  We report a single crystal neutron and x-ray diffraction study of the hybrid
improper multiferroic Ca3Mn1.9Ti0.1O7 (CMTO), a prototypical system where the
electric polarization arises from the condensation of two lattice distortion
modes.With increasing temperature (T ), the out-of-plane, antiphase tilt of
MnO6 decreases in amplitude while the in-plane, in-phase rotation remains
robust and experiences abrupt changes across the first-order structural
transition. Application of hydrostatic pressure (P) to CMTO at room temperature
shows a similar effect. The consistent behavior under both T and P reveals the
softness of antiphase tilt and highlights the role of the partially occupied d
orbital of the transition-metal ions in determining the stability of the
octahedral distortion. Polarized neutron analysis indicates the
symmetry-allowed canted ferromagnetic moment is less than the 0.04 {\mu}B/Mn
site, despite a substantial out-of-plane tilt of the MnO6 octahedra.
