We have developed a major new computer code, SHELL (available for distribution via the WWW – see http://dougal.chm.bris.ac.uk/programs/shell), which incorporates significant general advances in the methodology for the direct minimisation of the free energies of periodic solids and surfaces with possibly very large unit cells. This is achieved using a combination of lattice statics and dynamics together with analytic derivatives of the vibrational frequencies with respect to all external and internal variables, treating all of these on an equal footing.

We have used this new code, together with Monte Carlo, molecular dynamics and periodic Hartree-Fock calculations to examine the behaviour of a wide range of systems at high pressure and/or high temperature, including, for the first time, transition metal oxides. Applications range from understanding the negative thermal expansion of ZrW₂O₈ to the structures adopted by MnO and NiO at high pressure, which is prompting new experimental studies. We have calculated, also for the first time, defect free energies and surface free energies as a function of temperature, based on the full minimisation.

A new means of classification of the mechanisms of reconstructive phase transitions has been developed and we have published the first comparative study of the energetics of competing mechanisms in ionic solids. This opens up the exciting prospect of soon carrying out the first a priori dynamics study of the kinetics of a solid-state reaction.

The theory developed for the free-energy minimisation has also been applied to (a) the thermodynamics of solid solutions by averaging over a large number of different arrangements of the cations within a large supercell and where our lattice dynamics approach has the advantage that the Gibbs free energy can be calculated efficiently and accurately without resorting to lengthy thermodynamic integration (b) polymers, metals and alloys. Major advances have also been made in the derivation of the accurate interionic potentials from ab initio theory. In addition, with the help of a CASE student, our results for solids have been implemented into a hydrodynamic code used in the explosives industry, extracting the required input parameters where required from our simulations.

1. Free energy derivatives and structure optimisation within quasiharmonic lattice dynamics, M.B. Taylor, G.D. Barrera, N.L. Allan and T.H.K. Barron, Phys. Rev. B56, 14380-14390 (1997).

2. Quasiharmonic free energy and derivatives for three-body interactions, M.B. Taylor, N.L. Allan, J.A.O. Bruno and G.D. Barrera, Phys. Rev. B (submitted).

3. SHELL – a code for lattice dynamics and structure optimisation of ionic crystals, M.B. Taylor, G.D. Barrera, N.L. Allan, T.H.K. Barron and W.C. Mackrodt, Comp. Phys. Comm. 109, 135-143 (1998).

4. Surfaces at high temperatures – quasiharmonic free energy and derivatives for slabs, M.B. Taylor, C.E. Sims, N.L. Allan and W.C. Mackrodt (to be submitted to Phil. Mag.)

5. The zero static internal stress approximation in lattice dynamics and the calculation of isotope effects on molar volumes, N.L. Allan, T.H.K. Barron and J.A.O. Bruno, J. Chem. Phys. 105, 8300-8303 (1996).

6. Ionic solids at elevated temperatures and high pressures: MgF₂, G.D. Barrera, M.B. Taylor, N.L. Allan, T.H.K. Barron, L.N. Kantorovich and W.C. Mackrodt, J. Chem. Phys. 107, 4337-4344 (1997).

7. Polar solids at high temperatures and pressures: a-Al₂O₃, G.D. Barrera, N.L. Allan and W.C. Mackrodt (in preparation)

8. Ionic solids at high temperatures and pressures: ab initio, lattice dynamics and Monte Carlo studies, N.L. Allan, G.D. Barrera, W.C. Mackrodt, C.E. Sims and M.B. Taylor, J. Chem Soc., Faraday Trans (invited Feature Article) (submitted)

9. The free energy of formation of defects in polar solids, M.B. Taylor, G.D. Barrera, N.L. Allan, T.H.K. Barron, and W.C. Mackrodt, Faraday Discuss. 106, 377-387 (1997).

10. Understanding negative thermal expansion, J. Mat. Chem. (invited feature article) (in preparation)

11. The stability of polymorphs of MgCl₂ – an ab initio study, G.D. Barrera, N.L. Allan and M.R. Soriano, Chem. Phys. Lett. 278, 267-271 (1997); a further ms.:Interatomic potentials from ab initio theory: MgCl₂ will be submitted to the same journal.

12. A first-principles Hartree-Fock description of MnO at high pressures, W.C Mackrodt, E-A. Williamson, D. Williams and N.L. Allan, Phil. Mag. B77, 1063-1075 (1998).

13. The thermodynamics and mechanism of the B1-B2 phase transition in Group 1 Halides and Group 2 Oxides, C.E. Sims, G.D. Barrera, N.L. Allan and W.C. Mackrodt, Phys. Rev. B57, 11164-11172 (1998).

14. Modelling explosions and equations of state, C.E. Sims, M. Braithwaite, N.L. Allan and W.C. Mackrodt (in preparation)

15. Hybrid Monte Carlo and lattice dynamics simulations: the enthalpy of mixing of binary oxides, J.A. Purton, J.D. Blundy, M.B. Taylor, G.D. Barrera and N.L. Allan, Chem. Commun. 627-628 (1998).

16. Thermal expansion of polymers: mechanisms in orthorhombic polyethylene, J.A.O. Bruno, N.L. Allan, T.H.K. Barron and A.D. Turner, Phys. Rev. B (in press)

17. Lithium oxide and superionic behaviour – a study using potentials from periodic ab initio calculations, R.M. Fracchia, G.D. Barrera, N.L. Allan. T.H.K. Barron and W.C. Mackrodt, J. Phys. Chem. Solids 59, 435-445 (1998).

SHELL User Manual: for on-line version see http://dougal.chm.bris.ac.uk/programs/shell

Polar solids at high temperatures and pressures, C.E. Sims (PhD thesis. Bristol University, submission » Autumn 1998)

Seminar (NLA), Department of Physics, University of Keele, May 1995

Seminar, (NLA) Department of Chemistry, University of Buenos Aires, September 1995

Seminar (NLA) Department of Physical Chemistry, University of Oxford, May 1997.

Faraday Discussion 106 Solid-State Chemistry: New Opportunities from Computer Simulations (London) July 1997 (NLA)

Summer School, University of Torino, September 1995 (WCM)

Seminar, Department of Physical Chemistry, University of Hannover, February 1996(WCM)

Seminar, Laboratoire de Physique des Solides, University of Paris-Sud, June 1996 (WCM)

CECAM Conference, Palaiseau, September 1996 (WCM)

CCP5 Conference (Bristol) 1996 (MBT)

Third International Conference on Materials Chemistry (Exeter) 1997 (NLA)

CCP5 Conference (London) 1997 (NLA)

EURODIM 98 (Keele) to be given July 1998 (NLA)

Engineering Foundation Conference, Hawaii, 1998 (NLA)

In addition poster presentations have been given at several RSC (chiefly Polar Solids) and CCP5 conferences.

Contact: Dr. N.L. Allan, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS. Email: n.l.allan@bris.ac.uk . http://dougal.chm.bris.ac.uk/allan