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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...

Energy dispersive x-ray diffraction and Raman measurements show that the mineral wollastonite (CaSiO3) becomes amorphous at a pressure of 25.6 GPa at 300 K. A consideration of the high pressure/high temperature behavior of CaSiO3 shows that the amorphization occurs because the phase transition wollastonite→perovskite CaSiO3 is kinetically impeded. We suggest that the amorphous phase may be viewed as a defective long-period modulated wollastonite phase.

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.