  Understanding the interplay between the structural, chemical and physical
properties of nanomaterials is crucial for designing new devices with enhanced
performance. In this regards, doping of metal oxides is a general strategy to
tune size, morphology, charge, lattice, orbital and spin degrees of freedoms
and has been shown to affect nanomaterials properties for photoelectrochemical
water splitting, batteries, catalysis, magnetic applications and optics. Here
we report the role of lattice small polaron in driving the morphological
transition from nearly isotropic to nanowire crystals in Si doped hematite
($\alpha-Fe_2O_3$). Lattice small polaron formation is well evidenced by the
increase of hexagonal strain and degree of distortion of $FeO_6$ showing a
hyperbolic trend with increasing Si content. Local analysis via pair
distribution function highlights an unreported crossover from small to large
polarons, which affects the correlation length of the polaronic distortion from
short to average scales. Ferromagnetic double exchange interactions between
$Fe^{2+}/Fe^{3+}$ species is found to be the driving force of the crossover,
constraining the chaining of chemical bonds along the [110] crystallographic
direction. This promotes the increase in the reticular density of Fe atoms
along the hematite basal plane only, which boosts the anisotropic growth of
nanocrystals with more extended [110] facets. Our results show that magnetic
and electronic interactions drive preferential crystallographic growth in doped
metal oxides, thus providing a new route to design their functional properties.
