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

Abstract

The structural stability and phase transition behavior of tetragonal (I4/m) hollandite type KFeTiO have been investigated by in situ 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 (I2/m) 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 a ∼ a, b ∼ c, and c ∼ b. The pressure evolution of the unit cell parameters indicates anisotropic compression with β = β ≥ β in the tetragonal phase and becomes more anisotropic with β ≪ β < β 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 c-axis compared to the a- and b-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.

Novel Single-Crystal Hollandite K1.46Fe0.8Ti7.2O16 Microrods: Synthesis, Double Absorption, and Magnetism

“…These tunnels are occupied by K + ions at 2a (K1) and 4e (K2) Wyckoff sites. The Fe 3+ , Ti 4+ , and O 2– ions occupy 8h Wyckoff sites. , The KOH concentration is also crucial in the synthesis of pure KFTO as shown in Figure b. The KFTO phase is less and the mixture cannot get a good crystallinity under a low alkali concentration of 0.5 M. The content of KFTO increases with the KOH concentration, and pure KFTO is obtained at 1 M KOH at 300 °C for 80 min.…”

New Rubidium-Containing Mixed-Metal Titanium Hollandites

“…Another structure type that can be added to the list is hollandite, TiO 2 (H), which can be prepared by removing the A cation from the more typical hollandite composition, A x M 8 O 16 . Materials crystallizing in the hollandite structure type with compositions of A x M 8 O 16 are ubiquitous in nature due to their great structural stability and exhibit significant compositional variety as indicated by the presence of alkali, alkaline earth, and even heavy metal cations, such as Pb 2+ , in these materials. − Hollandites have the general formula A x M 8 O 16 , where the A site is usually occupied by a large alkali or alkaline earth cation, such as Cs, Sr, or Ba, with the octahedral M site occupied by a single metal cation or by a mixture of these. Hollandite titanates, A x Ti 8 O 16 , feature a microporous structure containing 2 × 2 channels that can accommodate a variety of cations as well as small molecules, such as water.…”