Anomalous thermal expansion behaviour of Ln2Mo4O15 (Ln = Y, Dy, Ho, Tm)

Based on a consideration of the crystal structures of well-known negative thermal expansion (NTE) materials, ZrV2O7 and ZrW2O8, we have identified a new series of tetramolybdates, Ln2Mo4O15 (Ln = Y, Dy, Ho, Tm), that exhibit an anomalous thermal expansion behaviour. Unlike ZrW2O8 and ZrV2O7 which are cubic, the tetramolybdates are monoclinic (space group P21/c). Nevertheless, the framework is similar to ZrV2O7/ZrW2O8 consisting of MoO4 tetrahedra that are weakly connected to Mo2O7 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) (Dy2Mo4O15) 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 ZrW2O8. Further detailed structural studies are essential to unravel the mechanism of anomalous thermal expansion behaviour of these molybdates.