Crystallography

Giant 11 metre gypsum crystals

GypsumGypsumThe giant gypsum crystals in Mexico's "Cueva de los Cristales" are a stunning natural wonder featuring crystals up to 11 metres long.

These giant gypsum (hydrated calcium sulphate) crystals in the “Cave of Crystals” in the Naica mine, Chihuahua, Mexico pose an interesting problem: how are they formed. A Spanish-Mexican team led by Prof García-Ruiz et al. propose that these crystals are derived from "a self-feeding mechanism driven by a solution-mediated, anhydrite-gypsum phase transition". The solution from which the crystals grew were maintained in a very narrow, stable temperature range. It seems likely that related features will be discovered in the future.1

See Geology: April, 2007, v. 35, no. 4, where the crystals are featured on the cover.

Polonium: did it kill Alexander Litvinenko?

Polonium metal structurePolonium metal structureIt is suggested that poisoning by polonium-210 may have caused the death of Alexander Litvinenko, said to be a former Russian spy, in November 2006. Following his death at the end of November 2006, traces of polonium were found at several places he had visited before becoming ill. Before his death it was thought that thallium, or even radiothallium, might have been the cause of his illness. At the time of writing it is not clear who killed him, but not surprisingly the Russians deny it. Polonium-210 decays through the emission of α-particles and these emissions are noramlly easy to stop, but they are very dangerous if the polonium is inside the body.

Polonium is radioactive and present only in extremely low abundances in the environment. It is quite metallic in nature despite its location beneath oxygen in the periodic table. It is made in very small quantities through a nuclear reaction of bismuth. Neutron irradiation of 209bismuth (atomic number 83) gives 210polonium (atomic number 84).

209Bi + 1n → 210Po + e-

Polonium-210, 210Po, transmutes into the lead isotope 206Pb by the emission of an α-particle. The half life for this process is just over 138 days meaning that after 138 days one-half of the original 210Po has disappeared and after 2 times 138 days 3/4 has gone.

21084Po → 20682Pb + 42He

The short half life of polonium-210 and the heat generated with the above radioactive decay means that polonium metal generates considerable heat (141 W), meaning that the metal and its compounds self-heat. This is a useful property and polonium can be used as a small heat source (if expensive!). It can be used in space satellites for this purpose and is especially desirable as there are no moving parts. It was also used in the lunar rovers to keep internal parts warm during the frigid lunar nights.

Polonium metal is unique in that it is the only element whose structure (known as the α-form) is a simple cubic array of atoms in which each atom is surrounded by six other polonium atoms. On gentle warming to 36°C, this converts into a second form known as the β-form.

Polonium dioxidePolonium dioxidePolonium dissolves in acids to form pink hydrated Po(II), presumably as[Po(OH2)6]2+. This seems to oxidize to yellow Po(IV) species perhaps as a consequence of oxidizing agents produced through the α-particle induced decay of water. The polonium(II) oxide PoO is known but this oxidizes easily to the Po(IV) oxide PoO2.

Polonium dichloridePolonium dichlorideThe Po(II) halides PoX2 (X = Cl, Br, I) are known (the chloride and bromide are particularly well characterised) while all the Po(IV) halides PoX4 (X = F, Cl, Br, I) are known.

There are few crystallographically characterised polonium compounds largely because not many researchers work with polonium and the difficulties associated with characterising such radioactive compounds. The 14-electron polonium(IV) anion [PoI6]2– is strictly octahedral meaning the lone pair is sterochemically inactive.

New form of carbon dioxide: amorphous

Only carbon from the Group 14 elements forms stable double bonds with oxygen under normal conditions. When frozen, carbon dioxide is known as "dry-ice". A non-molecular single-bonded crystalline form of carbon dioxide (phase V) exists at high pressure according to Italian and French researchers.1

Amorphous forms of silica (a-SiO2) and germania (a-GeO2) are known at ambient conditions but only recently has an amorphous, silica-like form of carbon dioxide, a-CO2. This is labelled a-carbonia and made by compression of CO2 at room temperature at pressures between 40 and 48 GPa (that's a staggering 400-500 thousand atmospheres).

During this compression, infrared spectra at temperatures up to 680 K show the progressive formation of C–O single bonds and the simultaneous disappearance of all infrared bands associated with molecular CO2. Raman and synchrotron X-ray diffraction measurements confirm the amorphous character of the CO2. Vibrational and diffraction data for a-SiO2 and a-GeO2 are closely related and calculations also suggest shows that a-CO2 is structurally homologous to a-silica (a-SiO2) and a-germania (a-GeO2).

This research helps to understanding the nature of the interiors of gas-giant planets where carbon dioxide may be squeezed at very high pressures. Maybe it could be used to make very hard glass because it is expected to be very stiff rather like diamond. The researchers ponder whether "small amounts of these new glasses could be of interest for technology applications like hard and resistant coatings for micro-electronics, for example."

Go to work on a terbium nitride buckyegg

BuckyEggBuckyEggAn egg-shaped fullerene, or "buckyball egg" has been made and characterized by chemists in America at UC Davis (California), Virginia Tech, and Emory and Henry College in Virginia. They were trying to encapsulate terbium atoms within fullerenes but instead encapsulated terbium nitride within an egg-shaped fullerene.1

The compound Tb3N@C84 was synthesized using an arc-discharge generator by vaporizing composite graphite rods containing a mixture of Tb4O7, graphite, and iron nitride as catalyst in a low-pressure He/N2atmosphere. This gave a complex mixture of products and chromatography gave seven terbium-containing fractions, the fourth fraction of which contained two isomers of Tb3N@C84. Crystallographc studies show the compound from one angle in particular seems very egg shaped! Remarkable! The Tb3N unit is clearly visible (terbium in green and nitrogen in blue).

Until the publication of this work it was normally accepted that no two pentagons can touch in a fullerene and are always surrounded by hexagons. However in this case there are two pentagons (the 8 atoms at the pointy part of the egg at the top of the attached image) linked as a bent pentalene fragment.

Superconductivity in diamond

Workers in Russia and Los Alamos, USA report in Nature1 superconductivity in boron-doped diamond synthesized at high pressure (nearly 100,000 atmospheres) and temperature (2500-2800 K). Electrical resistivity, magnetic susceptibility, specific heat and field-dependent resistance measurements show that boron-doped diamond (carbon) is a bulk, type-II superconductor below the superconducting transition temperature Tc 4 K.

Boron has one less electron than carbon and, because of its small atomic radius, is relatively easily incorporated into diamond. The boron acts as a charge acceptor and the resulting diamond is effectively hole-doped.

Conjuring Crystals

This is interesting. NASA scientists are examining a seemingly magical way to produce high-quality crystals.

Perhaps a NASA laboratory is an unlikely setting for a magic show. Nevertheless, this is where Frank Szofran and colleagues are growing high-quality crystals using a method as amazing as any conjuring trick. By carefully cooling a molten germanium-silicon mixture inside a cylindrical container, they coax it into forming a single large and extraordinarily well-ordered crystal. Such crystals have very few defects because, remarkably, they never touched the walls of the very container in which they grew.

WebElements: the periodic table on the WWW [http://www.webelements.com/]

Copyright 1993-2011 Mark Winter [The University of Sheffield and WebElements Ltd, UK]. All rights reserved.