  Recent experimental work has revealed that the unusually strong, isotropic
structural negative thermal expansion in cubic perovskite ionic insulator ScF3
occurs in excited states above a ground state tuned very near a structural
quantum phase transition, posing a question of fundamental interest as to
whether this special circumstance is related to the anomalous behavior. To test
this hypothesis, we report an elastic and inelastic X-ray scattering study of a
second system Hg2I2 also tuned near a structural quantum phase transition while
retaining stoichiometric composition and high crystallinity. We find similar
behavior and significant negative thermal expansion below 100K for dimensions
along the body-centered-tetragonal c axis, bolstering the connection between
negative thermal expansion and zero temperature structural transitions. We
identify the common traits between these systems and propose a set of materials
design principles that can guide discovery of new materials exhibiting negative
thermal expansion.
