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Bond volumes in crystals and glasses and a study of the germanate anomaly
Ist Teil von
Journal of non-crystalline solids, 1999-02, Vol.243 (2), p.220-232
Ort / Verlag
Amsterdam: Elsevier B.V
Erscheinungsjahr
1999
Quelle
Alma/SFX Local Collection
Beschreibungen/Notizen
The unit cell of a crystal is decomposed into partial volumes
V
b of its bonds.
V
b is described in terms of the bond length
d and the coordination number
N by
V
b
=V
0+α
∗(d
3/N)
. On the basis of structural data of about 170 binary and ternary compounds, including borates, silicates and germanates, it is demonstrated that
V
0=0 and
α
∗
comp
=12.25
define compact structures.
V
0≠0 is a signature of structures containing free volumes or voids. The universal character of
α
∗
comp
permits the determination of the free volume
V
0 in alkali-germanate glasses and thereby a quantitative test of the idea that a change of the germanium coordination is the origin of the germanate anomaly. The test shows that the idea does not work since the pure transition from
N
Ge=4 to
N
Ge=6 changes the volume of the glass only by a marginal amount. It is suggested that a redistribution of void sizes which is accompanied by a negative deformation of the network is the leading mechanism which is responsible for the anomaly.