Synthesis and Magnetism of Extended Solids Containing Transition-Metal Cations in Square-Planar, MO₄ Coordination Sites

Abstract

With the exception of systems containing Cu 2+ , complex metal oxides containing paramagnetic transitionmetal cations in square-planar coordination are rare. However, by either introducing chalcogenide, pnictide, or halide anions to form mixed-anion systems or by utilizing low-temperature topochemical reduction via anion deintercalation, an extensive range of phases containing square-planar MO 4 units can be prepared. The crystal chemistry of a series of transition-metal A 2 MO 2 X 2 oxyhalide and A 2 MO 2 B 2 X 2 oxychalcogenide and oxypnictide phases is reviewed along with how their magnetic behavior changes as a function of the transition-metal and electron count. As a contrast, these mixed-anion phases are then compared to a series of topochemically reduced, metastable transition-metal oxides that also contain square-planarcoordinated transition metals, to more fully illustrate the magnetic properties of extended frameworks of square-planar MO 4 units.

Proposed ordering of textured spin singlets in a bulk infinite-layer nickelate

“…The Ba and Se ions lie on 4e (0, 0, z) sites with z = 0.4136 and 0.1606, respectively. The NiO 2 layers with a large separation of 10 Å form a two-dimensional square lattice without apical oxygen, as in the d 9 nickelates NdNiO 2 and LaNiO 2 [12][13][14]. The (AgSe) 2 layer provides someẑ-axis coupling.…”

“…A nonmagnetic ground state is observed in other square planar d 8 nickelates [8,9], viz., BaNiO 2 [10] and monovalent LaNiO 2 (see references in Ref. [11]), so BNOAS is an anomalous case.…”

“…However, the recent discovery [6] and verification [7] of superconductivity in thin films of strained, hole-doped NdNiO 2 has reinvigorated interest in layered nickelates. The focus has been on d 9 , spin-half layers, due to their similarity to the cuprates, which may be (hole or electron) doped to produce high-temperature superconducting (HTS) phases. The complementary route to superconductivity, not formerly taken seriously, may arise in compounds with a Ni d 8 configuration that approach the same d 9 regime upon electron doping.…”

“…The focus has been on d 9 , spin-half layers, due to their similarity to the cuprates, which may be (hole or electron) doped to produce high-temperature superconducting (HTS) phases. The complementary route to superconductivity, not formerly taken seriously, may arise in compounds with a Ni d 8 configuration that approach the same d 9 regime upon electron doping. The d 8 configuration brings with it the competition between low-spin and high-spin configurations, which is expected to depend on strain applied to the NiO 2 layer by other components of the compound as well as crystal-field splittings, and may or may not promote HTS.…”

“…A major issue is the relative importance of low-spin S = 0 and high-spin S = 1 states of the ion, and (we will propose) the nature of the S = 0 ion. As mentioned, most known layered d 8 nickelates are nonmagnetic [8,9], S = 0, so the magnetic behavior (even without its strangeness) of these new nickelates presents new physics. Building on the underlying magnetism, one can anticipate that electron doping, moving the Ni formal valence toward the d 9 configuration, might induce superconductivity as has been found in the (Nd, Sr)NiO 2 system.…”