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We report the development of a high-temperature Seebeck coefficient Standard Reference Material (SRM) for use in instrument validation and interlaboratory data comparison in the temperature range of 295–900 K to support the research, development, and production of materials and devices related to thermoelectric-based energy conversion applications. We describe the synthesis, anneal–quench procedure, and physical characterization of a p-type boron-doped polycrystalline silicon–germanium alloy with a nominal composition of Si80Ge20. For the certification measurements, we describe the measurement protocols, statistical analysis, the certified Seebeck coefficient values, comprehensive uncertainty budgets, and metrological traceability. Our extensive efforts to identify, reduce, and quantify measurement uncertainties will be emphasized. This new SRM complements SRM 3451 Low-Temperature Seebeck Coefficient Standard (10–390 K) to provide certified reference materials traceable to the International System of Units (SI) for Seebeck coefficient measurements within the temperature range 10–900 K.

Layered perovskite Ca2.91Na0.09Ti2-xRhxO7 (x = 0.00, 0.02, 0.04, 0.06) were synthesized by a conventional solid-state reaction. Room temperature ferroelectricity has been confirmed. The remanent polarization increases with an increase of Rh content, which is due to a larger oxygen octahedral distortion by Rh doping. The coercive field increases with Rh doping as the pinning effect of oxygen vacancies reduce the mobility of domain wall. Remanent polarization and coercive field are caused by different mechanisms, so it is possible to modulate them independently to meet the requirement of application in ferroelectric field. The concentration of oxygen vacancy increased with Rh doping, leading to the significant increase of leakage current density. The bandgap of samples doped with Rh drastically decrease and the visible light response of the sample was improved by Rh doping due to the formation of impurity energy levels within the band gap.

Two-dimensional transition metal dichalcogenides (2D-TMDs) are of broadening research interest due to their novel physical, electrical, and thermoelectric properties. Having the chemical formula MX 2, where M is a transition metal and X is a chalcogen, there are many possible combinations to consider for materials-by-design exploration. By identifying novel compositions and utilizing the lower dimensionality, which allows for improved thermoelectric performance (e.g., increased Seebeck coefficients without sacrificing electron concentration), MX 2 materials are promising candidates for thermoelectric applications. However, to develop these materials into wide-scale use, it is crucial to comprehensively understand the compositional affects. This work investigates the structure, electronic, and phonon properties of 18 different MX 2 materials compositions as a benchmark to explore the impact of various elements. There is significant correlation between properties of constituent transition metals (atomic mass and radius) and the structure/properties of the corresponding 2D-TMDs. As the mass of M increases, the n-type power factor and phonon frequency gap increases. Similarly, increases in the radius of M lead to increased layer thickness and Seebeck coefficient S. Our results identify key factors to optimize MX 2 compositions for desired performance.

Comparative measurements of the Seebeck coefficient are challenging due to the diversity of instrumentation and measurement protocols. With the implementation of standardized measurement protocols and the use of Standard Reference Materials (SRMs®), for example, the recently certified National Institute of Standards and Technology (NIST) SRM® 3451 ‘‘Low Temperature Seebeck Coefficient Standard (10–390 K)’’, researchers can reliably analyze and compare data, both intra- and inter-laboratory, thereby accelerating the development of more efficient thermoelectric materials and devices. We present a comparative overview of commonly adopted Seebeck coefficient measurement practices. First, we examine the influence of asynchronous temporal and spatial measurement of electric potential and temperature. Temporal asynchronicity introduces error in the absolute Seebeck coefficient of the order of ≈10%, whereas spatial asynchronicity introduces error of the order of a few percent. Second, we examine the influence of poor thermal contact between the measurement probes and the sample. This is especially critical at high temperature, wherein the prevalent mode of measuring surface temperature is facilitated by pressure contact. Each topic will include the comparison of data measured using different measurement techniques and using different probe arrangements. We demonstrate that the probe arrangement is the primary limit to high accuracy, wherein the Seebeck coefficients measured by the 2-probe arrangement and those measured by the 4-probe arrangement diverge with the increase in temperature, approaching ≈14% at 900 K. Using these analyses, we provide recommended measurement protocols to guide members of the thermoelectric materials community in performing more accurate measurements and in evaluating more comprehensive uncertainty limits.

