Hamdi Ben Yahia

Synthesis and characterization of the crystal structure and magnetic properties of the ternary manganese vanadate NaMnVO4.

A new ternary manganese vanadate, NaMnVO(4), was synthesized by solid state reaction route, and its crystal structure and magnetic properties were characterized by X-ray diffraction, magnetic susceptibility and specific heat measurements, and by density functional calculations. NaMnVO(4) crystallizes in the maricite-type structure with space group Pnma, a = 9.563(1) A, b = 6.882(1) A, c = 5.316(1) A, and Z = 4. NaMnVO(4) contains MnO(4) chains made up of edge-sharing MnO(6) octahedra, and these chains are interlinked by VO(4) tetrahedra. The magnetic susceptibility has a broad maximum at T(max) = 24 K and follows the Curie-Weiss behavior above 70 K with θ = -62 K. NaMnVO(4) undergoes a three-dimensional antiferromagnetic ordering at T(N) = 11.8 K. The spin exchanges of NaMnVO(4) are dominated by the intrachain antiferromagnetic exchange, and the interchain spin exchanges are spin-frustrated. The most probable magnetic structure of the ordered magnetic state below T(N) was predicted on the basis of the extracted spin exchanges.

Synthesis and characterization of the crystal structure and magnetic properties of the new fluorophosphate LiNaCo[PO4]F.

The new compound LiNaCo[PO(4)]F was synthesized by a solid state reaction route, and its crystal structure was determined by single-crystal X-ray diffraction measurements. The magnetic properties of LiNaCo[PO(4)]F were characterized by magnetic susceptibility, specific heat, and neutron powder diffraction measurements and also by density functional calculations. LiNaCo[PO(4)]F crystallizes with orthorhombic symmetry, space group Pnma, with a = 10.9334(6), b = 6.2934(11), c = 11.3556(10) Å, and Z = 8. The structure consists of edge-sharing CoO(4)F(2) octahedra forming CoFO(3) chains running along the b axis. These chains are interlinked by PO(4) tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The magnetic susceptibility follows the Curie-Weiss behavior above 60 K with θ = -21 K. The specific heat and magnetization measurements show that LiNaCo[PO(4)]F undergoes a three-dimensional magnetic ordering at T(mag) = 10.2(5) K. The neutron powder diffraction measurements at 3 K show that the spins in each CoFO(3) chain along the b-direction are ferromagnetically coupled, while these FM chains are antiferromagnetically coupled along the a-direction but have a noncollinear arrangement along the c-direction. The noncollinear spin arrangement implies the presence of spin conflict along the c-direction. The observed magnetic structures are well explained by the spin exchange constants determined from density functional calculations.

Single crystal X-ray structure study of the Li2−xNaxNi[PO4]F system

The new compounds Li(2-x)Na(x)Ni[PO(4)]F (x = 0.7, 1, and 2) have been synthesized by a solid state reaction route. Their crystal structures were determined from single-crystal X-ray diffraction data. Li(1.3)Na(0.7)Ni[PO(4)]F crystallizes with the orthorhombic Li(2)Ni[PO(4)]F structure, space group Pnma, a = 10.7874(3), b = 6.2196(5), c = 11.1780(4) Å and Z = 8, LiNaNi[PO(4)]F crystallizes with a monoclinic pseudomerohedrally twinned structure, space group P2(1)/c, a = 6.772(4), b = 11.154(6), c = 5.021(3) Å, β = 90° and Z = 4, and Na(2)Ni[PO(4)]F crystallizes with a monoclinic twinned structure, space group P2(1)/c, a = 13.4581(8), b = 5.1991(3), c = 13.6978(16) Å, β = 120.58(1)° and Z = 8. For x = 0.7 and 1, the structures contain NiFO(3) chains made up of edge-sharing NiO(4)F(2) octahedra, whereas for x = 2 the chains are formed of dimer units (face-sharing octahedra) sharing corners. These chains are interlinked by PO(4) tetrahedra forming a 3D framework for x = 0.7 and different Ni[PO(4)]F layers for x = 1 and 2. A sodium/lithium disorder over three atomic positions is observed in Li(1.3)Na(0.7)Ni[PO(4)]F structure, whereas the alkali metal atoms are well ordered in between the layers in the LiNaNi[PO(4)]F and Na(2)Ni[PO(4)]F structures, which makes both compounds of great interest as potential positive electrodes for sodium cells.

