Chemistry of high-temperature superconductivity in the NbN family

The high transition temperatures Tc of the family of RY compounds (R = Ti, Zr, Hf, V, Nb, or Ta, Y = C, N, or O) are attributed to softness of the lattice against hydrostatic deformations, which gives rise to strong electron longitudinal-acoustic phonon coupling at short wavelengths. Chemical trends within the RY family provide evidence to support this explanation. The microscopic theory of superconductivity is used to predict from the value of Tc the form of the phonon spectrum of NbN. For the acoustic branch the predicted value of (ω2)1/2 is 150 °K. Also Tcmax in the RY family is estimated to be 22 °K.