J. L. Bernstein

Bio: J. L. Bernstein is an academic researcher. The author has contributed to research in topics: Crystal structure & Lattice constant. The author has an hindex of 6, co-authored 6 publications receiving 1355 citations.

Ferroelectric Tungsten Bronze‐Type Crystal Structures. I. Barium Strontium Niobate Ba0.27Sr0.75Nb2O5.78

Abstract: Ferroelectric Ba0.27Sr0.75Nb2O5.78, with Tc = 348° ± 15°K, is a tungsten bronze‐type structure crystallizing in the tetragonal system, with lattice constants a = 12.43024 ± 0.00002 and c = 3.91341 ± 0.00001 A at 298°K, space group P4bm, and five formulas in the unit cell. The integrated intensities of 6781 structure factors were measured with PEXRAD, 875 symmetry‐independent structure factors being significantly above background. The metal‐atom positions were determined from the three‐dimensional Patterson function and the oxygen atoms from subsequent Fourier series. The final agreement factor between measured and calculated structure factors is 0.0508. The structure consists of close‐packed slightly puckered layers of oxygen atoms separated by nearly c / 2. The Nb atoms are slightly displaced from one layer, the Ba and Sr atoms from the other and in the same sense. The oxygen atoms in the Ba and Sr layer are disordered. Neither of the two independent sites occupied by the Ba and Sr atoms is fully filled....

Ferroelectric Tungsten Bronze‐Type Crystal Structures. II. Barium Sodium Niobate Ba(4 + x)Na(2 − 2x)Nb10O30

Abstract: Ferroelectric Ba(4+x)Na(2 − 2x)Nb10O30, with a Curie temperature of 833°K, has a tungsten bronze‐type structure and crystallizes in the orthorhombic system, with subcell lattice constants a = 17.59182 ± 0.00001, b = 17.62560 ± 0.00005, and c = 3.994915 ± 0.000004 A at 298°K. The space group is Cmm2, and there are two formulas in the subcell. The c axis of the true cell is double that of the subcell. The integrated intensities of 6911 reflections within a reciprocal hemisphere of radius (sinθ)/λ = 1.02 A−1 were measured with PEXRAD, 1873 symmetry‐independent structure factors being significantly above background. The metal atom positions were determined from the three‐dimensional Patterson function and the oxygen atoms from metal‐phased Fourier series. The final agreement index between measured and calculated structure factors is 0.0579. The structure differs only in detail from previously determined tetragonal tungsten bronze structures. In the general formula (A1)2(A2)4C4(B1)2(B2)8O30, the B1 and B2 sites are fully occupied by Nb, the A2 sites by Ba and the A1 site by 87.0% Na and 6.5% Ba. Evidence from chemical analysis, x‐ray density calculations and the present determination suggests that the best approximation to the formula of the crystal studied is Ba4.13Na1.74Nb10O30. The Ba and O atoms at z ≃ 12 are disordered in a manner similar to the O atoms in the Ba layer in barium strontium niobate. The four crystallographically independent Nb atoms, each in octahedral coordination, are linked to O atoms by distances ranging from 1.765 ± 0.021 to 2.270 ± 0.021 A, with a mean value of 1.989 A. Ba is 10 coordinated, with Ba–O distances ranging upward from 2.671 ± 0.013 A. Na is 12 coordinated, with Na–O distances ranging from 2.660 ± 0.014 to 2.990 ± 0.015 A, with a mean of 2.788 A. The Nb‐atom displacements from the mean oxygen planes lie in the range 0.171–0.205 A; the parabolic relation with Curie temperature predicts a displacement of 0.204 A. The measured value of Ps at room temperature is 40 μC cm−2: the linear relation between displacement and polarization predicts a saturation value of 44–53 μC cm−2. All metal atoms are displaced from the oxygen planes in the sense given by the macroscopic positive polarity.

