Structure, phase transitions and ionic conductivity of K_3NdSi_6O_(15)·xH_2O. I. α-K_3NdSi_6O_(15)·2H_2O and its polymorphs

Hydrothermally grown crystals of α-K_3NdSi_6O_(15)·2H_2O, potassium neodymium silicate, have been studied by single-crystal X-ray methods. The compound crystallizes in space group Pbam, contains four formula units per unit cell and has lattice constants a = 16.008 (2), b = 15.004 (2) and c = 7.2794 (7) Å, giving a calculated density of 2.683 Mg m^(−3). Refinement was carried out with 2161 independent structure factors to a residual, R(F), of 0.0528 [wR(F^2) = 0.1562] using anisotropic temperature factors for all atoms other than those associated with water molecules. The structure is based on highly corrugated (Si_2O_5^(2−))_∞ layers which can be generated by the condensation of xonotlite-like ribbons, which can, in turn, be generated by the condensation of wollastonite-like chains. The silicate layers are connected by Nd octahedra to form a three-dimensional framework. Potassium ions and water molecules are located in interstitial sites within this framework, in particular, within channels that extend along [001]. Aging of as-grown crystals at room temperature for periods of six months or more results in an ordering phenomenon that causes the length of the c axis to double. In addition, two phase transitions were found to occur upon heating. The high-temperature transformations, investigated by differential scanning calorimetry, thermal gravimetric analysis and high-temperature X-ray diffraction, are reversible, suggesting displacive transformations in which the layers remain intact. Conductivity measurements along all three crystallographic axes showed the conductivity to be greatest along [001] and further suggest that the channels present in the room-temperature structure are preserved at high temperatures so as to serve as pathways for easy ion transport. Ion-exchange experiments revealed that silver can readily be incorporated into the structure

Structure, phase transitions and ionic conductivity of K_3NdSi_6O_(15)·xH_2O. II. Structure of β-K_3NdSi_6O_(15)

Hydrothermally grown crystals of β-K_3NdSi_6O_(15), potassium neodymium silicate, have been studied by single-crystal X-ray methods. Under appropriate conditions, the compound crystallizes in space group Bb2_1m and has lattice constants a = 14.370 (2), b = 15.518 (2) and c = 14.265 (2) Å. There are 30 atom sites in the asymmetric unit of the basic structure. With eight formula units per unit cell, the calculated density is 2.798 Mg m^(-3). Refinement was carried out to a residual, wR(F^2), of 0.1177 [R(F) = 0.0416] using anisotropic temperature factors for all atoms. The structure is based on (Si_2O_5^(2-))∞ layers, connected by Nd polyhedra to form a three-dimensional framework. Potassium ion sites, some of which are only partially occupied, are located within channels that run between the silicate layers. The silica-neodymia framework of β-K_3NdSi_6O_(15), in particular the linkages formed between the silicate layers and Nd polyhedra, bears some similarities to that of the essentially isocompositional phase α-K_3NdSi_6O_(15)·2H_2O. In both, the silicate layers are corrugated so as to accommodate a simple cubic array of NdO_6 octahedra with lattice constant 7.5 Å. Furthermore, the Si_2O_5 layers in β-K_3NdSi_6O_(15) are topologically identical to those of the mineral sazhinite, Na_2HCeSi_6O_(15). Although β-K_3NdSi_6O_(15) and sazhinite are not isostructural, the structures of each can be described as slight distortions of a high-symmetry parent structure with space group Pbmm

Structure, phase transitions and ionic conductivity of K_3NdSi_6O_(15)·xH_2O. I. α-K_3NdSi_6O_(15)·2H_2O and its polymorphs

Hydrothermally grown crystals of α-K_3NdSi_6O_(15)·2H_2O, potassium neodymium silicate, have been studied by single-crystal X-ray methods. The compound crystallizes in space group Pbam, contains four formula units per unit cell and has lattice constants a = 16.008 (2), b = 15.004 (2) and c = 7.2794 (7) Å, giving a calculated density of 2.683 Mg m^(−3). Refinement was carried out with 2161 independent structure factors to a residual, R(F), of 0.0528 [wR(F^2) = 0.1562] using anisotropic temperature factors for all atoms other than those associated with water molecules. The structure is based on highly corrugated (Si_2O_5^(2−))_∞ layers which can be generated by the condensation of xonotlite-like ribbons, which can, in turn, be generated by the condensation of wollastonite-like chains. The silicate layers are connected by Nd octahedra to form a three-dimensional framework. Potassium ions and water molecules are located in interstitial sites within this framework, in particular, within channels that extend along [001]. Aging of as-grown crystals at room temperature for periods of six months or more results in an ordering phenomenon that causes the length of the c axis to double. In addition, two phase transitions were found to occur upon heating. The high-temperature transformations, investigated by differential scanning calorimetry, thermal gravimetric analysis and high-temperature X-ray diffraction, are reversible, suggesting displacive transformations in which the layers remain intact. Conductivity measurements along all three crystallographic axes showed the conductivity to be greatest along [001] and further suggest that the channels present in the room-temperature structure are preserved at high temperatures so as to serve as pathways for easy ion transport. Ion-exchange experiments revealed that silver can readily be incorporated into the structure

Structure of Na_3NdSi_6O_(15)._2H_2O - A Layered Silicate with Paths for Possible Fast Ion Conductor

