Abstract
Abstract
Chemical substitutions offer a universal approach to tailor material properties, but the resulting chemical disorder can impose important structural responses that remain poorly understood. Here we perform a detailed investigation of the effect of chemical disorder in a low-symmetry prototype system A = Ca and Ba in Sr7-xAxMn4O15, through a combined high resolution X-ray and neutron powder diffraction study and first-principles density functional theory (DFT) simulations. We find that, despite the coordination environment across the 4 symmetry-unique Sr sites in the monoclinic P2_1/c structure being nominally the same, a large anisotropic and asymmetric solubility preference is observed for Ca2+ and Ba2+ which have significantly smaller and larger ionic radii, respectively. We contrast this anisotropic control parameter with both experimental and computational studies of the effect of isotropic high-pressure on the evolution of the structural parameters. We show that the complex evolution of lattice parameters (strain) in this series of compounds can only be understood to arise as a result of the coupling between anisotropic site substitution with the acoustic instabilities in this material. Mechanistically, this coupling is shown to be mediated by rigid unit modes, whose composition-dependent evolution accurately reproduces the non-trivial evolution of the lattice parameters across the entire series. Since the existence of coordination polyhedra is rather ubiquitous across functional solid state materials chemistry, the clarification of the interplay between chemical disorder, acoustic instabilities and rigid unit mode distortions represents an important step in the development of approaches for the targeted design of functional materials properties.