A suite of complementary high-throughput screening systems for combinatorial films was developed at National Institute of Standards and Technology to facilitate the search for efficient thermoelectric materials. These custom-designed capabilities include a facility for combinatorial thin film synthesis and a suite of tools for screening the Seebeck coefficient, electrical resistance (electrical resistivity), and thermal effusivity (thermal conductivity) of these films. The Seebeck coefficient and resistance are measured via custom-built automated apparatus at both ambient and high temperatures. Thermal effusivity is measured using a frequency domain thermoreflectance technique. This paper will discuss applications using these tools on representative thermoelectric materials, including combinatorial composition-spread films, conventional films, single crystals, and ribbons.

The graphitic carbon nitride (g-C3N4) which is a two-dimensional conjugated polymer has drawn broad interdisciplinary attention as a low-cost, metal-free, and visible-light-responsive photocatalyst in the area of environmental remediation. The g-C3N4-based materials have excellent electronic band structures, electron-rich properties, basic surface functionalities, high physicochemical stabilities and are “earth-abundant.” This review summarizes the latest progress related to the design and construction of g-C3N4-based materials and their applications including catalysis, sensing, imaging, and white-light-emitting diodes. An outlook on possible further developments in g-C3N4-based research for emerging properties and applications is also included.

We have successfully developed a Seebeck coefficient Standard Reference Material (SRM™), Bi2Te3, that is essential for interlaboratory data comparison and for instrument calibration. Certification measurements were performed using a differential steady-state technique on 10 samples (15 measurements) randomly selected from a batch of 390 bars. The certified Seebeck coefficient values are provided from 10 to 390 K, and they are further supported by transient measurements. The availability of this SRM will validate measurement results, leading to a better understanding of the structure/property relationships and underlying physics of potential high-efficiency thermoelectric materials.

A Standard Reference Material (SRM™) for the Seebeck coefficient is critical for inter-laboratory data comparison and for instrument calibration. To develop this SRM™, we have conducted an international round-robin measurement survey of two candidate materials—undoped Bi2Te3 and constantan (55% Cu and 45% Ni alloy). Measurements were performed in two rounds by twelve laboratories involved in active thermoelectric research using a number of commercial and custom-built measurement systems and techniques. We report the results of these measurements and the statistical analysis performed. Based on this extensive study, we have selected Bi2Te3 as the prototype standard material.

Phases that are in equilibrium with BaR2CuO6+x (R=lanthanides and Y), such as the “green-phase” and “brown-phase” structural variants of BaR2CuO5 in bulk samples, are attractive choices for flux-pinning for coated conductor applications because of the guaranteed chemical stability. In films, high-temperature x-ray diffraction studies of Ba2RCu3O6+x superconductor deposited on SrTiO3 substrate using the trifluoroacetate solution method demonstrate that while BaNd2CuO5 (“brown-phase” structure) develops at 735 °C and 100 Pa pO2, neither BaGd2CuO5 nor Ba(Nd1/3Eu1/3Gd1/3)2CuO5 (both green-phase structure) form at these conditions. As a result, Ba2(Nd1/3Eu1/3Gd1/3)Cu3O6+x in thin films is in equilibrium with the brown-phase, and Ba2GdCu3O6+x is in equilibrium with Gd2O3 in the Ba–Gd–Cu–O system, in contrast to the bulk systems. Different phase relationships in the vicinity of the Ba2RCu3O6+x phase imply different phases are available for flux-pinning applications. These differences will need to be considered carefully in designing optimized superconducting coated conductors.

For applications ranging from phase equilibria to the processing of second-generation high Tc superconductor-coated-conductors, phase diagrams constructed under carbonate-free conditions are needed. Subsolidus phase equilibria of BaO-R2O3-CuOz (R = Ho) have been investigated at \( {\hbox{p}}_{{{\rm{O}}_{{\rm{2}}} }} \hbox{\,=\,} 100\;{\hbox{Pa}} \) (810°C), 21 kPa (875°C) and 0.1 MPa (850 and 930°C) by applying controlled atmosphere methods to minimize the presence of carbonate and CO2 and H2O contamination. Under carbonate-free conditions, most of these phase diagrams are different from those reported in the literature. In this paper, we also review and compare the phase diagrams of ten BaO-R2O3-CuOz systems (R = Nd, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm and Yb) that were previously determined in this laboratory under \( {\hbox{p}}_{{{\rm{O}}_{{\rm{2}}} }} \hbox{\,=\,} 100\;{\hbox{Pa}}{\hbox{.}} \) Among these diagrams, a distinct trend of phase formation and tie-line relationships is observed.

