The structural behaviour of the nepheline family : (2) Distance Least Squares modelling of Sr and Ba aluminates, MAl2O4

Abstract

The Distance Least Squares (DLS) structure modelling technique has been used to model the compositional and thermal expansion changes of the Sr-Ba-aluminate (MAl2O4) solid solution series and the results analysed in terms of the two tilting systems previously reported (Henderson and Taylor, 1982). The room-temperature structures of monoclinic SrAl2O4 and the monoclinic members of the (Sr,Ba)Al2O4 series were satisfactorily modelled by adjusting the input variables and weighting parameters for the Al-Al distances taking account of the anisotropy of the cell parameter changes. The calculated and observed cell parameters for the monoclinic solid solution series at room temperature show close agreement. Calculated atomic coordinates were used to determine the tilt angles for both tilting systems. These angles show decreasing values with increasing molar volumes while Al-O-Al angles show an inverse relationship. The thermal expansion of monoclinic SrAl2O4 was also modelled satisfactorily. With increasing temperature the main changes are seen in the expansion of the Sr-O distance and increase in Al-O-Al angles but there seems to be little change in the tilt angles for both tilt systems. The different behaviour in the tilt angles for thermal expansion and expansion arising from compositional changes suggests that temperature and composition are not analogous variables in these compounds. In contrast to the results for the monoclinic compounds the modelling of the hexagonal members of the (Sr,Ba)Al2O4 series was relatively unsuccessful. The problems encountered may largely be due to constraints from the space groups, and hence symmetry relations, assumed in the modelling. Indeed the lack of a satisfactory result may indicate that the reported structure and space group may refer to an averaged structure. If this is correct it may only be possible to model the observed compositional and thermal behaviour when the true cell is known.