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The crystal structures of RbTiOAsO4, KTiO(Po.58,-AS0.42)O4, RbTiOPO4 and (Rbo.465,Ko.535)TiOPO4, all of which are orthorhombic (space group Pna2~)and isostructural with KTiOPO4, have been determined from single-crystal X-ray diffraction data and refined to R factors between 0.027 and 0.040. The results are used for an analysis of structural trends in crystals of the KTiOPO4 family by comparison of the room-temperature structures with that proposed for the high-temperature prototypic phase (space group Pnan). RbTiOAsO4, KTiO(Po.ss,Aso.42)O4, RbTiOPO4 AND (Rbo.465,Ko.535)TiOPO4 Crystal colour, shape and size Linear absorption coefficient (cm ') Diffractometer Radiation Scan Lattice parameters (A) Ranges of tl. k. 1

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BaFe12O19 is a popular M-type hexaferrite with a Néel temperature of 720 K and is of enormous commercial value ($3 billion/year). It is an incipient ferroelectric with an expected ferroelectric phase transition extrapolated to lie at 6 K but suppressed due to quantum fluctuations. The theory of quantum criticality for such uniaxial ferroelectrics predicts that the temperature dependence of the electric susceptibility χ diverges as 1/T3, in contrast to the 1/T2 dependence found in pseudo-cubic materials such as SrTiO3 or KTaO3. In this paper we present evidence of the susceptibility varying as 1/T3, i.e. with a critical exponent γ = 3. In general γ = (d + z – 2)/z, where the dynamical exponent for a ferroelectric z = 1 and the dimension is increased by 1 from deff = 3 + z to deff = 4 + z due to the effect of long-range dipole interactions in uniaxial as opposed to multiaxial ferroelectrics. The electric susceptibility of the incipient ferroelectric SrFe12O19, which is slightly further from the quantum phase transition is also found to vary as 1/T3.