T’-La₄Ni₃O₈ , a T’-type nickelate compound with mixed-valence Ni¹⁺/Ni²⁺ ions, the analog of the high-Tc cuprates with mixed-valence Cu²⁺/Cu³⁺,has attracted much interest. At ambient pressure, T’-La₄Ni₃O₈ undergoes an abrupt phase transition at Tt=105K, confirmed by transport and magnetic properties measurements. However,  no long-range spin ordering was observed below Tt. Density-function-theory calculation predicts a metallic low-spin phase with a shorter Ni-O bond length and an antiferromagnetic-insulator high-spin phase with a larger Ni-O distance. It was suggested that the transition at Tt actually is due to HS-LS spin-state transition. In order to understand the pressure effect on this structural transition, a high-pressure study up to 50 GPa was performed, with x-ray diffraction and magnetic and electric conductivity measurements as diagnostic tools.

 

Low-temperature resistivity and magnetization measurements show the strong suppression of the transition temperature Tt by pressure. Up to 50 GPa, the high-pressure phase remains an insulator from 50K to room temperature. Two phases coexist in the 2-6 GPa pressure range as observed from conductivity measurements. In-situ, high-pressure x-ray diffraction confirmed a structural transition at P > 21 GPa, although the technique is insufficiently sensitive to detect the second phase between 2 GPa and 6 GPa. The structure refinement shows that the pressure-induced isolated oxygen displacement occurred at 21 GPa. [See the figure for the crystalline structures of T’ (low pressure) and T₊ (high pressure) phases]. Isolated oxygen displacements occur in the T’ structure at pressure well lower than the pressure (21 GPa) where the T’ to T⁺ phase transition occurs. These displacements may stabilize the high spin state. The calculation that predicts the stability of the low-spin phase at high pressure did not considered this pressure-induced structural change.

 

The research was carried out in collaborations between EFree members Wenge Yang (HPSynC) and Guoyin Shen (HPCAT), and researchers at University of Texas-Austin, Florida State University, University of Tokyo, and the Chinese Academy of Sciences [J. Cheng et al., Phys. Rev. Lett. 108, 236403 (2012)].