For nickel oxide, our data show an almost linear dependence close to the literature data . At ambient pressure, metastable nanocrystalline rock-salt zinc oxide has a rather narrow temperature range of stability , and the scatter of the thermal expansion data is quite significant and determined by the chemical history of the precursor, i.e., w-ZnO nanopowder . Thus, in this case the powder microstructure -grain sizes, grain boundaries, etc. -make a significant contribution to the thermal expansion.