Structural and physical properties of trilayer nickelates <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>R</mml:mi><mml:mn>4</mml:mn></mml:msub><mml:msub><mml:mi>Ni</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>10</mml:mn></mml:msub></mml:mrow></mml:math> (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>R</mml:mi><mml:mo>=</mml:mo><mml:mi>La</mml:mi></mml:mrow></mml:math>, Pr, and Nd)

Abstract

We investigate in detail the low-temperature structural and physical properties of the trilayer nickelates ${R}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ ($R=\mathrm{La}$, Pr, and Nd), which crystallize with a monoclinic symmetry (space group $P{2}_{1}a,Z=4$) and undergo a metal-to-metal transition (MMT) near ${T}_{\mathrm{MMT}}$= $135\phantom{\rule{0.16em}{0ex}}\mathrm{K}\phantom{\rule{0.16em}{0ex}}(\mathrm{La}), 156\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (Pr), and $160\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (Nd). Using a high-resolution synchrotron powder x-ray diffraction technique, we show that the lattice parameters in all cases exhibit an anomalous behavior at ${T}_{\mathrm{MMT}}$, however, without any sign of change in the lattice symmetry. Unambiguous signature of MMT is also observed in the magnetic and transport data, suggesting a strong coupling between the electronic, magnetic, and structural degrees of freedom. Analysis of thermal expansion yields hydrostatic pressure dependence of MMT in close agreement with previous high-pressure experiments. In ${\mathrm{Pr}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$, the ${\mathrm{Pr}}^{3+}$ ions located in the rocksalt (RS) layers order magnetically near $5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is significantly suppressed compared to ${\ensuremath{\theta}}_{p}\ensuremath{\sim}\ensuremath{-}36\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In contrast, ${\mathrm{Pr}}^{3+}$ ions in the perovskite-block (PB) layers exhibit a crystal field (CF) induced nonmagnetic singlet ground state. In ${\mathrm{Nd}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$, on the other hand, the CF ground state of ${\mathrm{Nd}}^{3+}$ ions in both RS and PB layers is a Kramers doublet. The heat capacity of ${\mathrm{Nd}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ shows a pronounced Schottky-like anomaly near $40\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and a sharp upturn indicating short-range correlations between the Nd-moments below $10\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. However, no signs of long-range ordering of Nd-moments could be found down to $2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ despite a sizable value of ${\ensuremath{\theta}}_{p}\ensuremath{\sim}\ensuremath{-}40\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The strongly suppressed magnetic long-range ordering in both $R=\mathrm{Pr}$ and Nd suggests the presence of strong magnetic frustration in these compounds. In the presence of an overwhelming Schottky contribution, the electronic term in the specific heat of ${\mathrm{Pr}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ and ${\mathrm{Nd}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$ appears highly inflated, which can be falsely interpreted as a sign of heavy fermion behavior as is done in a recent study on ${\mathrm{Nd}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10}$. Accordingly, the low-temperature resistivity of these compounds is found to follow a $\ensuremath{-}{T}^{0.5}$ rather than a $\ensuremath{-}\mathrm{ln}T$ dependence.

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