Crystal Structure of Piezoelectric Bismuth Germanium Oxide Bi12GeO20

Abstract: Piezoelectric Bi12GeO20 crystallizes in the cubic system with space group I23 and lattice constant a = 10.1455±0.0008 A at 298°K. The complete x‐ray scattering pattern within a reciprocal lattice hemisphere of radius (sinθ)/λ = 1.02 A−1 was determined with PEXRAD. A total of 4812 reflections were measured, resulting in 631 independent and significantly nonzero Fmeas. The crystal structure was solved using three‐dimensional Patterson and Fourier series, and refined by the method of least squares. The final agreement factor R is 0.062. The germanium atoms occupy geometrically regular tetrahedra, with Ge–O=1.717±0.028 A. The bismuth atoms are heptacoordinated: five oxygen atoms form an incomplete octahedral arrangement, with Bi–O distances ranging from 2.076±0.027 to 2.640±0.028 A. The remaining two oxygen atoms are electrostatically coordinated, on either side of the 6s2 inert electron pair in Bi3+, at distances of 3.082 and 3.170 A from bismuth. The Bi atom vibrates anisotropically. The absolute configuration of the atomic arrangement has been determined. The positive piezoelectric polarization induced by compressive stress applied along 〈111〉 is given by the normal to a triangular GeO4 tetrahedral face in the direction from the Ge atom to the face.

Crystal Structure of the Transition‐Metal Molybdates and Tungstates. II. Diamagnetic Sc2(WO4)3

Abstract: Sc2(WO4)3, diamagnetic above 30°K, crystallizes in the orthorhombic system, Space Group Pnca, with lattice constants a=9.596±0.004, b=13.330±0.003, and c=9.512±0.004 A at 298°K. The complete x‐ray scattering pattern within a reciprocal lattice hemisphere of radius (sinθ)/λ=1.02 A−1 was measured with PEXRAD. The crystal structure was solved by use of three‐dimen sional Patterson and Fourier series and refined by the method of least squares, using 1731 independent structure factors. The final agreement factor R is 0.0622. Scandium atoms occupy slightly distorted octahedra, with average Sc–O=2.063 A and Sc–O distances ranging from 2.026±0.015 to 2.124±0.010 A. Two crystallographically independent W atoms are surrounded by somewhat distorted tetrahedra: the W–O distances vary from 1.695±0.009 to 1.829±0.016 A, the average being 1.761 A. The thermal vibrations are significantly anisotropic. Sc2(WO4)3 forms the structure type for 23 trivalent metal tungstates and molybdates, including the nine smaller rare‐earth tungstates. The larger rare‐earth tungstates, crystallizing in the Eu2(WO4)3 structure type, have 8 coordination about the rare‐earth ion; the smaller have 6 coordination. A simple correlation is found between the variation in radius ratio due to the lanthanide contraction and the change in coordination.

Ferroelectric Tungsten Bronze‐Type Crystal Structures. III. Potassium Lithium Niobate K(6−x−y)Li(4+x)Nb(10+y)O30

Abstract: Ferroelectric K(6−x−y)Li(4+x)Nb(10+y)O30, with x ≃ 0.07, y ≃ 0.23, and a Curie temperature of 613°K, crystallizes with a tungsten bronze‐type structure in the tetragonal system. The lattice constants are a = 12.5764 ± 0.0002 and c = 4.0149 ± 0.0001 A at 298°K. The space group is P4bm. There is one formula per unit cell. The integrated intensities of 6578 structure factors, inside a reciprocal hemisphere of radius (sinθ) / λ = 1.02 A−1, were measured with PEXRAD. There are 998 symmetry‐independent Fmeas significantly above background. Isomorphism with BaxSr(5−x)Nb10O30 allowed the metal atom positions to be deduced. Oxygen atom positions were obtained by Fourier series methods. Refinement of structural parameters by the method of least squares resulted in a final agreement factor R = 0.0401. The structure is generally like that of other tungsten bronzes of formula (A1)2(A2)4C4(B1)2(B2)8O30, with the A1 site occupied by 87% K and 13% Li, the A2 site by 99% K and 1% Li, and the C site by 94% Li and 6% Nb. Th...

