Bryan C. Chakoumakos

Localized vibrational modes in metallic solids

Filled skutterudite antimonides, are cubic compounds with the formula RM4Sb12, where R is a rare-earth element (such as La or Ce), and M is a transition metal (for example, Fe or Co). The rare-earth ion is weakly bound in an oversized atomic cage formed by the other atoms. Its presence has been shown to cause a dramatic reduction in the lattice component of the thermal conductivity, while having little effect on the electronic properties of the compound. This combination of properties makes filled skutterudites of interest as thermoelectric materials. It has been suggested that localized, incoherent vibrations of the rare-earth ion are responsible for the reduction in thermal conductivity, but no direct evidence for these local vibrational modes exists. Here we report the observation of local modes in La-filled skutterudites, using heat capacity, elastic constant and inelastic neutron scattering measurements. The La atoms show unusual thermodynamic behaviour, characterized by the presence of two low-energy localized modes. Our results suggest that consideration of local modes will play an important role in the design of the next generation of thermoelectric materials.

Phase transitions in perovskite at elevated temperatures - a powder neutron diffraction study

The structure of CaTiO3 has been studied at high temperatures by powder neutron diffraction methods. From inspection of the diffraction data two phase transitions are evident, with an intermediate tetragonal (I/mcm) structure forming near 1500 K and a primitive cubic structure above 1580 K. Detailed Rietveld analyses of the data suggest there may also be a phase transition from the room temperature Pbnm structure to an orthorhombic Cmcm structure around 1380 K. A remarkable feature of the results is the regular variation in the out-of-phase octahedral tilt angle over the entire temperature range.

Neutron powder diffraction study of rhombohedral rare-earth aluminates and the rhombohedral to cubic phase transition

Neutron powder diffraction has been used to examine the structural changes of the rare-earth aluminates LaAlO3 , PrAlO3 and NdAlO3 over a wide range of temperatures. At room temperature, all three aluminates adopt the rhombohedral perovskite structure in space group R c (a = 5.3647(1) A, c = 13.1114(3) A for LaAlO3 , a = 5.3337(2) A, c = 12.9842(4) A for PrAlO3 , a = 5.3223(2) A, c = 12.9292(5) A for NdAlO3 ). The rhombohedral structure is characterized by rotation of the oxygen atom octahedra about the threefold axis, and compression of these octahedra parallel to the same axis. As the temperature is increased, the rotation angle and the compression decrease, indicative of an approach to the cubic symmetry of the ideal perovskite. Only for LaAlO3 , however, was the transition at a low enough temperature to unequivocally obtain the cubic phase. For PrAlO3 the transition was closely approached before the sample can failed, but for NdAlO3 the transition appeared to be inaccessible within the available temperature range. The rotation angle is taken to represent the order parameter, and its temperature variation is well described by a generalized mean field approach. Such a description suggests the transitions are continuous, being at 820 K and second order for the transition in LaAlO3 , and at 1768 K and tricritical for the transition in PrAlO3 . In the proximity of the phase transition, the octahedral compression varies with the square of the rotation angle, though this description is inadequate remote from the transition, and the constant of proportionality is different for the different compounds.

Alpha-Decay—Induced Fracturing in Zircon: The Transition from the Crystalline to the Metamict State

A natural single crystal of zircon, ZrSiO4, from Sri Lanka exhibited zonation due to alpha-decay damage. The zones vary in thickness on a scale from one to hundreds of micrometers. The uranium and thorium concentrations vary from zone to zone such that the alpha-decay dose is between 0.2 x 1016 and 0.8 x 1016 alpha-events per milligram (0.15 to 0.60 displacement per atom). The transition from the crystalline to the aperiodic metamict state occurs over this dose range. Differential expansion of individual layers due to variations in their alpha-decay dose caused a systematic pattern of fractures that do not propagate across aperiodic layers. High-resolution transmission electron microscopy revealed a systematic change in the microstructure from a periodic atomic array to an aperiodic array with increasing alpha-decay dose. At doses greater than 0.8 x 1016 alpha-events per milligram there is no evidence for long-range order. This type of damage will accumulate in actinide-bearing, ceramic nuclear waste forms. The systematic pattern of fractures would occur in crystalline phases that are zoned with respect to actinide radionuclides.

Synthesis of superparamagnetic MgFe2O4 nanoparticles by coprecipitation

MgFe2O4 nanoparticles have been synthesized by coprecipitation method. Magnetic measurements in combination with neutron diffraction have determined the existence of a superparamagnetic state in this metal oxide system. The superparamagnetic relaxation of magnetization in these nanoparticles has been studied by using Mossbauer spectroscopy. The relaxation time has been correlated with the particle size and temperature and is consistent with Neel theory.

