A.W. Sleight

p-type conductivity in CuCr1−xMgxO2 films and powders

CuCr1−xMgxO2, a wide band gap semiconductor with the delafossite structure, has been synthesized in bulk and thin-film form. Bulk undoped CuCrO2 is almost black and has moderate conductivity with p-type carriers. Upon doping with 5% Mg, the conductivity increases by a factor of 1000. In films, the best p-type conductivity is 220 S cm−1 in CuCr0.95Mg0.05O2, a factor of 7 higher than previously reported for Cu-based p-type delafossites. Undoped films have a conductivity of order 1 S cm−1. Films are usually polycrystalline on amorphous substrates, but undoped films can be c-axis oriented if deposited at or above 650 °C. Optical and ultraviolet transmission data indicate a direct band gap of 3.1 eV.

Rapid synthesis of ZrW2O8 and related phases, and structure refinement of ZrWMoO8

Abstract Six different synthesis routes to ZrW 2 O 8 and related phases are described and compared. A new combustion route requires a synthesis time of less than 15 min and a furnace temperature of 773 K. The reactants were ammonium tungstate, zirconium oxynitrate, 3 N nitric acid, and urea as a fuel. Synthesis by a coprecipitation route is complete in just a few hours but requires a final calcination temperature of about 1425 K. The sol–gel route gives the desired crystalline phases with calcination temperatures of about 875 K, but this route requires more than 1 week for good results. A fourth hydrothermal route also starts with solution precursors. Finally, for purposes of comparison, syntheses were performed by direct calcination of ZrO 2 and WO 3 or zirconium oxynitrate and ammonium metatungstate. In addition to ZrW 2 O 8 , samples of isostructural HfW 2 O 8 , Zr 0.5 Hf 0.5 W 2 O 8 , ZrW 1.5 Mo 0.5 O 8 , HfW 1.5 Mo 0.5 O 8 , and ZrWMoO 8 were prepared by the combustion route. Sintered pellets of ZrW 2 O 8 prepared from powders produced by the combustion or hydrothermal syntheses were 90% dense. A room-temperature structural refinement of ZrWMoO 8 using a combination of X-ray and neutron powder diffraction data confirmed that it has the β-ZrW 2 O 8 structure.

Room-Temperature Superstructure of ZrV2O7

The structure of ZrV 2 O 7 , zirconium pyrovanadate, has been refined from single-crystal synchrotron X-ray data. As with other phases in the AM 2 O 7 family, ZrV 2 O 7 shows a set of strong reflections, which can be explained on the basis of a cubic unit cell with a = 8.765 A, and a family of much weaker reflections due to a 3 × 3 × 3 superstructure. The superstructure has been refined to R F = 0.036 (a = 26.296 A, Pa3, 6972 reflections) and contains highly regular ZrO 6 and VO 4 polyhedra. Of the six unique V 2 O 7 groups, two are constrained by symmetry to contain linear V-O-V linkages, while the remaining four are free to bend away from 180°. The structural distortions from the ideal high-symmetry structure to the observed room-temperature structure are described.

Pressure-induced cubic-to-orthorhombic phase transformation in the negative thermal expansion material HfW2O8

The effect of pressure on the crystal structure of HfW2O8 has been investigated by neutron powder diffraction. At a hydrostatic pressure of 0.62 GPa at room temperature the cubic material transforms, with a 5% reduction in volume, to the same orthorhombic phase that is seen in the isostructural compound ZrW2O8 above 0.21 GPa. The transformation is sluggish, requiring about 24 h to complete at constant pressure. Once formed, the orthorhombic phase is retained upon release of pressure. Upon heating to 360 K, the metastable orthorhombic phase transforms back to the cubic phase. The substantially higher pressure for the cubic-to-orthorhombic transition in HfW2O8, compared to ZrW2O8, may be important for the application of this material in composites with controlled thermal expansion because rather large local pressures can occur in such composites.

Synthesis and structure of bismuth copper arsenate, BiCu2AsO6

Abstract A new compound with the formula BiCu 2 AsO 6 has been prepared. This dark green compound crystallizes in space group Pnma with a =12.253(1) A, b =5.280(1) A, c =7.577(1) A, and Z=4. The structure contains (BiO 2 ) − chains and (AsO 4 ) 3− tetrahedra. Square pyramidal coordination occurs for Cu 2+ with the pyramids sharing edges of their bases in pairs to form Cu 2 O 8 dimers. An unusual feature of the Bi 3+ coordination is five short bonds to oxygen.

