Terrell A. Vanderah

Phase transitions and microwave dielectric properties in the perovskite-like Ca(Al0.5Nb0.5)O3−CaTiO3 system

Phase transitions and microwave dielectric properties in the (1−x)Ca(Al0.5Nb0.5)O3–xCaTiO3 system were analyzed using x-ray and neutron powder diffraction, transmission electron microscopy, Raman spectroscopy, and dielectric measurements at microwave frequencies (2–8 GHz). Rietveld structural refinements demonstrated that both end compounds exhibit similar octahedral tilted frameworks, while in Ca(Al0.5Nb0.5)O3, tilting is superimposed onto NaCl-type ordering of Al and Nb on the B sites. Accordingly, the room-temperature structures of CaTiO3 and Ca(Al0.5Nb0.5)O3 are described by orthorhombic Pbnm and monoclinic P21/n symmetries, respectively, with similar lattice parameters, √2ac×√2ac×2ac (where ac is the lattice parameter of cubic perovskite). The (1−x)Ca(Al0.5Nb0.5)O3–xCaTiO3 system features both cation ordering and octahedral tilting phase transitions. The Ca(Al0.5Nb0.5)O3 structure remains ordered at least up to 1625 °C. However, the temperature of the order/disorder transition decreases rapidly with ...

Phase equilibria, crystal structures, and dielectric anomaly in the BaZrO3-CaZrO3 system

Abstract Phase equilibria in the (1−x)BaZrO3–xCaZrO3 system were analyzed using a combination of X-ray and neutron powder diffraction, and transmission electron microscopy. The proposed phase diagram features two extended two-phase fields containing mixtures of a Ba-rich cubic phase and a tetragonal, or orthorhombic Ca-rich phase, all having perovskite-related structures. The symmetry differences in the Ca-rich phases are caused by different tilting patterns of the [ZrO6] octahedra. In specimens quenched from 1650°C, CaZrO3 dissolves only a few percent of Ba, whereas the solubility of Ca in BaZrO3 is approximately 30 at % . The BaZrO3–CaZrO3 system features at least two tilting phase transitions, Pm3m→I4/mcm and I4/mcm→Pbnm. Rietveld refinements of the Ba0.8Ca0.2ZrO3 structure using variable-temperature neutron powder diffraction data confirmed that the Pm3m→I4/mcm transition corresponds to a rotation of octahedra about one of the cubic axes; successive octahedra along this axis rotate in opposite directions. In situ variable-temperature electron diffraction studies indicated that the transition temperature increases with increasing Ca-substitution on the A-sites, from approximately −120°C at 5 at % Ca to 225°C at 20 at % Ca. Dielectric measurements revealed that the permittivity increases monotonically from 36 for BaZrO3 to 53 for Ba0.9Ca0.1ZrO3, and then decreases to 50 for Ba0.8Ca0.2ZrO3. This later specimen was the Ca-richest composition for which pellets could be quenched from the single-phase cubic field with presently available equipment. Strongly non-monotonic behavior was also observed for the temperature coefficient of resonant frequency; however, in this case, the maximum occurred at a lower Ca concentration, 0.05⩽x⩽0.1. The non-linear behavior of the dielectric properties was attributed to two competing structural effects: a positive effect associated with substitution of relatively small Ca cations on the A-sites, resulting in stretched Ca–O bonds, and a negative effect, related to the distortion of the A-site environment (bond strain relaxation) upon octahedral tilting.

Crystal Structure of the Compound Bi2Zn2/3Nb4/3O7

The crystal structure of Bi 2 Zn 2/3 Nb 4/3 O 7 was determined using a combination of electron, x-ray, and neutron powder diffraction. The compound crystallizes with a monoclinic zirconolite-like structure [C2/c (No.15) space group, a = 13.1037(9) A, b = 7.6735(3) A, c = 12.1584(6) A, β = 101.318(5)°]. According to structural refinement using neutron diffraction data, Nb preferentially occupies six-fold coordinated sites in octahedral sheets parallel to the (001) planes, while Zn is statistically distributed between two half-occupied (5 + 1)-fold coordinated sites near the centers of six-membered rings of [Nb(Zn)O 6 ] octahedra. The Nb/Zn cation layers alternate along the c -axis with Bi-layers, in which Bi cations occupy both eight- and seven-fold coordinated sites. The eight-fold coordinated Bi atoms exhibited strongly anisotropic thermal displacements with an abnormally large component directed approximately along the c -axis (normal to the octahedral layers).

