The rapid microwave-assisted hydrothermal synthesis of NASICON-structured Na3V2O2x (PO4)2F3-2x (0 < x ≤ 1) cathode materials for Na-ion batteries.

NASICON-structured Na3V2O2x (PO4)2F3-2x (0 < x ≤ 1) solid solutions have been prepared using a microwave-assisted hydrothermal (MW-HT) technique. Well-crystallized phases were obtained for x = 1 and 0.4 by reacting V2O5, NH4H2PO4, and NaF precursors at temperatures as low as 180-200 °C for less than 15 min. Various available and inexpensive reducing agents were used to control the vanadium oxidation state and final product morphology. The vanadium oxidation state and O/F ratios were assessed using electron energy loss spectroscopy and infrared spectroscopy. According to electron diffraction and powder X-ray diffraction, the Na3V2O2x (PO4)2F3-2x solid solutions crystallized in a metastable disordered I4/mmm structure (a = 6.38643(4) Å, c = 10.62375(8) Å for Na3V2O2(PO4)2F and a = 6.39455(5) Å, c = 10.6988(2) Å for Na3V2O0.8(PO4)2F2.2). With respect to electrochemical Na+ (de)insertion as positive electrodes (cathodes) for Na-ion batteries, the as-synthesized materials displayed two sloping plateaus upon charge and discharge, centered near 3.5-3.6 V and 4.0-4.1 V vs. Na+/Na, respectively, with a reversible capacity of ∼110 mA h g-1. The application of a conducting carbon coating through the surface polymerization of dopamine with subsequent annealing at 500 °C improved both the rate capability (∼55 mA h g-1 at a discharge rate of 10C) and capacity retention (∼93% after 50 cycles at a discharge rate of C/2).

Homologous series of layered perovskites A(n+1)B(n)O(3n-1)Cl: crystal and magnetic structure of a new oxychloride Pb4BiFe4O11Cl.

The nuclear and magnetic structure of a novel oxychloride Pb(4)BiFe(4)O(11)Cl has been studied over the temperature range 1.5-700 K using a combination of transmission electron microscopy and synchrotron and neutron powder diffraction [space group P4/mbm, a = 5.5311(1) Å, c = 19.586(1) Å, T = 300 K]. Pb(4)BiFe(4)O(11)Cl is built of truncated (Pb,Bi)(3)Fe(4)O(11) quadruple perovskite blocks separated by CsCl-type (Pb,Bi)(2)Cl slabs. The perovskite blocks consist of two layers of FeO(6) octahedra located between two layers of FeO(5) tetragonal pyramids. The FeO(6) octahedra rotate about the c axis, resulting in a √2a(p) × âˆš2a(p) × c superstructure. Below T(N) = 595(17) K, Pb(4)BiFe(4)O(11)Cl adopts a G-type antiferromagnetic structure with the iron magnetic moments confined to the ab plane. The ordered magnetic moments at 1.5 K are 3.93(3) and 3.62(4) µ(B) on the octahedral and square-pyramidal iron sites, respectively. Pb(4)BiFe(4)O(11)Cl can be considered a member of the perovskite-based A(n+1)B(n)O(3n-1)Cl homologous series (A = Pb/Bi; B = Fe) with n = 4. The formation of a subsequent member of the series with n = 5 is also demonstrated.

Expanding the Ruddlesden-Popper manganite family: the N = 3 La(3.2)Ba(0.8)Mn3O10 member.

La(3.2)Ba(0.8)Mn(3)O(10), a representative of the rare n = 3 members of the Ruddlesden-Popper manganites A(n+1)Mn(n)O(3n+1), was synthesized in an evacuated sealed silica tube. Its crystal structure was refined from a combination of powder X-ray diffraction (PXD) and precession electron diffraction (PED) data, with the rotations of the MnO(6) octahedra described within the symmetry-adapted mode approach (space group Cccm, a = 29.068(1) Å, b = 5.5504(5) Å, c = 5.5412(5) Å; PXD R(F) = 0.053, R(P) = 0.026; PED R(F) = 0.248). The perovskite block in La(3.2)Ba(0.8)Mn(3)O(10) features an octahedral tilting distortion with out-of-phase rotations of the MnO(6) octahedra according to the (Φ,Φ,0)(Φ,Φ,0) mode, observed for the first time in the n = 3 Ruddlesden-Popper structures. The MnO(6) octahedra demonstrate a noticeable deformation with the elongation of two apical Mn-O bonds due to the Jahn-Teller effect in the Mn(3+) cations. The relationships between the octahedral tilting distortion, the ionic radii of the cations at the A- and B-positions, and the mismatch between the perovskite and rock-salt blocks of the Ruddlesden-Popper structure are discussed. At low temperatures, La(3.2)Ba(0.8)Mn(3)O(10) reveals a sizable remnant magnetization of about 1.3 µ(B)/Mn at 2 K, and shows signatures of spin freezing below 150 K.

Slicing the perovskite structure with crystallographic shear planes: the A(n)B(n)O(3n-2) homologous series.

