  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.
