An electron diffraction study of the incommensurately modulated α'L-phase of distrontium silicate, Sr2SiO4

A careful electron diffraction study of the incommensurately modulated alpha ' L -phase of Sr 2 SiO 4 has revealed the character of the atomic displacement pattern associated with each of the experimentally observed modulation wave-vectors. The phonon eigenvector associated with the incommensurate modulation with wave-vector q 1 =(0.303+or-0.005) b* is found to transform with Sigma 2 symmetry, while that associated with wave-vector q 2 =(0.396+or-0.005) b* is found to transform with Sigma 4 symmetry. A heating experiment revealed the presence of a reversible alpha ' L to or from alpha ' H phase transition at about 500 degrees C. The incommensurate modulation wave-vectors showed no temperature dependence within the alpha ' L -phase and disappeared altogether at the weakly first-order reversible alpha ' L to or from beta phase transition at about 90 degrees C. A Landau-like free-energy expansion is proposed to describe the low temperature modulated phases; this reproduces qualitatively the experimental observations.