Anomalous thermal expansion behaviour of $Ln_2Mo_4O_{15}$ (Ln = Y, Dy, Ho, Tm)

Sebastian, L and Sumithra, S and Manjanna, J and Umarji, AM and Gopalakrishnan, J (2003) Anomalous thermal expansion behaviour of $Ln_2Mo_4O_{15}$ (Ln = Y, Dy, Ho, Tm). In: Materials Science and Engineering B, 103 (3). pp. 289-296.

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Abstract

Based on a consideration of the crystal structures of well-known negative thermal expansion (NTE) materials, $ZrV_2O_7$ and $ZrW_2O_8$, we have identified a new series of tetramolybdates, $Ln_2Mo_4O_{15}$ (Ln = Y, Dy, Ho, Tm), that exhibit an anomalous thermal expansion behaviour. Unlike $ZrW_2O_8$ and $ZrV_2O_7$ which are cubic, the tetramolybdates are monoclinic (space group $P2_1/c$). Nevertheless, the framework is similar to $ZrV_2O_7/ZrW_2O_8$ consisting of $MoO_4$ tetrahedra that are weakly connected to $Mo_2O_7$ anions, which bind the Ln in a seven-fold anion coordination. An exploratory investigation of these materials using dilatometry, differential scanning calorimetry (DSC) and high-temperature powder X-ray diffraction (XRD) $(Dy_2Mo_4O_{15})$ reveals a negative or low thermal expansion in the 30–200 °C temperature range. Since there are no major structural changes in the high-temperature XRD and there is no first-order phase transition in DSC in the 30–500 °C range, we believe the mechanism of anomalous thermal expansion in these molybdates likely involves second-order structural changes of the molybdate oxygens, in a manner similar to the second-order changes found in $ZrW_2O_8$. Further detailed structural studies are essential to unravel the mechanism of anomalous thermal expansion behaviour of these molybdates.

Item Type:	Journal Article
Publication:	Materials Science and Engineering B
Publisher:	Elsevier
Additional Information:	The Copyright belongs to Elsevier.
Keywords:	Negative thermal expansion;Tetramolybdates;Ln2Mo4O15;Second-order phase transition
Department/Centre:	Division of Chemical Sciences > Materials Research Centre Division of Chemical Sciences > Solid State & Structural Chemistry Unit
Date Deposited:	17 May 2006
Last Modified:	19 Sep 2010 04:26
URI:	http://eprints.iisc.ac.in/id/eprint/6721

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