Several superconducting transition temperatures in the range of 30-40 K were reported in the recently discovered intercalated FeSe sytem (A1-xFe2-ySe2, A = K, Rb, Cs, Tl). Although the superconducting phases were not yet conclusively decided, more than one magnetic phase with particular orders of iron vacancy and/or potassium vacancy were identified, and some were argued to be the parent phase. Here we show the discovery of the presence and ordering of iron vacancy in nonintercalated FeSe (PbO-type tetragonal {\beta}-Fe1-xSe). Three types of iron-vacancy order were found through analytical electron microscopy, and one was identified to be nonsuperconducting and magnetic at low temperature. This discovery suggests that the rich-phases found in A1-xFe2-ySe2 are not exclusive in Fe-Se related superconductors. In addition, the magnetic {\beta}-Fe1-xSe phases with particular iron-vacancy orders are more likely to be the parent phase of FeSe superconducting system, instead of the previously assigned {\beta}-Fe1+{\delta}Te.
@article{arxiv.1312.7255,
title = {Fe-vacancy order and superconductivity in PbO-type tetragonal {\beta}-Fe1-xSe},
author = {Ta-Kun Chen and Chung-Chieh Chang and Hsian-Hong Chang and Ai-Hua Fang and Chih-Han Wang and Wei-Hsiang Chao and Chuan-Ming Tseng and Yung-Chi Lee and Yu-Ruei Wu and Min-Hsueh Wen and Hsin-Yu Tang and Fu-Rong Chen and Ming-Jye Wang and Maw-Kuen Wu and Dirk Van Dyck},
journal= {arXiv preprint arXiv:1312.7255},
year = {2015}
}