Novel spin-dimer system of a microporous polyborate NaCuB 7 O 12 .nH 2 O
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1032-I10-06
Novel spin-dimer system of a microporous polyborate NaCuB7O12.nH2O Jing Ju, Juns Sasaki, Naoki Toyota, and Katsumi Tanigaki Tohoku University, Sendai, 9808578, Japan ABSTRACT A new microporous polyborate, NaCuB7O12.nH2O, is synthesized using boric acid as a flux. It contains a 14-membered ring framework built up by CuO6 octahedra, BO4 tetrahedra and BO3 triangles. The spin-1/2 dimers of Cu2+ ions are connected by BO3 to form an infinite ladder along the b-axis. Magnetic susceptibility data between 2 and 330 K are presented, and the data are accurately fitted by theoretical predictions of Bleaney-Bowers for the spin-1/2 dimer, with J/kB = -153.8 K, and g = 1.98, where J is the intradimer exchange interaction constant and g is the Landé g factor.
INTRODUCTION An emergence of a variety of new open-framework materials has prompted the applications in different fields such as molecular sieves, ion-exchangers and catalysts.1,2 In terms of potential applications, the incorporation of d-elements into frameworks is considerably interesting because it opens the possibility to obtain porous materials with additional electrical, optical or magnetic properties. Among them, the search for magnetic open-framework structures has been challenging due to their potential applications in the development of low-density magnets, magnetic sensors and intelligent or multifunctional materials.3,4 Todorokite family of manganese oxides is a well-known example among the show magnetic frameworks,5-8 which interesting spin glass like behavior at low temperature. Other inorganic magnetic openframeworks are mostly based upon phosphates with a large list of metal ions (iron,9-11 cobalt,12-14 manganese,15 vanadium,16 etc.). In 1(a) Schematic view of the crystal structure of NaCuB7O12.nH2O addition, a big branch of organic molecules as Fig. along the b-axis, (b) the S-1/2 dimer of Cu2+ ions encapsulated in the 10direct constituents of the porous structures has membered ring cluster formed by 4BO4 and 6BO3. Superscript (i) refers been developed.4,17-19 Recently a series of to the equivalent position (-x, -y, z). borate-based transition metal open frameworks have been reported including, for example, Zn6B12O24,20 Na2Co2B12O2121 and NaCuB7O12.nH2O,22 which respectively contain one-dimensional 8-, 12- and 14-membered ring channels. These channels are formed by transition metal polyhedra, ZnO4, CoO6 or CuO6, respectively, while tetrahedral BO4 and triangular BO3 are the common units for these frameworks. The geometric flexibility of boron coordination endows these materials various topologies of open frameworks, which may result in distinct magnetic properties among them. Here we report on the magnetic properties of the orthorhombic compound NaCuB7O12.nH2O containing a spin-1/2 dimer of Cu2+ ions.
The structural framework of NaCuB7O12.nH2O is composed of octahedral CuO6, tetrahedral BO4 and triangular BO3.22 The borate groups (10BO3 and 4BO4) form a onedimensional channel of 14-membered ellipsoidal rings along the b-axis as shown in Fig. 1
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