Modelling of aluminophosphates : a study of the methods and forcefields used in the simulation of their mechanical properties

Abstract

Aluminophosphates are inorganic, crystalline, microporous materials similar m structure to certain aluminosilicates (also known as zeolites). The basic structure of aluminophosphates consists of alternating alumina and phosphate units. Similar to certain zeolites, aluminophosphates have a high internal surface area due to the presence of pores and channels, and find a number of uses as molecular sieves. Unlike zeolites, however, the bonding is considered to be 'molecular ionic' i.e. they are made up of discrete aluminium and phosphate ions. Forcefield based simulations have also suggested that certain aluminophosphates may have auxetic properties, that is they would have negative Poisson' s ratios in certain planes. This dissertation examines various forcefields and minimisation methods that can be used to model aluminophosphates. In particular, three minimisation are inter-compared where in the first method, symmetry constraints are imposed throughout (SYM), the second where constraints are removed from the beginning (P1), and the third where a few minimisation steps are carried out with symmetry but the minimisation is continued to convergence without symmetry (SYM-Pl). Various force-fields namely the BKS, Burchart, CVFF and Universal force-fields found within the Cerius2 modelling package and the Catlow and Sauer force fields found within GULP were also compared. Modelling using the Cerius2 force-field followed by a statistical analysis suggested that there was no significant difference in accuracy between the methods in the case of the BKS, Burchart, and Universal forcefields. In the case of the CVFF forcefield it was suggested that minimisation with symmetry constraints was found to be more accurate. However, not all structures tended to follow the 'general' trend. A comparison of all the various Cerius2 and GULP forcefields was also carried out, and it was found from statistical analysis that the best results are obtained with the Burchart, Catlow, or Sauer forcefields. As in the previous case some of the structures did not follow this trend, hence it might be better to treat structures individually. From an investigation into the mechanical properties of two aluminophosphates, AFN and Feldspar, it was suggested that the method of minimisation may affect the mechanical IV properties. It was shown that minimisation under symmetry may suggest that a structure is auxetic while minimisation of the same structure in the absence of symmetry would suggest otherwise thus indicating that extra care should be taken when imposing symmetry to avoid situations where auxeticity is artificially manifested. It was also confirmed that Feldspar is predicted to exhibit auxetic behaviour, particularly in the (001) plane were all three forcefields used (i.e. Burchart, Catlow and Sauer) suggested that either negative or very close to zero Poisson's ratio when loading in certain directions.