Timothy W. Darling

Critical examination of heat capacity measurements made on a Quantum Design physical property measurement system

Abstract We examine the operation and performance of an automated heat-capacity measurement system manufactured by Quantum Design (QD). QD’s physical properties measurement system (PPMS) employs a thermal-relaxation calorimeter that operates in the temperature range of 1.8–395 K. The accuracy of the PPMS specific-heat data is determined here by comparing data measured on copper and synthetic sapphire samples with standard literature values. The system exhibits an overall accuracy of better than 1% for temperatures between 100 and 300 K, while the accuracy diminishes at lower temperatures. These data confirm that the system operates within the ±5% accuracy specified by QD. Measurements on gold samples with masses of 4.5 and 88 mg indicate that accuracy of ±3% or better can be achieved below 4 K by using samples with heat capacities that are half or greater than the calorimeter addenda heat capacity. The ability of a PPMS calorimeter to accurately measure sharp features in Cp(T) near phase transitions is determined by measuring the specific heat in the vicinity of the first-order antiferromagnetic transition in Sm2IrIn8 (T0=14 K) and the second-order hidden order (HO) transition in URu2Si2 (TN=17 K). While the PPMS measures Cp(T) near the second-order transition accurately, it is unable to do so in the vicinity of the first-order transition. We show that the specific heat near a first-order transition can be determined from the PPMS-measured decay curves by using an alternate analytical approach. This correction is required because the latent heat liberated/absorbed at the transition results in temperature–decay curves that cannot be described by a single relaxation time constant. Lastly, we test the ability of the PPMS to measure the specific heat of Mg11B2, a superconductor of current interest to many research groups, that has an unusually strong field-dependent specific heat in the mixed state. At the critical temperature the discontinuity in the specific heat is nearly 15% lower than measurements made on the same sample using a semi-adiabatic calorimeter at Lawrence Berkeley National Laboratory.

Direct observation of the formation of polar nanoregions in Pb(Mg1/3Nb2/3)O3 using neutron pair distribution function analysis

Using neutron pair distribution function analysis over the temperature range from 1000 to 15 K, we demonstrate the existence of local polarization and the formation of medium-range, polar nanoregions (PNRs) with local rhombohedral order in a prototypical relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O3. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0% to a maximum of approximately 30% as the temperature decreases from 650 to 15 K. Below T approximately 200 K the volume fraction of the PNRs becomes significant, and PNRs freeze into the spin-glass-like state.

Resonant ultrasound spectroscopy for materials studies and non-destructive testing

Abstract The use of mechanical resonances to test properties of materials is older than the industrial revolution. Early documented cases of British railroad engineers tapping the wheels of a train and using the sound to detect cracks perhaps marks the first real use of resonances to test the integrity of high-performance alloys. Attempts were made in the following years to understand the resonances of solids mathematically, based on shape and composition. But Nobel Laureate Lord Rayleigh best summarized the state of affairs in 1894, stating ‘the problem has, for the most part, resisted attack’. More recently, modern computers and electronics have enabled Anderson and co-workers, with their work on minerals, and our work at Los Alamos on new materials and manufactured components, to advance the use of resonances to a precision non-destructive testing tool that makes anisotropic modulus measurements, defect detection and geometry error detection routine. The result is that resonances can achieve the highest absolute accuracy for any routine dynamic modulus measurement technique, can be used on the smallest samples, and can also enable detection of errors in certain classes of precision manufactured components faster and more accurately than any other technique.

Acoustic dissipation associated with phase transitions in lawsonite, CaAl2Si2O7(OH)2·H2O

Abstract Resonant ultrasound spectra of a single crystal and a polycrystalline sample of lawsonite [CaAl2Si2O2(OH)2·H2O] have been measured at room temperature and at low temperatures in the region 20-300 K. The influence of known phase transitions at 125 and 270 K is seen in the frequency variations of the resonance peaks, which are indicative of elastic stiffening, and in values for the quality factor QQF, which are indicative of dissipation. Two dissipation peaks, at ~250 and ~210 K, are interpreted as being due to the proton order-disorder processes associated with the two species of hydrogen atoms in the structure: one in hydroxyl OH groups and one in the H2O molecules. These occur below the Cmcm ↔ Pmcn transition point but coincide with changes in the shear elastic constants and in features of IR spectra reported elsewhere. A third, much smaller, dissipation peak occurs immediately below the Pmcn ↔ P21cn transition point. The combination of these anomalies in acoustic dissipation and in elastic constants is consistent with the view that the Cmcm ↔ Pmcn transition is driven both by displacive and proton ordering effects. For the Pmcn ↔ P21cn transition, dissipation and the transition are more closely related, consistent with the view that the transition is driven essentially by proton ordering.

Role of the lattice in the gamma-->alpha phase transition of Ce: a high-pressure neutron and x-ray diffraction study.

