High-pressure growth of NaMn{sub 7}O{sub 12} crystals

• Published in: Journal of solid state chemistry Vol. 179; no. 12
• Main Authors: Gilioli, Edi, Calestani, Gianluca, Licci, Francesca, Paorici, Carlo, Gauzzi, Andrea, Bolzoni, Fulvio, Prodi, Andrea
• Format: Journal Article
• Language: English
• Published: United States 15.12.2006
• Subjects: COOLING; CRYSTAL GROWTH; CRYSTALLIZATION; ELECTRON SPIN RESONANCE; GRAIN BOUNDARIES; HEATING; MANGANESE OXIDES; MATERIALS SCIENCE; MONOCRYSTALS; NEUTRON DIFFRACTION; POLYCRYSTALS; SCANNING ELECTRON MICROSCOPY; SODIUM OXIDES
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2006.07.036

With the aim of producing large crystals of metastable NaMn{sub 7}O{sub 12} manganite, suitable for physical measurements (i.e.: RXS, Raman, EPR, STS, single-crystal neutron diffraction), we carried out a systematic investigation of the parameters controlling the growth of crystals, including the thermodynamic variables (T, P, and reagent composition) and the kinetic factors, such as reaction time and heating/cooling rate. By varying each parameter while maintaining constant the other ones, we found the thermodynamic conditions under which an optimum equilibrium is reached between the competing nucleation and growth rates. They were found to range between 400 and 700 {sup o}C (T) and between 20 and 60 Kbars (P), respectively. Under these conditions, we further optimized the growth process, by establishing the most appropriate growth duration (several hours), reagent type (pre-reacted precursor) and composition (presence of 0.4 mole% water and of 5% Na excess with respect to the stoichiometric composition). Typical crystals having several hundreds {mu}m in linear sizes were reproducibly obtained, while the largest sample was about 800 {mu}m. A description of the crystal growth mechanism, based on the experimental results, is also presented and discussed. It assumes that two different mechanisms control the crystal growth, depending on whether the crystallization is taking place outside the stability field, i.e. in presence of native reagents, or inside it, i.e. in a polycrystalline NaMn{sub 7}O{sub 12} phase matrix. In the first case, large crystal growth occurs thanks to the low nucleation and high diffusion rates, while in the second one the crystallization is due to the soltate mechanism based on the free energy reduction caused by grain boundary migration. - Graphical abstract: Optical (a) and SEM images (b) of NaMn{sub 7}O{sub 12} crystals. Note the markers: 300 {mu}m, top-right corner (a) and 40 {mu}m, bottom left (b)