Nanosized rare-earth hexaborides: Low-temperature preparation and microstructural analysis Alessia Aprea a, * , Angelo Maspero a , Norberto Masciocchi a , Antonietta Guagliardi a, b , Alessandro Figini Albisetti c , Giovanni Giunchi c a Dipartimento di Scienza e Alta Tecnologia and To.Sca.Lab, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy b Istituto di Cristallografia and To.Sca.Lab, Consiglio Nazionale delle Ricerche, via Lucini 3, 22100 Como, Italy c Edison SpA, Divisione Ricerca e Sviluppo, Foro Buonaparte 31, 20121 Milano, Italy article info Article history: Received 21 December 2012 Received in revised form 25 February 2013 Accepted 3 April 2013 Available online 18 April 2013 Keywords: Rare-earth boride X-ray diffraction Nanostructured materials Solid state reaction Total scattering techniques abstract A versatile, rapid and easy synthesis of pure rare-earth-(RE) hexaboride powders was developed, without resorting to hazardous precursors or generating undesired, ineliminable, side products. To this purpose, we employed a metathesis reaction, typically starting from a mixture of a hydrated rare earth trichloride and MgB 2 , kept at 650  C for 1 h under vacuum. This methodology affords nanosized RE hexaborides, with average crystallite (domain) sizes down to a few nanometers, useful for tailoring the functional performances of the MgB 2 superconducting phase produced by the reactive liquid infiltration method. For the powders showing the lowest average domain sizes (YbB 6 and EuB 6 ), an unconventional micro- structural analysis, based on Total Scattering methods and on the Debye Function Approach, was also performed, which provided the complete nanocrystal size distributions. Ó 2013 Elsevier Masson SAS. All rights reserved. 1. Introduction Metallic borides and, in particular, rare-earth hexaborides (REB 6 ) have recently attracted the attention of many scientists and technologists, either at the academic or at the industrial level, and have been the subject of many theoretical and experimental studies [1]. Although known since long, the renewed interest for this class of materials is bound to their appealing functional properties and unique features, such as high chemical and thermal stability, high mechanical strength, high melting point, low work function and low volatility at high temperature, all of which are fundamental for their use in advanced technological applications. As one example above all, they can provide excellent thermionic electron sources or cathodic materials in electron microscopy [2], possessing high brightness and long service life. REB 6 species crystallize in the cubic Pm-3m space group sym- metry [3], and can be considered isomorphous to the archetypical structure of CsCl, with the chloride atoms (at the origin) replaced by the octahedral B 6 -cluster and the rare-earth metal ion at ½,½,½. The main features of these materials are normally attributed to the presence of a rigid boron closo-cluster stabilized by strong covalent bonds; however, using different rare-earth metals, small, but non- negligible, differences in the hexaborides properties [2,4,5] can be appreciated. To prepare pure REB 6 powders or monoliths, a variety of different methodologies have been employed: i) direct solid-phase combi- nation of the corresponding elements (1800  C) [6], ii) carbothermal reduction of rare-earth oxides and boron (1500  C) [7], iii) electro- chemical synthesis in molten salts (850  C) [8], iv) floating zone method of rare-earth oxides with boron (1700  C) [9], v) solution reactions in molten aluminum (1300  C) [10], vi) chemical vapor deposition (1150  C) [11] and vii) pulsed laser deposition (850  C) [12]. More recently, REB 6 materials have begun to be prepared by less invasive methods, using (poly-hydrated) metal chlorides and NaBH 4 (or H 3 BO 3 ) as cheap metal and boron sources, respectively, often assisted by the presence of metallic magnesium, I 2 or LiCl/KCl melts. For all the latter cases, the reactions require a sealed auto- clave under a static Ar atmosphere and temperature ranging from 400 up to ca. 900  C [13]. RE-acetates, coupled with NaBH 4 at 900  C, have also been used, but the final REB 6 products typically contain undesired C impurities [2b]. If the use of highly toxic boron sources is allowed, then REB 6 can also be prepared using BCl 3 ,B 2 H 6 or B 10 H 14 and rare-earth oxides powders, at temperatures ranging from 800 to 1150  C [14]. A very recent review article on boride synthesis (among other reactions) in autoclaves has also appeared [15]. * Corresponding author. Tel.: þ39 031 238 6636; fax: þ39 031 238 6630. E-mail address: alessia.aprea@uninsubria.it (A. Aprea). Contents lists available at SciVerse ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie 1293-2558/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2013.04.001 Solid State Sciences 21 (2013) 32e36