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Title: Phase behaviour, thermal expansion and compressibility of SnMo 2 O 8

Abstract

The phase behaviour and thermoelastic properties of SnMo2O8, derived from variable temperature and pressure synchrotron powder diffraction data, are reported. SnMo2O8 is a member of the AM2O8 family of negative thermal expansion (NTE) materials, but unexpectedly, has positive thermal expansion. Over the P-T space explored (298–513 K, ambient to 310 MPa) four different forms of SnMo2O8 are observed: α, β, γ and γ'. The γ to β transition is temperature-, pressure-, and time-dependent. SnMo2O8 is a much softer material (α and γ form have BT = 29 and 26 GPa at 298 K) than other members of the AM2O8 family. Counter-intuitively, its high temperature β phase becomes stiffer with increasing temperature (BT ~36 GPa at 490 K). The pressure dependence of the thermal expansion for each phase is reported.

Authors:
; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org.:
DOE - BASIC ENERGY SCIENCESFOREIGN
OSTI Identifier:
1418057
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Solid State Chemistry; Journal Volume: 258; Journal Issue: C
Country of Publication:
United States
Language:
ENGLISH
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Araujo, Luiza R., Gallington, Leighanne C., Wilkinson, Angus P., and Evans, John S. O. Phase behaviour, thermal expansion and compressibility of SnMo 2 O 8. United States: N. p., 2018. Web. doi:10.1016/j.jssc.2017.08.040.
Araujo, Luiza R., Gallington, Leighanne C., Wilkinson, Angus P., & Evans, John S. O. Phase behaviour, thermal expansion and compressibility of SnMo 2 O 8. United States. doi:10.1016/j.jssc.2017.08.040.
Araujo, Luiza R., Gallington, Leighanne C., Wilkinson, Angus P., and Evans, John S. O. Thu . "Phase behaviour, thermal expansion and compressibility of SnMo 2 O 8". United States. doi:10.1016/j.jssc.2017.08.040.
@article{osti_1418057,
title = {Phase behaviour, thermal expansion and compressibility of SnMo 2 O 8},
author = {Araujo, Luiza R. and Gallington, Leighanne C. and Wilkinson, Angus P. and Evans, John S. O.},
abstractNote = {The phase behaviour and thermoelastic properties of SnMo2O8, derived from variable temperature and pressure synchrotron powder diffraction data, are reported. SnMo2O8 is a member of the AM2O8 family of negative thermal expansion (NTE) materials, but unexpectedly, has positive thermal expansion. Over the P-T space explored (298–513 K, ambient to 310 MPa) four different forms of SnMo2O8 are observed: α, β, γ and γ'. The γ to β transition is temperature-, pressure-, and time-dependent. SnMo2O8 is a much softer material (α and γ form have BT = 29 and 26 GPa at 298 K) than other members of the AM2O8 family. Counter-intuitively, its high temperature β phase becomes stiffer with increasing temperature (BT ~36 GPa at 490 K). The pressure dependence of the thermal expansion for each phase is reported.},
doi = {10.1016/j.jssc.2017.08.040},
journal = {Journal of Solid State Chemistry},
number = C,
volume = 258,
place = {United States},
year = {Thu Feb 01 00:00:00 EST 2018},
month = {Thu Feb 01 00:00:00 EST 2018}
}
  • No abstract prepared.
  • We have performed thermal expansion and compressibility measurements on the recently discovered superconducting material Na{sub x}CoO{sub 2}{center_dot}4xD{sub 2}O (x{approx_equal}1/3) using neutron powder diffraction over the temperature range 10-295 K and the pressure range 0-0.6 GPa. Pressure measurements were done in a helium-gas pressure cell. Both the thermal expansion and compressibility are very anisotropic, with the largest effects along the c axis, as would be expected for a layered material with weak hydrogen bonding nominally along the c axis. Near room temperature, the anisotropies of the thermal expansion and compressibility of the hexagonal crystal structure are nearly the same [({delta}c/c)/({delta}a/a){approx_equal}3-4], withmore » a 100 deg. C change in temperature being roughly equivalent to 0.2 GPa pressure. It might be then inferred that changes in atom position parameters are also the same, but this is not the case. While the effects of temperature on the atom positions are essentially what one might expect, the effects of pressure are surprising. With increasing pressure, the thickness of the CoO{sub 2} layer increases, due to the combined effects of an increasing Co-O bond length and changes in the O-Co-O angles of the CoO{sub 6} octahedra. We conclude that this unusual effect results from pressure-induced strengthening of the hydrogen bonding between the Na{sub x}(D{sub 2}O){sub 4x} layers and the CoO{sub 2} layers. The strengthening of these hydrogen bonds requires that charge be moved from elsewhere in the structure; hence, there is a pressure-induced charge redistribution that weakens (lengthens) the Co-O bonds and changes the electronic structure of the superconducting CoO{sub 2} layers.« less
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  • The thermal expansion of the orthorhombic {gamma} phase of ZrW{sub 2}O{sub 8} has been measured using neutron powder diffraction from 4.6 to 410 K, where it transforms to the cubic {alpha} phase. At low temperature, {gamma}-ZrW{sub 2}O{sub 8} has a negative thermal expansion, but the thermal expansion becomes less negative with increasing temperature and is slightly positive at room temperature. This behavior can be explained in terms of the contributing phonon modes: At low temperature, the vibrational modes lead to a negative thermal expansion, but additional modes that become active upon increasing temperature add positive contributions. Above room temperature, themore » a and b axes increase more sharply while the c axis reverses its behavior and decreases with increasing temperature. This unusual behavior can be explained in terms of a thermally activated process, presumed to result from oxygen-atom migration, that makes an additional contribution to the thermal expansion. (c) 1999 The American Physical Society.« less
  • The effect of pressure on the crystal structure of HfW{sub 2}O{sub 8} has been investigated by neutron powder diffraction. At a hydrostatic pressure of 0.62 GPa at room temperature the cubic material transforms, with a 5% reduction in volume, to the same orthorhombic phase that is seen in the isostructural compound ZrW{sub 2}O{sub 8} above 0.21 GPa. The transformation is sluggish, requiring about 24 h to complete at constant pressure. Once formed, the orthorhombic phase is retained upon release of pressure. Upon heating to 360 K, the metastable orthorhombic phase transforms back to the cubic phase. The substantially higher pressuremore » for the cubic-to-orthorhombic transition in HfW{sub 2}O{sub 8}, compared to ZrW{sub 2}O{sub 8}, may be important for the application of this material in composites with controlled thermal expansion because rather large local pressures can occur in such composites.« less