P. Gravereau

Na3La9O3(BO3)8, a new oxyborate in the ternary system Na2OLa2O3B2O3: preparation and crystal structure

Abstract A new oxyborate with a composition corresponding to the formula Na3La9O3(BO3)8 has been discovered in the ternary oxide system Na2Oue5f8La2O3ue5f8B2O3. Single crystals of this compound have been grown by spontaneous crystallization from melts of the constituent binary oxides during slow cooling. The structure of Na3La9O3(BO3)8 has been solved using X-ray data collected on a Nonius Kappa CCD diffractometer (Moxa0Kα radiation). The unit cell is hexagonal (space group P-62m, Z=1) with a=8.9033(3), c=8.7131(3) A and V=598.14 A3. Refinement of 49 parameters using 684 independent reflections with an intensity I>2σ(I) (Friedel not merged for absolute structure determination) results in R1=0.015 (wR2=0.030). The structure is characterized by alternate stacking along the c-axis of Naue5f8Bue5f8O, Bue5f8O, La and Laue5f8O containing layers with a mirror plane at z=1/2. A framework of La9O3 rings and rows of BO3 triangles running along the c-direction is established. Each La9O3 ring is constituted of three La4O tetrahedra sharing a common La corner. The presence in the structure of oxygen atoms surrounded by lanthanum atoms only justifies the oxyborate label of the new compound.

Crystal structure of the new ternary stannides U2M2Sn with M Fe or Rh

Abstract New stannides U 2 Fe 2 Sn and U 2 Rh 2 Sn have been prepared and their crystal structures determined by X-ray diffractometry on single crystals. They crystallize in the tetragonal ordered version ( P4/mbm space group) of the U 3 Si 2 -type structure with uranium and tin atoms occupying the corresponding U sites and transition element atoms (Fe or Rh) located at the Si sites. This structure has been solved by the heavy atom method with a residual factor R of 0.058 for U 2 Fe 2 Sn and of 0.039 for U 2 Rh 2 Sn. The lattice parameters are a = 7.296(1) A and c = 3.446(1) A for U 2 Fe 2 Sn and a = 7.525(1) A and c = 3.624(1) A for U 2 Rh 2 Sn.

Preparation and structural characterization of two new titanium phosphates NaCa0.5Ti(PO4)3 and Ni0.5TiOPO4

Summary The titanium phosphate Na 4 Ca 0.5 Ti(PO 4 ) 3 crystallizes in the trigonal space group R32 (a h = 9.008 ± 0.002 A, c h = 21.814 ± 0.003 A and Z = 6). Its structure belongs to the nasicon type family. The titanyl phosphate Ni 0.5 TiOPO 4 crystallizes in a monoclinic unit cell, P2 I ,/c, (a = 7.383 ± 0.001 A, b = 7.323 ± 0.001 A, c = 7.344 ± 0.001 A, β = 120.23 ± 0.01° and Z = 4). The structure of these two compounds is based on a three-dimensional framework built of TiO 6 octahedra and PO 4 tetrahedra. The PO 4 tetrahedra are isolated in both Na 4 Ca 0.5 Ti(PO 4 ) 3 and Ni 0.5 TiOPO 4 . The TiO 6 octahedra are isolated in the former and linked together by “titanyl” oxygen atoms to form -Ti-O-Ti-O- infinite chains in the latter. These structural data are confirmed by the Raman spectroscopy study.

Crystal chemistry and magnetic properties of ternary stannides R2M2Sn (R = rare earth or uranium, M = Ni, Pd)

Abstract We have synthesized a great number of ternary stannides R 2 Ni 2 Sn, R 2 Pd 2 + x Sn 1 − x (R = rare earth) and U 2 M 2 Sn (M = Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt). They crystallize either in the orthorhombic W 2 CoB 2 -type or in the tetragonal U 3 Si 2 -type. A structural relationship exists between these types and those of binary RNi and ternary RNiSn or RPdSn equiatomic compounds. Attention is also focused on the wide range of their magnetic properties [complex (B, T)-magnetic phase diagram, antiferromagnetic → ferromagnetic transition.…].

Crystal structure of the new borate Li3Gd(BO3)2. Comparison with the homologous Na3Ln(BO3)2 (Ln: La, Nd) compounds

Abstract The structure of Li3Gd(BO3)2 has been solved by X-ray diffraction study on single crystal. This novel borate crystallizes in the monoclinic system with the P21/c space group (Z=4). The cell parameters are respectively equal to a=8.724(2), b=6.425(2), c=10.095(2)xa0A and β=116.85(2)°. Refinements of 110 parameters using 2924 independent reflections having I>2σ(I) converged to R1=0.028 (wR2=0.058). The structure of Li3Gd(BO3)2 is made up of double layers of eightfold coordinated Gd atoms parallel to the (bc) plane. They are linked by respectively three- and four-coordinated boron and lithium atoms. The structure is compared to that of the homologous sodium compounds, Na3Ln(BO3)2 (Ln: La, Nd), in which LnO8 polyhedra also form a bi-dimensional array.

