Negative thermal expansion effects, spectroscopic properties and electronic structure transformations in Tm2Zr(MoO4)5

Abstract

New noncentrosymmetric molybdate Tm₂Zr(MoO₄)₅ was synthesized. The crystal structure variation with temperature was determined by Rietveld analysis. At 303 K, the monoclinic structure was determined in space group P2₁. A reversible phase transition P2₁↔Cmc2₁ was found at 390 K. From 423 to 520 K, nearly a ZTE (α = 0.7 × 10⁻⁶ K⁻¹) behavior was revealed. In the range of 520–720 K, the negative thermal expansion (NTE) effect (α = -2.3 × 10⁻⁶ K⁻¹) is evident for the cell volume. Tm₂Zr(MoO₄)₅ is stable up to 1100 K. Electronic structures of monoclinic and orthorhombic Tm₂Zr(MoO₄)₅ were evaluated by DFT methods. The bandgap values determined for monoclinic Tm₂Zr(MoO₄)₅ are E_{g direct} = 3.83 eV and E_{g indirect} = 3.43 eV. The vibrational properties of monoclinic phase were characterized by Raman spectroscopy. The photoluminescence emission in monoclinic Tm₂Zr(MoO₄)₅ at 303 K is dominated by a narrower band at 650 nm due to the ¹G₄ - ³F₄ transition.