Peculiarities in the Raman spectra of ZrB12 and LuB12 single crystals
Graphical abstract
Raman spectra of LunatB12, Lu11B12 and ZrnatB12.
Introduction
Metal–boron refractory compounds are widely used in modern techniques due to their unique combination of properties (see [1], [2]), such as high melting points, hardness, thermal as well as chemical stability. Moreover, these compounds are model substances for numerous physical investigations, for example to determine the different boron sub-lattices in MB2, MB4, MB6, MB12 and MB66 phases (M=metal) with respect to the influence of the nature of the various metals or metal ions on the properties of the boride framework.
In this context the consideration of structural imperfections is very important. In solids with periodic structures defects are usually taken as weak deviations from the ideal atomic arrangement. However, for some complex boron structures (β-rhombohedral boron and boron carbide) it was proven that disregarding structural defects in band structure calculations leads to fundamental errors in the determination of electronic properties. In extreme cases, e.g. that of boron carbide, metallic character instead of semiconducting behavior was predicted (see [3], [4]). At present, it remains to be investigated if in binary boron compounds like metal hexaborides and dodecaborides the influences of structural imperfections, defects or distortions has similarly dramatic effects on the electronic properties.
The investigated boron compounds MB12 (M=Lu, Zr) may be considered as clathrate-like systems [5] comprising two sub-structures—the very rigid boron cages and the relatively weakly bonded metal atoms with atomic diameters, which are often distinctly smaller than the accommodating voids of the anionic boron framework. Nevertheless, while the rigid boron network essentially determines mechanical and related properties, the metal atoms and their oxidation states preferably determine electronic transport and magnetic properties. For instance, the electrical conductivity of MB6 phases can vary from insulating (CaB6) to metallic behavior (LaB6 and some others). Hardness and chemical resistance of the boride phases usually increase with the boron content increasing. After the recent discovery of high-T superconductivity in the otherwise well-known MgB2 [6], all boride phases newly attracted the attention of physicists, inorganic chemists and material scientists.
The same holds for rare-earth metal dodecaborides REB12, (RE=rare-earth metal), for which overviews on properties have been accomplished [7], [8], [9], [10]. Zirconium dodecaboride ZrB12 is a member of this structure family and it was synthesized as early as 1952; however, apart from the crystal structure, the knowledge of properties has remained very limited [11], [12], [13], [14], [15], although the Debye temperature was determined to correspond to 1040 K [16]. Band structure calculations were performed by Shein and Ivanovski [17], and metallic conductivity and superconductivity (Tc approximately 6 K) are known [18]. Crystal structure and composition imply a similarity to YB12, UB12, REB12, (RE=rare-earth metal) which are characterized by unusual physical properties like mixed valency, heavy fermion behavior, superconductivity, and specific magnetic transitions.
For the stability range of ZrB12 controversial data are given in the literature: A very large temperature range extending from ambient conditions to the melting point has been reported, but also a very narrow one [19], [20]. According to Massalski's constitutional diagram of the Zr–B binary system it is even limited to temperatures between 1696 and 2082 °C only, and the phase has a peritectic melting character [21]. Meanwhile, the preparation of ZrB12 single crystals, which are stable at ambient conditions [16], [22], solved this controversy.
We report on Raman spectra of ZrB12 single crystals for the first time and compare these to measurements on LuB12. According to the electron configuration 4d2 5s2 for Zr and 5d1 6s2 for Lu, different electron transfers to the boron substructures may be realized. The effect of these differences on Raman spectra will be taken into account.
Section snippets
Sample material
Large ZrB12 single crystals with a diameter of 6 mm and a length up to 40 mm [16] are prepared using pure natural boron (isotope distribution, 18.83% 10B, 81.17% 11B). X-ray diffraction diagrams of the crushed rod revealed only the presence of ZrB12. Moreover, diagrams obtained from both ends of the large rod evidence single-crystalline character, and the cell parameter agrees well within the estimated experimental error with the value of 740.75(1) pm, which was recently reported
Experimental
In order to gain plain sample surfaces, different preparation methods like cutting, lapping, polishing and etching were tested. The results of Raman investigations of differently treated specimen evidence that freshly broken samples manufactured from plates, which are cut from single crystals yield exactly reproducible Raman spectra and reliable spectral positions of the Raman lines. Accordingly, this preparation method was finally used for all investigated samples.
The Raman measurements were
Results and discussion
The crystal structure of the investigated specimens (UB12 type) can be described in terms of a modified fcc unit cell with the metal atoms in the center of regular B24 truncated octahedra, with B atoms at each of their 24 vertices. An alternative description is a modified NaCl-type structure formed by metal atoms and B12 cubo-octahedra (see [10] and references therein). The atomic pattern bears a certain similarity to clathrate-type arrangements and may, thus, be considered to consist of two
Conclusion
Despite the good quality of the investigated ZrB12 and LuB12 single crystals, both compounds contain considerable amount of structural imperfections lifting the phonon selection rules. This leads to numerous detectable Raman peaks in spectral ranges, where only Raman-inactive vibrations are expected. In this context, we like to remark that the presence of structural defects (B vacancies) is compatible with the density measurements (see above). Another independent confirmation of structural
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