Cation exchange as a mechanism to engineer polarity in layered perovskites

Cation-exchange reactions performed on the n = 2 Dion-Jacobson phases RbNdNb2O7 and RbNdTa2O7, using LiNO3 and NaNO3, yield the corresponding LiNdM2O7 and NaNdM2O7 (M = Nb, Ta) phases. Synchrotron X-ray and neutron powder diffraction data, in combination with second-harmonic generation data and supported by first-principles DFT calculations, reveal that the LiNdM2O7 phases adopt n = 2 Ruddlesden-Popper type structures with an a-a-c+/-(a-a-c+) distortion described in the polar space group B2cm. In contrast, the NaNdM2O7 phases adopt n = 2 Ruddlesden-Popper type structures with an a-b0c0/b0a-c0 distortion, described in the centrosymmetric space group P42/mnm. The differing structures adopted by the LiNdM2O7 and NaNdM2O7 phases are rationalized on the basis of a competition between (i) optimizing the size of the Li/Na coordination site via octahedral tilting and (ii) ordering the Na/Li cations within the (Li/Na)O2 sheets to minimize cation-cation repulsion - the former appears to be the dominant factor for the Li phases, and the latter factor dominates for the Na phases. The strong A′-cation dependence of the tilting distortions adopted by the A′NdM2O7 phases suggests that by careful selection of the substituting cation the tilting distortions of layered perovskite phases can be rationally tuned to adopt polar configurations, and thus new ferroelectric phases can be synthesized

Synthesis and characterization of Se4Nb2O13: A new ternary Se4+-Nb5+-oxide with monoselenite and diselenite groups

A new noncentrosymmetric ternary selenite, Se4Nb2O13, has been synthesized from SeO2 and Nb2O5. Crystal data: Se4Nb2O13, Mr = 709.64, monoclinic, space group Pa (No. 7), a = 7.555(6) Å, b = 6.637(8) Å, c = 11.377(5) Å, β= 109.23(3)°, V = 538.64(2) Å3 (T = 200 K), Z = 2, R(F) = 6.44%, RW(F) = 7.12%. The compound consists of rows of corner-linked NbO6 octahedra that are also connected through monoselenite, SeO3, and diselenite, Se2O5, groups

A new three-dimensional vanadium selenite, (VO)(2)(SeO3)(3), with isolated and edge-shared VO6 octahedra

Single crystals of (VO)2(SeO3)3 have been prepared from SeO2 and V2O5. The crystal structure has been determined by single-crystal X-ray diffraction. Crystal data: (VO)2(SeO3)3, Mr = 514.75; monoclinic, space group P21a (No. 14); cell parameters a = 9.151(1) Å, b = 6.353(1) Å, c = 14.992(1)Å, β = 93.538(5)°, and Z = 4. The structure consists of VO6 octahedra and SeO3 groups. The vanadium octahedra either are edge-sharing and form [V2O6.66]3.33- groups or are corner-linked through an [SeO3/2]+ cation. The structure may be rationalized in terms of connecting layers of {2[SeO2/2O1/3]4/3+[VO1/1O 4/2O1/3]5/3-}+ cations and {[SeO3/2]+[VO1/1O5/2]2-}- anions. © 1997 American Chemical Society

Hydro(solvo)thermal synthesis and structure of a three-dimensional zinc fluorophosphate: Zn-2(4,4'-bipy)(PO3F)(2)

A new three-dimensional zinc fluorophosphate is prepared by using hydro(solvo)thermal conditions with ZnO, (HF)x·H3PO4 and 4,4′-bipyridyl; the single-crystal structure is determined

The first fully fluorinated organically templated materials: Synthesis, structures, and physical properties of [H3N(CH2)(3)NH3]U2F10 center dot 2H(2)O, [H3N(CH2)(4)NH3]U2F10 center dot 3H(2)O, [H3N(CH2)(6)NH3]U2F10 center dot 2H(2)O, and [HN(CH2CH2NH3)(3)]U5F24

A series of new layered uranium(IV) fluorides have been prepared under hydrothermal conditions from UO2, HF, and H3PO4 by using H2N(CH2)nNH2 (n = 3, 4, or 6), and N(CH2-CH2NH2)3 as structure-directing agents. [H3N(CH2)3NH3]U2F 10·2H2O (UFO-1), [H3N(CH2)4-NH3]U2F 10·3H2O (UFO-2), [H3N(CH2)6NH3]U2F 10·2H2O (UFO-3), and [HN(CH2CH2NH3)3]-U5F 24 (UFO-4) have been characterized by single-crystal X-ray diffraction, thermogravimetric analysis, BET isotherms, and magnetic susceptibility measurements. UFO-1, 2, 3, and 4 contain negatively charged uranium fluoride (UF) layers constructed from linked UFn polyhedra separated by charge balancing organic cations and occluded water molecules. In UFO-1, 2, and 3, the layers are constructed from equivalent UF9 tricapped trigonal prisms that share three edges and two corners, whereas UFO-4 contains both UF8 bicapped trigonal prisms and UF9 tricapped trigonal prisms. We have demonstrated that the interlamellar organic cations in these materials can be ion-exchanged with a large variety of alkali, alkaline earth, and transition metal cations, thus providing a low-temperature route to new condensed mixed metal uranium fluorides. The magnetic susceptibility data indicate that UFO-1, 2, and 3 exhibit Curie-Wiess behavior between room temperature and 20 K. [Crystal data: UFO-1, monoclinic, space group P21/c (no. 14), a = 10.715(1) Å, b = 7.097(1) Å, c = 8.767(1) Å, β = 93.804(6)°, V = 665.21(1) Å3 (T = 150 K), Z = 2, R(F) = 5.42%, Rw(F) = 6.79%; UFO-2, triclinic, space group P1 (no. 2), a = 12.024(2) Å, b = 7.149(3) Å, c = 8.765(2) Å, α = 90.063(4)°, β = 107.133(5)°, γ = 92.098(3)°, V = 719.49(3) Å3 (T = 150 K), Z = 2, R(F) = 8.05%, Rw(F) = 10.11%; UFO-3, monoclinic, space group P21/c (no. 14), a = 13.800(2) Å, b = 7.056(1) Å, c = 8.720(1) Å, β = 108.338(6)°, V = 805.97(2) Å3 (T = 150 K), Z = 2, R(F) = 3.64%, Rw(F) = 4.65%; UFO-4, triclinic, space group P1 (no. 2), a = 11.534(2) Å, b = 11.532-(4) Å, c = 9.634(2) Å, α = 104.034(4)°, β = 101.112(8)°, γ = 93.922(4)°, V = 1210.9(5) Å3 (T = 150 K), Z = 2, R(F) = 6.12%, Rw(F) = 8.19%.]