Jeremiah P. Tidey | Searcharxiv

arXiv2026

Magnetism and 3D Electron Diffraction Solution of Hydrated Rubidium-Ruthenium Oxide Rb$_2$Ru$_2$O$_7$.H$_2$O

The crystal structure of Rb$_2$Ru$_2$O$_7$.H$_2$O was determined by three-dimensional electron diffraction from the individual crystallites of a solid-state powder product. Rb$_2$Ru$_2$O$_7$.H$_2$O crystallizes in space group \textit{C}2/\textit{c} ($a=7.841(3)$ Å, $b=12.500(3)$ Å, $c=8.392(2)$ Å, $β=93.57(4)^\circ$, Z=4). The structure contains infinite chains that run normal to the (101) plane and consist of alternating RuO$_6$ octahedra and square-pyramidal RuO$_5$ units connected via shared O-O edges. Magnetic properties were measured on the bulk powder, showing a diamagnetic baseline from 300 to 60 K with a small Curie tail below 55 K. The magnetic moment, calculated from the 1.8 K isotherm, saturates at $M=4.4\times10^{-3}\,μ_\mathrm{B}\,\mathrm{Ru}^{-1}$, much less than would be expected for $S=1$ ruthenium. Bond-valence-sum analysis indicates high-valent Ru, and the near-diamagnetic response is consistent with the edge-sharing Ru-Ru motif, where weak direct Ru-Ru overlap yields a local singlet.

cond-mat.mtrl-sci↗

arXiv2021

Structural origins of the infamous "Low Temperature Orthorhombic" to "Low Temperature Tetragonal" phase transition in high-Tc cuprates

We undertake a detailed high-resolution diffraction study of a novel plain band insulator, La$_2$MgO$_4$, which may be viewed as a structural surrogate system of the undoped end-member of the high-T$_c$ superconductors, La$_{2-x-y}$A$^{2+}_x$RE$^{3+}_y$CuO$_{4}$ (A = Ba, Sr, RE= Rare Earth). We find that La$_2$MgO$_4$ exhibits the infamous low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) phase transition that has been linked to the suppression of superconductivity in a variety of underdoped cuprates, including the well known La$_{2-x}$Ba$_{x}$CuO$_4$ ($x=0.125$). Furthermore, we find that the LTO-to-LTT phase transition in La$_2$MgO$_4$ occurs for an octahedral tilt angle in the 4 $^{\circ}$ to 5 $^{\circ}$ range, similar to that which has previously been identified as a critical tipping point for superconductivity in these systems. We show that this phase transition, occurring in a system lacking spin correlations and competing electronic states such as charge-density waves and superconductivity, can be understood by simply navigating the density-functional theory ground-state energy landscape as a function of the order parameter amplitude. This result calls for a careful re-investigation of the origins of the phase transitions in high-T$_c$ superconductors based on the hole-doped, $n = 1$ Ruddelsden-Popper lanthanum cuprates.

cond-mat.supr-con↗