J. Y. Pivan

Nouveau type structural dans la chimie des phosphures : Le compose ternaire Ni20Zr6P13. Preparation, structure et proprietes

Abstract Two new compounds Ni 12 Zr 2 P 7 and Ni 20 Zr 6 P 13 were synthesized in the NiZrP system by reacting the constituent elements. Ni 12 Zr 2 P 7 is of the Fe 12 Zr 2 P 7 -type while Ni 20 Zr 6 P 13 appears as a new structural type in the chemistry of transition metal phosphides. Its unit cell is hexagonal with space group P6 and contains one formula unit. The X-ray structure was studied from three-dimensional single-crystal counter data and was refined down to R = 0.040 for 221 independent reflections. The structure of Ni 20 Zr 6 P 13 can be described as built up by two groups of three phosphorus trigonal prisms occupied by the zirconium atoms. In each group, the |ZrP 6 | prisms are linked together by common edges in order to generate triangular phosphorus sites occupied by nickel atoms. In addition, nickel atoms are also in tetrahedral and square-planar pyramidal phosphorus sites. A comparative study with the Fe 2 P- and Co 4 Hf 2 P 3 -type structures having the same metal/non metal ratio as in Ni 12 Zr 2 P 7 and Ni 20 Zr 6 P 13 is also discussed. A nearly temperature independent paramagetism and a metallic conduction deducted from magnetic and electrical measurements exhibit the metallic behavior for these new compounds.

A new classification scheme to describe and predict structure types in pnictide and silicide chemistry

Structural homogeneity (symmetry, metal/nonmetal ratio, and atomic coordination) in ternary phosphides of lanthanide and transition metals suggested that the anti-AlB2, Fe2P, Zr2Fe12P7, Zr6Ni20P13, (La, Ce)12Rh30P21, Ho20Ni66P43, and La18Rh96P51 types are members of a structural series. A new method based on triangular units is proposed to describe these structures. The stacking of the units in different modes enables us to describe numerous other structure types, to predict new ones, and to establish a classification scheme in pnictide and silicide chemistry.

Polymorphism and physical properties of LnNi2As2 compounds (Ln = La → Gd)

Abstract Ternary BaAl 4 -type derivative structures are frequently found with lanthanoid transition metal silicides, germanides and pnictides with the Ln atoms occupying the Ba sites and the other atoms distributed in an ordered fashion on the Al sites. The ternary phase diagrams LnNiAs were investigated and the compounds LnNi 2 As 2 were obtained with Ln = La → Gd. They crystallize either with the body-centered tetragonal ThCr 2 Si 2 -type (L.T. form), space group I4/mmm, or primitive tetragonal CaBe 2 Ge 2 -type structure (H.T. form), space group P4/nmm. The X-ray structures of LaNi 2 As 2 and CeNi 2 As 2 were refined down to residual values of 0.059 and 0.053 in their L.T. and of 0.028 and 0.060 in their H.T. forms. Electrical and magnetic measurements were performed on single crystals; they exhibit a metallic conductivity and a magnetic ordering at low temperature when Ln = Ce, Pr, Sm, Eu. Above 50 K, the compounds have a normal C.W. behavior with magnetic moments consistent with those of the free Ln 3+ ions, except for Eu which is divalent. LaNi 2 As 2 (L.T. and H.T.) is paramagnetic slightly temperature dependent.

Synthesis and structure determination of (La,Ce)12Rh30P21

Abstract Single crystals of the new phosphide (La,Ce)12Rh30P21 were obtained by reacting the elemental components at high temperature. The unit cell is hexagonal with lattice constants a = 17.475(3) A , c = 3.948(1) A , space group P6 3 m , Z = 1. The structure has been determined from X-ray data and refined to a residual R value of 0.049 for 595 independent reflections and 40 variable parameters. This structure is of a new type and appears as the fourth term of the structural series of Fe2P, Zr2Fe12P7 and Zr6Ni20P13 types with the general formula An(n−1)B(n + 1)(n + 2)Cn(n + 1) + 1. Rare earth atoms are in trigonal prismatic phosphorus coordination while rhodium atoms occupy three different phosphorus polyhedra: “square-planar” pyramids, tetrahedra and triangles. For the first time in pnictide chemistry a 4d transition element exhibits a triangular non-metal coordination.

