Incommensurately modulated α′-Sr₂SiO₄

Abstract

The solid-state chemistry of superficially mundane A₂BX₄ compounds with β-K₂SO₄-related structures is ‘a can of worms’, characterized by prolific polymorphism and, despite careful single-crystal X-ray diffraction studies, persistent uncertainty about exact space groups and atom positions. This is true of Ca₂SiO₄^1–7, a major constituent of cement, and Sr₂SiO₄^8–10 despite 50 years of (confusing) literature. On the other hand, the solid-state physics of some other A₂BX₄ compounds (for example, K₂SeO₄), as well as more exotic charge-density-wave types, involves modulated structures. Similar behaviour could conceivably resolve the structural confusion cited above (wrong structural models may have been refined); to investigate this possibility¹¹ we have studied the transition β- → α′-Sr₂SiO₄ by beam-heating the β-polymorph in an electron microscope. Electron diffraction patterns reveal two sets of incommensurate satellite reflections, indicating distinct modulated structures. Their origin is an asymmetric —Sr—O—Sr—O— chain (common to β and α′), with alternating bond lengths which tend to equality as T increases, but at a cost of reduced total bond strength and energy.