  Superconductivity in BaFe2(As1-xPx)2 iron pnictides emerges when its in-plane
two-dimensional (2D) orthorhombic lattice distortion associated with nematic
phase at Ts and three-dimensional (3D) collinear antiferromagnetic (AF) order
at TN (Ts = TN) are gradually suppressed with increasing x, reaching optimal
superconductivity around x = 0.30 with Tc $\approx$ 30 K. Here we show that a
moderate uniaxial pressure along the c-axis in BaFe2(As0.70P0.30)2
spontaneously induces a 3D collinear AF order with TN = Ts > 30 K, while only
slightly suppresses Tc. Although a ~ 400 MPa pressure compresses the c-axis
lattice while expanding the in-plane lattice and increasing the
nearest-neighbor Fe-Fe distance, it barely changes the average iron-pnictogen
height in BaFe2(As0.70P0.30)2. Therefore, the pressure- induced AF order must
arise from a strong in-plane magnetoelastic coupling, suggesting that the 2D
nematic phase is a competing state with superconductivity.
