Neutron powder diffraction study of the orthorhombic to monoclinic transition in Sc2W3O12 on compression
Graphical abstract
Introduction
Orthorhombic Sc2W3O12 can be regarded as the structural prototype for a large family of compounds O12 (A=variety of +3 cations, ) with interesting anisotropic thermal expansion behavior that includes negative volume thermal expansion for many compositions [1], [2], [3], [4], [5], [6]. Sc2W3O12 (Pnca) consists of corner sharing ScO6 octahedra and WO4 tetrahedra. On heating, the a- and c-axes contract and the b-axis expands leading to an overall volume contraction corresponding to an average intrinsic linear coefficient of thermal expansion of [7]. The structural changes associated with this volume contraction in Sc2W3O12 were examined initially by Evans et al. [7] and further studied by Weller and co-workers [8]. It was found that the volume contraction was associated with a coupled tilting of the quasi-rigid polyhedra that make up the framework [7]. There have also been variable temperature crystal structure studies for Y2W3O12 [3], [5], Al2W3O12 [5], and Sc2Mo3O12 [6] where significant trends in AOM bond angles with temperature have also been observed.
The behavior of negative thermal expansion (NTE) materials on compression has received considerable recent attention due to a combination of practical considerations and fundamental interest in the variety of crystalline to crystalline, and crystalline to amorphous (pressure induced amorphization or PIA [9], [10], [11]), transformations that are seen. From a practical standpoint, these transformation often occur in a pressure range that could be achieved during the fabrication of composites containing NTE materials [12], [13] and they typically lead to a loss, or dramatic reduction, in negative thermal expansion, which would degrade the properties of the composite.
Within the O12 family of materials, there have been high pressure studies of Y2W3O12, Sc2W3O12, Sc2Mo3O12, Al2W3O12 and Lu2W3O12 [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. This work has included the examination of electrical properties under pressure [15], [17], [18], [24], [25], pressure induced amorphization [14], [16], [22], [28], decomposition at high pressure and temperature [22], [23], crystalline to crystalline and crystalline to amorphous transitions by Raman spectroscopy [19], [20], [21], [22], phase transitions by X-ray diffraction [24], [26], [27], and compressibility by X-ray diffraction [22], [26], [27], [28]. To the best of our knowledge, there have been no high pressure crystallographic studies of members of this family.
In the present paper, we report a high pressure crystallographic study of Sc2W3O12 using Time-of-Flight (TOF) powder neutron diffraction. The structural changes that occur on compression are contrasted with those that occur for similar unit cell volumes during negative thermal expansion and compared to those seen at the orthorhombic to monoclinic phase transition in Sc2Mo3O12.
Section snippets
Sample preparation
Sc2W3O12 powder was prepared from Sc2O3 (Strem Chemicals, Newburyport, MA) and WO3 (Aldrich, Milwaukee, WI). Stoichiometric amounts of the two oxides were thoroughly mixed and ground. The mixture was initially heated at 1000 °C for 5 hours and after regrinding it was heated at 1200 °C for an additional 12 hours in air.
Neutron diffraction data collection and analyses
TOF powder diffraction data were collected using the Special Environment Powder Diffractometer (SEPD) [29], at the Intense Pulsed Neutron Source (IPNS), Argonne National
Results and discussion
Compounds with the orthorhombic Sc2W3O12 structure often undergo a volume reducing ferroelastic phase transition to a monoclinic structure on cooling [35], although this is not seen >10 K for Sc2W3O12 at ambient pressure [7]. This transition can also be induced by compression at ambient temperature. It has been observed for Sc2Mo3O12 [20] and Al2W3O12 [19] by high pressure Raman spectroscopy, and high pressure X-ray studies have confirmed the occurrence of this transition for these two
Acknowledgements
A.P.W. is grateful for support under National Science Foundation grant DMR-0203342. The work at Argonne National Laboratory was supported by the U.S. Department of Energy, Division of Materials Sciences – Basic Energy Sciences under contract W-31-109-Eng-38. We would like to thank Dr. Robert Downs for helpful discussions regarding the use of XtalDraw [39].
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