Due to their high technological and geological relevance, silicates are one of the most studied classes of inorganic compounds. Under ambient conditions, the silicon in silicates is almost exclusively coordinated by four oxygen atoms, while high-pressure treatment normally results in an increase in the coordination from four- to sixfold. Reported here is a high-pressure single-crystal X-ray diffraction study of danburite, CaB₂Si₂O₈, the first compound showing a step-wise transition of Si coordination from tetrahedral to octahedral through a trigonal bipyramid. Along the compression, the Si₂O₇groups of danburite first transform into chains of vertice-sharing SiO₅trigonal bipyramids (danburite-II) and later into chains of edge-sharing SiO₆octahedra (danburite-III). It is suggested that the unusual formation of an SiO₅configuration is a consequence of filling up the pentacoordinated voids in the distorted hexagonal close packing of danburite-II.

The region of the electromagnetic spectrum between 0.3 and 20 THz is a “frontier region” of spectroscopy in physics, chemistry, biology, material sciences, and medicine [ 1 Opportunities in THz Science, Report of a DOE-NSF-NIH Workshop, Arlington, VA (USA), Feb. 12-14, 2004 http://www.sc.doe.gov/bes/reports/list.html [Google Scholar] ]. Radiation in the THz range allows the investigation and excitation of phenomena with time constants of about 1 ps. Examples on this time scale are atomic electron orbits in highly excited Rydberg states, rotations of small molecules and modes of collective oscillations of proteins and polar liquids like water. Resonances of electrons in semiconductor nanostructures and band gaps of superconductors can equally be studied with THz radiation. The THz region lies above the frequencies of traditional electronics but below the range of optical and infrared generators. Until now, possibilities to generate sufficiently intense and brilliant radiation at the wavelengths in question were scarce and the region was therefore named the “THz gap” (Figure 1). In accelerators such highly intense coherent THz radiation can be generated under special conditions when the electron bunch length is comparable to the wavelength of the emitted radiation [ 2 Carr, G.L. 2001. Observation of coherent synchrotron radiation from the NSLS VUV ring. Nucl. Instr. & Methods in Physics Research, A463: 387 [Google Scholar] , 3 Abo-Bakr, M. 2000. Coherent mm-Radiation Experiments at the BESSY II Storage Ring EPAC [Google Scholar] ]. Since the emission of long wavelengths is suppressed by shielding effects of the vacuum chamber, the bunch length needs to be sufficiently short in order to yield observable intensities.

The pressure dependence of structural behavior in the orthorhombic (Pccn, PI) and monoclinic (P2₁/c, PII) polymorphs of the compound [Fe(PM-BiA)₂(NCS)₂], where PM–BiA = (N–(2′–pyridylmethylene)–4-amino–bi–pheynyl), is studied with synchrotron single-crystal X-ray diffraction and vibrational spectroscopy. Both polymorphs are stable up to ∼1.5 GPa, with a spin state transition occurring only in polymorph PII under hydrostatic conditions as documented by single-crystal synchrotron diffraction. The diffraction data also provide evidence of the formation of superstructures for both PI, with a doubled c axis, and PII, with a doubled b axis, on applying pressures above 2 GPa. The LS and HS states seem to coexist at high-pressures for both polymorphs studied with synchrotron infrared spectroscopy at quasi-hydrostatic conditions. Such results indicate that the occurrence of spin-crossover transformations in [Fe(PM-BiA)₂(NCS)₂] might strongly depend on the stress in the sample.

The fragile quantum states of low-temperature quantum applications require protection from infrared radiation caused by higher-temperature stages or other sources. In particular, signal lines have to be manufactured to prevent infrared photons entering through dielectric openings while maintaining low microwave loss. We propose a material system that can efficiently block radiation up to the optical range while transmitting photons at low gigahertz frequencies. It is based on the effect that incident photons are strongly scattered when their wavelength is comparable to the size of particles embedded in a weakly absorbing medium (Mie scattering). The goal of this work is to tailor the absorption and transmission spectrum of an non-magnetic epoxy resin containing sapphire spheres by simulating its dependence on the size distribution. Additionally, we fabricate several material compositions, characterize them, as well as other materials, at optical, infrared, and gigahertz frequencies. In the infrared region (stop band), the attenuation of the Mie scattering optimized material is high and comparable to that of other commonly used filter materials. At gigahertz frequencies (passband), the prototype filter exhibits a high transmission at millikelvin temperatures, with an insertion loss of less than 0.4 dB below 10 GHz.