M. Vlasse

Synthesis and structural study of the new rare earth magnesium borates LnMgB5O10 (Ln = La, …, Er)

Abstract To obtain rare earth luminescent materials with weak concentration quenching, the B2O3-rich portion of the ternary diagram Ln2O3MgOB2O3 (Ln = rare earth) has been investigated. A ternary phase of composition LnMgB5O10 has been found for Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er. These compounds all crystallize in the monoclinic space group P2 1 c . The structure has been determined on a LaMgB5O10 crystal. A full-matrix least-squares refinement leads to R = 0.039. The structure can be described as being made of (B5O105−)n two-dimensional layers linked together by the lanthanum and magnesium ions. The rare earth atom coordination polyhedra form isolated chains. These borates are isostructural with some rare earth cobalt borates.

The crystal structure of a new high -Nd- concentration laser material: Na3Nd(PO4)2

The crystal structure of Na3Nd(PO4)2 has been determined from three-dimensional Mo-Kα diffractometer data. The space group is Pbc21, the lattice constants are: a = 15.874(8) Ab = 13.952(8) Ac = 18.470(9) A and there are 24 formula units per unit cell, giving a Nd concentration of 5.8 1021cm−3. The final R-factor with 2329 independent reflections is 0.060. The structure of Na3Nd(PO4)2, very closely related to that of Na3La(VO4)2, is made up of isolated PO4 tetrahedra and of sodium and neodymium atoms arranged in an ordered way. The tripling of the a parameter results only from the distortion of the PO4 tetrahedra. The NdOy polyhedra are isolated from one another and the shortest Nd-Nd distance is 4.65 A.

Crystal structure of tetragonal boron related to α-AlB12

Abstract Single crystals of the so-called β-tetragonal (or tetragonal II or III) boron modification have been obtained from boron deposits prepared by hydrogen reduction of BBr3 on tantalum filaments at 1200°C. Chemical analysis of the samples shows that this phase can be regarded as a true modification of pure elemental boron in contrast to α-tetragonal phases which require small amounts of foreign atoms to stabilize their boron framework. The lattice parameters (a = 10.14(1)A; c = 14.17(1)A) were obtained and refined from single crystal data. The unit cell contains four chemical units, B21 · 2B12 · B2.5 resulting in dc = 2.34 g cm−3 (dm = 2.36(2) g cm−3). The systematic extinctions are compatible with space group P41 or P43. The structure was determined from 1009 independent reflexions using a model derived from the recently solved structure of α - AlB12 (a = 10.161A; c = 14.283A; space group P41212 or P43212). The final R value (unweighted data) is 9.6%. Basically, the structure of this tetragonal form of boron consists of the same three-dimensional boron skeleton, built upon simple and twinned icosahedra, as that of α-AlB12. However, the defective twinned icosahedral B19 units in α-AlB12 are now completed (B21 units) in the related tetragonal boron. A number of interstitial sites, located at positions different from those occupied by aluminum in α-AlB12, are totally or partially filled by boron atoms and very probably increase the stability of the boron framework.

The structures of the Na3Ln(XO4)2 phases (Ln = rare earth, X = P, V, As)

A complete and detailed structural description of sodium rare-earth orthophosphates, orthovanadates and orthoarsenates, with general formula Na3Ln(XO4)2 (Ln = rare-earth, X = P, V, As), is presented. Their structural evolution has been studied not only as a function of the Ln3+ ion size and the (XO4)3− group, but also as a function of temperature.

Preparation, properties and crystal structure of TlxV6S8

Abstract A new sulfide of vanadium and thallium TlxV6S8 (x = 0.52±0.2) has been prepared. Its unit cell has hexagonal symmetry, space group P63 and dimensions: a = 9.187(7) A , c = 3.298(4) A , z = 1. The structure is characterize by infinite triple rutile chains along the c axis containing short V-V (2.860 A) distances. The compound shows metallic behavior.

Structure, superconductivity and magnetism of new rare earth-rhodium silicides RE2Rh3Si5 of U2Co3Si5-type

Abstract New ternary silicides RE 2 Rh 3 Si 5 (RE = Y, La, Nd, Sm, Gd, Tb, Dy, Ho, Er) have been prepared. They crystalize in the U 2 Co 3 Si 5 -type structure. Only the silicides containing the diamagnetic rare earths Y and La show a superconducting transition at T Cr = 4.4 ± 0.2K and T Cr = 2.7 ± 0.1K respectively, those with magnetic rare earths order antiferromagnetically at low temperature.

The boron-silicon solid solution: A structural study of the SiB∼36 composition

Abstract The structure of a crystal of a solid solution of silicon in β-rhombohedral boron has been studied by single-crystal X-ray diffractometry. The final R value was 5.4% for 815 reflections. The space group is R 3 m and hexagonal lattice parameters are a = 11.01(1), c = 23.90(2) A. The cell contains 305.51 boron and 8.443 silicon atoms. The boron framework of β-boron is only slightly changed. Two Si atoms occupy interstitial holes. A third silicon substitutes partially for a framework boron atom. The results obtained are discussed.

The sodium ytterbium orthophosphate Na3(1+x)Yb(2-x)(PO4)3

A new sodium ytterbium orthophosphate with general formula Na3(1+x)Yb(2-x)(PO4)3 (0.07 ⩽ x ⩽ 0.50) has been prepared and characterized. Its crystal structure has been determined from a single crystal for x = 0.50. The space group is R3c, the lattice constants are : a = 9.12(1) A, c = 21.81(6) A. The structure of Na4.50Yb1.50(PO4)3 is related to that of NaZr2(PO4)3. The PO4 tetrahedra and the (Yb,Na)O6 octahedra form a three-dimensional skeleton in which the remaining sodium atoms are inserted. This structural type is also found for the phases Na4.50Ln1.50(PO4)3 (Ln = Tm, Lu) and Na4.50Ln1.50(AsO4)3 (Ln = Er, Tm, Yb, Lu).

The α-AlB12 structure

Abstract The crystal structure of α-AlB 12 , reported recently by Higashi, Sakurai, and Atoda is confirmed by an independent investigation of a different crystal. The space group is P 4 1 2 1 2 (or P 4 3 2 1 2) and our lattice parameters are a = 10.161(7)A, c = 14.283(8)A. The structure was partially solved by Patterson methods, when the full structure was communicated to us by Higashi et al. Utilizing 2393 reflections, our structure refinement yields an R value of 2.6% with very low standard deviations for structural parameters (0.0001 for atomic coordinates of boron atoms). Within the standard deviations there is excellent agreement with all parameters determined by Higashi et al. It is found, nevertheless, that the structural parameters of α-AlB 12 do not serve to account for the reported Debye-Scherrer patterns of “BeB 6 ,” “LiB 6 ,” or β-tetragonal boron, all of which have the same cell dimensions and space group as α-AlB 12 .

Crystal structure and magnetic properties of new rare earth ternary equiatomic silicides RERhSi

Abstract New ternary silicides RERhSi (RE=Y, Gd, Tb, Dy, Ho, Er) have been prepared by arc melting from the elements and annealed for for several days at 800°C. Single crystal studies show these materials to be of orthorhombic symmetry and isostructural with TiNiSi (anti-PbCl 2 structure). Gd, Tb and Dy compounds show a spontaneous magnetization and their ordering consists probably of a non-colinear arrangement of the moments. HoRhSi and ErRhSi order antiferromagnetically at 8K and 7.5K respectively. At 4.2K HoRhSi undergoes a metamagnetic transition above 6kOe. A field transition appears also at 4.2K for ErRhSi above 12kOe.