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

Abstract

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.