First-order antiferromagnetic transitions of SrMn2P2 and CaMn2P2 single crystals containing corrugated-honeycomb Mn sublattices.

SrMn2P2 and CaMn2P2 are insulators that adopt the trigonal CaAl2Si2-type structure containing corrugated Mn honeycomb layers. Magnetic susceptibility χ and heat capacity versus temperature T data reveal a weak first-order antiferromagnetic (AFM) transition at the Néel temperature [Formula: see text] K for SrMn2P2 and a strong first-order AFM transition at [Formula: see text] K for CaMn2P2 Both compounds exhibit isotropic and nearly T-independent [Formula: see text], suggesting magnetic structures in which nearest-neighbor moments are aligned at [Formula: see text] to each other. The 31P NMR measurements confirm the strong first-order transition in CaMn2P2 but show critical slowing down above [Formula: see text] for SrMn2P2, thus also evidencing second-order character. The 31P NMR measurements indicate that the AFM structure of CaMn2P2 is commensurate with the lattice whereas that of SrMn2P2 is incommensurate. These first-order AFM transitions are unique among the class of (Ca, Sr, Ba)Mn2 (P, As, Sb, Bi)2 compounds that otherwise exhibit second-order AFM transitions. This result challenges our understanding of the circumstances under which first-order AFM transitions occur.

CsMn4As3: A Layered Tetragonal Transition-Metal Pnictide Compound with an Antiferromagnetic Ground State.

We report the synthesis and properties of a new layered tetragonal ternary compound CsMn4As3 (structure type, KCu4S3; space group, P4/ mmm, no. 123; and Z = 2). The material is a small band gap semiconductor and exhibits an antiferromagnetic ground state associated with Mn spins. The compound exhibits a signature of a distinct magnetic moment canting event at 150(5) K with a canting angle ≈ 0.3°. Although some features of the magnetic characteristics of this new compound are qualitatively similar to those of the related BaMn2As2, the underlying Mn sublattices of the two materials are quite different. While the Mn square-lattice layers in BaMn2As2 are equally spaced along the c-direction with the interlayer distance dL Ba = 6.7341(4) Å, the Mn sublattice forms bilayers in CsMn4As3 with the interlayer distance within a bilayer being dL Cs = 3.1661(6) Å; the distance between the two adjacent bilayers is dB = 7.290(6) Å. This difference in the Mn sublattice is bound to significantly alter the energy balance among the J1, J2, and J c exchange interactions within the J1- J2- J c model compared to those in BaMn2As2 and the other related 122 compounds, including the well-known iron-arsenide superconductor parent compound BaFe2As2. Owing to the novelty of its transition-metal sublattice, this new addition to the family of tetragonal materials related to the iron-based superconductors brings prospects for doping and pressure studies in the search of new superconducting phases as well as other exciting correlated electron properties.

Low-energy electronic structure in the unconventional charge-ordered state of ScV6Sn6.

Kagome vanadates AV3Sb5 display unusual low-temperature electronic properties including charge density waves (CDW), whose microscopic origin remains unsettled. Recently, CDW order has been discovered in a new material ScV6Sn6, providing an opportunity to explore whether the onset of CDW leads to unusual electronic properties. Here, we study this question using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). The ARPES measurements show minimal changes to the electronic structure after the onset of CDW. However, STM quasiparticle interference (QPI) measurements show strong dispersing features related to the CDW ordering vectors. A plausible explanation is the presence of a strong momentum-dependent scattering potential peaked at the CDW wavevector, associated with the existence of competing CDW instabilities. Our STM results further indicate that the bands most affected by the CDW are near vHS, analogous to the case of AV3Sb5 despite very different CDW wavevectors.

Y3MnAu5: three distinctive d-metal functions in an intergrown cluster phase.

The new Y(3)MnAu(5) intermetallic phase is obtained from the arc-melted elements in virtually quantitative yields after annealing at 1000 °C for ~3 d. Its remarkable structure [rhombohedral, R3, Z = 6; a = 8.489(1) Å, c = 18.144(2) Å] consists of a 2:1 cubic-close-packed intergrowth between edge-shared Mn-centered Au rhombohedra (Mn@Au(8)) with gold-centered antiprismatic (Au@Y(6)) clusters via a common gold network. Magnetic susceptibility (χ) data for Y(3)MnAu(5) were fitted by a Curie-Weiss law. The Curie constant indicates a large effective moment corresponding to nearly localized Mn spins S = 5/2, and the Weiss temperature demonstrates the dominance of ferromagnetic (FM) interactions. An antiferromagnetic (AFM) transition at T(N) = 75 K and a possible spin reorientation transition at 65 K were observed. Analysis of the χ data for T < T(N) suggests a planar noncollinear helical AFM structure that arises from competing AFM interactions between FM-aligned layers of spins in the ab-plane with a turn angle of 69° between the spins along the helix c-axis. A magnetic field-induced spin flop transition is observed below T(N). Spin-polarized LMTO-LSDA calculations indicate an ~2 eV splitting of the Mn 3d states and a metallic ground state, and their COHP analyses demonstrate that ~81% of the total Hamilton populations originate from heteroatomic polar Y-Au and Mn-Au bonding. The Mn 3d, Y 4d, and Au 5d characteristics are remarkably diverse: localized and magnetically polarized for Mn; reducing and cationic for Y; and relativistically strongly bonded and oxidizing for Au, bonding of the latter two being broadly delocalized.

{Mo24Fe12} macrocycles: anion templation with large polyoxometalate guests.

POM and circumstance: Nanometer-sized polyoxometalates (POMs) bring a new direction to anion-templated supramolecular chemistry. The Keggin (left) and Dawson-type (right) polyoxoanions direct the assembly of giant metallomacrocycles through an array of weak hydrogen-bonding interactions. The concerted action of multiple hydrogen bonds keeps the templating guests embedded within the hosts, even in the solution state.

Short-range ferromagnetic order due to Ir substitutions in single-crystallineBa(Co1-xIrx)2As2(0 ≤ x ≤ 0.25).

The ternary-arsenide compound BaCo2As2was previously proposed to be in proximity to a quantum-critical point where long-range ferromagnetic (FM) order is suppressed by quantum fluctuations. Here we report the effect of Ir substitutions for Co on the magnetic and thermal properties of Ba(Co1-xIrx)2As2(0 ≤ x ≤ 0.25) single crystals. These compositions all crystallize in an uncollapsed body-centered-tetragonal ThCr2Si2structure with space groupI4/mmm. Magnetic susceptibility measurements reveal clear signatures of short-range FM ordering for x ≥ 0.11 below a nearly composition-independent characteristic temperatureTcl≈ 13 K. The small variation ofTclwith x, thermomagnetic irreversibility between zero-field-cooled and field-cooled magnetic susceptibility versusT, the occurrence of hysteresis in magnetization versus field isotherms at low field and temperature, and very small spontaneous and remanent magnetizations < 0.01 µB/f.u. together indicate that the FM response arises from short-range FM ordering of FM spin clusters as previously inferred to occur in Ca(Co1-xIrx)2-yAs2. Heat-capacityCp(T) data do not exhibit any clear feature aroundTcl, consistent with the very small moments of the FM clusters. TheCp(T) in the paramagnetic temperature regime 25-300 K is well described by the sum of a Sommerfeld electronic contribution and Debye and Einstein lattice contributions where the latter lattice contribution suggests the presence of low-frequency optic modes associated with the heavy Ba atoms in the crystals.