A Variable Temperature Synchrotron X-ray Diffraction Study of Colossal Magnetoresistant NdMnAsO0.95F0.05

Observation of an exotic insulator to insulator transition upon electron doping the Mott insulator CeMnAsO

A promising route to discover exotic electronic states in correlated electron systems is to vary the hole or electron doping away from a Mott insulating state. Important examples include quantum criticality and high-temperature superconductivity in cuprates. Here, we report the surprising discovery of a quantum insulating state upon electron doping the Mott insulator CeMnAsO, which emerges below a distinct critical transition temperature, TII. The insulator-insulator transition is accompanied by a significant reduction in electron mobility as well as a colossal Seebeck effect and slow dynamics due to decoupling of the electrons from the lattice phonons. The origin of the transition is tentatively interpreted in terms of many-body localization, which has not been observed previously in a solid-state material.

Electronic and Magnetic Properties of Cation Ordered Sr2Mn2.23Cr0.77As2O2

Several different mechanisms of magnetoresistance (MR) have been observed in 1111 LnMnAsO_1-xF_x oxypnictides (Ln = lanthanide) as a result of magnetic coupling between the Mn and Ln. Such phases also exhibit interesting magnetic phase transitions upon cooling. Sr₂Mn₂CrAs₂O₂ has been synthesized to investigate if it is possible to observe MR and/or magnetic phase transitions as a result of magnetic coupling between the Mn and Cr. Sr₂Mn₂CrAs₂O₂ crystallizes in the tetragonal space group I4/mmm containing alternating MO₂^2- and M'₂As₂^2- layers, and neutron diffraction results demonstrate that the actual stoichiometry is Sr₂Mn_2.23Cr_0.77As₂O₂. Cation order is present between Mn and Cr, with Cr predominantly occupying the square planar MO₂^2- site. Below 410 K, the magnetic moments of the Mn/Cr ions in the M'₂As₂^2- sublattice exhibit G-type antiferromagnetic order. The Mn/Cr moments within the MO₂^2- layer order below 167 K with a K₂NiF₄-type antiferromagnetic structure that simultaneously induces a spin flip of the magnetic moments in the M'₂As₂^2- layers from a G-type to a C-type antiferromagnetic arrangement. The results demonstrate that the superexchange interactions are finely balanced in Sr₂Mn_2.23Cr_0.77As₂O₂. Sr₂Mn_2.23Cr_0.77As₂O₂ is semiconducting, and there is no evidence of MR.

Layered Compounds BaFMgPn (Pn = P, As, Sb, and Bi), Transition-Metal-Free Representatives of the 1111 Structure Type

Four new transition metal-free pnictide representatives of the LaOAgS structure type were predicted by DFT calculations and found in the BaFMgPn (Pn = P, As, Sb and Bi) family. The compounds adopt the tetragonal space group P4 /nmm with the unit cell parameters a/ c 4.3097(1) Å/9.5032(1) Å, 4.3855(1) Å/9.5918(1) Å, 4.5733(1) Å/9.8184(1) Å, and 4.6359(1) Å/9.8599(1) Å, respectively. According to the DFT calculations, these new compounds are semiconductors with band gaps steadily decreasing from Pn = P ( ca. 2 eV) to Pn = Bi ( ca. 1 eV). The corresponding strontium fluoride and rare-earth oxide analogs are unlikely to exist and have not been observed yet. The trends of the stability within 1111 and structurally and/or chemically related compounds based on a combined consideration of geometry and DFT calculations are discussed.