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.

A structural study of the layered perovskite-derived Sr n (Ti, Nb) n O3n +2 compounds by transmission electron microscopy

Abstract A series of Sr n (Nb, Ti) n O3n+2 structures with n = 4, 4.5, 5, 6 and 7 were studied by transmission electron microscopy. These structures are composed of infinite two-dimensional slabs of the distorted perovskite structure that are n (Ti, Nb)O6 octahedra thick and extend parallel to the {110}perrovskite plane. The slabs are displaced with respect to each other by the translation vector ½[011]perovskite. All members of the Sr n (Nb, Ti) n O3n+2 series have an orthorhombic basic lattice with a ≈ a perovskite and c ≈ 2 1/2 a perovskite, while the long b axis increases systematically with increasing n value. The compounds with n = 4, 5, 6 and 7 were observed to undergo a commensurate ← incommensurate phase transition on cooling in the temperature range 150–250°C. The wave-vector of the incommensurate modulation is parallel to [100] direction of the basic orthorhombic lattice and is close but not exactly equal to ½a*. The n = 5 incommensurate phase further transformed at 180°C to a monoclinic structure with the space group P1121/b (No. 14). For the n = 4. 6 and 7 compounds, no lock-in transition was observed down to −170°C. For the compound with n = 4.5, the transition from orthorhombic to monoclinic structure P1121/b occurred on cooling at 390°C. All transitions observed in the Sr n (Nb, Ti) n O3n+2 compounds were attributed to tilting of the (Ti, Nb)O6 octahedra.

Phase Equilibria and Crystallography of Ceramic Oxides

Research in phase equilibria and crystallography has been a tradition in the Ceramics Division at National Bureau of Standards/National Institute of Standatrds and Technology (NBS/NIST) since the early thirties. In the early years, effort was concentrated in areas of Portland cement, ceramic glazes and glasses, instrument bearings, and battery materials. In the past 40 years, a large portion of the work was related to electronic materials, including ferroelectrics, piezoelectrics, ionic conductors, dielectrics, microwave dielectrics, and high-temperature superconductors. As a result of the phase equilibria studies, many new compounds have been discovered. Some of these discoveries have had a significant impact on US industry. Structure determinations of these new phases have often been carried out as a joint effort among NBS/NIST colleagues and also with outside collaborators using both single crystal and neutron and x-ray powder diffraction techniques. All phase equilibria diagrams were included in Phase Diagrams for Ceramists, which are collaborative publications between The American Ceramic Society (ACerS) and NBS/NIST. All x-ray powder diffraction patterns have been included in the Powder Diffraction File (PDF). This article gives a brief account of the history of the development of the phase equilibria and crystallographic research on ceramic oxides in the Ceramics Division. Represented systems, particularly electronic materials, are highlighted.

Subsolidus Phase Relations and Dielectric Properties in the SrO-Al2O3-Nb2O5 System

