Date: 16:00-18:00, Friday, July 11, 2025
Speaker: Katsutoshi Aoki
Title: Neutron Diffraction Study of Magnetic Phase Boundary of Iron
Abstract:
Iron, a prototypical itinerant ferromagnetic electron system, has long been a major research target in condensed matter physics because of its rich phase behaviour at high temperatures and high pressures. Four crystalline phases are present: the ambient-condition α-phase (bcc), the high-temperature γ-phase (fcc), the high-pressure ε-phase (hcp), and the second high-temperature δ-phase (bcc) stable within a limited temperature and pressure range. Upon heating at ambient pressure, α-iron undergoes a ferromagnetic–paramagnetic transition at the Curie temperature (TC =1043 K), followed by a structural transition to γ-iron at the α–γ transition temperature (Tag = 1180 K). This transition sequence has been interpreted in terms of the magnetic stabilization of the bcc structure relative to the close-packed fcc structure.
The structural and magnetic phase transitions have been investigated under high-temperature and high-pressureconditions to elucidate the contribution of the magnetism to the phase stability. The α–γ transition temperature decreases by approximately 500 K as pressure increases from 0.1 MPa to the triple point pressure around 8 GPa. In contrast, the Curie temperature remains nearly constant at 1043 K up to 1.8 GPa, at which the α–γ structural transition occurs. No magnetic characterization along the α–γ phase boundary has been performed beyond 1.8 GPa.
We performed in situ neutron powder diffraction (NPD) measurements on α-iron at high temperatures and high pressures to extrapolate the magnetic phase boundary into the stability region of γ-iron. Neutrons, which are scattered by both atomic nuclei and magnetic moments, provide a powerful tool for simultaneously probing crystal and magnetic structures. Curie temperatures were determined from the temperature dependence of the magnetic moment (μₘ) measured under quasi-isobaric conditions. Using the obtained Curie temperatures, we inferred an extended magnetic phase boundary spanning both the α- and γ-phase regions. This extended boundary enabled calculation of the magnetic moment along the α–γ phase boundary, providing a foundation for future theoretical studies on the role of magnetism in the phase stability of α-iron.
Speaker: Hiroki Kobayashi
Title: Access to dense metastable ice phases via deeply supercooled water
Abstract:
We conducted comprehensive crystallisation experiments for emulsified water to report that deeply supercooled water crystallises at the homogeneous nucleation temperature into dense metastable phases, including two new phases we proposed to name ices XXI and XXII, in the pressure region of 0.6–2.4 GPa. The crystal structures of these new phases were solved by powder x-ray/neutron diffraction methods coupled with molecular dynamics simulations. Ice XXI has a large unit cell of Z = 152 with the I-42d space group, and transforms into an orientationally ordered low-temperature counterpart named ice XXIII associated with symmetry reduction to P212121. The structure of ice XXI is identical to that of a computationally predicted phase called ice T2. This is the very first example of the experimental discovery of a “computational ice polymorph” under similar p–T conditions. Interestingly, we revealed by our MD simulations that the hydrogen-bonded network structure around W6 molecules differs depending on the choice of the force field. The best Rietveld fit to the neutron diffraction data was given by the structure suggested by TIP4P/Ice, whilst TIP5P did not reproduce the experimental hydrogen-atom positions correctly. It is remarkable that neutron diffraction experiments brought us even such a computational insight, where TIP4P/Ice worked better than TIP5P in terms of reproducing the structure of ice XXI. Ice XXII has an even larger unit cell of Z = 304 with the Fdd2 symmetry, where several screw-shaped units are connected at “intersection” molecules.
I like metastable phases because of their exotic flavour. Over the years (since I started to study ice IV in 2021 for my BSc studies), however, I have received several criticisms from a number of people that it is purposeless and fruitless to intensively study metastable phases. I’ve not been energetic enough to stay resistant to such criticisms, but I hope I provided nicer answers by bridging experiments and theoretical calculations via ice XXI – at least better than ‘exotic flavour’?