Negative thermal expansion near two structural quantum phase transitions

Abstract

Negative thermal expansion (NTE), a material's propensity to shrink when heated, is an unusual physical effect occurring in some crystalline materials. The authors present a combined inelastic x-ray and x-ray diffraction study of thermal expansion and soft-mode dynamics of two materials (ScF${}_{3}$ and Hg${}_{2}$I${}_{2}$) and show they harbor extremely soft lattice degrees of freedom, suggestive that each material lies in close proximity to a structural quantum phase transition. A side-by-side comparative study of the mercurous halides and the $3d$ transition metal trifluorides elucidates the common origins of NTE in these systems and how NTE can arise from structural phase competition near zero temperature.