The temperature and pressure dependence of the thermal displacements and lattice parameters were obtained across the gamma-->alpha phase transition of Ce using high-pressure, high-resolution neutron and synchrotron x-ray powder diffraction. The estimated vibrational entropy change per atom in the gamma-->alpha phase transition, DeltaS(gamma-alpha)(vib) approximately (0.75+/-0.15)k(B), is about half of the total entropy change. The bulk modulus follows a power-law pressure dependence that is well described using the framework of electron-phonon coupling. These results clearly demonstrate the importance of lattice vibrations, in addition to the spin and charge degrees of freedom, for a complete description of the gamma-->alpha phase transition in elemental Ce.

Neutron diffraction study of the contribution of grain contacts to nonlinear stress‐strain behavior

[1] Repeatable, hysteretic loops in quasi-static loading measurements on rocks are well known; the fundamental processes responsible for them are not. The grain contact region is usually treated as the site of these processes, but there is little supporting experimental evidence. We have performed simultaneous neutron diffraction and quasi-static loadingexperimentsonaselectionofrockstoexperimentally isolate the response of these contact regions. Neutron diffraction measures strain in the lattice planes of the bulk of the grain material, so differences between this strain and the macroscopic response yield information about grain contact behavior. We find the lattice responds linearly to stress in all cases, oblivious to the macroscopic unrecoverable strains, curvature, and hysteresis, localizing these effects to the contacts. Neutron diffraction shows that the more granular rocks appear to distribute stresses so that the same strain appears in all the grains, independent of crystallographic orientation. INDEX TERMS: 3909 Mineral Physics: Elasticity and anelasticity; 3902 Mineral Physics: Creep anddeformation;3954MineralPhysics:Xray,neutron,andelectron spectroscopy and diffraction; 3994 Mineral Physics: Instruments and techniques; 3694 Mineralogy and Petrology: Instruments and techniques.Citation: Darling, T. W., J. A. TenCate, D. W. Brown, B. Clausen, and S. C. Vogel (2004), Neutron diffraction study of the contribution of grain contacts to nonlinear stress-strain behavior, Geophys. Res. Lett., 31, L16604, doi:10.1029/2004GL020463.

Elastic relaxations associated with the Pm\bar {3}m –R\bar {3}c transition in LaAlO3: III. Superattenuation of acoustic resonances

Resonant ultrasound spectroscopy has been used to characterize elastic softening and anelastic dissipation processes associated with the Pm3m <--> R3c transition in single crystal and ceramic samples of LaAlO(3). Softening of the cubic structure ahead of the transition point is not accompanied by an increase in dissipation but follows different temperature dependences for the bulk modulus, (1/3)(C(11) + C(12)), and the shear components, (1/2)(C(11) + C(12)) and C(44), as if the tilting instability contains two slightly different critical temperatures. The transition itself is marked by the complete disappearance of resonance peaks (superattenuation), which then reappear below ∼700 K in spectra from single crystals. Comparisons with low frequency, high stress data from the literature indicate that the dissipation is not due to macroscopic displacement of needle twins. An alternative mechanism, local bowing of twin walls under low dynamic stress, is postulated. Pinning of the walls with respect to this displacement process occurs below ∼350 K. Anelasticity maps, analogous to plastic deformation mechanism maps, are proposed to display dispersion relations and temperature/frequency/stress fields for different twin wall related dissipation mechanisms. These allow comparisons to be made of anelastic loss mechanisms under mechanical stress with elastic behaviour observed by means of Brillouin scattering at high frequencies which might also be related to microstructure.

Atomic pair distribution function analysis of materials containing crystalline and amorphous phases

Abstract The atomic pair distribution function (PDF) approach has been used to study the local structure of liquids, glasses and disordered crystalline materials. In this paper, we demonstrate the use of the PDF method to investigate systems containing a crystalline and an amorphous structural phase. We present two examples: Bulk metallic glass with crystalline reinforcements and Fontainebleau sandstone, where an unexpected glassy phase was discovered. In this paper we also discuss the refinement methods used in detail.

Beryllium’s monocrystal and polycrystal elastic constants

Using resonant-ultrasound spectroscopy, we measured beryllium’s elastic constants for both a monocrystal and a polycrystal. Thus, we consider the monocrystal–polycrystal elastic-constant relationship for hexagonal symmetry. Beside the Cij, we report the Debye characteristic temperature Θ and the Gruneisen parameter γ. We comment on beryllium’s chemical bonding and its remarkably low Poisson ratio.

Quantifying amorphous and crystalline phase content with the atomic pair distribution function

Pair distribution function (PDF) analysis is a long-established technique for studying the local structure of amorphous and disordered crystalline materials. In todays increasingly complex materials landscape, the coexistence of amorphous and crystalline phases within single samples is not uncommon. Though a couple of reports have been published studying samples with amorphous and crystalline phases utilizing PDF analysis, to date little has been done to determine the sensitivity that the method currently has in resolving such contributions. This article reports a series of experiments that have been conducted on samples with known ratios of crystalline quartz and amorphous glassy silica to examine this question in detail. Systematic methods are proposed to obtain the best possible resolution in samples with unknown phase ratios and some problems that one might encounter during analysis are discussed.