Abstract Subsolidus phase equilibria in the SrO–Al2O3–Nb2O5 system were determined by synthesis of 75 compositions in air in the temperature range 1200–1600°C. Phase assemblages were determined by X-ray powder diffraction at room temperature. Two new ternary compounds, Sr4AlNbO8 and Sr5.7Al0.7Nb9.3O30, were observed to form in addition to the known double perovskite, Sr2AlNbO6 ( Fm 3 m , a=7.7791(1) A). Sr4AlNbO8 crystallizes with a monoclinic unit cell (P21/c; a=7.1728(2), b=5.8024(2), c=19.733(1) A; β=97.332(3)°) determined by electron diffraction studies; the lattice parameters were refined using X-ray powder diffraction data, which are given. This compound decomposes above 1525°C; attempts to grow single crystals from neat partial melts, or using a strontium borate flux, were unsuccessful. The phase Sr5.7Al0.7Nb9.3O30 (Sr6−xAl1−xNb9+xO30, x=0.3) forms with the tetragonal tungsten bronze structure (P4bm; a=12.374(1), c=3.8785(1) A), melts incongruently near 1425°C, and occurs essentially as a point compound, with little or no range of x-values; indexed X-ray powder diffraction data are given. The tungsten bronze structure exhibits a narrow region of stability in the SrO–Al2O3–Nb2O5 system, which is probably related to the small size of Al3+. The existence of an extensive cryolite-type solid solution, Sr3(Sr1+xNb2−x)O9−3/2x, occurring between Sr4Nb2O9 (x=0) and Sr6Nb2O11 (x=0.5), was confirmed, with cubic lattice parameters ranging from 8.268(2) to 8.303(1) A, respectively. The dielectric properties of the three ternary compounds occurring in the system were measured using the specimen as a TE011 or TE0γδ dielectric resonator: Sr2AlNbO6: er=25, τf=−3 ppm/°C, tan δ=1.9×10−3 (7.7 GHz); Sr4AlNbO8: er=27, tan δ=2.8×10−3 (10.5 GHz); Sr5.7Al0.7Nb9.3O30: er=168, tan δ=3.8×10−2 (3.1 GHz). Sr2AlNbO6, when sintered in 1 atm oxygen, exhibited a reduced permittivity (er=21) and a significantly improved dielectric loss tangent (tan δ=5.2×10−4, 8.3 GHz), resulting in a four-fold increase in Q×f as compared to the specimen sintered in air.

On the crystal structure of KInS2-I

Abstract The ambient-pressure form of KInS2 crystallizes in the monoclinic space group C2/c; a = 10.981(3), b = 10.979(3), c = 15.010(5) A, β = 100.55(2)°, Vol = 1779.2(9)A3, Z = 16, Dx = 3.257g/cm3 for MF = 218.04. An X-ray single-crystal structure determination has confirmed that KInS2-I has the TlGaSe2 structure. The bonding in KInS2-I is highly covalent and exhibits both two-dimensional and three-dimensional features. The structure is comprised of layers of vertex-sharing [In4S10] adamantane-like units composed of four [InS4] tetrahedra. The stacking arrangement of these layers creates channels that contain the potassium ions in distorted trigonal prismatic sites that provide the three-dimensionality of the structure. The strength of the interlayer potassium-sulfur bonding is reflected in the nonmicaceous morphology and water-stability of the transparent light-yellow crystals.

Phase Equilibria and Dielectric Properties in Perovskite-like (1 − x)LaCa0.5Zr0.5O3– xATiO3 (A = Ca, Sr) Ceramics

Phase equilibria and dielectric properties were analyzed for selected compositions in both LaCa 0 . 5 Zr 0 . 5 O 3 -CaTiO 3 (LCZ-CT) and LaCa 0 . 5 Zr 0 . 5 O 3 -SrTiO 3 (LCZ-ST) systems using x-ray powder diffraction and transmission electron microscopy. The end-member LaCa 0 . 5 Zr 0 . 5 O 3 does not occur as a single phase but rather as a mixture of a perovskite-type phase with approximate composition La 0 . 9 4 Ca 0 . 5 3 Zr 0 . 5 3 O 3 plus a minor amount of La 2 O 3 . This perovskite phase exhibited a combination of 1:1 ordering of Ca and Zr on the B-sites and octahedral tilting. In the (I - x)LCZ-xCT system, the compositions x = 1/3 and x = ½ yielded single phases with perovskite-like structures featuring similar 1:1 B-site ordering superimposed onto octahedral tilting. The x = ½ composition in the LCZ-ST system resides in a two-phase field and contains a major perovskite phase and La 2 O 3 ; the B-cations in the perovskite phase remain disordered at all temperatures. The approximate boundaries of perovskite-like phase fields in the La 2 O 3 -ATiO 3 -CaZrO 3 (A = Ca, Sr) systems were outlined, as well as a schematic diagram for perovskite B-cation ordering transitions in the LCZ-CT system. The dielectric properties of the compositions investigated were measured at microwave frequencies and were correlated with the observed structural behavior.