“…5,9,10 Using ISOTROPY to couple the ⌫ 4 Ϫ and R 4 ϩ IR's, we obtain three possible antiphase tilt systems that can combine with the Cm ferroelectric structure: a Ϫ a Ϫ c 0 , a Ϫ a Ϫ c Ϫ , a Ϫ b Ϫ c Ϫ , which result in space-groups Cm, Cc, P1, respectively. The tilt system we are seeking is thus a Ϫ a Ϫ c Ϫ .…”