Abdelaali Daidouh

Synthesis, structure and conductivity study of new monovalent phosphates with the langbeinite structure

Abstract This paper reports the synthesis, structure and the conductivity study of the new monovalent phosphates of the formula Na 2 MTi(PO 4 ) 3 , M=Fe or Cr. The structures of these compounds have been refined from X-ray powder diffraction by the Rietveld method. Their reflections are indexed in the cubic system with langbeinite-type structure in the P2 1 3 space group. This structure can be best described in terms of [M 2 P 3 O 18 ] structural units composed of two MO 6 octahedra linked to each other through three PO 4 tetrahedra. Curie-type behaviour is observed in the magnetic susceptibility, which confirms the trivalent state of M metals and suggest no magnetic interactions. The electrical behaviour of these phases show that the activation energies are relatively high and they could be related to the previous structural analysis

Structural and electrical study of the alluaudites (Ag1−xNax)2FeMn2(PO4)3 (x=0, 0.5 and 1)

Abstract Three compounds with the alluaudite-type structure (Ag 1− x Na x ) 2 FeMn 2 (PO 4 ) 3 , with x =0, 0.5 and 1 have been synthesised by solid state reactions. These phases crystallise in the monoclinic system (space group C2/c, and Z =4), and the structure is formed by layers of edge-sharing FeO 6 and MnO 6 octahedra linked together via common corners of PO 4 tetrahedra, yielding to 1D channels in which monovalent cations are located. Complex impedance measurements were made in the frequency range from 1 Hz to 10 MHz, at temperatures between 373 and 873 K, and reveal the predominately ionic character of the electrical conductivity in all these compounds. The activation energies show that the silver derivatives ( x =0 and 0.5) are better conductors than the sodium one ( x =1) and a phase transition about 625 K is observed for the first of them.

New polymorphs of A2VP2O8 (A=Na, Rb):: Structure determination and ionic conductivity

Abstract New polimorphs of the compounds Na 2 VP 2 O 8 and Rb 2 VP 2 O 8 have been prepared by a direct fusion of the reactants. Na 2 VP 2 O 8 (NVP) crystallises in the tetragonal P4mb space group with a =8.108(4) A and c =4.943(6) A. Rb 2 VP 2 O 8 (RVP) is orthorhombic, space group Pnma, a =12.790(3) A, b =7.09(3)(8) A and c =9.1786 A. Both compounds are formed by the association of P 2 O 7 groups and VO 5 square pyramids. NVP is isotypical with K 2 VP 2 O 8 and consists of [VP 2 O 8 ] ∞ layers parallel to the ab plane intercalated by Na atoms that are located into pentagonal tunnels running along the c -axis. RVP is isotypical with Cs 2 VP 2 O 8 and formed by the assemblage of [VP 2 O 10 ] units leading to the two-dimensional [VP 2 O 7 ] ∞ layers laying in the bc plane; the Rb + are located between the layers in the interstitial spaces defined by the seven-sided windows which do not give rise to tunnels. The second part of this paper is devoted to the ionic conductivity properties of these materials using the complex impedance method, the measurements were made in the frequency range 10 MHz–1 Hz. The activation energies calculated were E a (NVP)=0.49 eV and E a (RVP)=0.85 eV.

Structure determination of the new layered compound Cs2TiP2O8 and ionic conductivity of Cs2MP2O8 (M = Ti, V)

Abstract A new titanium pyrophosphate Cs 2 TiP 2 O 8 with a layer structure and isotypic with Cs 2 VP 2 O 8 has been synthesised using solid state reactions. The structure determination of this material, from X-ray powder diffraction, shows that it crystallises in the orthorhombic space group Pnma with the parameters a =13.521(0) A, b =7.253(2) A and c =9.439(9) A. The basic structure of this new compound consist in [Ti 2 P 4 O 16 ] ∞ layers parallel to the (100) plane sandwiching Cs atoms. These layers built up from TiO 5 square pyramids and P 2 O 7 groups forming [TiP 2 O 8 ] ∞ and [TiPO7] ∞ chains running along [010] and [001] directions respectively. The ionic conductivity was measured and compared for the compounds Cs 2 MP 2 O 8 (M=Ti, V) over the frequency range 1 Hz–10 MHz, and temperatures between 523 K and 873 K for Cs 2 TiP 2 O 8 , and between 423 and 703 K for Cs 2 VP 2 O 8 , using complex impedance diagrams. The conductivities were analysed by an Arrhenius equation of the form σ = σ 0 exp(− E a / k T ). The activation energy calculated were E a (Ti)=0.80 eV and E a (V)=0.88 eV.

Structure features and ionic conductivity of the AM4(PO4)3 orthophosphates : (A = Na, K, Rb; M = Ni, Mn)

Abstract Two new orthophosphates KNi 4 (PO 4 ) 3 and RbMn 4 (PO 4 ) 3 , with tunnel structure, have been synthesized. Both compounds crystallize in the orthorhombic space group Pmnn , Z=4 whose parameters are a =6.152(2), b =16.214(2), c =9.484(3) for KNi 4 (PO 4 ) 3 and a =6.450(4), b =16.730(3), c =9.870(3) for RbMn 4 (PO 4 ) 3 . Their three-dimensional framework [M 4 P 3 O 12 ] ∞ consists of edge-sharing trioctahedral units [M 3 O 13 ] and square pyramidal groups M 2 O 8 , and all of them are joined by the isolated PO 4 tetrahedra. This 3D lattice possesses large pseudo-hexagonal tunnels running along the c -direction where the alkali cations are located. Ionic conductivity in these materials as well as in the isostructural phases NaNi 4 (PO 4 ) 3 and KMn 4 (PO 4 ) 3 , was undertaken by impedance spectroscopy, and this behaviour is due to the alkali ions mobility through the tunnels. These results are analyzed on the basis of the structural characteristics and alkali cations sizes.

A Comparison of the Magnetic Structures of KMn4(PO4)3 and KCo4(PO4)3 Based on the Connectivity of the Coordination Polyhedra

The crystal structures of KMn(4)(PO(4))(3) and KCo(4)(PO(4))(3) have been determined by neutron diffraction at room temperature. Both compounds are orthorhombic with similar cell parameters, but they crystallize in different space groups, Pnam for the Mn phosphate and Pnnm for the Co analogue. On the basis of the metal cation polyhedra and their connectivity, the crystal structures have been rationalised, which allow interpretation of the main magnetic interactions between them. Magnetic measurements show ferromagnetic behaviour for the Co compound, whereas in the Mn derivative antiferromagnetism is observed. Both magnetic structures are described and qualitatively analysed in terms of superexchange and super-superexchange interactions.