Hydrothermal investigations of the high silica region of the Na₂0-Nd₂0₃-SiO₂ system, carried out in the search for new fast-ion conductors (FIC's), yielded the compound Na₃NdSi₆O₁₅.2H₂0 (sodium neodymium silicate). Single-crystal X-ray methods provided lattice constants of a=7.385(2), b=30.831(7) and c= 7.1168 (13)Å, space group Cmm2, and 22 atoms in the asymmetric unit. With four formula units per unit cell, the calculated density is 2.68gcm⁻³. Refinement was carried out with 1113 independent structure factors to a weighted residual wR(F) of 2.63% [8.09% for wR(F²)] using anisotropic temperature factors for all atoms. The structure, based on corrugated Si₆O₁₅ layers containing four-, five-, six- and eight-membered rings, is related to that of a model previously reported for a compound assigned the composition NaNdSi₆O₁₃(OH)₂.nH₂0. Our structure differs in the placement of sodium ions and water molecules, and contains no hydroxyl groups. We believe that both studies examined the same phase

Three-Dimensional Analysis of Wakefields Generated by Flat Electron Beams in Planar Dielectric-Loaded Structures

An electron bunch passing through dielectric-lined waveguide generates $\check{C}$erenkov radiation that can result in high-peak axial electric field suitable for acceleration of a subsequent bunch. Axial field beyond Gigavolt-per-meter are attainable in structures with sub-mm sizes depending on the achievement of suitable electron bunch parameters. A promising configuration consists of using planar dielectric structure driven by flat electron bunches. In this paper we present a three-dimensional analysis of wakefields produced by flat beams in planar dielectric structures thereby extending the work of Reference [A. Tremaine, J. Rosenzweig, and P. Schoessow, Phys. Rev. E 56, No. 6, 7204 (1997)] on the topic. We especially provide closed-form expressions for the normal frequencies and field amplitudes of the excited modes and benchmark these analytical results with finite-difference time-domain particle-in-cell numerical simulations. Finally, we implement a semi-analytical algorithm into a popular particle tracking program thereby enabling start-to-end high-fidelity modeling of linear accelerators based on dielectric-lined planar waveguides.Comment: 12 pages, 2 tables, 10 figure

Structure of Na_3YSi_6O_(15) - a Unique Silicate Based on Discrete Si_6O_(15) Units, and a Possible Fast-Ion Conductor

Hydrothermal investigations in the high silica region of the Na₂O-Y₂0₃-Si₆O₁₅ system, carried out in a search for novel fast-ion conductors (FIC's), yielded the new compound trisodium yttrium hexasilicate, Na₃YSi₆O₁₅. Single-crystal X-ray methods revealed that Na₃YSi₆O₁₅ crystallizes in space group Ibmm, has lattice constants a=10.468 (2), b=15.2467 (13) and c=8.3855 (6) Å, Z=4, and 11 atoms in the asymmetric unit. Refinement was carried out to a weighted residual of 3.53% using anisotropic temperature factors for all atoms. The structure is unique in that the silica tetrahedra form isolated SiSi₆O₁₅⁶⁻ double dreier-rings, rather than layers as might be expected from the Si to O ratio of 0.4. No isomorphs to Na₃YSi₆O₁₅ have been reported

Calculation of wakefields in a 17 GHz beam-driven photonic band-gap accelerator structure

We present the theoretical analysis and computer simulation of the wakefields in a 17 GHz photonic band-gap (PBG) structure for accelerator applications. Using the commercial code CST Particle Studio, the fundamental accelerating mode and dipole modes are excited by passing an 18 MeV electron beam through a seven-cell traveling-wave PBG structure. The characteristics of the longitudinal and transverse wakefields, wake potential spectrum, dipole mode distribution, and their quality factors are calculated and analyzed theoretically. Unlike in conventional disk-loaded waveguide (DLW) structures, three dipole modes (TM[subscript 11]-like, TM[subscript 12]-like, and TM[subscript 13]-like) are excited in the PBG structure with comparable initial amplitudes. These modes are separated by less than 4 GHz in frequency and are damped quickly due to low radiative Q factors. Simulations verify that a PBG structure provides wakefield damping relative to a DLW structure. Simulations were done with both single-bunch excitation to determine the frequency spectrum of the wakefields and multibunch excitation to compare to wakefield measurements taken at MIT using a 17 GHz bunch train. These simulation results will guide the design of next-generation high-gradient accelerator PBG structures.United States. Dept. of Energy. High Energy Physics Division (Contract DEFG02- 91ER40648)China. Fundamental Research Funds for the Central Universities (Contract ZYGX 2010J055

Results from the CLIC Test Facility

In order to study the principle of the Compact Linear Collider (CLIC) based on the Two Beam Acceleration (TBA) scheme at high frequency, a CLIC Test Facility (CTF) has been set-up at CERN. After four years of successful running, the experimental programme is now fully completed and all its objectives reached, particularly the generation of a high intensity drive beam with short bunches by a photo-injector, the production of 30 GHz RF power and the acceleration of a probe beam by 30 GHz structures. A summary of the CTF results and their impact on linear collider design is given. This covers 30 GHz high power testing, study of intense, short single bunches; as well as RF-Gun, photocathode and beam diagnostic developments. A second phase of the test facility (CTF2) is presently being installed to demonstrate the feasibility of the TBA scheme by constructing a fully engineered, 10 m long, test section very similar to the CLIC drive and main linacs, producing up to 480 MW of peak RF power at 30 GHz and accelerating the beam up to 320 MeV. The present status of CTF2 is reported