We report the synthesis and crystal structure of a new high-temperature form of Ca₃P₂. The crystal structure was determined through Rietveld refinements of synchrotron powder x-ray diffraction data. This form of Ca₃P₂ has a crystal structure of the hexagonal Mn₅Si₃ type, with a Ca ion deficiency compared to the ideal 5:3 stoichiometry. This yields a stable, charge-balanced compound of Ca²⁺ and P³⁻. We also report the observation of a secondary hydride phase, Ca₅P₃H, which again is a charge-balanced compound. The calculated band structure of Ca₃P₂ indicates that it is a three-dimensional Dirac semimetal with a highly unusual ring of Dirac nodes at the Fermi level. The Dirac states are protected against gap opening by a mirror plane in a manner analogous to what is seen for graphene.

Significance: Although polymorphs of a substance can often have dramatically different physical properties, polytypes, which occur when the geometry of a structural layer is maintained but the number of layers in the layer-stacking sequence is changed, rarely do. Here we find, using random substitution of Te for some of the Se to induce structural changes in TaSe ₂ , a classic layered dichalcogenide, so that the transition temperature to superconductivity ( T _c ) is significantly different for different polytypes and polymorphs and especially differs when going from one polytype to another. This observation implies either a surprising sensitivity of T _c to the layer-stacking sequence or a similarly surprising sensitivity of T _c to the small changes in layer geometry that accompany the change in polytype.

The recent discovery of superconductivity in the Ruddlesden-Popper bilayer nickelate, specifically La₃Ni₂O₇, has generated significant interest in the exploration of high-temperature superconductivity within this material family. In this study, we present the crystallographic and electrical resistivity properties of two distinct Ruddlesden-Popper nickelates: the bilayer La₃Ni₂O₇ (referred to as 2222-phase) and a previously uncharacterized phase, La₃Ni₂O₇ (1313-phase). The 2222-phase is characterized by a pseudo F-centered orthorhombic lattice, featuring bilayer perovskite [LaNiO₃] layers interspaced by rock salt [LaO] layers, forming a repeated ...2222... sequence. Intriguingly, the 1313-phase, which displays semiconducting properties, crystallizes in the Cmmm space group and exhibits a pronounced predilection for a C-centered orthorhombic lattice. Within this structure, the perovskite [LaNiO₃] layers exhibit a distinctive long-range ordered arrangement, alternating between single- and trilayer configurations, resulting in a ...1313... sequence. This report contributes to novel insights into the crystallography and the structure-property relationship of Ruddlesden-Popper nickelates, paving the way for further investigations into their unique physical properties.

Magnetism, when combined with an unconventional electronic band structure, can give rise to forefront electronic properties such as the quantum anomalous Hall effect, axion electrodynamics, and Majorana fermions. Here we report the characterization of high-quality crystals of EuSn₂P₂, a new quantum material specifically designed to engender unconventional electronic states plus magnetism. EuSn₂P₂ has a layered, Bi₂Te₃-type structure. Ferromagnetic interactions dominate the Curie–Weiss susceptibility, but a transition to antiferromagnetic ordering occurs near 30 K. Neutron diffraction reveals that this is due to two-dimensional ferromagnetic spin alignment within individual Eu layers and antiferromagnetic alignment between layers—this magnetic state surrounds the Sn–P layers at low temperatures. The bulk electrical resistivity is sensitive to the magnetism. Electronic structure calculations reveal that EuSn₂P₂ might be a strong topological insulator, which can be a new magnetic topological quantum material (MTQM) candidate. The calculations show that surface states should be present, and they are indeed observed by angle-resolved photoelectron spectroscopy (ARPES) measurements.

A solid-state zinc ion battery based on a cellulose nanofiber–polyacrylamide hydrogel electrolyte and a Mg_0.23V₂O₅·1.0H₂O cathode.