No abstract

Energy dispersive x-ray diffraction measurements show that the T phase of ND2O5 becomes x-ray amorphous at a pressure of 19.2 GPa at 300 K. This pressure-induced amorphization is novel since the oxide is simultaneously reduced as it becomes amorphous. The amorphization occurs because the pressure-induced reduction r-Nb20s-* crystalline Nbi2C>29 is kinetically impeded. A consideration of the ambient pressure atomic mechanism of the oxidation of NbO* compounds (x < 2.5) suggests that the pressure-induced amorphous state is formed by oxygen defects in the r-Nb20s lattice. PACS numbers: 61.42.+h, 62.50.+p, 78.30.Ly, 8I.30.HdZiman begins his book Models of Disorder with the following sentence: "Disorder is not mere chaos: it implies defective order" 111. How defective this order can be is just beginning to be quantified: Kruger and Jeanloz discovered that AIPO4 becomes amorphous when compressed, yet when the pressure is released the crystalline state returns with its initial crystalline orientation [2,3]. This indicates a close structural relationship between the ambient pressure crystalline state and the high-pressure amorphous state. Pressure-induced amorphization has been observed for several classes of materials; e.g., a-quartz, berlinite, Ca(N03)2/NaN03, SnU, anorthite, and Ca(OH)2 [2,4,5]. Pressure-amorphized materials can have properties different from those of their melt-quenched counterparts; a recent example is the anisotropicity of pressure-amorphized a-quartz [6]. Understanding the mechanism and driving force behind these pressure-induced transformations is seminal in revealing the "order" in amorphous solids. Several possibilities, not necessarily mutually exclusive, have been suggested as the driving force for solid-state amorphizations. These include melting, mechanical instabilities, a kinetically impeded phase transition, and the generation of many defects or dislocations [2,4,7-11].Kikuchi et al. studied the effect of shock compression on the T phase of pentavalent niobium oxide (r-Nb2C>5) [12]. They found that r-Nb20s reduced to a form of NbC>2 via an intermediate step: shocked to shocked to 7-Nb 2 0 5 -• cryst. Nb| 2 0 2 9 -* cryst. Nb 0 94O 2 . 30.0 GPa 40.0 GPa It is well known that compression can drive solid-state oxidation or reduction and that transitions that are sluggish with static pressure can be facile with shock compression [13-15]. Atomic mobility occurs because shock waves cause microscopic shear and strain in the sample; also, the temperature rise associated with shock compression can enhance the transformation [15]. These considerations led us to investigate the effect of static compression on r-Nb2C>5 to see whether the transition observed by Kikuchi et al. could be impeded and result in an amorphous state. The r-Nb2C>5 samples were prepared by heating niobia aerogels [16]. The x-ray diffraction patterns of the samples agree with those reported for T-Nb205 [17]. We pressurized the samples using a Merrill-Bassett style diamond anvil cell. We monitored the pressure within the cell us...

A central question in the study of amorphous materials is the extent to which they are ordered. When the crystalline intermetallic R-Al(5)Li(3)Cu is compressed to 23.2 gigapascals at ambient temperature, an amorphous phase is produced whose order can be described as defects in a curved-space crystal. This result supports a structural relation between quasi-crystals and amorphous metals based on icosahedral ordering. This result also shows that a metallic crystal can be made amorphous by compression.

a-quartz becomes x-ray amorphous when compressed, at ambient temperature, by pressures of 25 to 30 GPa [Hemley et al.. Nature 334, 5 (1988)1. The relationship between pressure-amorphized and melt-quenched silica is of great interest. The most fundamental characteristic of melt-quenched silica is its lack of periodicity at the atomic level. We report here that high-resolution electron microscopy shows that a-quartz, pressure amorphized at 30.5 GPa is, as is conventional melt-quenched silica, amorphous at the atomic level.PACS numbers: 61.42.+h, 62.50.+p, 78.30.Ly, 81.30.Hd A novel method for producing amorphous solids is by compression at ambient temperature [1-10]. A central unanswered question is the relationship between pressure-induced amorphous solids and amorphous materials produced by conventional methods. While there are many aspects to consider, e.g., the existence of a glass transition, anisotropicity, the defining characteristic is the structure at the atomic level. Melt-quenched amorphous solids have no long-range order or translational periodicity [l 1-13]. It is common to assume that amorphous materials prepared by nonconventional methods are microcrystalline. Chen and Spaepan have stated this tendency best: "Invariably [a microcrystalline model] is resurrected when a new type of amorphous material is discovered, only to be put aside in most cases in favor of a truly amorphous structure (one without lattice periodicity).[14]" Our goal in this Letter is to show definitively that pressure-amorphized materials can be truly amorphous. Determining the length scale over which the translational order is absent depends on the structural probe used. For example, materials that appear amorphous by conventional x-ray diffraction methods may contain crystallites with an average diameter below --20 A [15]. Electron diffraction techniques probe order on a scale much smaller than X rays: High-resolution transmission electron microscopy (HRTEM) measurements have a resolution on the order of an angstrom [16].The most highly studied pressure-induced amorphization is of a-quartz [17][18][19]. Raman and energy dispersive x-ray diffraction show that it becomes amorphous between 25 and 30 GPa, while single-crystal studies detect the onset of amorphization at -15 GPa [1,2,20]. We have used HRTEM to study the pressure-induced x-ray amorphous state of a-quartz. We studied a sample of aquartz before (20.0 GPa) and after (30.5 GPa) amorphization.The sample of a-quartz powder (from Johnson Matthey, Alpha Products, 99.999%) was pressurized using a Merrill-Bassett style diamond anvil cell [21]. A 4:1 mixture of methanol and ethanol was used as the pressure-transmitting fluid. While our experiments are at pressures above the hydrostaticity limit of this medium,

No abstract