The non-centrosymmetric heavy fermion compound CePt3Si orders antiferromag-netically at TN = 2.2K, followed by a superconducting transition, where Ts = 0.45 K for high quality samples and Ts = 0.75K for lower quality samples. We have used neutron Larmor diffraction to measure the temperature dependence and distribution of the lattice constants in a single crystal of CePt3Si with Ts = 0.75K. In our study we observe an unusually wide range of lattice parameters for the a-axis and c-axis with Δa/a approx Δc/c approx 10‑3. Small angle neutron scattering suggests an abundance of defects along the lattice planes. When taking into account the pressure dependence of Ts our study attributes the increased value of Ts in low quality samples to an effective negative pressure.