Interfacial reactions between the Ba2YCu3O6+x superconductor and the CeO2 buffer layers employed in coated conductors have been modeled experimentally by investigating the kinetics of the reaction between Ba2YCu3O6+x films and CeO2 substrates. At 810°C, the Ba2YCu3O6+x-CeO2 join within the BaO-Y2O3-CeO2-CuOx quaternary system is nonbinary, thereby establishing the phase diagram topology that governs the Ba2YCu3O6+x/CeO2 reaction. At a mole ratio of Ba2YCu3O6+x:CeO2 of 40:60, a phase boundary was found to separate two four-phase regions. On the Ba2YCu3O6+x-rich side of the join, the four-phase region consists of Ba2YCu3O6 +x, Ba(Ce1−zYz)O3−x, BaY2CuO5, and CuOx; on the CeO2 rich side, the four phases were determined to be Ba(Ce1−zYz) O3−x, BaY2CuO5, CuOx and CeO2. The Ba2YCu3O6+x/CeO2 reaction is limited by solid-state diffusion, and the reaction kinetics obey the parabolic rule, x = Kt1/2, where x = thickness of the reaction layer, t = time, and K = a constant related to the rate constant; K was determined to be 1.6 × 10−3 μm/s1/2 at 790°C and 4.7 × 10−3 μm/s1/2 at 830°C. The activation energy for the reaction was determined to be Eact = 2.67 × 105 J/mol using the Arrhenius equation.

Partial-melt processing of Bi2+xSr2-x-yCa1+yCu2O8+δ(Bi-2212) thick-film conductors with additions of nanophase Al2O3was studied for dual purposes of increasing flux pinning and inhibiting Sr—Ca—Cu—O phase defect formation. Nanophase Al2O3(2Sr2.38Ca1.15Cu2.92O9.7+δ) closer to the ideal Bi-2223 composition. The effect of Al2O3addition on film microstructural and superconducting properties was studied for a range of partial-melt temperatures (865 to 900 °C). Results were compared to Al2O3-free films with compositions lying within the single-phase solid-solution 2212 region. Nanophase Al2O3reacted with 2212-type precursors to form a composite of micron size or smaller particles of solid-solution (Sr,Ca)3Al2O6in a solid-solution 2212 superconducting matrix. The Ca content of the (Sr,Ca)3Al2O6particles formed approximated that of the 2212 precursor (≤6% mole fraction difference). Addition of 6–25% volume fraction of (Sr,Ca)3Al2O6to Bi-2212 (by reaction between Al2O3and Bi-2212) only slightly reduced superconducting transition temperatures andc-axis texturing; however this addition improved film quality by reducing Sr—Ca—Cu—O defect volume fraction by factors of 2 to 6 and significantly increased the critical current density by over one order of magnitude for 0 to 2 T applied fields at 20 to 30 K.

The subsolidus equilibria and the primary phase field (crystallization field) of the 110 K high-Tc (Bi,Pb)-2223 ([Bi,Pb]:Sr:Ca:Cu) phase have been determined in the presence of Ag under a 92.5% Ar/7.5% O2 atmosphere (volume fraction). A total of 29 six-phase volumes that include both the (Bi,Pb)-2223 and Ag phases was observed. These subsolidus volumes are similar to those observed without the presence of Ag. The compositional range of initial melts of these volumes (mole fraction basis) covers BiO1.5 from 5.6% to 25.3%, PbO from 0.4% to 13.8%, SrO from 8.4% to 31.9%, CaO from 12.2% to 33.3%, CuO from 21.7% to 40.9%, and AgO0.5 from 1.2% to 6.3%. Based on these data, the primary crystallization field for the (Bi,Pb)-2223 phase in the presence of Ag was constructed using the convex hull technique. A section through this “volume” was portrayed by holding the AgO0.5, SrO, and CaO components at the median value of the 29 compositions while allowing projection on the other three axes (BiO1.5, PbO, and CuO). The net effect of Ag on the melt composition is a reduction in the PbO concentration and an increase in the SrO content. Applications of the liquidus data are also discussed.

