A²⁺ Cation Control of Chiral Domain Formation in A(TiO)Cu₄(PO₄)₄ (A = Ba, Sr)

Single crystals of two novel tetragonal chiral materials, A­(TiO)­Cu4(PO4)4 (A = Ba, Sr), were grown from Na2Mo2O7 flux, and their crystal and chiral domain structures were characterized. Polarized-light microscopy studies of the chiral domain structures in the crystals show that Ba­(TiO)­Cu4(PO4)4 mostly hosts a multidomain state, while a monodomain state predominantly appears in Sr­(TiO)­Cu4(PO4)4. To explain this striking difference, we quantified the chirality strength of these materials by comparing atomic positions in the chiral and nearest-achiral crystal structures, revealing larger chirality strength in Sr­(TiO)­Cu4(PO4)4 than in Ba­(TiO)­Cu4(PO4)4. Our proposed mechanisms linking the chirality strength and domain formation can account for the different occurrence frequency of chiral domains in this system.