INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 16 (2004) S2771–S2779 PII: S0953-8984(04)70766-3 A computational study of the structure, lattice and defect properties of pure and doped F - and OH - -topaz R A Jackson 1,3 and M E G Valerio 2 1 Lennard-Jones Laboratories, School of Chemistry and Physics, Keele University, Keele, Staffs ST5 5BG, UK 2 Departamento de F´ ısica, Universidade Federal de Sergipe, 49.100-000 S˜ ao Cristov˜ ao, SE, Brazil E-mail: r.a.jackson@chem.keele.ac.uk and mvalerio@fisica.ufs.br Received 16 October 2003 Published 25 June 2004 Online at stacks.iop.org/JPhysCM/16/S2771 doi:10.1088/0953-8984/16/27/012 Abstract This paper describes a computer modelling study of the F − and OH − forms of topaz, Al 2 SiO 4 (OH, F) 2 . A potential model is developed, and used to calculate the perfect lattice and defect properties of both forms of the mineral. Excellent agreement between experimental and calculated phonon frequencies is obtained. Predictions are made of the probable intrinsic defect structure and of the location of dopant ions incorporated in ion implantation. 1. Introduction Topaz is an aluminium fluoro-/hydroxy-silicate with fairly constant chemical composition Al 2 SiO 4 (OH, F) 2 . The only major difference found between samples is the F/OH ratio. The structure consists of [SiO 4 } 4− groups linking octahedral chains of Al[O 4 (F, OH) 2 ] in a zigzag fashion parallel to the c-axis. Four of the six anions surrounding the Al 3+ ion belong to [SiO 4 ] 4− tetrahedra; the remaining two are either F − or OH − groups. The structure is orthorhombic, with space group Pbnm [1]. Topaz has important potential applications in dosimeter devices, in which the F/OH ratio has been found to be of significance, particularly as it shows tissue equivalent dosimetry behaviour [2]. This paper describes a computational approach to the study of topaz. Computer modelling methods (see [3] for a recent mineralogical application) are applied using a combination of potentials transferred from previous aluminosilicate simulations [4], and newly obtained Al–O and Al–F potentials, fitted to the F − -topaz structure. Calculated perfect lattice properties for F − - and OH − -topaz are presented and compared with experimental values of infrared and Raman frequencies. Defect formation energies are reported for intrinsic defects in both materials,and conclusions drawn about the probable defect structure 3 Author to whom any correspondence should be addressed. 0953-8984/04/272771+09$30.00 © 2004 IOP Publishing Ltd Printed in the UK S2771