Tetragonal CuO: A new end member of the 3d transition metal monoxides
  Monoclinic CuO is anomalous both structurally as well as electronically in the 3$d$ transition metal oxide series. All the others have the cubic rock salt structure. Here we report the synthesis and electronic property determination of a tetragonal (elongated rock salt) form of CuO created using an epitaxial thin film deposition approach. In situ photoelectron spectroscopy suggests an enhanced charge transfer gap $\Delta$ with the overall bonding more ionic. As an end member of the 3d transition monoxides, its magnetic properties should be that of a high $T_N$ antiferromagnet. 
Wolter Siemons, Gertjan Koster, Dave H.A. Blank, Robert H. Hammond,   Theodore H. Geballe, Malcolm R. Beasley