  We report the results of a systematic investigation of the phase diagram of
the iron-based superconductor, Ba1-xKxFe2As2, from x = 0 to x = 1.0 using high
resolution neutron and x-ray diffraction and magnetization measurements. The
polycrystalline samples were prepared with an estimated compositional variation
of \Deltax <~ 0.01, allowing a more precise estimate of the phase boundaries
than reported so far. At room temperature, Ba1-xKxFe2As2 crystallizes in a
tetragonal structure with the space group symmetry of I4/mmm, but at low
doping, the samples undergo a coincident first-order structural and magnetic
phase transition to an orthorhombic (O) structure with space group Fmmm and a
striped antiferromagnet (AF) with space group Fcmm'm'. The transition
temperature falls from a maximum of 139K in the undoped compound to 0K at x =
0.252, with a critical exponent as a function of doping of 0.25(2) and 0.12(1)
for the structural and magnetic order parameters, respectively. The onset of
superconductivity occurs at a critical concentration of x = 0.130(3) and the
superconducting transition temperature grows linearly with x until it crosses
the AF/O phase boundary. Below this concentration, there is microscopic phase
coexistence of the AF/O and superconducting order parameters, although a slight
suppression of the AF/O order is evidence that the phases are competing. At
higher doping, superconductivity has a maximum Tc of 38 K at x = 0.4 falling to
3 K at x = 1.0. We discuss reasons for the suppression of the spin-density-wave
order and the electron-hole asymmetry in the phase diagram.
