Resolving the controversies about the 'nearly cubic' and other phases of Sr<SUB>1-x</SUB>Ca<SUB>x</SUB>TiO<SUB>3</SUB> (0≤x≤1): II. Comparison of phase transition behaviours for x = 0.40 and 0.43

The phase transition behaviour of two 'nearly cubic' compositions, x = 0.40 and 0.43, of Sr<SUB>1-x</SUB>Ca<SUB>x</SUB>TiO<SUB>3</SUB>, representative of Pbnm and Pbcm space group structures, has been investigated using temperature dependent dielectric, x-ray diffraction and Raman scattering techniques. For x = 0.40, a first order antiferroelectric phase transition occurs around 364 K followed by an antiferrodistortive phase transition involving the R point (q=1/2 1/2 1/2) instability at ~800 K. For x = 0.43, on the other hand, there is no antiferroelectric phase transition. Instead, two antiferrodistortive transitions around 463 and 850 K linked with the M (q=1/2 1/2 0 ) and R (q=1/2 1/2 1/2) point instabilities, respectively, are observed. Both the compositions also exhibit qualitatively different types of phase transitions below room temperature. Evidence for a new phase transition at low temperatures is presented for x = 0.43