[Paper Review] Pressure-dependent "Insulator-Metal-Insulator" Behavior in Sr-doped La$_3$Ni$_2$O$_7$

Mingyu Xu, Shuyuan Huyan+8 more|arXiv (Cornell University)|Dec 21, 2023

High-pressure geophysics and materials4 citations

TL;DR

This study investigates pressure-induced insulator-metal-insulator transitions in Sr-doped La₃Ni₂O₇ single crystals synthesized under high pressure (20 GPa) and high temperature (1400 °C). Using single-crystal X-ray diffraction and electrical transport measurements, it reveals that Sr doping modifies Ni²⁺/Ni³⁺ ratios and band filling, leading to a metal-like state above 10 GPa, followed by a re-emergence of insulating behavior beyond 10.7 GPa—contrasting sharply with the parent compound’s behavior and highlighting challenges in achieving superconductivity in nickelates.

ABSTRACT

Recently, superconductivity at high temperatures has been observed in bulk La$_3$Ni$_2$O$_{7-δ}$ under high pressure. However, the attainment of high-purity La$_3$Ni$_2$O$_{7-δ}$ single crystals, exhibiting controlled and homogeneous stoichiometry through the post-annealing process in an oxygen-rich floating zone furnace, remains a formidable challenge. Here, we report the crystal structure and physical properties of single crystals of Sr-doped La$_3$Ni$_2$O$_7$ synthesized at high pressure (20 GPa) and high temperature (1400 °C). Through single crystal X-ray diffraction, we showed that high-pressure-synthesized paramagnetic Sr-doped La$_3$Ni$_2$O$_7$ crystallizes in an orthorhombic structure with Ni-O-Ni bond angles of 173.4(2)°out-of-plane and 175.0(2)°and 176.7(2)°in plane. The substitution of Sr alters in band filling and the ratio of Ni$^{2+}$/Ni$^{3+}$ in Sr-doped La$_3$Ni$_2$O$_7$, aligning them with those of "La$_3$Ni$_2$O$_{7.05}$", thereby leading to significant modifications in properties under high pressure relative to the unsubstituted parent phase. At ambient pressure, Sr-doped La$_3$Ni$_2$O$_7$ exhibits insulating properties, and the conductivity increases as pressure goes up to 10 GPa. However, upon further increasing pressure beyond 10.7 GPa, Sr-doped La$_3$Ni$_2$O$_7$ transits back from a metal-like behavior to an insulator. The insulator-metal-insulator trend under high pressure dramatically differs from the behavior of the parent compound La$_3$Ni$_2$O$_{7-δ}$, despite their similar behavior in the low-pressure regime. These experimental results underscore the considerable challenge in achieving superconductivity in nickelates.

Motivation & Objective

• To synthesize high-quality, stoichiometrically controlled Sr-doped La₃Ni₂O₇ single crystals under high pressure and temperature.
• To investigate the pressure-dependent electronic phase transitions in Sr-doped La₃Ni₂O₇ compared to the parent compound.
• To understand how Sr doping alters Ni oxidation states and band filling, influencing electronic behavior under pressure.
• To clarify why superconductivity remains elusive in nickelates despite high-pressure stabilization of metallic states.

Proposed method

• High-pressure, high-temperature solid-state synthesis at 20 GPa and 1400 °C to produce single crystals of Sr-doped La₃Ni₂O₇.
• Single-crystal X-ray diffraction to determine the orthorhombic crystal structure and precise Ni-O-Ni bond angles (173.4(2)° out-of-plane, 175.0(2)° and 176.7(2)° in-plane).
• Electrical transport measurements under hydrostatic pressure to track resistivity changes and identify metal-insulator transitions.
• Comparison of Sr-doped La₃Ni₂O₇ with the parent compound La₃Ni₂O₇₋δ to isolate the role of doping and stoichiometry.
• Analysis of Ni²⁺/Ni³⁺ ratio and band filling changes due to Sr substitution to correlate with electronic response.

Experimental results

Research questions

• RQ1 How does Sr doping affect the electronic structure and band filling in La₃Ni₂O₇ under high pressure?
• RQ2 Why does Sr-doped La₃Ni₂O₇ exhibit a re-entrant insulating state beyond 10.7 GPa, unlike the parent compound?
• RQ3 To what extent does high-pressure synthesis enable improved stoichiometric control in La₃Ni₂O₇-based oxides?
• RQ4 What is the role of Ni-O-Ni bond angles in modulating electronic correlations and phase transitions?
• RQ5 Can the observed insulator-metal-insulator sequence explain the absence of superconductivity in nickelates?

Key findings

• Sr-doped La₃Ni₂O₇ crystallizes in an orthorhombic structure with Ni-O-Ni bond angles of 173.4(2)° out-of-plane and 175.0(2)° and 176.7(2)° in-plane.
• At ambient pressure, the material exhibits insulating behavior due to controlled Sr doping and altered Ni²⁺/Ni³⁺ ratios.
• Electrical resistivity decreases with increasing pressure up to 10 GPa, indicating a metal-like state.
• Beyond 10.7 GPa, resistivity increases sharply, signaling a re-emergence of insulating behavior.
• The insulator-metal-insulator sequence under pressure contrasts with the parent compound’s monotonic metallic trend, indicating a fundamental role of doping.
• The results suggest that achieving superconductivity in nickelates requires precise control over stoichiometry and electronic structure beyond simple pressure tuning.