Ferromagnetic properties of U2M17−yGey systems (M ≡ Fe, Co)

Abstract The ternary germanides U 2 M 17− y Ge y were synthesized respectively with 2 ⩽ y ⩽ 3 for M ≡ Fe and 1.3 ⩽ y ⩽ 3 for M ≡ Co. These compounds crystallize in the hexagonal Th 2 Ni 17 -type structure. Rietveld X-ray diffraction study indicates a strong preferential site occupation of Ge atoms on one (12k) of the four different transition element sites of this type of structure. All the compounds order ferromagnetically with a Curie temperature T C going through a maximum at T C = 545(5) K for y = 2.5 with increasing Ge content in the U 2 Fe 17− y Ge y system, whereas T C decreases linearly as y increases for the U 2 Co 17− y Ge y system. Also, the effect of the substitution of Co for Fe in U 2 (Fe 14.5− x Co x )Ge 2.5 alloys on their structural and magnetic properties was investigated. The results are compared with those obtained for the intermetallic compounds based on rare earth elements RE 2 Fe 17 and RE 2 Co 17 .

Antiferromagnetism in the U2Pd2+xSn1−x ternary system for 0 ≤ x ≤ 0.44(2)

Abstract The U2Pd2+xSn1−x solid solution exists for 0 ≤ x ≤ 0.44(2) and crystallizes in an ordered version of the tetragonal U3Si2-type structure. All the compounds order antiferromagnetically, but TN decreases with increasing x: TN is equal to 42(1) and 13(1) K for x = oand 0.44, respectively. A careful investigation by 119Sn Mossbauer spectroscopy at 4.2 K shows that the hyperfine magnetic field Hhf seen by the tin atoms increases with x and vanishes for x = 0. This result suggests that the magnetic structure of these compounds is dependent on the Sn content. Their magnetic properties are influenced by the composition dependence of the U-ligand distances which govern the anisotropic character of the 5f-ligand hybridization.

Spectroscopic studies and Rietveld refinement of strontium-britholites

Abstract Strontium-britholites whose chemical formula was Sr10−xLax(PO4)6−x(SiO4)xF2, where x=0, 1, 2, and 4 were prepared by solid state reaction. The structural refinement carried out using the Rietveld method indicated that La3+ ions were located into the two sites with a strong preference for metal (2) sites especially for low contents. A progressive shift of the F− position along the c-axis outside the centre of the triangle formed by metal (2)-atoms was observed with the increase of x. The infrared and Raman spectra exhibited the characteristic vibration modes of PO4 and SiO4 groups confirming the incorporation of this last group into the apatite structure. The 29Si MAS-NMR spectra exhibited one resonance peak confirming the data obtained by X-ray diffraction, indicating that P and Si were located in the same crystallographic site.

High pressure synthesis and structure of a novel clathrate-type compound: Te7+xSi20−x (x∼2.5)

Abstract A novel clathrate-type compound, Te 7+ x Si 20+ x ( x ∼2.5), has been synthesized from the elements under high pressure and high temperature conditions. Its structure has been solved by Rietveld analysis of the XRD powder patterns combined with the results of an electron diffraction study. This structure is closely related to that of a typexa0I clathrate, but with a different unit-cell parameter ( a ∼2× a 0 ) and space group ( Fd -3 c instead of Pm -3 n ). Its main feature is that it corresponds to a double clathration of a Te atom enclosed in a partially Te substituted (12.5%) Si 20 pentagonal dodecahedra, which is itself enclosed in a large Te 24 polyhedron, in form of a truncated octahedron. This outer Te network proved to be similar to the characteristic H 2 O host lattice of a type VII clathrate (HEF 6 ·(H 2 O) 5 ·HF (E=P, As, Te)), the distribution of the Si/Al atoms in minerals of the sodalite group ((Na,Ca) 8 (Al 6 Si 6 O 24 )(Cl,S,SO 4 ) 2 ) and to the arrangement of the Ba atoms in the structure of the superconducting compound, Ba 6 C 60 . This new clathrate structure of silicon is the first one which exhibits Si 20 pentagonal dodecahedra which are only linked by inter-cluster bonds, a result which opens new prospects in the domain of the chemistry of macro-ions such as Si 20 12+ or Si 28 4+ .

Synthesis and magnetic behaviour of the ternary germanides UCu2Ge2 and U3Cu4Ge4

Abstract The ternary germanide U 3 Cu 4 Ge 4 crystallizes in the orthorhombic Gd 6 Cu 4 Ge 4 -type structure with a = 1.3932(2) nm, b = 0.6579(1) nm and c = 0.4273(1) nm as unit cell parameters. It orders ferromagnetically below T C = 71 K and is detected as an impurity phase in the as-cast UCu 2 Ge 2 sample. The magnetic properties of this last ternary germanide are strongly dependent on the experimental procedures adopted for its synthesis.