Synthesis and characterization of Cd2Ru2O7

Cubic Cd{sub 2}Ru{sub 2}O{sub 7} was prepared hydrothermally at 700 C under 3 kbars pressure. Refinement of single crystal X-ray diffraction data verified the pyrochlore structure with a cell edge of 10.129(1) and a Ru-O distance of 1.927(1). Magnetic, DSC, electrical, and X-ray data indicate a transition at about 100 K that is electronic without any detectable structural change.

Strong anisotropic thermal expansion in oxides

Abstract The strong anisotropic thermal expansion behavior found for cordierite ((Mg 2 Al 4 Si 5 O 15 ), β-eucryptite (LiAlSiO 4 ) and NZP (NaZr 2 P 3 O 12 ) is qualitatively rationalized using distance least squares (DLS) modeling. In this approach, the thermal expansion is driven by the ionic bonds of Mg 2+ , Li + or Na + . Due to constraints imposed by shared polyhedra edges or faces, thermal expansion of the ionic bonds expands the lattice in only one or two dimensions. Due to the connectivity in these structures, this expansion in some directions causes contraction in the other directions. The thermal expansion of β-eucryptite was determined from powder neutron diffraction data over the temperature range 10–809 K. This revealed that the volume thermal expansion of β-eucryptite becomes substantially more negative below room temperature than it is above room temperature. The structure was refined by the Rietveld method from data collected at 12 different temperatures. DLS modeling studies suggest that Li–O bond expansion plus movement of Li from tetrahedral to octahedral sites can explain the thermal expansion behavior above room temperature. However, such an approach cannot explain the more pronounced low-temperature negative thermal expansion, which is most likely attributable to rocking motions of AlO 4 and SiO 4 tetrahedra.

New phases in the ZrO2–Bi2O3 and HfO2–Bi2O3 systems

Abstract New phases of the type M 1−x Bi x O 2−x/2 with a defect fluorite structure have been prepared where M is Zr or Hf. These apparently metastable phases were prepared by precipitation from solution followed by calcination at 600°C. For the Hf 1−x Bi x O 2−x/2 system, x ranges from x = 0.40 to x = 0.75. For the Zr 1−x Bi x O 2−x/2 system, x ranges from x = 0.50 to x = 0.75. Heating the defect fluorite Hf 1−x Bi x O 2−x/2 phases to temperatures of 700 to 950°C leads to the formation of Bi 2 Hf 2 O 7 with the Bi 2 Sn 2 O 7 structure. Thus, Bi 2 Hf 2 O 7 is pseudocubic, but apparently face centered tetragonal with a = 21.66 A and c = 21.84 A. When Bi 2 Hf 2 O 7 is heated to 1000°C, it decomposes into HfO 2 and Bi 1.94 Hf 0.06 O 3.03 with the β-Bi 2 O 3 structure. Heating the defect fluorite Zr 1−x Bi x O 2−x/2 phases to higher temperatures never produced a Bi 2 Zr 2 O 7 phase with the Bi 2 Sn 2 O 7 structure. Instead, the compositional range of the defect fluorite structure steadily decreases with increasing temperature, and this phase had completely decomposed at 750°C into ZrO 2 and Bi 1.84 Zr 0.16 O 3.08 with the β-Bi 2 O 3 structure.

Synthesis and structure of Bi2CaV2O9

Abstract A new compound in the Bi/Ca/V/O system has been found, and its structure determined from single crystal X-ray diffraction data. The new compound, Bi2CaV2O9, is monoclinic with a=7.096(2), b=12.400(2), c=9.397(1) A, β=107.19(2)°, and Z=4. The two crystallographically distinct VO4 tetrahedra do not share corners. Thus, there is one oxygen which is not part of a VO4 tetrahedron, and the structural formula may be written as Bi2Ca(VO4)2O. Mixing of Bi3+ and Ca2+ occurs on three sites where coordination to oxygen is seven or eight.

Cubic phase in the La1−xSrxAlO3−x/2 system

Abstract A series of La 1−x Sr x AlO 3−x/2 phases were prepared from x = 0.00 to 0.35 at 1550°C in air. Rhombohedral symmetry was observed for values of x up to 0.27, but the La 0.65 Sr 0.35 AlO 2.725 sample appears to be cubic. Dieletric measurements on sintered pellets show the dielectric constant decreasing from about 24 to 19 with Sr substitution, whereas dielectric loss increases from 10 −4 to 2.5 × 10 −3 .