Superconducting State and Phase Transitions
  From the days when superconductivity was discovered its science was entangled by the unresolved problem of the relationship between superconductive state, its crystal structure and its phase transitions. The problem was exacerbated by the adjacent scientific area - solid state phase transitions - that offered to divide phase transitions into first and second order. Adding to that the conclusion (turned out erroneous)on the structural identity of the superconductor and normal phases, the whole issue was uncertain and confusing. This article straights it out. 
Yury Mnyukh, Vitaly Vodyanoy
cond-mat.supr-con