Synthesis and characterization of the crystal structure, the magnetic and the electrochemical properties of the new fluorophosphate LiNaFe[PO4]F

The new compound LiNaFe[PO(4)]F was synthesized by a solid state reaction route, and its crystal structure was determined using neutron powder diffraction data. LiNaFe[PO(4)]F was characterized by (57)Fe Mössbauer spectroscopy, magnetic susceptibility, specific heat capacity, and electrochemical measurements. LiNaFe[PO(4)]F crystallizes with orthorhombic symmetry, space group Pnma, with a = 10.9568(6) Å, b = 6.3959(3) Å, c = 11.4400(7) Å, V = 801.7(1) Å(3) and Z = 8. The structure consists of edge-sharing FeO(4)F(2) octahedra forming FeFO(3) chains running along the b axis. These chains are interlinked by PO(4) tetrahedra forming a three-dimensional framework with the tunnels and the cavities filled by the well-ordered sodium and lithium atoms, respectively. The specific heat and magnetization measurements show that LiNaFe[PO(4)]F undergoes a three-dimensional antiferromagnetic ordering at T(N) = 20 K. The neutron powder diffraction measurements at 3 K show that each FeFO(3) chain along the b-direction is ferromagnetic (FM), while these FM chains are antiferromagnetically coupled along the a and c-directions with a non-collinear spin arrangement. The galvanometric cycling showed that without any optimization, one mole of alkali metal is extractable between 1.0 V and 5.0 V vs. Li(+)/Li with a discharge capacity between 135 and 145 mAh g(-1).

Synthesis and Characterization of the Crystal and Magnetic Structures and Properties of the Hydroxyfluorides Fe(OH)F and Co(OH)F

The title compounds were synthesized by a hydrothermal route from a 1:1 molar ratio of lithium fluoride and transition-metal acetate in an excess of water. The crystal structures were determined using a combination of powder and/or single-crystal X-ray and neutron powder diffraction (NPD) measurements. The magnetic structure and properties of Co(OH)F were characterized by magnetic susceptibility and low-temperature NPD measurements. M(OH)F (M = Fe and Co) crystallizes with structures related to diaspore-type α-AlOOH, with the Pnma space group, Z = 4, a = 10.471(3) Å, b = 3.2059(10) Å, and c = 4.6977(14) Å and a = 10.2753(3) Å, b = 3.11813(7) Å, and c = 4.68437(14) Å for the iron and cobalt phases, respectively. The structures consist of double chains of edge-sharing M(F,O)6 octahedra running along the b axis. These infinite chains share corners and give rise to channels. The protons are located in the channels and form O-H···F bent hydrogen bonds. The magnetic susceptibility indicates an antiferromagnetic ordering at ∼40 K, and the NPD measurements at 3 K show that the ferromagnetic rutile-type chains with spins parallel to the short b axis are antiferromagnetically coupled to each other, similarly to the magnetic structure of goethite α-FeOOH.

Incommensurate crystal structure of LiCd4(VO4)3

Abstract Single crystals of LiCd4(VO4)3 were prepared by melting a powder sample of LiCd4(VO4)3 at 950 °C followed by slow cooling at 5 °C/hours down to room temperature. The crystal structure was determined by single crystal X-ray diffraction using the 4D superspace formalism. The superspace group is Cmcm(10γ) with unit cell parameters a = 5.8513(3) Å, b = 9.027(5) Å, c = 6.7721(7) Å and modulation vector q = a* + 0.7034c*. The LiCd4(VO4)3 structure is related to the Na2CrO4 structure type, which contains two independent tetrahedral sites that are occupied by sodium and chromium atoms and one octahedral site that is occupied by the second sodium atom. In the LiCd4(VO4)3 structure, the octahedral site is fully occupied by a cadmium atom, and one of the two tetrahedral sites is fully occupied by a vanadium atom. The second tetrahedral site is only partially filled: one third lithium, one third cadmium and one third vacant. To reflect the site occupancies, the overall formula can be expressed as Li1/3Cd1/3[ ]1/3CdVO4. One of the unusual features of the structure is the large anisotropy in the modulated occupancy of the tetrahedral site by lithium and cadmium. Structural relationships with the olivine structure type are discussed.

Crystal structures of the new fluorophosphates Li9Mg3[PO4]4F3 and Li2Mg[PO4]F and ionic conductivities of selected compositions

The new compounds Li2Mg[PO4]F and Li9Mg3[PO4]4F3 have been synthesized by a solid state reaction route. The crystal structures were determined from single-crystal X-ray diffraction data. Li2Mg[PO4]F crystallizes with the orthorhombic Li2Ni[PO4]F structure, space group Pnma, a = 10.7874(3), b = 6.2196(5), c = 11.1780(4) A, V = 721.13(10) A3, and Z = 8, whereas Li9Mg3[PO4]4F3 crystallizes with hexagonal symmetry, space group P63, with a = 12.6159(6), c = 5.0082(4) A, V = 690.32(7) A3, and Z = 2. A merohedral twinning was taken into account for its structural refinement. The structure of Li2Mg[PO4]F contains MgO3F chains made up of edge-sharing MgO4F2 octahedra. These chains are interlinked by PO4 tetrahedra forming a 3D-Mg[PO4]F framework. The lithium atoms occupy mainly three distinct crystallographic sites. The structure of Li9Mg3[PO4]4F3 consists of corner-sharing MgO4F2 octahedra forming MgO4F chains running along the c axis. These chains are interlinked by PO4 tetrahedra forming a 3D-Mg3[PO4]4F3 framework with hexagonal and pentagonal tunnels, in which are located the Li atoms. This study reveals also a strong relationship between Li2Mg[PO4]F-, Mg1−xFexAl3[BO3][SiO4]O2- and P21/c-Li5V[PO4]2F2- structures; and between P63-Li9Mg3[PO4]4F3 and P21/c-Na2Mn[PO4]F. The ionic conductivities σ of the composite material Li6Mg4[PO4]3[SO4]F3 and Li9Mg3[PO4]4F3, estimated using electrochemical impedance spectroscopic analyses at 300 °C, are 3.9 × 10−5 and 10−4 S cm−1 with activation energies of 0.524 eV and 0.835 eV, respectively.

