Inelastic neutron measurements of the high temperature lattice excitations in NaI show that in thermal equilibrium at 555 K an intrinsic mode, localized in three dimensions, occurs at a single frequency near the center of the spectral phonon gap, polarized along [111]. At higher temperatures the intrinsic localized mode gains intensity. Higher energy inelastic neutron and x-ray scattering measurements on a room temperature NaI crystal indicate that the creation energy of the ground state of the intrinsic localized mode is 299 meV.

Intrinsic localized modes (ILMs) – also known as discrete breathers – are localized excitations that form without structural defects in discrete nonlinear lattices. For crystals in thermal equilibrium ILMs were proposed to form randomly, an idea used to interpret temperature activated signatures of ILMs in α-U and NaI. Here, however, we report neutron scattering measurements of lattice vibrations in NaI that provide evidence of an underlying organization: (i) with small temperature changes ILMs move as a unit back-and-forth between [111] and [011] orientations, and (ii) when [011] ILMs lock in at 636 K the transverse optic (TO) mode splits into three modes with symmetry-breaking dynamical structure resembling that of a superlattice, but there are no superlattice Bragg reflections and the pattern itself has crystal momentum. We conclude that this dynamical pattern is not derived from the rearrangement of atoms but from a coherent arrangement of ILMs decorating the crystal lattice in equilibrium.

Measurements of the phonon density of states by inelastic neutron time-of-flight scattering and specific heat measurements along with first principles calculations, provide compelling evidence for the existence of an Einstein oscillator ("rattler ") at ωE1 ≈ 5.0 meV in the filled skutterudite Yb0.2Co4Sb12. Multiple dispersionless modes in the measured density of states of Yb0.2Co4Sb12 at intermediate transfer energies (14 meV ≤ ω ≤ 20 meV) are exhibited in both the experimental and theoretical density-of-states of the Yb-filled specimen. A peak at 12.4 meV is shown to coincide with a second Einstein mode at ωE2 ≈ 12.8 meV obtained from heat capacity data. The local modes at intermediate transfer energies are attributed to altered properties of the host CoSb3 cage as a result of Yb-filling. It is suggested that these modes are owed to a complementary mechanism for the scattering of heat-carrying phonons in addition to the mode observed at ωE1 ≈ 5.0 meV. Our observations offer a plausible explanation for the significantly-higher dimensionless figures of merit of filled skutterudites, compared to their parent compounds.

Enthalpy and entropy are thermodynamic quantities critical to determining how and at what temperature a phase transition occurs. At a phase transition, the enthalpy and temperature-weighted entropy differences between two phases are equal (∆H = T ∆S), but there are materials where this balance has not been experimentally or theoretically realized, leading to the idea of hidden order and disorder. In a Pu-1.9 at.% Ga alloy, the δ phase is retained as a metastable state at room temperature, but at low temperatures, the δ phase yields to a mixed-phase microstructure of δand α-Pu. The previously measured sources of entropy associated with the α → δ transformation fail to sum to the entropy predicted theoretically. We report an experimental measurement of the entropy of the α → δ transformation that corroborates the theoretical prediction, and implies that only about 65% of the entropy stabilizing the δ phase is accounted for, leaving a missing entropy of about 0.5 kB/atom. Some previously proposed mechanisms for generating entropy are discussed, but none seem capable of providing the necessary disorder to stabilize the δ phase. This hidden disorder represents multiple accessible states per atom within the δ phase of Pu that may not be included in our current understanding of the properties and phase stability of δ-Pu.

Phonon densities of states (DOS) for the high performing thermoelectric material, AgPb m SbTe 2+m m = 16, 18,and 20), were extracted from time-of-flight inelastic neutron scattering measurements. The phonon DOS of LAST-18 differs remarkably from LAST-16 and LAST-20 by exhibiting a dramatic broadening of its acoustic modes that increases on heating. This broadening coincides with a minimum in the thermal conductivity, a maximum in the electrical conductivity and Seebeck coefficient, and a related peak in thermoelectric performance. We argue that the anomalous broadening originates with scattering enhanced by modifications to Te-Ag(Sb) bonds caused by their resonant electronic states falling near the Fermi energy for m = 18.