In recent years, particular interest has been addressed by researchers in the synthesis and study of  enstatite (MgSiO₃). This is related with the good physical, chemical, electrical, mechanical properties of enstatite. In fact, it is useful for several technological applications such as substrates in electronics, as insulators at high frequencies, as materials for thermal insulation in applications at high temperatures, and mainly for laser technology luminescence. In view of its very good properties, mainly luminescence and to find others applications, in recent years syntheses and property studies have been conducted on enstatite doped with varying amounts of Cr³⁺, Li⁺Sc³⁺, Ti⁴⁺ and Ni²⁺ [1]. In this contest, growth of Mn-doped enstatite was carried out in order to elucidate the best conditions of its formation and to assess their potential in advanced technological applications. Mn- doped enstatite single crystals were grown in lithium-vanadomolybdate flux as melting agent, the following nutrients were used: SiO₂ (granular quartz), MgO (periclase), MnO (manganese oxide), before appropriately treated to make them more reactive. Several starting mixtures, with different MnO concentrations, were first held to 1350 °C, 1050 °C and 950 °C and then slowly cooled down to 700 °C or 600 °C with three different cooling rate (3.75 °C/h, 2.1 °C/h and 1.7 °C/h). The products were characterized and studied by stereo binocular microscopy, powder crystal X-ray diffraction (XRPD), and scanning electron microscopy with energy-dispersive spectrometry (SEM/EDS). Several crystals were further characterized by single-crystal X-ray diffraction (XRD), micro-Raman and cathodoluminescence (CL). Mn-doped enstatite crystals were magenta in colour and prismatic (Fig.1), and all appeared to be euhedral and elongate parallel to c-axis. Optical inspection and XRPD analyses of products revealed that hausmannite (Mn²⁺Mn³⁺₂O₃) and quartz were also found in some runs in addition to Mn-doped enstatite. According to the XRPD patterns, the major diffraction peaks of Mn-doped enstatite consisted of clinoenstatite associated with some orthoenstatite. The maximum size of Mn-doped enstatite was 8 mm in length when the cooling rate was 1.7 °C/h, becoming smaller when the cooling rate was faster. For all runs, the doped amount of  MnO (wt %) in enstatite crystals ranged from 2.20 wt% to 12.60 wt%  as calculated from EDS/SEM analyses.  However, in some runs, enstatite crystals were zoned and had not the same MnO (wt%) dopant content, indicating crystallization in non equilibrium conditions. When the Mg/Mn molar ratio was > 0.43, no Mn-doped enstatite was produced. Raman spectrum of pure enstatite single crystal was compared with Mn-doped enstatite spectra in order to study the effect of the increasing doping on the position of  Raman bands. For two different MnO values, about 6 wt% and 10 wt%, the following trends were observed in the Raman spectra: 1) a down shifting of the peak positions; 2) a widening of the peaks; 3) a general decrease of Raman intensity. The last effect is due to the increase in surface reflection as the MnO concentration increases. The other ones indicates changes in vibrational modes because of the increasing presence of MnO. In particular the  Raman band at about 686 cm^-1 in the pure enstatite spectrum, shifts down to 680 cm^-1 for 10 wt% Mn doping, in good agreement with the data reported by Stalder et al. [2] for similar crystals doped with Fe. At room temperature, the CL spectrum of Mn-doped enstatite contains a broad emission located at 677 nm. As confirmed by Lin Lin et al. [3] this broad band is attributed to the transition ⁴T_1g(G)→⁶A_1g(S) of Mn²⁺ substitutional to Mg²⁺ position in enstatite. The comparison of the CL spectrum of Mn-doped enstatite with the analogous spectrum of pure enstatite shows that the broad emission bands which appear in Mn-doped enstatite are completely absent in the CL spectrum of the no-doped enstatite.

[1] A. Bloise, V. Pingitore, D. Miriello, C. Apollaro, D. Armentano, E. Barrese, A. Oliva, J. Cryst. Growth 329 (2011) 86.

[2] R. Stalder, A. Kronz,  B. C. Schmidt, Eur. J. Mineral. 21 (2009) 27.

[3] L. Lin, Y. Min, S. Chaoshu, Z. Weiping, Y. Baogui, J. Rare Earths 24 (2006) 104.

1. POSTER: Flux growth and characterization of Mn- doped enstatite single crystals, JPEG image data, JFIF standard 1.02, 0.2MB

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