Metal to Semimetal Transition in CaMgSi Crystals Grown from Mg−Al Flux

详细信息    查看全文

• 作者：Jeffrey B. Whalen ; Julia V. Zaikina ; Randall Achey ; Ryan Stillwell ; Haidong Zhou ; Christopher R. Wiebe ; Susan E. Latturner
• 刊名：Chemistry of Materials
• 出版年：2010
• 出版时间：March 9, 2010
• 年：2010
• 卷：22
• 期：5
• 页码：1846-1853
• 全文大小：828K
• 年卷期：v.22,no.5(March 9, 2010)
• ISSN：1520-5002

文摘

Single crystals of CaMgSi were produced using the metal flux synthesis method in a Mg/Al 1:1 mixture. The large rod-shaped crystals measure up to 7 mm in length. This phase crystallizes with the orthorhombic TiNiSi structure type (space group Pnma; a = 7.4752(2) Å, b = 4.42720(10) Å, c = 8.3149(2) Å; R₁ = 0.021). Despite its relationship to semiconducting Zintl phases Mg₂Si and Ca₂Si, CaMgSi is metallic at room temperature; this produces a positive (160 ppm) ²⁹Si MAS NMR chemical shift and is supported by DOS calculations. A metal to semimetal electronic transition at around 50 K is evident in the resistivity, magnetic susceptibility, and electron paramagnetic resonance measurements. Low temperature powder X-ray diffraction data indicates that a structural distortion accompanies this transition. The electronic heat capacity coefficient (0.4695 mJ/mol·K²) determined from low temperature heat capacity data supports the designation of CaMgSi as a semimetal at low temperature. The hydrogen storage capacity of this phase is negligible (≤0.5 wt % hydrogen), although exposure to hydrogen does destabilize the structure, inducing decomposition at 500 °C.