The chemical reactions and the compositional characteristics of liquids which lead to the phase formation of the Bi(Pb)-2223 [(Bi, Pb): Sr: Ca: Cu: O] superconductor have been studied for a precursor composition of Bi1.8Pb0.4Sr2Ca2.2Cu3Ox. The combined techniques of quenching, powder x-ray diffraction, differential thermal/thermogravimetric analysis (DTA/TGA), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDS) were used to characterize the subsolidus phases and the presence of liquid. Samples were annealed under purified air and under a volume fraction mixture of 7.5% O2/92.5% Ar. The effects of Ag in both the pure air and the 7.5% O2/92.5% Ar experiments were also studied. Results are discussed with respect to their processing implications.

The subsolidus phase relationships of the high-Tc 2223 superconductor in the (Bi, Pb)–Sr–Ca–Cu–O (BSCCO) system have been examined at 810–820 °C. All experiments were carried out at ambient pressure in a 7.5% O2 (92.5% Ar) atmosphere. Eleven phases were found to exist in equilibrium with the 2223 phase. These 11 phases include CuO and 10 oxide solid solutions. From among these phases, a total of 48 five-phase combinations including the 2223 and 2212 phases were investigated experimentally, and 16 equilibrium assemblages were found which define a multicomponent compositional space corresponding to the 2223 + 2212 solid-state compatibility region. The subsolidus data form a partial basis for future investigation of the Pb-2223 primary phase field.

Modified lead titanate of 0.9PbTiO3 −0.1Pb(Mg0.5 W0.5)O3 thin films have been deposited onto Pt-coated Si substrates by pulsed laser deposition. Films were crystallized in situ during deposition or by post-depositional heat treatment (post-annealing). Compositional and structural characterization showed that the phase formation and microstructure of the films were highly sensitive to deposition conditions. Perovskite single phase films were formed in situ at 650 °C, PO2 = 40 Pa as well as by post-annealing amorphous films at 650 °C. In the post-annealing process, the amorphous as-deposited phase was crystallized to perovskite and/or pyrochlore, and the ratio of perovskite to pyrochlore was found to be influenced by the depositional PO2. Depending on the deposition temperature, the grain structures of the crystallized films were columnar or equiaxed. A relatively homogeneous surface morphology was obtained by deposition at a lower pressure (PO2 = 13 Pa). The in situ crystallized films showed variable crystallographic orientation. The more (111) oriented films had the lowest remanent polarizations and the highest coercive fields. A method for preparing randomly oriented films, via a two-step deposition process with intermediate annealing, is believed to give the most consistent results and the best ferroelectric properties at the present level of development.

The influence of incorporating nanoparticulate additions into Ca3Co4O9 (CCO) thin films prepared by pulsed laser deposition using composite targets of CCO and CCO + 3wt% BaZrO3 (BZO) on Si and LaAlO3 substrates is investigated. X-ray data and high-resolution scanning electron microscopy reveal preferred c-axis orientation of the films deposited on Si substrates with the formation of nanoparticles between ∼ 10 – 50 nm. Preliminary thermoelectric behavior shows an enhancement of the power factor α2/ρ at room temperature. The microstructure and thermoelectric behavior of the CCO films are compared to the BZO-doped films.

The interplay of melting equilibria and reaction kinetics is important during formation of the Ba2YCu3O6+x (Y-213) phase from starting materials in the quaternary reciprocal system Ba,Y,Cu//O,F. For experimental investigation of the process we used a combination of differential thermal analysis (DTA) for study of melting equilibria, and in-situ high-temperature x-ray diffraction (HTXRD) for study of the phase formation and reaction kinetics. DTA investigation of compositions spaced along compositional vectors extending from the oxide end to the fluoride end of the reciprocal system have given evidence of low melting liquids (∼600 °C) near the fluorine-rich end. Work is continuing to determine whether similar thermal events observed in the interior of the system also indicate low temperature liquids, and on the extent to which low-melting liquids could be involved in Y-213 phase formation. HTXRD investigations have been initiated on the conversion of 0.3 μm and 1.0 μm thick BaF2-Y-Cu precursor films to Y-213 in the presence of water vapor. Preliminary results indicated that the thickness of film has a strong influence on the texture of the Y-213 film: a 0.3 μm film showed mainly (001) texture, whereas a 1.0 μm film showed a greater volume fraction of (h00) texture. While the HTXRD method cannot directly reveal the presence of liquid, we are working to combine DTA and HTXRD data for a unified picture of Y-213 phase formation during the “BaF2 ex-situ” process for coated-conductor fabrication.

