High-pressure behavior of Fe₂O₃

High pressure behavior of Fe₂O₃ has been a long-standing subject of research due to its high importance for understanding Earth's interiors. At pressures from 40 to 60 GPa it undergoes a series of transformations, such as structural changes with a large volume discontinuity (~10 %), a drop of the resistivity, a spin crossover of Fe³⁺, and a disappearance of the ordered magnetic state. The crystal structure of the phase(s) observed on compression at ambient temperature above 50 GPa is still under question since only powder X-ray diffraction (XRD) data were available so far. Mössbauer and Raman spectroscopy studies cannot provide definitive structural information. Applying laser heating to Fe₂O₃, compressed up to 70 GPa and above, results in a distinct reconstructive phase transition to the CaIrO₃-type structure, according to powder XRD. Poverty of the available structural data encouraged us to perform a series of high-pressure and high-temperature XRD experiments on single crystals of Fe₂O₃ in diamond anvil cells. We have studied the behavior of Fe₂O₃ at pressures up to 100 GPa and temperatures up to 2500 K. Here we report crystal structures of two novel high-pressure Fe₂O₃ polymorphs, as well as the relations between a spin state of iron atoms and the crystal chemistry of the iron compound. In our compression experiments initially hematite-structured Fe₂O₃ transformed to a new phase at ~54 GPa with 10 % of the volume reduction. This phase has a triclinic distorted perovskite-type structure. The second reconstructive transition occurred at 66-70 GPa with 3 % of the volume discontinuity and resulted in formation of an orthorhombic phase. Laser heating to ~2100¹100 K at pressures above 70 GPa promoted a transition to a Cmcm CaIrO₃-type phase, whose crystal structure was refined by means of single crystal XRD to R₁ ~ 9.7 %. Decompression experiments showed that the Cmcm phase transforms back to hematite at pressures between ~25 and 15 GPa.

Transition between two Kosterlitz-Thouless phases in Sc-doped TiOCl

The compound TiOCl is a quasi-1-dimensional (1D) quantum magnet (Seidel et al., 2003). Upon cooling, TiOCl undergoes a phase transition at Tc2 = 90 K towards a state with incommensurate magnetic order, followed by a second phase transition at Tc1 = 67 K towards a spin-Peierls state (Seidel et al., 2003; Shaz et al., 2005; van Smaalen et al., 2005). Both low-temperature phases involve structural distortions that have been characterized by x-ray diffraction. The absence of any phase transitions has been reported for scandium-doped TiOCl with doping levels 0.01 < x < 0.1 for ScxTi1-xOCl (Glancy et al., 2008, 2010; Zhang et al., 2010; Aczel et al., 2011). We have synthesized ScxTi1-xOCl for x = 0.005. Based on temperature-dependent x-ray diffraction experiments and specific-hear measurements, we have found that the x = 0.005 compound transforms into incommensurate and spin-Peierls-like phases on cooling. Despite apparent large correlation lengths, these phases lack long-range order. A sluggish transformation is thus found between states of ScxTi1-xOCl that support different kinds of fluctuations.

Pressure-induced normal-incommensurate-commensurate phase transitions in TiPO₄

The complex interplay between spin, charge, orbital, and lattice degrees of freedom has made low-dimensional quantum spin magnets with strong antiferromagnetic (AF) spin-exchange coupling prime candidates for studying unusual magnetic phenomena. A progressive spin-lattice dimerization in one-dimensional AF Heisenberg chains, which occurs below a critical temperature and induces a singlet ground state with a magnetic gap, is commonly referred to as spin-Peierls (SP) transition. Recently, the compounds TiOX (X = Cl, Br) and TiPO₄ have been intensively investigated due to their unconventional behavior [1,2]. Unlike standard SP systems, TiOX and TiPO₄ undergo a sequence of normal-incommensurate-commensurate phase transitions on cooling at remarkably high transition temperatures. The transition temperatures are related to the direct exchange interactions between Ti ions, which increases strongly with decreasing the distance between the Ti ions, and therefore is very sensitive to the applied hydrostatic pressure. We have performed pressure-dependent single-crystal X-ray diffraction of TiPO₄ using synchrotron radiation. TiPO₄ undergoes a pressure-induced pahse transiton towards an incommensurate phase already below 10 GPa. This transformation is followed by the lock-in phase transition to the dimerized SP phase. Both structures are analogous to those at low temperatures, but reveal significantly larger modulation amplitudes. In this contribution we will present the detailed discussion of the high-pressure structures of TiPO₄ and their behavior on compression. Furthermore, similarities and differences of high-pressure phase diagrams of TiOCl and TiPO₄ and discrepancies between predicted and observed structures will be considered.