We report on synthesis, structural characterization, resistivity, magnetic and thermal expansion measurements on the as yet unexplored $delta$-phase of FeSe$_{1-x}$, here synthesized under ambient- (AP) and high-pressure (HP) conditions. We show that
in contrast to $beta$-FeSe$_{1-x}$, monophasic superconducting $delta$-FeSe$_{1-x}$ can be obtained in off-stoichiometric samples with excess Fe atoms preferentially residing in the van der Waals gap between the FeSe layers. The AP $delta$-FeSe$_{1-x}$ sample studied here ($T_c$ $simeq$ 8.5,K) possesses an unprecedented residual resistivity ratio RRR $simeq$ 16. Thermal expansion data reveal a small feature around $sim$90,K, which resembles the anomaly observed at the structural and magnetic transitions for other Fe-based superconductors, suggesting that some kind of magnetic state is formed also in FeSe. %indicative of a fluctuating magnetic ordering. For HP samples (RRR $simeq$ 3), the disorder within the FeSe layers is enhanced through the introduction of vacancies, the saturated magnetic moment of Fe is reduced and only spurious superconductivity is observed.
