Ferroelectric compounds of the type AB2(X04)3

This class of compounds has a generalized formula, AB2(XO4), where A = Li, Na, K, Rb, Cs, Cu, Ag; B = U, Th, Ce; and X = P, As, or V. Crystals of K-, Na-, Li-U2(PO4)3; Li-, Na-, K-, Cu-, Ag-, Rb-, Cs-Th2(PO4)3; NaTh2(Po.5 As0.5O4)3 and Na(Th, Ce)(PO4)3 were successfully grown from stoichiometric mixtures in B2O3 fluxes cooled from 1200% at 4°/hr. Reducing and oxidizing conditions were necessary for valence control of U and Ce, respectively. The compounds have high chemical and thermal stability, neither being dissolved by mineral acids nor decomposing below 1000%. Monoclinic indexing of powder data was based on C2/c (Cc) and least-squares refined lattice constants were obtained for a number of compounds. Large flattened platelets of Na-, Li-U2(PO4)3; Li-, Na-, Rb-Th2(PO4)3; Na(ThCe)(PO4)3 and NaTh2(P0.5As0.5O4)3 were grown. These platelets were flattened on 100 which facilitated depositing electrodes on as-grown crystals for measuring the polar axis which lies in the {100{ plane. Ferroelectric hysteresis loops were observed for NaU2(PO4)3; NaTh2(PO4)3; Na(Th2Ce)(PO4)3 and NaTh2(P0.5 As0.5O4)3. Polarization varies on the order of 0-10 μC/cm2but high fields are required, E > 10 kV cm-1. The lack of detectable ferroelectric response in compounds where the A-cation radius exceeds 1.3 A is explained on the basis of structure-size relations.Optical measurements of these larger crystals showed biaxial positive character, 2 V ∼ 90°, medium birefringence, and refractive indices around 1.70.