Negative thermal expansion in ZnF$_2$
  We have investigated temperature dependence of the lattice parameters and the unit cell volume of ZnF$_2$ by neutron diffraction and have discovered negative thermal expansion (NTE) at low temperature. To understand why this simple compound exhibits NTE we performed first principle calculations. These calculations reproduce qualitatively the experimental temperature dependence of volume. 
Tapan Chatterji, Mohamed Zbiri, and Thomas C. Hansen
cond-mat.mtrl-sci