Abstract

Negative thermal expansion (NTE) materials which shrink on heating attracts the keen attention because these can compensate for the thermal expansion of structural materials by making composites and solve the critical problems caused by the thermal expansion. We utilize 6s² lone pair activity of Pb²⁺ and Bi³⁺ and valence skipping nature of these ions for exploration of NTE materials [1]. PbVO₃ is a PbTiO₃-type compound with an enhanced polar tetragonal structure (c/a = 1.23) owing to dxy orbital ordering of V⁴⁺ (d¹). Hole doping by Bi³⁺ substitution for Pb²⁺ decreases the polar distortion and enables temperature induced polar-nonpolar transition accompanied by ~ 9 % volume shrinkage [2, 3]. Similarly ferroelectric transition temperature of BiFeO₃ can be reduced by A- and B-site substitutions and NTE has been achieved [4]. BiNiO₃ is a perovskite compound stabilized by high-pressure (HP) synthesis at 6 GPa. It has a characteristic valence distribution of \( Bi^{3+}_{0.5}Bi^{5+}_{0.5}Ni^{2+}O_{3} \) and undergoes a pressure induced intermetallic charge transfer transition resulting in Bi³⁺Ni³⁺O₃ HP phase above 4 GPa. This transfer causes Ni’s valence to change from Ni²⁺ to Ni³⁺, leading to the Ni–O bond contracting and unit cell volume de-creasing by 2.5% [5]. In the case of BiNi_1-xFe_xO₃, the charge transfer transition between Bi⁵⁺ and Ni²⁺ can be induced by heating at ambient pressure (AP), leading to an NTE [6, 7]. Similarly, PbCrO₃ exists in a \( Pb_{0.5}^{2+} Pb_{0.5}^{4+}Cr^{3+}O_{3} \) valence distribution at AP and exhibits a 9.8% pressure-induced volume collapse [8]. We investigated the phase relation of PbCrO₃ in the pressure-temperature space and found that, contrary to BiNiO₃, PbCrO₃ returns to the ambient pressure phase when the temperature is increased under pressure. The slope of the phase boundary in the P-T phase diagram of BiNiO₃ is negative because the metallic Bi³⁺Ni³⁺O₃ HP-LT phase has higher entropy than \( Pb_{0.5}^{2+} Pb_{0.5}^{4+}Cr^{3+}O_{3} \). On the other hand, glassy distribution of Pb²⁺ and Pb⁴⁺ enhances the entropy of the \( Pb_{0.5}^{2+} Pb_{0.5}^{4+}Cr^{3+}O_{3} \)phase and the phase boundary has a positive slope [9]. Large thermal expansion rather than NTE is expected in PbCrO₃ if the high-pressure phase is stabilized by chemical substitutions and indeed, Pb_0.7Ca_0.3CrO₃ exhibits approximately 12% unit cell volume expansion on heating between 300 – 400 K [9].

References

[1] M. Azuma et al., Annu. Rev. Mater. Res. 51, 329 (2021).
[2] H. Yamamoto et al., Angew. Chem. Int. Ed. 57, 8170 (2018).
[3] T. Nishikubo et al., Chem. Mater. 35, 870 (2023).
[4] K. Hatayama et al., J. Am. Chem. Soc., 147, 44845 (2025).
[5] M. Azuma et al., Nat. Commun. 2, 347 (2011).
[6] K. Nabetani et al., Appl. Phys. Lett. 106, 061912 (2015).
[7] T. Nishikubo et al., J. Am. Chem. Soc. 141 19397 (2019).
[8] R. Yu et al., J. Am. Chem. Soc. 137, 12719 (2015).
[9] Q Liu et al., Chem. Mater., 37, 3305 (2025).

Biography

Professor Masaki Azuma obtained his Ph.D. from Kyoto University, Japan in 1995. From 2004 to 2010, he served as an associate professor at the Institute for Chemical Research, Kyoto University. Currently, he is a professor at Institute of Science Tokyo, Japan. He is the chairman of the World Research  Hub Initiative at Institute of Science Tokyo, and a council  member of the High Pressure Science and Technology Advanced Research Institute (HPSTAR) in Japan. He is a senior researcher in the field of solid-state physics and chemistry. He has discovered various functional new materials such as spin ladder compounds, ferromagnetic ferroelectrics, lead-free piezoelectric materials, and negative thermal expansion materials by high-pressure synthesis, and clarified their functional manifestation mechanisms by synchrotron radiation X-ray diffraction and spectroscopy.

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