  We used temperature dependent high-resolution x-ray powder diffraction and
magnetization measurements to investigate structural, magnetic and electronic
degrees of freedom across the ferromagnetic magneto-elastic phase transition in
Mn1Fe1P0.6-wSi0.4Bw (w = 0, 0.02, 0.04, 0.06, 0.08). The magnetic transition
was gradually tuned from a strong first-order (w = 0) towards a second-order
magnetic transition by substituting P by B. Increasing the B content leads to a
systematic increase in the magnetic transition temperature and a decrease in
thermal hysteresis, which completely vanishes for w = 0.08. Furthermore, the
largest changes in lattice parameter across the magnetic transition occur for w
= 0, which systematically becomes smaller approaching the samples with w =
0.08. Electron density plots show a strong directional preference of the
electronic distribution on the Fe site, which indicates the forming of bonds
between Fe atoms and Fe and P/Si in the paramagnetic phase. On the other hand,
the Mn-site shows no preferred directions resembling the behaviour of a free
electron gas. Due to the low B concentrations (w = 0 - 0.08), distortions of
the lattice are limited. However, even small amounts of B strongly disturb the
overall topology of the electron density across the unit cell. Samples
containing B show a strongly reduced variation in the electron density compared
to the parent compound without B.
