Pressure and Temperature Dependent Structural Studies on Hollandite Type Ferrotitanate and Crystal Structure of a High Pressure Phase

The structural stability and phase transition behavior of tetragonal (<i>I</i>4/<i>m</i>) hollandite type K₂Fe₂Ti₆O₁₆ have been investigated by <i>in situ</i> high pressure X-ray diffraction using synchrotron radiation and a diamond anvil cell as well as by variable temperature powder neutron and X-ray diffraction. The tetragonal phase is found to be stable in a wider range of temperatures, while it reversibly transforms to a monoclinic (<i>I</i>2/<i>m</i>) structure at a moderate pressure, viz. 3.6 GPa. The pressure induced phase transition occurs with only a marginal change in structural arrangements. The unit cell parameters of ambient (t) and high pressure (m) phases can be related as <i>a</i>_<i>m</i> ∼ <i>a</i>_<i>t</i>, <i>b</i>_<i>m</i> ∼ <i>c</i>_<i>t</i>, and <i>c</i>_<i>m</i> ∼ <i>b</i>_<i>t</i>. The pressure evolution of the unit cell parameters indicates anisotropic compression with β_a = β_b ≥ β_c in the tetragonal phase and becomes more anisotropic with β_a ≪ β_b < β_c in the monoclinic phase. The pressure–volume equations of state of both phases have been obtained by second order Birch–Murnaghan equations of state, and the bulk moduli are 122 and 127 GPa for tetragonal and monoclinic phases, respectively. The temperature dependent unit cell parameters show nearly isotropic expansion, with marginally higher expansion along the <i>c</i>-axis compared to the <i>a-</i> and <i>b-</i>axes. The tetragonal to monoclinic phase transition occurs with a reduction of unit cell volume of about 1.1% while the reduction of unit cell volume up to 6 K is only about 0.6%. The fitting of temperature dependent unit cell volume by using the Einstein model of phonons indicates the Einstein temperature is about 266(18) K