Perspective on the Development of Lead-free Piezoceramics

Abstract: A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Pyroelectric devices and materials

Abstract: The physics of pyroelectric detectors is reviewed, including a discussion of response and electronic noise and their dependence on device design and material parameters Other sources of noise are described, particularly as generated by environmental effects such as microphony, together with techniques for their minimisation The range of ferroelectric materials which have been assessed for use in pyroelectric detectors is reviewed and their properties compared, particularly from the aspect of application to different types of devices Finally, an account is given of the wide range of applications for which pyroelectric detectors have been used, including a detailed description of both the pyroelectric vidicon and pyroelectric arrays and their application to thermal imagers

Oxygen‐Octahedra Ferroelectrics. I. Theory of Electro‐optical and Nonlinear optical Effects

Abstract: A microscopic tensor theory of the electro‐optical and nonlinear optical effects in oxygen‐octahedra ferroelectrics is presented. The theory stresses the importance of the basic BO6 octahedron building block in this class of materials. This common structural unit leads to similarities in band structure and similarities in polarization‐induced, Stark‐like energy band shifts. Using the polarization‐potential tensor concept to describe these shifts, we relate the quadratic electro‐optic g coefficients and the nonlinear optic δ coefficients to static and optical‐frequency polarization‐potential tensors, respectively. These tensors are found to be nearly the same in all oxygen‐octahedra ferroelectrics, leading us to conclude that these materials possess the same fundamental electro‐optical and nonlinear optical properties. The physical origin of both effects is shown to be related to polarization‐induced modulation of the (pdπ) energy overlap integral. The resulting static and optical‐frequency polarization potentials are found to be almost, but not exactly, equal. We also show that in the ferroelectric phase the linear electro‐optic effect is fundamentally a quadratic effect biased by the spontaneous polarization, which enables us to calculate the important r coefficients. An analysis of optical‐refractive‐index‐dispersion data shows that oxygen‐octahedra ferroelectrics, and many other materials as well, have nearly the same dispersion behavior described by the parameter e0/S0=6±0.5×10−14 eV·m2 where e0 is an average interband‐oscillator energy in eV and S0 is an average interband‐oscillator strength defined by a single‐term Sellmeier description of optical index data.

Lead-Free Relaxor Ferroelectrics

Abstract: Feature size is a natural determinant of material properties. Its design offers the technological perspectives for material improvement. Grain size, crystallite size, domain width, and structural defects of different nature constitute the classical design elements. Common ferroelectric ceramics contain micrometer grain sizes and submicrometer domain widths. Domain wall mobility is a major contribution to their macroscopic material properties providing approximately half of the macroscopic output in optimized materials. The extension into the dynamic nanoworld is provided by relaxor ferroelectrics. Ionic and nanoscale field disorders form the base to a state with natural nanometer-size polar structures even in bulk materials. These polar structures are highly mobile and can dynamically change over several orders of magnitude in time and space being extremely sensitive to external stimuli. This yields among the largest dielectric and piezoelectric constants known. In this feature article, we want to outline how lead-free relaxors will offer a route to an environmentally safer option in this outstanding material class. Properties of uniaxial, planar, and volumetric relaxor compositions will be discussed. They provide a broader and more interesting scope of physical properties and features than the classical lead-containing relaxor compositions.

Investigation of the Electrical Properties of Sr1−xBaxNb2O6 with Special Reference to Pyroelectric Detection

Abstract: The dielectric constants, electrical conductivity, specific heat, and pyroelectric coefficients of ferro‐electric Sr1−xBaxNb2O6 (SBN) are investigated as a function of temperature in the range 10°–500°K, and as a function of the Sr/Ba composition of the material. A simple technique for measuring absolute pyroelectric coefficients and spontaneous polarizations of ferroelectrics is described. The electric field and frequency dependence of the dielectric properties are also investigated. The theory of pyroelectric detection is discussed from a materials point of view, and the experimental data are considered in terms of the usefulness of SBN as a pyroelectric detector of electromagnetic radiation.