Synthesis and characterization of the crystal structure and magnetic properties of the hydroxyfluoride MnF2-x(OH)x (x ∼ 0.8)

The new compound MnF(2-x)(OH)x (x ~ 0.8) was synthesized by a hydrothermal route from a 1 : 1 molar ratio of lithium fluoride and manganese acetate in an excess of water. The crystal structure was determined using the combination of single crystal X-ray and neutron powder diffraction measurements. The magnetic properties of the title compound were characterized by magnetic susceptibility and low-temperature neutron powder diffraction measurements. MnF(2-x)(OH)x (x ~ 0.8) crystallizes with orthorhombic symmetry, space group Pnn2 (no. 34), a = 4.7127(18), b = 5.203(2), c = 3.2439(13) Å, V = 79.54(5) Å(3) and Z = 2. The crystal structure is a distorted rutile-type with [Mn(F,O)4] infinite edge-sharing chains along the c-direction. The protons are located in the channels and form O-HF bent hydrogen bonds. The magnetic susceptibility measurements indicate an antiferromagnetic ordering at ~70 K and the neutron powder diffraction measurements at 3 K show that the ferromagnetic chains with spins parallel to the c-axis are antiferromagnetically coupled to each other, similarly to the magnetic structure of tetragonal rutile-type MnF2 with isoelectronic Mn(2+). MnF(2-x)(OH)x (x ~ 0.8) is expected to be of great interest as a positive electrode for Li cells if the protons could be exchanged for lithium.

Crystal Structures of New Pyrovanadates A2MnV2O7 (A = Rb, K)

Abstract The new compounds A2MnV2O7 (A = K, Rb) with structures related to the melilite-type have been synthesized by a solid state reaction route. The crystal structures of K2MnV2O7, Rb2MnV2O7 and KRbMnV2O7 have been determined using X-ray single crystal diffraction data. The compound K2MnV2O7 crystallizes with a melilite-type structure with tetragonal unit cell parameters a = 8.609(3), c = 5.538(4) Å and space group P4̅21m. The structures of Rb2MnV2O7 and KRbMnV2O7 are derived from the melilite-type structure with space group P4̅2/mnm and unit cell parameters a = 8.577(6), c = 11.809(6) Å , and a = 8.530(6), c = 11.466(5) Å , respectively. The three structures consist of [MnV2O7]2− layers perpendicular to the c axis separated by A+ layers. The [MnV2O7]2− layers feature corner-sharing MnO4 tetrahedra and V2O7 pyrovanadate units, the linkage leading to rings of five tetrahedra. The doubling of the c parameter for Rb2MnV2O7 or RbKMnV2O7 is explained by the existence of a mirror plane perpendicular to the [001] direction between two [MnV2O7]2− layers. The A+ alkali cations occupy distorted square antiprisms of oxygen atoms in K2MnV2O7 and distorted square prisms of oxygen atoms in Rb2MnV2O7and RbKMnV2O7.

X-Ray diffraction and SAED characterisations of RE4O4[PO4]Cl (RE = La, Pr, and Nd) and photoluminescence properties of Eu3+-doped La4O4[PO4]Cl

The new compounds RE4O4[PO4]Cl (RE = La, Nd, Sm and Gd) were synthesised by solid state reaction via a salt flux route and selected compositions were investigated by SAED, photoluminescence, and powder- and/or single crystal-X-ray diffraction. All the crystal structures consist of an alternation of [RE4O4]4+ and [ClPO4]4− layers. The [RE4O4]4+ layer contains ORE4/4 tetrahedra which share common edges. In the tetragonal RE4O4[PO4]Cl (RE = Pr and Nd) compounds, the PO43− and Cl− anions are located between these layers in a disordered manner. This is reflected by the presence of diffuse streaks parallel to the c* axis on the SAED patterns. These lines disappear for larger rare earth metals and keep the place to well defined superstructure reflections in La4O4[PO4]Cl, with a*superstructure = 4/5a*tetragonal − 2/5b*tetragonal and b*superstructure = 2/5a*tetragonal + 4/5b*tetragonal. The presence of these superstructure reflections is due to a new ordering of the PO43− and Cl− anions between the [RE4O4]4+ layers. The Eu3+-doped La3PO7 and La4O4[PO4]Cl samples show the characteristic Eu3+-related 4f → 4f lines and intense emission of red light upon excitation via the O2− → Eu3+ charge-transfer band at 280 nm. Due to the efficient energy transfer from the charge-transfer state to the 4f-levels, especially, La4O4[PO4]Cl:Eu3+ (5 mol%) may be of potential interest as a red phosphor.