High-temperature X-ray investigation of natural columbites

Structural investigations at high temperature were carried out on natural columbite samples across the join Fe(Nb0.95 Ta0.05)2O6-Mn(Nb0.95 Ta0.05)2O6. The samples were preliminarily annealed to attain the complete cation-ordered state and avoid the superimposition of the effects of cation ordering during high-temperature studies. Unit-cell parameters of three columbites with different XFe content were measured at regular intervals in the temperature range 25-900 °C using single-crystal X-ray diffraction techniques. The crystal structures of completely ordered ferrocolumbite and manganocolumbite were also refined from intensity data collected at room temperature, 300 and 600 °C. Structural thermal expansion coefficients show positive, linear expansion of a, b, c lattice constants and cell volume. In general, slightly higher expansion occurs along a and c directions. However, anisotropy decreases sharply with decreasing Fe content. Reversibility of thermal expansion in the investigated temperature range was checked by high-temperature diffraction studies under heating-up and cooling-down conditions. Impurities do not play an important role in thermal expansion of columbites; expansion coefficients measured on two crystals of the same sample characterized by different Ti content are in fact almost identical. Structural changes with temperature essentially affect bond lenghts: volumes of both A and B octahedral sites increase linearly with temperature, whereas interpolyhedral geometrical parameters do not vary significantly

The compressional behaviour of the columbite-group minerals along the FeNb2O6-MnNb2O6 solid solution

Abstract 3.3.e