Differential thermal analysis and thermogravimetric analysis (DTA/TGA) experiments have been completed in the BaF2-BaO-Y2O3-CuOx-H2O system at various oxygen pressures to determine the effect of adding solid BaF2and gaseous H2O (at pH2O = 2.5 kPa) on the eutectic melting in the system BaO-Y2O3-CuOx. We have investigated the eutectic melting temperature over the range of pO2 = 20 Pa to pO2 = 0.1 MPa under the following conditions: 1) without BaF2 or H2O; 2) with H2O only; 3) with BaF2 only; 4) with both BaF2 and H2O. Results indicate that without BaF2 or H2O, eutectic melting is depressed from 917°C at pO2 = 0.1 MPa to 838°C at pO2 = 1 kPa. At oxygen pressures below 1 kPa, down to pO2 = 20 Pa, no further lowering of eutectic melting temperature was observed. The topologies of the temperature vs. log pO2 curves for various combinations of BaF2 and H2O were similar to that for the experiments without BaF2 or H2O, but all curves were shifted to lower temperatures. For the experiments with addition of BaF2 only, H2O only, and BaF2 with H2O, the eutectic melting temperatures as a function of pO2 were lowered by, respectively, 5°C - 15°C, 15°C - 25°C and 20°C - 30°C. Application of results to the “BaF2ex-situ” method of processing Ba2YCu3Ox coated conductors is discussed.

Stimulable phosphor thin films are being investigated for use as optical data storage media. We have successfully applied atomic force microscopy (AFM) to the measurement of the surface texture of these films. Determination of the surface texture of the films is important for evaluating the effect of surface quality on optical scatter. In other thin film material systems it has been found that the surface “bumps” revealed by AFM correspond to grains in the film. This is not the case for the stimulable phosphor films used in our study. We have determined the grain size of our phosphor films by transmission electron microscopy (TEM) and x-ray diffraction (XRD). The grain size from TEM and XRD does not correlate with the size of the AFM surface “bumps.” For example, in two of the five films studied, the XRD derived grain size varies by a factor of two but the size of the surface “bumps” remains the same. We conclude that the texture of the film surface is not directly determined by the grain size of the phosphor material.

Thepreparation of modified lead titanate (0.PbTiO3-Pb(Mg0.5W0.5)O3) thin films by pulsed excimer laser deposition on Pt-coated Si substrates has been studied. Films were prepared with substrate temperatures of 300–650°C, and ambient oxygen pressures of 2.7–4OPa. Phase formation and morphology of the films varied according to deposition conditions. At 650°C, 40Pa, well-crystallized perovskite single phase films were obtained. Most films were ferroelectric and showed preferred orientation which could be correlated with the texture of the underlying platinum. The effect of preferred orientation on the dielectric properties of the films is discussed.

Thin films of BaTiO3 were deposited on platinum-coated silicon substrates using pulsed laser deposition and characterized using electron microscopy, powder x-ray diffraction and electrical measurements. The microstructure consisted of columnar BaTiO3 grains oriented normal to the substrate. Two preferred orientations were observed, with either the (001) or (111) planes of BaTiO3 being parallel to the substrate. The electrical properties of two films were measured and it was found that the (111) film was ferroelectric and the (001) film was not. Possible reasons for this are discussed.

X-ray diffraction techniques were used to study the crystallography of PZT thin films prepared by the laser deposition technique. This investigation included identification of phases formed during the annealing process and also the analysis of the profiles of selected diffraction peaks. The PZT films annealed below 800°C typically showed powder x-ray diffraction patterns corresponding to a cubic structure (i.e no peak splitting) instead of the tetragonal patterns characteristic of the target materials. The upper bound contribution of the macro and micro strain to the observed X-ray peak profile and positions was estimated. It was believed that the combined effect of small crystallite size together with residual strain, and possible local inhomogeneity gave rise to the broadening and displacement of the x-ray peaks, which subsequently masked off the splittings. At this stage the physical effect of high temperature annealing is not known. It is possible that as the annealing temperature increased, grain growth took place along with relaxation of residual strain, allowing peak splitting to be observed.

