Neutron powder diffraction study of the orthorhombic to monoclinic transition in Sc2W3O12 on compression

doi:10.1016/j.solidstatesciences.2006.02.022

Solid State Sciences

Volume 8, Issues 3–4, March–April 2006, Pages 289-295

https://doi.org/10.1016/j.solidstatesciences.2006.02.022 Get rights and content

Abstract

Time-of-flight powder neutron diffraction combined with Rietveld analysis was used to study Sc₂W₃O₁₂ at pressures of up to 0.6 GPa in a large volume helium gas cell. Orthorhombic (Pnca) Sc₂W₃O₁₂, bulk modulus 32(3) GPa, transforms to a softer monoclinic (P2₁/a) structure, 11.8(8) GPa, on compression at between 0.25 and 0.30 GPa. This transition shows significant hysteresis and involves a volume reduction of ∼1.5%. The volume reduction during compression of the orthorhombic phase is much less anisotropic than that induced by heating this negative thermal expansion phase at ambient pressure. While the volume reduction on heating orthorhombic Sc₂W₃O₁₂ is associated with pronounced changes in Sc single bond OW angles, there are no major changes in these angles on compression to similar densities. The structural changes seen at the pressure induced orthorhombic to monoclinic transition in Sc₂W₃O₁₂ are similar, although not identical, to those seen for the equivalent thermally induced transition in Sc₂Mo₃O₁₂.

Graphical abstract

Introduction

Orthorhombic Sc₂W₃O₁₂ can be regarded as the structural prototype for a large family of compounds $A_{2} M_{3}$ O₁₂ (A=variety of +3 cations, $M = W, Mo$ ) 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 $- 2.2 \times 10^{−6} K^{−1}$ [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 A single bond OM 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 $A_{2} M_{3}$ 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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Neutron powder diffraction study of the orthorhombic to monoclinic transition in Sc2W3O12 on compression

Abstract

Graphical abstract

Introduction

Section snippets

Sample preparation

Neutron diffraction data collection and analyses

Results and discussion

Acknowledgements

J. Solid State Chem.

J. Solid State Chem.

Int. J. Inorg. Mater.

J. Solid State Chem.

Int. J. Inorg. Mater.

J. Solid State Chem.

Mater. Sci. Rep.

Prog. Mater. Sci.

Solid State Commun.

J. Phys. Chem. Solids

J. Solid State Chem.

Solid State Commun.

J. Phys. Chem. Solids

J. Solid State Chem.

Physica C

J. Solid State Chem.

J. Mater. Res.

J. Phys. Chem. B

Eur. J. Mineral.

Metall. Mater. Trans. A

Neutron powder diffraction study of the orthorhombic to monoclinic transition in Sc₂W₃O₁₂ on compression