Thermal expansion of LaAlO3 and (La,Sr)(Al,Ta)O3, substrate materials for superconducting thin-film device applications

The thermal expansion for the perovskite (La,Sr)(Al,Ta)O3, i.e., LSAT, grown from the formulation 0.29(LaAlO3):0.35(Sr2AlTaO6), was determined by Rietveld refinement of neutron powder diffraction data over the temperature range of 15–1200 K. In comparison to LaAlO3 the relative volume thermal expansion is the same, although the cell volume of LSAT is slightly larger. Site occupation refinement for LSAT gives a structural formula of (La0.29(5)Sr0.71(5))A site(Al0.65(1)Ta0.35(1))B siteO3. At and below 150 K, LSAT shows a small distortion from cubic symmetry. Unlike the cubic-to-rhombohedral transition (800 K) observed in LaAlO3, the low temperature structural phase transition in LSAT appears to be cubic-to-tetragonal or cubic-to-orthorhombic. The rms displacement of the A site in LSAT is significantly larger than that for LaAlO3, and about half of the difference can be accounted for by a static displacement component.

Structural disorder and thermal conductivity of the semiconducting clathrate Sr8Ga16Ge30

Abstract The temperature dependence of the atomic displacement parameters for Sr8Ga16Ge30 determined from refinements of neutron powder and single-crystal diffraction data shows that the anomalously large values for one of the two unique Sr atoms persist from 295 to 11 K. Its position is better described by a fractionally occupied four-fold split site, but the rms displacement remains the largest of all of the atoms in the structure. Difference Fourier maps of this Sr site show a residual nuclear density with lobes in the directions of the split-atom positions. The Ga and Ge atoms appear to be fully disordered on the three distinct framework sites. The measured atomic displacement parameters are used to derive estimates of the following thermodynamic related quantities: Debye temperature, 271 K; mean velocity of sound, 2600 m/s; temperature of the Einstein “rattler”, 85 K; mean free path of heat-carrying phonons, 5.36 A; and lattice thermal conductivity, 0.008 W/cm-K.

Enhanced current density Jc and extended irreversibility in single‐crystal Bi2Sr2Ca1Cu2O8 via linear defects from heavy ion irradiation

Large enhancements in the critical current density Jc were produced in single crystals of the high‐temperature superconductor Bi2Sr2Ca1Cu2O8 by irradiation with high energy Sn ions. In addition, the irreversibility line was moved to considerably higher magnetic fields. In contrast with analogous studies on Y1Ba2Cu3O7, there was little, if any, selective pinning when the magnetizing field was applied parallel to the linear, ion‐damage‐produced tracks.

The Metamict State

Recently, the words “metamict” or “metamictization” have been increasingly used instead of “amorphous” or “amorphization” in acknowledgment of the fact that the first recognized example of the transition from the crystalline to the aperiodic state was in natural materials—minerals. The term (originally “metamikte” from Greek for “mix otherwise” because of their complex compositions) was first defined by Broegger in a Danish encyclopedia as one of three classes of amorphous substances: porodine (= colloidal), hyaline, and metamikte. Minerals considered to be metamict were judged amorphous because of their conchoidal fracture and isotropic optical properties; however, well-developed crystal faces evidenced the prior crystalline state. Hamberg was the first to suggest that metamictization is a radiation-induced, periodic-to-aperiodic phase transition caused by alpha particles which originate from constituent radionuclides in the uranium and thorium decay series. Rinne and Vegard confirmed by x-ray diffraction studies that metamict minerals were either amorphous or finely crystalline. Later work supported the idea of a radiation-induced transformation. The work of Stackelberg and Rottenbach established that the decrease in density, refractive indices, and birefringence correlated with the breakdown of the structure with increasing alpha-decay dose. Stackelberg and Rottenback tried to test this hypothesis directly by bombarding a thin slab of zircon with alpha particles. The results were inconclusive because the slab fractured, but this must have been one of the first experiments in which an “ion beam” was used to “modify” a material.

Structural phase transition of the spinel-type oxide LiMn2O4

Abstract The crystal structure of the low-temperature (LT) form of spinel-type LiMn2O4 has been studied by neutron and X-ray powder diffraction as well as electron diffraction. Reflections of the LT phase in its neutron and X-ray diffraction patterns were indexed on the basis of not a tetragonal unit cell but roughly an orthorhombic one (space group Fddd) with lattice parameters a=8.2797(2), b=8.2444(3) and c=8.1981(2) A. However, the presence of extra reflections which cannot be indexed with this unit cell shows that the actual lattice parameters are different from the above ones. Superlattice reflections, which were missing in powder diffraction patterns, with tripled periodicity were observed in electron diffraction patterns of the LT phase.