Rh12As7, a host structure for rare earth elements: Crystal structures of Rh12As7 and Ho2Rh12As7

The binary compound Rh12As7 and the related ternary compounds M2Rh12X7 (M ≡ rare earth, Zr, Y; X ≡ As, P) were synthesized. The X-ray structures of Rh12As7 and Ho2Rh12As7 were studied using three-dimensional single-crystal counter data and were refined in the centrosymmetric space group P63m down to R values of 0.049 and 0.039 for 216 and 241 independent reflections respectively. The structure of Rh12As7 is derived from the Cr12P7 (or V12P7) type in which the arsenic atom that statistically occupies the 4e position is disordered along the sixfold axis. The filling by holmium atoms of the two trigonal prismatic arsenic voids in Rh12As7, correlated with a displacement of about c2 of all rhodium atoms, leads to the structure Ho2Rh12As7. The phenomenon of metalloid disorder along the c axis in Rh12As7 and Ho2Rh12As7 is compared with that observed in the Cr12P7, Zr2Fe12P7, Zr2Rh12P7 and Rh20Si13 structures.

Ternary rare earth transition metal phosphides II. Synthesis and structure determination of Ho20Ni66P43

Abstract Single crystals of Ho 20 Ni 66 P 43 were obtained by reacting the elemental components at high temperature. The unit cell is hexagonal with lattice constants a = 23.095 (7) A , c = 3.742 (4) A ; the space group is P6 3 /m with Z = 1. The structure has been determined from X-ray data to a final R value of 0.054 for 850 reflections (I > 1 σ (I)). This structure is of a new type and belongs to the structural family of Zr 2 Fe 12 P 7 - and Zr 6 Ni 20 P 13 -types. The coordination polyhedra and structural comparison with closely related structure-types are discussed.

Ternary rare-earth transition metal phosphides. I. Synthesis and crystal structure of Ho5Ni19P12

Single crystals of the new phosphide Ho5Ni19P12 were obtained by reacting the elemental components at high temperature. The compound crystallizes in the hexagonal space-group P62m with the cell constants: a = 12.288 (2), c = 3.762 (2) A, Z = 1. The structure has been determined from X-ray data and refined to a final R value of 0.049 for 271 independent reflections. It is derivative of the Hf2Co4P3- type structure. Holmium atoms are in a trigonal- prismatic phosphorus coordination while nickel atoms occupy three different phosphorus polyhedra: triangle, tetrahedra and ‘square-planar’ pyramids. The atomic arrangement of Ho5Ni19P12 appears as a combination of those observed in Zr2Fe12P7 and Zr6Ni20P13-type structures.

Tetrahedral Ni4 clusters in a marcasite-type host structure: The preparation and crystal structure of MNi4X2 compounds (X ≡ P, As; M ≡ Zr, Hf, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu)

Abstract Ternary pnictides MNi4X2 (M ≡ Zr, Hf; X ≡ As, P) and the corresponding rare earth arsenides LnNi4As2 (Ln = Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) have been synthesized from their elements. They all have isotypic tetragonal crystal structures, space group P42/mnm with two formulae per unit cell. The X-ray structure, performed on single crystals of ZrNi4P2 (R = 0.028, 122 independent reflections) exhibits tetrahedral Ni4 clusters in a marcasite-type host structure. The Ni4 clusters with common edges develop as infinite linear chains along the ¦011¦ direction. While most of the compounds of marcasite-type are semiconducting, first electrical measurements reveal a metallic conduction. Preliminary magnetic measurements on rare-earth compounds indicate a Curie-Weiss paramagnetism without magnetic ordering above 4 K.

New ternary pnictides Ln2NiX2 (X = P, As) with a filled tip-type structure

Abstract New ternary Ln 2 NiX 2 (Ln = Tb, Dy, Ho, Er and Zr; X = P, As) have been synthesized. They have isotypic hexagonal crystal structures, space group P6 3 /mmc with two formulae per unit cell. The X-ray structures of Ho 2 NiAs 2 and Zr 2 NiAs 2 were refined, from three-dimensional single-crystal intensity data, down to final R values of 0.039 and 0.032 respectively. In these new compounds, the Ln atoms show a somewhat unusual octahedral X-coordination and the nickel atoms are in triangular sites. There are no X-X bondings while strong Ln-Ni interactions occur to generate a 2D-metallic hexagonal network quasi parallel to the (001) plane. Structural relationship with TiP, NiMoP 2 types are discussed.

Crystal structures and properties of new rare earth rhodium phosphides : La6Rh32P17 and Ce6Rh32P17

Abstract La6Rh32P17 and Ce6Rh32P17 crystallize in the hexagonal system, space group P 6 3 m o with a = 27.054(6) A , c = 3.944(1) A and a = 26.941(5) A , c = 3.926(1) A respectively, Z = 3. The structures were solved from single crystal X-ray data and exhibit a new type which is closely related to the structural series of Fe2P, Zr2Fe12P7 and Zr6Ni20P13 types. The main features of the structure result from phosphorus vacancies which imply face-shared “LnRh8” clusters running in a direction parallel to the c-axis. The La compound shows a Pauli paramagnetism while the Ce compound, which does not fit a Curie-Weiss law, suggests strong crystal field or Kondo effects. Both compounds are good metallic conductors.