Coexisting Displacive and Order–Disorder Lattice Instabilities in the Antiferroelectric Titanite CaTiSiO₅

We investigate the phase transition mechanism in the titanite-type compound CaTiSiO5, a promising antiferroelectric (AFE) candidate, using a combination of density functional theory (DFT) and temperature-dependent synchrotron X-ray diffraction (XRD). Phonon calculations identify soft-modes at both the Γ and Y points of the Brillouin zone, indicating that the system is close to both ferroelectric and AFE instabilities. Anisotropic diffuse scattering reveals strong one-dimensional (1D) correlation of atomic displacement along the c-axis. Anisotropic atomic displacement parameters (ADPs) of Ti and Ca exhibit contrasting temperature evolutions: Ti shows divergent behavior consistent with a displacive transition, whereas Ca exhibits step-like changes characteristic of an order-disorder transition. These findings highlight the coexistence of displacive and disorder-type instabilities in CaTiSiO5 and offer new insight into the design of functional antiferroelectrics via lattice-mode engineering in titanite-type compounds.