Amorphous is the fourth structural state of matter -next to crystalline, liquid and gaseous. It is frequently the outcome of directed material synthesis and less often the result of a phase transition. Metals have a propensity to amorphize upon fast cooling, producing ribbons and more recently bulk glasses; semiconductors and organics less so.
Industrial applications are sparse, not because these materials are inherently less promising, but rather because not as much effort has been spent on addressing their potential usefulness and characterizing their structures. Amongst semiconductors amorphous silicon is extensively used because it can easily be deposited as thin film on large surfaces, at temperatures as low as 75 degrees C, which permits deposition on glass and plastics. The small solar cells used in pocket calculators have been made with a-Si for many years. Amorphous chalcogenides are also used in inexpensive solar cells, and in modified form for CD/RW computer memory disks. Zr-based bulk amorphous alloys have recently been acclaimed for their superior mechanical characteristics (high yield strength, high hardness, high wear resistance, elevated elastic limit). Applications of these alloys range from ski cores, tennis rackets, to industrial coatings and classified defense devices. And the saga goes on... In May 2005 two papers appeared in Physical Review Letters (PRL 94, 205502; PRL 94, 205501) which described two amorphous, bulk metallic alloys which promise to combine strength with the versatility of plastic. The discovery (possibly invention) of these materials highlight the need to develop a structural probe which allows one to accurately determine the atomic structure of amorphous materials -thereby opening the capacity to fine-tune their exciting properties. A recent article in Ultramicroscopy 98 (2002) 27-42 hinted at on-going development of electron microscopy for amorphous solids.
We have entered the field through our interest in pressure-induced amorphization. The first generally accepted occurrence of pressure-induced amorphization was reported by Mishima, Calvert and Whalley (1984) in hexagonal ice, from quenched samples. The first in-situ evidence of a crystalline-to-amorphous (c-a) phase transition under pressure was discovered in SnI4 by Fuji et al. from laboratory XRD data and by Sugai from Raman measurements, both in 1985. The discovery of such a novel and exciting class of material immediately triggered a wide-ranging search for similar phases in other chemical systems. While the first materials had been obtained solely on compression of an initially crystalline material, later amorphous materials were also obtained from de-compression of an initially crystalline material synthesized at high-pressure. By the early 1990 a large number of compounds were reported to become amorphous under pressure; the phenomenon seemed to be common-place, and, as a result, after a while, interest in pressure-induced amorphization waned, even though in many cases the c-a mechanism remained unexplained
In the late 1990’s, i.e. very recently, with the advent of more powerful experimental techniques such as the combination of synchrotron radiation, diamond-anvil cell and area detector, a closer look at some purportedly key compounds such as berlinite (AlPO4) suddenly failed to reveal an amorphous phase, in particular if the incident x-ray beam was small enough to avoid sample regions with pressure gradients or if hydrostaticity was unquestionably assured by the use of a gas as pressure medium. As a result, the field of pressure-induced amorphization studies is now wide open again and ripe for another experimental assault, at an increased level of technical sophistication - as compared to the past.