Unusual 180 degrees P-O-P Bond Angles in ZrP(2)O(7)

The structure of cubic ZrP(2)O(7) at room temperature has been solved and refined using a combination of modeling and high-resolution neutron powder diffraction data. The cell edge is 24.74 ?, the space group is Pa&thremacr;, and Z is 108. For those P(2)O(7) units not on a 3-fold axis, the P-O-P angles range from 134 degrees to 162 degrees. Two crystallographically distinct P(2)O(7) groups are on three fold axes with P-O-P angles thus constrained to be 180 degrees on average. The structure of cubic ZrP(2)O(7) was also refined from data taken at 227, 290, 371, 435, and 610 degrees C. The 3 x 3 x 3 superstructure present at room temperature disappears at about 290 degrees C, and all P-O-P angles of P(2)O(7) are then constrained by symmetry to be 180 degrees on average. The exceptionally low thermal expansion shown by ZrP(2)O(7) above 290 degrees C is likely related to the unusual P-O-P angle.     

Refined cell parameters of the Ln2WO6-type rare earth tungstates

Rare earth tungstates of the stoichiometry Ln₂WO₆ where Ln = Ce to Lu have been prepared and their lattice parameters were refined by a least-squares method. They have two structures: the $\text{C}\text{2}\text{c}$ symmetry for the compositions Ce₂WO₆ through Ho₂WO₆, and a monoclinic group from Er₂WO₆ to Lu₂WO₆ crystallizing most likely in the $\text{P}\text{2}\text{m}$ or $\text{P2}{}_1\text{m}$ structure. High pressure modifications are described for Dy₂WO₆ and Ho₂WO₆.     

Synthesis and Crystal Structure of a New Mixed Orthovanadate-Pyrovanadate Series: MBa2V3O11 or MBa2V2PO11 with M = Bi, In, or a Rare Earth

A new series of vanadates with the general formula M Ba₂V₃O₁₁, where M may be Bi, In, or a rare earth, has been synthesized and structurally characterized by single crystal X-ray diffraction and powder X-ray diffraction. The general formula may be rewritten as M Ba₂(VO₄)(V₂O₇) to emphasize that there is one orthovanadate group and one pyrovanadate group in each formula unit. Up to one-third of the vanadium may be replaced by phosphorous, leading to the general formula M Ba₂V₂PO₁₁. However, phosphorous shows no preference between the ortho and pyro groups. Both M Ba₂V₃O₁₁ and M Ba₂V₂PO₁₁ crystallize in the monoclinic system with the space group P2₁/_c and Z = 4. The cell parameters from single crystal X-ray data of BiBa₂V₃O₁₁ are a = 12.332(4) Å, b = 7.750(4) Å, c = 11.279(4) Å, β = 103.22(3)°, V = 1049(1) ų; and for BiBa₂PO₁₁ are a = 12.266(2) Å, b = 7.615(2) Å, c = 11.312(2) Å, β = 103.32(2)°, V = 1028.2(2) ų. The Bi atom coordinates to six oxygen atoms forming a distorted octahedron, and the edge sharing of BiO₆ octahedra results in a BiO₄ chain along the b axis. There are two types of Ba atoms with coordination numbers of 10 and 11. There are three types of tetrahedral (T) atoms in these structures. The nonequivalent T atoms of the pyro group give T-O-T angles of 167 and 171° in BiBa₂V₃O₁₁ and BiBa₂V₂PO₁₁, respectively. Isostructural M Ba₂V₃O₁₁ compounds were prepared in which M is In, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, or Lu.     

Low-Temperature Reaction of Aurivillius Phases with Halides

Kijima et al. [T. Kijima, S. Kimura, Y. Kawahara, K. Ohe, M. Yada, and M. Machida, J. Solid State Chem. 146, 60 (1999)] recently reported a reaction between Bi₄Ti₃O₁₂ and LiI in the presence of iodine at 350°C. Their product was never obtained free of Bi₄Ti₃O₁₂, but their claim was that their product was Bi₄Ti₃O₁₂ intercalated with I and Li. We have repeated this reaction and found conditions under which the reaction goes to completion, i.e., Bi₄Ti₃O₁₂ is completely consumed. The dominant crystalline product is apparently identical to the product reported by Kijima et al. However, we conclude that no intercalation of Bi₄Ti₃O₁₂ has occurred. This dominant crystalline phase is in fact BiOI. The remainder of the product is poorly crystalline. Analogous reactions occur at low temperature using other halides such as NaCl and other Aurivillius phases such as Bi₂WO₆.     

An improved error bound for a finite element approximation of a model for phase separation of a multi-component alloy

Using the approach of Rulla (1996 SIAM J. Numer. Anal. 33, 68-87)for analysing the time discretization error and assuming moreregularity on the initial data, we improve on the error boundderived by Barrett and Blowey (1996 IMA J. Numer. Anal. 16,257-287) for a fully practical piecewise linear finite elementapproximation with a backward Euler time discretization of amodel for phase separation of a multi-component alloy.     

Semiconductor-metal transition in novel Cd2Os2O7

Cd₂Os₂O₇ has been prepared for the first time and has the pyrochlore structure with a cubic cell edge of 10.17 Å at room temperature. Electrical, magnetic, and DSC measurements on single crystals of this compound show a sharp transition at 225 K which we interpret to be an electronic, second-order, metal-semiconductor transition. The low-temperature semiconducting phase is probably antiferromagnetic.