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Study resolves long-standing debate on low-pressure phase transitions in hafnium oxide

Researchers from the Institute of Solid State Â鶹ÒùÔºics, the Hefei Institutes of Â鶹ÒùÔºical Science of the Chinese Academy of Sciences, in collaboration with Southwest Jiaotong University, have combined high-pressure electrical transport experiments, high-pressure Raman spectroscopy, and first-principles calculations to reveal the structural phase transition behavior of hafnium oxide (HfO₂) under high pressure and its evolution mechanism in electrical properties.

The paper is in the journal Â鶹ÒùÔºical Review B.

"This study resolves the previous controversies regarding the phase transitions of HfO₂ in the low-pressure region," said Pan Xiaomei, a member of the team.

HfO₂ is a promising ferroelectric material compatible with complementary metal-oxide-semiconductor technology, with potential applications in memory and low-power devices. While it shows no ferroelectricity under normal conditions, strain or doping can induce a non-centrosymmetric orthorhombic phase, leading to ferroelectric behavior—highlighting the close link between structure and properties. High pressure is an effective tool to study and control such structural changes.

However, past studies have reported conflicting results about low-pressure phase transitions in HfO₂, especially between Raman spectroscopy and X-ray diffraction findings, making it difficult to fully understand its structure and properties.

To address this issue, the research team employed another high-pressure experimental technique—electrical transport—alongside high-pressure Raman spectroscopy and density functional theory calculations. The high-pressure experiments were conducted using a diamond anvil cell, enabling in-situ monitoring of pressure changes through ruby fluorescence.

In the electrical transport experiments, a sandwich structure of Au/HfO₂/Au was used, with insulating treatment applied to both sides of the steel gasket to prevent short circuits. Raman measurements were carried out using a 532 nm laser in a backscattering geometry.

The results showed that HfO₂ undergoes a clear phase transition from the monoclinic phase to an orthorhombic-I phase at around 3.5 ± 0.5 GPa, followed by a second transition to an orthorhombic-II phase at approximately 15.2 ± 0.6 GPa. Furthermore, when HfO₂ was doped with 5% yttrium, the transition pressures were found to decrease, indicating that doping plays a key role in modifying structure.

This study provided valuable insights for a deeper understanding of the relationship between its structure and electrical transport properties, according to the team.