Muon spin rotation spectroscopy reveals localized electron states in the geometrically frustrated metallic pyrochlore Cd2Re2O7 at temperatures from 2 to 300 K in transverse magnetic fields up to 7 T. Two distinctive types of localized states, with characteristic radii of about 0.5 and 0.15 nm, are detected at high and low temperature, respectively. These states may be spin polarons, formed due to strong exchange interaction between itinerant electrons and the magnetic 5d electrons of Re ions, which may determine the peculiar electronic and magnetic properties of Cd2Re2O7.

Cd 2 Re 2 O 7 single crystals exhibit an amorphouslike lattice thermal conductivity, latt . At temperatures about twice the Debye temperature, the phonon mean-free path approaches the value of the average Cd-Cd spacing while the specific heat is at values notably lower than the classical Dulong-Petit limit. We show that a model that treats Cd ions as Einstein oscillators embedded in a Debye host with strong optical-acoustic phonon coupling and some very high-frequency optical phonon modes in the system, can account for these features. Despite no structural change, latt undergoes a smooth slope change at ϳ340 K, above which the resistivity adopts a weak power-law behavior. The combined results of resistivity, thermopower, specific-heat, thermalconductivity, and Hall coefficient measurements corroborate that a phonon-mediated electron-hole scattering mechanism sets in above room temperature.

The defect stability in a prototypical perovskite oxide superlattice consisting of SrTiO3 and PbTiO3 (STO/PTO) is determined using first principles density functional theory calculations. Specifically, the oxygen vacancy formation energies Ev in the paraelectric and ferroelectric phases of a superlattice with four atomic layers of STO and four layers of PTO (4STO/4PTO) are determined and compared. The effects of charge state, octahedral rotation, polarization, and interfaces on the Ev are examined. The formation energies vary layer-by-layer in the superlattices, with Ev being higher in the ferroelectric phase than that in the paraelectric phase. The two interfaces constructed in these oxide superlattices, which are symmetrically equivalent in the paraelectric systems, exhibit very different formation energies in the ferroelectric superlattices and this can be seen to be driven by the coupling of ferroelectric and rotational modes. At equivalent lattice sites, Ev of charged vacancies is generally lower than that of neutral vacancies. Octahedral rotations (a 0 a 0 c -) in the FE superlattice have a significant effect on the Ev, increasing the formation energy of vacancies located near the interface but decreasing the formation energy of the oxygen vacancies located in the bulk-like regions of the STO and PTO constituent parts. The formation energy variations among different layers are found to be primarily caused by the difference in the local relaxation at each layer. These fundamental insights into the defect stability in perovskite superlattices can be used to tune defect properties via controlling the constituent materials of superlattices and interface engineering.

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Double perovskites A2FeReO6 (A = Ba, Ca) have been synthesized and studied using a combination of resonant ultrasound spectroscopy, magnetization, and magnetostriction measurements. A large softening in C44 over a wide temperature range is observed for Ba2FeReO6 below its Curie temperature (TC ≈ 311 K), which is indicative of a structural distortion. This softening is partially suppressed upon application of a low magnetic field. For Ca2FeReO6, both the longitudinal and shear moduli show a softening starting at T ≈ 160 K. In addition, magnetoelastic coupling constants have been estimated from elastic constants and magnetostriction data, which provide direct evidence of pronounced coupling between magnetism and the lattice degrees of freedom present in both compounds.