinated within a square-based
pyramid. The second difference function, which gives directly the Ti-Ti distribution, shows a first Ti-Ti
distance at 3.5 Å, with second and third Ti neighbors at 6 Å and 8 Å. These values correspond to a nonho-
mogeneous distribution of Ti in the glass structure and are consistent with corner-sharing TiO
5
pyramids. This
explains how this peculiar TiO
5
site, with one nonbridging O at 1.68 Å and four O at 1.96 Å, allows Ti to play
both a former and a modifier role in the glass structure. The environment of potassium is consistent with a
charge compensating role. The presence of Ti-enriched regions in the glass, which is in agreement with recent
reverse Monte Carlo simulations, is favored by the potassium atoms acting as charge compensators around the
short titanyl bond. S0163-18299807038-6
I. INTRODUCTION
The nature and extent of medium range order MRO in
glasses has been recently the subject of numerous studies by
EXAFS, neutron diffraction with isotopic substitution and
anomalous wide angle x-ray scattering.
1–3
These works have
shown that low charge cations are not homogeneously dis-
tributed into the glass structure but locally concentrated in
channels or ordered domains interlacing with the silicate
framework. A pioneering work to investigate the environ-
ment around a high charge cation, Ti
4+
, was carried out by
Wright et al. in a silicate glass by neutron diffraction with
isotopic substitution but the medium range organization
around titanium was poorly defined.
4
However, recent stud-
ies have shown that titanium oxide in alkali silicate glasses
has important effects on some thermodynamic properties.
5–7
For instance, the variation of the heat capacity at the glass
transition temperature can be 50% higher for the Ti-bearing
glasses than for the Ti-free glasses, in relation to an ‘‘ex-
cess’’ configurational entropy in titanosilicate systems. The
Cp anomaly increases with Na or K content and with the
depolymerization of the network rising to a maximum at the
trisilicate composition (K
2
TiSi
2
O
7
). The addition of TiO
2
into silicate glasses has also drastic effects on other physical
properties like thermal expansion,
8
nucleation rate and
viscosity.
9–12
It is thus important to clarify the structural role
of Ti in order to understand the influence of this element on
the physical properties of these glasses.
In early works, x-ray diffraction was used to study the
introduction of TiO
2
in alkali silicate glasses Ref. 13 and
the polymerization of the network has been investigated by
Raman spectroscopy, but little information has been gained
on the medium-range environment around Ti.
12,14
In binary
TiO
2
-SiO
2
glasses, Ti is 4-coordinated,
15
but several XAS
and x-ray absorption near edge structure XANES measure-
ments show that Ti-containing silicate glasses can be consid-
ered as composition-dependent mixtures of 4-, 5- and 6-
coordinated Ti.
16,17
The most comprehensive investigation of this system was
carried out by Wright et al. and Yarker et al.
4,18
The Ti site
geometry in alkali silicate glasses was determined in a K
2
O
TiO
2
2SiO
2
KTS2 glass by neutron diffraction with isoto-
pic substitution of Ti, combined with extended x-ray absorp-
tion fine structure EXAFS and XANES measurements on
the same glass. This work shows the presence of 5-
coordinated Ti,
5
Ti, with approximately one Ti-O distance
shorter than the other four. This is consistent with the pres-
ence of a titanyl group—a square-based pyramid,
TiO
5
—which is observed in crystalline Na
2
TiSiO
5
in con-
trast to many other crystalline alkali-titanium silicates, where
octahedral coordination of Ti is found. This neutron investi-
gation was pioneering in the sense that differences between
the scattered intensity measured for glasses with different Ti
isotopes were used to obtain information on the environment
around Ti. This includes an estimate of the Ti-Ti correlation
function by performing a double difference, which showed a
PHYSICAL REVIEW B 1 NOVEMBER 1998-I VOLUME 58, NUMBER 17
PRB 58 0163-1829/98/5817/113229/$15.00 11 322 ©1998 The American Physical Society