Amorphization and decomposition of scandium molybdate at high pressure

Abstract

The behavior of negative thermal-expansion material scandium molybdate Sc 2 ͑MoO 4 ͒ 3 is investigated at high pressure (HP) and high temperature (HT) using x-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The compound exhibits unusually high compressibility (bulk modulus ϳ6 GPa) and undergoes amorphization at 12 GPa. On the other hand, in situ laser heating of amorphous samples inside the diamond-anvil cell is found to result in crystalline diffraction pattern and Raman spectrum different from those of the original compound. Upon release of the pressure subsequent to laser heating, the Raman spectrum and the diffraction pattern remain unchanged. Matching of several of the diffraction lines and Raman peaks in the laser-heated samples with those of MoO 3 suggests a solid-state decomposition of the parent compound under HP-HT conditions into MoO 3 and other compounds. Other diffraction lines are found to correspond to Sc 2 Mo 2 O 9 , Sc 2 O 3 , and the parent compound. Quantitative analysis of the characteristic x-ray emission from different regions of the sample during scanning electron microscopic observations is used for obtaining the compositions of the daughter compounds. The stoichiometries of two main phases are found to be close to those of MoO 3 and Sc 2 Mo 2 O 9 . These results support the model that the pressure-induced amorphization occurred in this system because a pressure-induced decomposition was kinetically constrained.

Two-stage amorphization of scandium molybdate at high pressure

“…Note that initially there is a rapid decrease of intensity up to 4 GPa and then the decrease is much slower showing the persistence of diffraction from large d-spacings up to 11 GPa. It may be pointed out that we found no evidence of any structural phase transition around 4 GPa, as all the diffraction peaks exhibited monotonic shift as a function of pressure and no discontinuity in the P-V data was found [30]. Thus the decrease of diffraction intensities of large d-spacings also occurs in two stages: an initial rapid decrease up to 4 GPa and subsequent much slower decay till the completion of amorphization at 12 GPa.…”

Evolution of pressure-amorphized zirconium tungstate upon annealing

“…This is understandable since recrystallization requires small local rearrangements whereas decomposition involves diffusion over macroscopic length scales. It is worth pointing out that pressureinduced decomposition has been reported at high-temperature in several molybdates ZrMo 2 O 8 [29], Sm 2 (MoO 4 ) 3 , Eu 2 (MoO 4 ) 3 , Gd 2 (MoO 4 ) 3 [30], and Sc 2 (MoO 4 ) 3 [31] having similar network structure.…”

Polyamorphism in network structures

“…On the other hand, the asymmetric stretching mode (ν 3 ) shows only a weak dependence on pressure. It may be pointed out that the symmetric stretching vibration of tetrahedral ions such as SO 4 and WO 4 depends strongly on the bondlength and the mode frequency is known to increase at high pressure due to compression of the bond [24,25]. A decrease in ν 1 is anomalous [3] and would correspond to increase in the mean bonddistance.…”

“…This is believed to arise due to kinetic hindrance of transformation to final equilibrium state [2] that could be either a new high-density polymorph or a mixture of dense-packed daughter compounds [3,4]. In analogy with the crystalline solids, it likely that an amorphous solid (or a glass) could also undergo a structural transition from one non-crystalline state to another, i.e.…”