Crystal and Magnetic Structures and Physical Properties of a New Pyroxene NaMnGe2O6 Synthesized under High Pressure

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• 作者：Jinguang Cheng ; Wei Tian ; Jianshi Zhou ; Vincent M. Lynch ; Hugo Steinfink ; Arumugam Manthiram ; Andrew F. May ; Vasile O. Garlea ; Joerg C. Neuefeind ; Jiaqiang Yan
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：February 20, 2013
• 年：2013
• 卷：135
• 期：7
• 页码：2776-2786
• 全文大小：607K
• 年卷期：v.135,no.7(February 20, 2013)
• ISSN：1520-5126

文摘

A new pyroxene compound, NaMnGe₂O₆, has been synthesized at 3 GPa and 800 掳C and fully characterized by X-ray single-crystal diffraction, neutron powder diffraction, and measurements of magnetization and specific heat. NaMnGe₂O₆ crystallizes into a monoclinic C2/c structure with unit-cell parameters a = 9.859(2) 脜, b = 8.7507(18) 脜, c = 5.5724(11) 脜, and 尾 = 105.64(3)掳 at 153 K. A cooperative Jahn鈥揟eller distortion is formed by an ordering of the longest Mn鈥揙 bonds between two neighboring octahedra along the chain direction. This feature distinguishes NaMnGe₂O₆ from other pyroxene compounds without Jahn鈥揟eller active cations and suggests that the Jahn鈥揟eller distortion competes with the intrinsic local distortion in the pyroxene structure. No orbital order鈥揹isorder transition has been found up to 750 K. Like other alkali-metal pyroxenes with S > ¹/₂, NaMnGe₂O₆ (S = 2) was found to undergo a long-range antiferromagnetic (AF) ordering at T_N = 7 K due to intrachain and interchain exchange interactions. Due to the peculiar structural features and the corresponding magnetic coupling, the weak AF spin ordering gives way to a ferromagnetic-like state at a sufficiently high magnetic field. Specific-heat measurements demonstrated that a large portion of the magnetic entropy, >60%, has been removed above T_N as a result of strong spin correlations within the quasi-one-dimensional Mn³⁺-spin chains. The Reitveld refinement of neutron powder diffraction data gives a commensurate magnetic structure defined by k = [0 0 0.5] with Mn moments aligned mainly along the c-axis with a small component along both a- and b-axes.