Low-temperature investigations of the open-framework material HfMgMo₃O₁₂

详细信息    查看全文

• 作者：Kimberly J. Miller ; Michel B. Johnson ; Mary Anne White ; Bojan A. Marinkovic
• 关键词：A. Ceramic ; D. Phase transition ; E. Thermal conductivity ; E. Heat capacity
• 刊名：Solid State Communications
• 出版年：2012
• 出版时间：September, 2012
• 年：2012
• 卷：152
• 期：18
• 页码：1748-1752
• 全文大小：544 K

文摘

The A₂Mo₃O₁₂ family, where A³⁺ is a large trivalent cation, can show interesting thermal properties such as negative thermal expansion. One member of this family, HfMgMo₃O₁₂, where the two A³⁺ cations have been replaced by Hf⁴⁺ and Mg²⁺, has been shown to have a low positive coefficient of thermal expansion above room temperature. This property makes HfMgMo₃O₁₂ an attractive candidate as a component for solid solutions with near-zero thermal expansion. However, its properties below room temperature were unexplored. In this work we report the phase transition from orthorhombic Pnma to monoclinic P2₁/a at T¡«175 K with an enthalpy change of 0.27 kJ mol^?1. Relaxation calorimetry, from 5 K to 300 K, show only the small anomaly associated with this transition. The thermal conductivity, determined from 2 K to 300 K, was low, but not as low as some other materials exhibiting negative thermal expansion. Analysis of the low-temperature heat capacity indicates the presence of low-energy phonon modes in HfMgMo₃O₁₂, consistent with the low thermal conductivity. The upper bound of the Young's modulus, estimated from the effective Debye temperature derived from the low-temperature heat capacity, is 20 GPa, a relatively low value due to the flexibility of the framework structure.