A new A(n)B(n)O(3n-2) homologous series of anion-deficient perovskites has been evidenced by preparation of the members with n = 5 (Pb(2.9)Ba(2.1)Fe(4)TiO(13)) and n = 6 (Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)) in a single phase form. The crystal structures of these compounds were determined using a combination of transmission electron microscopy and X-ray and neutron powder diffraction (S.G. Ammm, a = 5.74313(7), b = 3.98402(4), c = 26.8378(4) Å, R(I) = 0.035, R(P) = 0.042 for Pb(2.9)Ba(2.1)Fe(4)TiO(13) and S.G. Imma, a = 5.7199(1), b = 3.97066(7), c = 32.5245(8) Å, R(I) = 0.032, R(P) = 0.037 for Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)). The crystal structures of the A(n)B(n)O(3n-2) homologues are formed by slicing the perovskite structure with (101)(p) crystallographic shear (CS) planes. The shear planes remove a layer of oxygen atoms and displace the perovskite blocks with respect to each other by the 1/2[110](p) vector. The CS planes introduce edge-sharing connections of the transition metal-oxygen polyhedra at the interface between the perovskite blocks. This results in intrinsically frustrated magnetic couplings between the perovskite blocks due to a competition of the exchange interactions between the edge- and the corner-sharing metal-oxygen polyhedra. Despite the magnetic frustration, neutron powder diffraction and Mössbauer spectroscopy reveal that Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) are antiferromagnetically ordered below T(N) = 407 and 343 K, respectively. The Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) compounds are in a paraelectric state in the 5-300 K temperature range.

Crystal structure and phase transitions in Sr3WO6.

The crystal structures of the beta and gamma polymorphs of Sr(3)WO(6) and the gamma<-->beta phase transition have been investigated using electron diffraction, synchrotron X-ray powder diffraction, and neutron powder diffraction. The gamma-Sr(3)WO(6) polymorph is stable above T(c) approximately 470 K and adopts a monoclinically distorted double perovskite A(2)BB'O(6) = Sr(2)SrWO(6) structure (space group Cc, a = 10.2363(1)A, b = 17.9007(1)A, c = 11.9717(1)A, beta = 125.585(1)(o) at T = 1373 K, Z = 12, corresponding to a = a(p) + 1/2b(p) - 1/2c(p), b = 3/2b(p) + 3/2c(p), c = -b(p) + c(p), a(p),b(p), c(p), lattice vectors of the parent Fm3m double perovskite structure). Upon cooling it undergoes a continuous phase transition into the triclinically distorted beta-Sr(3)WO(6) phase (space group C1, a = 10.09497(3)A, b = 17.64748(5)A, c = 11.81400(3)A, alpha = 89.5470(2)(o), beta = 125.4529(2)(o), gamma = 90.2889(2)(o) at T = 300 K). Both crystal structures of Sr(3)WO(6) belong to a family of double perovskites with broken corner sharing connectivity of the octahedral framework. A remarkable feature of the gamma-Sr(3)WO(6) structure is a non-cooperative rotation of the WO(6) octahedra. One third of the WO(6) octahedra are rotated by approximately 45 degrees about either the b(p) or the c(p) axis of the parent double perovskite structure. As a result, the WO(6) octahedra do not share corners but instead share edges with the coordination polyhedra of the Sr cations at the B positions increasing their coordination number from 6 to 7 or 8. The crystal structure of the beta-phase is very close to the structure of the gamma-phase; decreasing symmetry upon the gamma-->beta transformation occurs because of unequal octahedral rotation angles about the b(p) and c(p) axes and increasing distortions of the WO(6) octahedra.

The crystal structure of alpha-K3AlF6: elpasolites and double perovskites with broken corner-sharing connectivity of the octahedral framework.

The crystal structure of alpha-K(3)AlF(6) was solved and refined from a combination of powder X-ray and neutron diffraction data (a = 18.8385(3)A, c = 33.9644(6)A, S.G. I4(1)/a, Z = 80, R(P)(X-ray) = 0.037, R(P)(neutron) = 0.053). The crystal structure is of the A(2)BB'X(6) elpasolite type with the a = b approximately a(e) square root(5), c = 4a(e) superstructure (a(e), parameter of the elpasolite subcell) and rock-salt-type ordering of the K and Al cations over the B and B' positions, respectively. The remarkable feature of alpha-K(3)AlF(6) is a rotation of 2/5 of the AlF(6) octahedra by approximately pi/4 around one of the crystal axes of the elpasolite subcell, coinciding with the 4-fold symmetry axes of the AlF(6) octahedra. The rotation of the AlF(6) octahedra replaces the corner-sharing between the K and Al polyhedra by edge-sharing, resulting in an increase of coordination numbers of the K cations at the B positions up to 7 and 8. Due to significant deformations of the K polyhedra, the corner-sharing connectivity of the octahedral elpasolite framework is broken and the rotations of the AlF(6) octahedra do not have a cooperative character. Elpasolites and double perovskites with similar structural organization are discussed. The difference in ionic radii of the B and B' cations as well as the tolerance factor are proposed to be the parameters governing the formation of elpasolites and double perovskites with broken corner-sharing connectivity of the octahedral framework.