R.S. Roth

Low Temperature Coefficient Bulk Dielectrics in the Ca2Nb2O7–Ca2Ta2O7 System

Abstract The dielectric properties of bulk polycrystalline ceramics in the binary Ca 2 Ta 2 O 7 –Ca 2 Nb 2 O 7 system are reported. Measurements of the dielectric constant at 1 MHz were made at temperatures between 0 and 100°C. The large negative (for the niobate) and positive (for the tantalate) temperature coefficients for the dielectric constants in the pure materials in the vicinity of room temperature can be balanced in a two-phase region near x = 0.36 in Ca 2 Ta 2−x Nb x O 7 . At this composition, the dielectric constant is approximately 30, and the temperature coefficient of the dielectric constant is approximately 2 ppm/°C. The Q in our unoptimized polycrystalline ceramic at 1 MHz is approximately 5000.

Preparation and crystal structure of Sr5TiNb4O17

Abstract The compound Sr 5 TiNb 4 O 17 was prepared and its crystal structure determined by single-crystal X-ray diffraction. This compound crystallizes with an orthorhombic unit cell (space group Pnnm (No. 58); a = 5.6614(4), b = 32.515(7), c = 3.9525(3)A; Z = 2). The structure consists of alternating perovskite-like slabs offset with respect to each other by a 2 and c 2 . The Sr 2+ ions occupy two 12-fold coordinated sites within the slabs and a distorted (7 + 1)-fold coordinated site in the gap separating the slabs. Nb 5+ and Ti 4+ are distributed among the octahedral sites of the sperovskite slabs with preferential ordering of Ti 4+ on octahedral sites in the center of the slabs and Nb 5+ in the octahedral sites at the edges of the slabs adjacent to the gap. This compound is the n = 5 member of a structural series A n B n O 3n+2 , (A = Sr; B = Ti,Nb) where n is the number of perovskite layers within each slab.

Preparation and crystal structure of Sr6TiNb4O18

Abstract The crystal structure of newly prepared Sr6TiNb4O18 was determined by single-crystal X-ray diffraction and is described here. This compound crystallizes in space group R3m (No. 160; a=5.6437(5), c=41.347(3)A, Z=3). The structure consists of infinite perovskite-type slabs, five octahedra in thickness, extending parallel to (111)perovskite. All Sr ions are 12-coordinated; Nb5+ and Ti4+ are distributed among the octahedral M sites. The cation coordination spheres in and bordering the gaps between the perovskite-type slabs are highly distorted with bond valence sums indicative of residual chemical strain: SrO bonds are stretched, MO bonds compressed. Nb5+ preferentially occupies the distorted octahedral sites that border the gaps; the least amount of the higher valent metal is found in octahedral sites embedded in the center of the slabs. Similar crystal-chemical effects were reported for isostructural Ba6TiNb4O18 and for Sr5TiNb4O17, also a perovskite-slab-type structure but with slices cut parallel to (110)perovskite.

Crystal structure and properties of Ba5Fe4Ti10O31

Abstract Using single-crystal and powder X-ray diffraction methods, the crystal structure of Ba 5 Fe 4 Ti 10 O 31 was determined and its magnetic and dielectric properties were measured. Ba 5 Fe 4 Ti 10 O 31 crystallizes in space group P6 3 /mcm (No. 193) ( a =9.9886(2), c =42.226(2) A: Z =6: ρ calc =5.15 g cm −3 ) and exhibits an 18-layer close-packed structure built from vacancy-free [O,(Ba,O)] layers. The structure features octahedral sites occupied by a mixture of Fe 3+ and Ti 4+ , with some preferential ordering, and tetrahedral sites occupied by Fe 3+ , one of which shares faces and is half occupied. Some Ba ions in the structure display (9+3) coordination with three unusually long Ba–O bond distances. Computation of bond valence sums about the cation sites using the observed bond distances reveals significant deviations from the valence sum rule, indicating that the structure of Ba 5 Fe 4 Ti 10 O 31 contains residual strain not relieved by distortion. Indexed experimental X-ray powder diffraction data for Ba 5 Fe 4 Ti 10 O 31 are given and are in excellent agreement with those calculated from the results of the single-crystal structure determination. The compound exhibits approximately paramagnetic behavior that deviates somewhat from the Curie Law. Application of this formalism to the 1/ χ vs. T data above 250 K yields an effective moment consistent with the presence of high-spin Fe 3+ ( S =5/2), and a negative Weiss constant (≈−190 K) indicating cooperative magnetic interactions that are overall antiferromagnetic. The relative permittivity and dielectric loss tangent of a sintered polycrystalline disk of Ba 5 Fe 4 Ti 10 O 31 were measured between 7.1 and 7.7 GHz, yielding values (corrected for theoretical density) of 32 and 3.3(±0.3)×10 −3 , respectively, that were essentially independent of frequency.

Cs[sub 8. 5]W[sub 15]O[sub 48] and CsW[sub 2]O[sub 6]: Members of a new homologous series of cesium tungsten oxides

The crystal structures of two new reduced cesium tungsten oxides are reported. Along with the previously reported compound Cs[sub 6]W[sub 11]O[sub 36], they represent several members of a homologous series of layer compounds between the hexagonal tungsten bronze and pyrochlore structure types. The series formula is [A] Cs[sub 2n]W[sub 4n[minus]1]O[sub 12n], where n[minus]1 is the number of pyrochlore layers. 12 refs., 5 figs., 5 tabs.

Cs8.5W15O48 and CsW2O6

The crystal structures of two new reduced cesium tungsten oxides are reported. Along with the previously reported compound Cs[sub 6]W[sub 11]O[sub 36], they represent several members of a homologous series of layer compounds between the hexagonal tungsten bronze and pyrochlore structure types. The series formula is [A] Cs[sub 2n]W[sub 4n[minus]1]O[sub 12n], where n[minus]1 is the number of pyrochlore layers. 12 refs., 5 figs., 5 tabs.