Nonstoichiometric Control of Tunnel-Filling Order, Thermal Expansion, and Dielectric Relaxation in Tetragonal Tungsten Bronzes Ba0.5鈥?i>xTaO3鈥?i>x

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• 作者：Fengjuan Pan ; Xiaohui Li ; Fengqi Lu ; Xiaoming Wang ; Jiang Cao ; Xiaojun Kuang ; Emmanuel V茅ron ; Florence Porcher ; Matthew R. Suchomel ; Jing Wang ; Mathieu Allix
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：September 21, 2015
• 年：2015
• 卷：54
• 期：18
• 页码：8978-8986
• 全文大小：553K
• ISSN：1520-510X

文摘

Ordering of interpolated Ba²⁺ chains and alternate Ta鈥揙 rows (TaO)³⁺ in the pentagonal tunnels of tetragonal tungsten bronzes (TTB) is controlled by the nonstoichiometry in the highly nonstoichiometric Ba_0.5鈥?i>xTaO_3鈥?i>x system. In Ba_0.22TaO_2.72, the filling of Ba²⁺ and (TaO)³⁺ groups is partially ordered along the ab-plane of the simple TTB structure, resulting in a 鈭?-type TTB superstructure (Pbmm), while in Ba_0.175TaO_2.675, the pentagonal tunnel filling is completely ordered along the b-axis of the simple TTB structure, leading to a triple TTB superstructure (P2₁2₁2). Both superstructures show completely empty square tunnels favoring Ba²⁺ conduction and feature unusual accommodation of Ta⁵⁺ cations in the small triangular tunnels. In contrast with stoichiometric Ba₆GaTa₉O₃₀, which shows linear thermal expansion of the cell parameters and monotonic decrease of permittivity with temperature within 100鈥?00 K, these TTB superstructures and slightly nonstoichiometric simple TTB Ba_0.4TaO_2.9 display abnormally broad and frequency-dependent extrinsic dielectric relaxations in 10³鈥?0⁵ Hz above room temperature, a linear deviation of the c-axis thermal expansion around 600 K, and high dielectric permittivity 鈭?0鈥?5 at 1 MHz at room temperature.