Anomalous thermal expansion behaviour of Ln 2Mo 4O 15 (Ln = Y, Dy, Ho, Tm)

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.