Dense smooth lead zirconate-titanate thin films have been prepared by excimer laser deposition. The as-deposited films are amorphous as indicated by x-ray powder patterns. Differential scanning calorimetry studies show that the film has a glass transition at 301 °C, and the amorphous to crystalline transformation takes place above 350°C to 650°C. Phase formation as a result of post-deposition heat treatment is described.

Thin films of BaTiO3 and PZT (lead zirconate titanate, 47%PbTiO3, 53%PbZrO3) have been produced by laser irradiation of the appropriate ceramic targets and deposition of the ejected and vaporized material on planar substrates. The microstructural changes during thermal processing of these films have been studied by scanning electron microscopy/energy dispersive x-ray spectrometry (SEM/EDX), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), x-ray diffraction (XRD), and by measurement of electrical properties. Films have been deposited using both Nd/YAG and excimer lasers and on unheated as well as heated substrates. Excimer films are considerably smoother than the Nd/YAG films, and the uniformity of the as-deposited microstructures is promoted by substrate heating. However, ferroelectric hysteresis loops were only observed for the considerably less smooth Nd/YAG PZT films; thermal treatment did little to improve the smoothness of these films. An excimer BaTiO3 film deposited on a heated substrate showed crystallographic alignment and had a dielectric constant of −100. Efforts are underway to combine the best features of films produced by both methods.

A structural phase transition study of Ba2YCu3O6+x (x = 0 to 1) has been conducted on a series of 13 quenched samples. These samples were prepared from an orthorhombic material by annealing at temperatures from 400 to 1000 °C in air, followed by rapid quenching. All quenchings were performed by using a liquid-nitrogen-cooled copper cold well with a continuous flow of cooled helium gas. Various measurements including x-ray diffraction, thermogravimetric analysis, Meissner effect, and scanning electron microscopy were carried out in order to correlate the nature of the phase transition with erystallographic data, superconductivity, and annealing temperature. The phase transition from Ba2YCu3,O7 to Ba2YCu3O6 appears to involve two orthorhombic regions: region A with a

Phase assemblages in two Sr-Bi-Pb-Ca-Cu-O glasses and their crystallization into glass-ceramics through a sequence of heat treatments are described. Samples were heat treated at various temperatures based on differential thermal analysis (DTA) analysis of the as-quenched glasses. X-ray powder diffraction was used to identify the compounds in each sample; and the different phases formed in these two samples were compared. Results of microstructural characterization of selected samples using scanning electron microscopy are discussed.

The influence of the ionic size of the lanthanides R on melting relations of Ba2RCu3O6+x, where R=Y, Eu and Nd, was studied and compared with that of a high Tc superconductor mixed‐lanthanide phase Ba2(Y.75Eu.125Nd 125)Cu3O6+xThese materials have been characterized by a variety of methods including differential thermogravimetric analysis (DTA), scanning electron microscopy (SEM) with energy dispersive X‐ray spectroscopy (EDX) and X‐ray powder diffraction. Single phase samples of Ba2(Y.75Eu.125Nd.125)Cu3O6+x were annealed at 1004, 1040, 1052, 1060, 1078, 1107 and 1160°C and quenched into a helium gas container cooled by liquid nitrogen. The SEM micrographs of these samples showed the progressive chnages in features of the microstructures from sintering and grain growth through melting and then recrystallization from the melt. The addition of the SEM technique in conjunction with X‐ray diffraction has been helpful in the study of phase equilibria in this system.

Results are presented for 850°C oxygen annealing experiments (up to 62 hr) on the 2122 (Tl:Ca:Ba:Cu) oxide phase, with and without intermediate grinding. Samples were characterized by powder X-ray diffraction and a.c. magnetic susceptibility. The 2122 phase appears to be thermodynamically stable in oxygen under conditions of both extensive grinding, and, annealing of long duration. However, onset Tc's vary from 71 K to 109 K for apparently single-phase 2122 samples with nearly identical X-ray powder patterns, depending upon the heat treatment.

We have conducted structural and magnetic investigations on thermomechanically-detwinned YBa2Cu3O6+x single crystals. Single crystal x-ray diffraction studies on a fully untwinned crystal with a superconducting onset temperature of 54 K have revealed that oxygen atoms in the basal plane are offset from the crystallographic mirror plane in the a direction, leading to “zig-zag” Cu-O chains. Magnetic measurements on untwinned and twinned crystals at 77 K indicate low levels of flux pinning in both crystals, with a slightly larger amount of pinning in the twinned crystal.