The discovery of a light-induced spin transition at cryogenic temperatures in a series of iron(II) spin-crossover compounds in 1984 has had an enormous impact on spin-crossover research. Apart from being an interesting photophysical phenomenon in its own right, it provided the means of studying the dynamics of the intersystem crossing process between the high-spin and the low-spin state in a series of compounds and over a large temperature range. It could thus be firmly established that intersystem crossing in spin-crossover compounds is a tunnelling process, with a limiting low-temperature lifetime below 50 K and a thermally activated region above 100 K. This review begins with an elucidation of the mechanism of the light-induced spin transition, followed by an in depth discussion of the chemical and physical factors, including cooperative effects, governing the lifetimes of the light-induced metastable states.

Light-induced spin crossover and the high-spin→low-spin relaxation

Here, we present a structural study on the origin of ferroelectricity in Gd doped HfO2 thin films. We apply aberration corrected high-angle annular dark-field scanning transmission electron microscopy to directly determine the underlying lattice type using projected atom positions and measured lattice parameters. Furthermore, we apply nanoscale electron diffraction methods to visualize the crystal symmetry elements. Combined, the experimental results provide unambiguous evidence for the existence of a non-centrosymmetric orthorhombic phase that can support spontaneous polarization, resolving the origin of ferroelectricity in HfO2 thin films.

On the structural origins of ferroelectricity in HfO2 thin films

A geometrical model of the active phase of hydrotreating catalysts

Passive equipments operating in the 30-300 GHz (millimeter wave) band are compared to those in the 300 GHz- 3 THz (submillimeter band). Equipments operating in the submillimeter band can measure distance and also spectral information and have been used to address new opportunities in security. Solid state spectral information is available in the submillimeter region making it possible to identify materials, whereas in millimeter region bulk optical properties determine the image contrast. The optical properties in the region from 30 GHz to 3 THz are discussed for some typical inorganic and organic solids. In the millimeter-wave region of the spectrum, obscurants such as poor weather, dust, and smoke can be penetrated and useful imagery generated for surveillance. In the 30 GHz-3 THz region dielectrics such as plastic and cloth are also transparent and the detection of contraband hidden under clothing is possible. A passive millimeter-wave imaging concept based on a folded Schmidt camera has been devel- oped and applied to poor weather navigation and security. The optical design uses a rotating mirror and is folded using polarization techniques. The design is very well corrected over a wide field of view making it ideal for surveillance and security. This produces a relatively compact imager which minimizes the receiver count.

Millimeter-Wave and Submillimeter-Wave Imaging for Security and Surveillance Explosives hidden under clothing can be imaged by submillimeter waves, but millimeter waves are better suited for guiding helicopter navigation in poor weather.

Passive equipments operating in the 30-300 GHz (millimeter wave) band are compared to those in the 300 GHz-3 THz (submillimeter band). Equipments operating in the submillimeter band can measure distance and also spectral information and have been used to address new opportunities in security. Solid state spectral information is available in the submillimeter region making it possible to identify materials, whereas in millimeter region bulk optical properties determine the image contrast. The optical properties in the region from 30 GHz to 3 THz are discussed for some typical inorganic and organic solids. In the millimeter-wave region of the spectrum, obscurants such as poor weather, dust, and smoke can be penetrated and useful imagery generated for surveillance. In the 30 GHz-3 THz region dielectrics such as plastic and cloth are also transparent and the detection of contraband hidden under clothing is possible. A passive millimeter-wave imaging concept based on a folded Schmidt camera has been developed and applied to poor weather navigation and security. The optical design uses a rotating mirror and is folded using polarization techniques. The design is very well corrected over a wide field of view making it ideal for surveillance and security. This produces a relatively compact imager which minimizes the receiver count.

Millimeter-Wave and Submillimeter-Wave Imaging for Security and Surveillance

X-ray diffraction study of Hafnia under high pressure using synchrotron radiation

We have studied the structural behavior of PbO at high pressure by powder neturon diffraction in a McWhan cell, and by energy-dispersive powder x-ray diffraction and Raman spectroscopy in a diamond anvil cell. A phase (γ-PbO) occurs at room temperature between ∼0.7 and ∼2.5 GPa pressure, between the stability fields of litharge (<0.7 GPa) and massicot (2.5-10.1 GPa). There is presumably a triple point in the system at a few kbar and 0-200 °C. The phase is related to litharge by a reversible second-order transition. We infer that this is associated with the collapse of the eu acoustic mode. Unit-cell data at 1.6 GPa are Pm21n, a=4.027(3) , b=3.950(3) , c=4.767(4) , and Z=2. The pressure evolution of the spontaneous strain follows a simple Landau model. There are four distinct solid-state transformation paths between litharge and massicot that maintain the known topotactic relationship between the phases, maintain the translational symmetry common to both, and make use of continuous transitions between group-subgroup related structural intermediates. Both the γ phase and the modulated low-temperature phase of PbO are closely related to one step on one of these paths. Although there is evidence to suggest that the intermediate states do have a transient existence, several paths appear to be utilized. A transition to a γ-like phase also occurs in SnO, at 2.5 GPa, although there is no evidence of a massicotlike polymorph of this compound. The orthorhombic phase is stable to at least 7.5 GPa.

Second-order phase transition in PbO and SnO at high pressure: Implications for the litharge-massicot phase transformation.

Using oriented single crystals of beryl and dioptase, almost the full number of bands predicted by factor-group analysis has been observed in each case. The same [Si6O18]12– ring occurs in both minerals; in dioptase, slight distortions caused by the need to provide Cu2+ with a tetragonal environment have a profound effect on the spectrum which bears little resemblance to that of beryl. The bonding in both crystals is discussed.

Single-crystal vibrational spectra of beryl and dioptase

Single-crystal Raman spectra are reported for the three title compounds, and i.r.-reflectance parameters for apatite. An essentially complete assignment is deduced for apatite, including lattice modes, and is in close agreement with the predictions of factor-group analysis. The main features of the Raman spectra of vanadinite and mimetite are similarly accounted for. Factors causing broadening of some low-frequency Raman bands are discussed.

David M. Adams

Papers

X-ray diffraction study of Hafnia under high pressure using synchrotron radiation

Second-order phase transition in PbO and SnO at high pressure: Implications for the litharge-massicot phase transformation.

Single-crystal vibrational spectra of beryl and dioptase

Single-crystal vibrational spectra of apatite, vanadinite, and mimetite

Reassignment of the vibrational spectra of π-cyclopentadienyl- and methyl-π-cyclopentadienyl-manganese tricarbonyl