  Recent superconducting transition temperatures (Tc) over 100 K for monolayer
FeSe on SrTiO3 have renewed interest in the bulk parent compound. In KCl:AlCl3
flux-transport-grown crystals of FeSe0.94Be0.06, FeSe0.97Be0.03 and, for
comparison, FeSe, this work reports doping of FeSe using Be, among the smallest
of possible dopants, corresponding to an effective chemical pressure. According
to lattice parameter measurements, 6% Be doping shrank the tetragonal FeSe
lattice equivalent to a physical pressure of 0.75 GPa. Using this
flux-transport method of sample preparation, 6% of Be was the maximum amount of
dopant achievable. At this maximal composition of FeSe0.94Be0.06, the lattice
unit cell shrinks by 2.4%, Tc - measured in the bulk via specific heat -
increases by almost 10%, the Tc vs pressure behavior shifts its peak Tconset
downwards by ~1 GPa, the high temperature structural transition around TS = 89
K increases by 1.9 K (in contrast to other dopants in FeSe which uniformly
depress TS), and the low temperature specific heat gamma increases by 10 %
compared to pure FeSe. Also, upon doping by 6% Be the residual resistivity
ratio, rho(300K)/rho(T->0), increases by almost a factor of four, while
rho(300K)/rho(T=Tc+) increases by 50%.
