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中国科技论文在线
Computer modeling of properties of topaz by CLAYFF
Niu Jinan
*
, Qiang Yinghuai, Wang Zhihui
School of Materals Science and Engineering, China University of Mining and Technology,
Jiangsu Xuzhou (221116)
E-mail:njn0516@cumt.edu.cn yhqiang@cumt.edu.cn diao2830@163.com,
Abstract
Clay force field (CLAYFF) is supplemented by F potential parameters deriving from experimental
structures and used to model various topazes, cell parameters and frequencies calculated agree with
experimental results well. Calculation for structures with different H1/H2 ratio from OH-topaz reveals
that the maximum of lattice energy occurs at 0.35/0.65. The results calculated for structures with
different F contents from OH-topaz show that lattice parameters especially c value deviate the simple
linear correlations with F contents. Hydrogen bond calculations reveal more F in structure, the stronger
H bonds interaction of hydroxyl hydrogen.
Keywords: topaz; modeling; lattice; defect; frequency
0 Introduction
Topaz is a fluoroaluminosilicate mineral with the composition of Al
2
(SiO
4
)(OH,F)
2
.The only
major difference between samples is the F/OH ratio. The structure consists of Silicon-oxygen
tetrahedra linking aluminum-oxygen/fluorin octahedral, four of the six anions surrounding Al ion
belong to silicon-oxygen tetrahedral, and the remaining two are either fluorin or hydroxyl oxygen.
The structure is orthorhombic with space group Pbnm
[1,2]
. Topaz has important applications in
radition dosimetry
[3]
, and its optical protical properties may be modified by ion radiation
[4,5]
,
Marques et al
[6]
also investigated this effect.
Computer modeling methods have been increasingly used in the past decade to simulate a wide
range of materials and to evaluate their microscopic structure, physical, and the thermodynamic
properties. Some force fields have been developed successfully for aluminosilicates and related
minerals, such as Teppen’s covalent force field
[7],
Bougeard’s Zeolite-based force field
[8],
Sainz-Diaz’s force field
[9]
and Cygan’s clay force field (CLAYFF)
[10].
CLAYFF differs from
most others in that most interatomic potentials are treated as nonbonded, i.e. no bond interaction
between metal cations and anions, this allows us to effectively use the force field for relatively
large and highly disordered systems while keeping the number of parameters small. CLAYFF has
been successfully used to reproduce boehmite, portlandite, kaolinite, pyrophyllite, montmorillonite
and the vibrational spectra of more complicated hydrotalcite which agreed with experimental
result well, so CLAYFF has obvious advantage for aluminosilicates in contrast to other fields
[10]
.
Earlier, Jackson et al
[2,5,11]
used a supplemented force field transferred from their previous zeolite
simulation to study the perfect lattice and defect properties of two end members of topazes :
Al
2
(SiO
4
)F
2
(F-topaz) and Al
2
(SiO
4
)(OH)
2
(OH-topaz); in this paper, CLAYFF supplemented by F
potential is applied to study the lattice properties of topaz at different F/OH ratios and OH-topaz at
different H1/H2 ratios with the aim of complementing Jackson’s previous studies; at the same
time F- and OH-topaz defect and vibration properties are also calculated to compare with
Jackson’s results
[2]
.
Support by the Special Foundation for Science and Technology Research of China University of Mining and
Technology (ZK0005).