  High-resolution thermal-expansion and specific-heat measurements were
performed on single crystalline BaFe2(As1-xPx)2 (0 < x < 0.33, x = 1). The
observation of clear anomalies allows to establish the thermodynamic phase
diagram which features a small coexistence region of SDW and superconductivity
with a steep rise of Tc on the underdoped side. Samples that undergo the
tetragonal-orthorhombic structural transition are detwinned in situ, and the
response of the sample length to the magneto-structural and superconducting
transitions is studied for all three crystallographic directions. It is shown
that a reduction of the magnetic order by superconductivity is reflected in all
lattice parameters. On the overdoped side, superconductivity affects the
lattice parameters in much the same way as the SDW on the underdoped side,
suggesting an intimate relation between the two types of order. Moreover, the
uniaxial pressure derivatives of Tc are calculated using the Ehrenfest relation
and are found to be large and anisotropic. A correspondence between
substitution and uniaxial pressure is established, i.e., uniaxial pressure
along the b-axis (c-axis) corresponds to a decrease (increase) of the P
content. By studying the electronic contribution to the thermal expansion we
find evidence for a maximum of the electronic density of states at optimal
doping.
