Abstract: The structural relations of solid solutions in the series (Sr_1−xBa_x)Al₂O₄ were studied using room- and high-temperature X-ray methods, infra-red spectroscopy, and DTA. At room temperature, SrAl₂O₄ and solid solutions with x up to 0.31 are monoclinic, between x = 0.31 and 0.43 monoclinic and hexagonal forms coexist, and between x = 0.43 and 1.0 only hexagonal forms occur. On heating, a member of the monoclinic series of solid solutions transforms to hexagonal symmetry over a range of temperature within which both monoclinic and hexagonal forms coexist. The proportion of the hexagonal form increases instantaneously as the temperature is raised. The transformation temperature decreases with increasing BaAl₂O₄ in solid solution and, in addition, the temperature width of the region of coexistence is markedly enlarged. SrA1204 transforms over the range 665–705 °C and (Sr_0.7Ba_0.3)Al₂O₄ over 170–405 °C. The DTA trace for SrAl₂O₄ shows a peak at 677 °C. On cooling, the transformations show hysteresis of 15 to 25 °C.

The coexisting monoclinic and hexagonal forms are believed to be isochemical, and discontinuities in cell parameters occur within the region of coexistence both in the compositional series at room temperature and in the elevated temperature transformation experiments. The low-to-high transformation is accompanied by a volume change of −0.2 to −0.3 %, and is believed to be first-order displacive with additional characteristics similar to those of martensitic transformations.

The thermal expansion behaviour of structures in the (Sr,Ba)Al₂O₄, series indicates that two tilt systems are operative: co-operative rotation of tetrahedra about the c-axis, and tilting of tetrahedra relative to the 0001 plane.

The results for the (Sr,Ba)Al₂O₄ series are shown to be invaluable in reinterpreting the structural behaviour of members of the nepheline and leucite groups of minerals.