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Limits of Precision in the Determination of Lattice Parameters and Stresses by the Debye-Scherrer Method

Description: Note an investigation regarding the limits of precision in the determination of lattice parameters. Experiments were performed with copper radiation on zinc samples. Even when the crystal-grain orientation is perfectly uniform, the recording of the intensity distribution will entail errors.
Date: October 1947
Creator: Ekstein, Hans & Siegel, Stanley
Partner: UNT Libraries Government Documents Department

Lattice disorder and magnetism in f-electron intermetallics

Description: Real materials can have real differences compared to ideal systems. For instance, non-Fermi liquid (NFL) behavior was initially thought to be due to chemical disorder, since the first such materials were all substituted. Although several nominally well-ordered NFL's have been discovered and extensively studied, the effect of disorder on the magnetic properties of f-electron intermetallic systems remains poorly understood. Disorder in NFL systems is reviewed from an experimental, local structure point of view, including a discussion of results on the nominally ordered U{sub 3}Ni{sub 3}Sn{sub 4} and CeCoIn{sub 5} systems, and the chemically disordered UCu{sub 4}Pd and CeRhRuSi{sub 2} systems.
Date: July 29, 2004
Creator: Booth, C.H.; Han, S.-W.; Skanthakumar, S. & Sarrao, J.L.
Partner: UNT Libraries Government Documents Department

Solid Solution Phases in the Olivine-Type LiMnPO4/MnPO4 System

Description: Nonstoichiometry is reported in the LiMnPO{sub 4}/MnPO{sub 4} system for the first time. As lithium is removed from crystalline LiMnPO{sub 4} by chemical or electrochemical methods, the resulting two phase mixture consists of stoichiometric LiMnPO{sub 4} and a delithiated phase, Li{sub y}MnPO{sub 4}, whose lattice parameters depend upon the global extent of delithiation and on the crystalline domain size of the delithiated phase. This behavior is reproduced during electrochemical insertion of lithium. Again, no evidence for nonstoichiometry was found in the vicinity of LiMnPO{sub 4}. Attempts to create single phase solid solutions by heating mixtures of the two phases failed due to the thermal instability of Li{sub y}MnPO{sub 4}.
Date: April 7, 2009
Creator: Chen, Guoying & Richardson, Thomas J.
Partner: UNT Libraries Government Documents Department

Deceleration Orbit Improvements

Description: During the accelerator studies period of 12/90-1/91 much study time was dedicated to improving the E760 deceleration ramps. 4 general goals were in mind: (1) Reduce the relative orbit deviations from the nominal reference orbit as much as possible. This reduces the potential error in the orbit length calculation - which is the primary source of error in the beam energy calculation. (2) Maximize the transverse apertures. This minimizes beam loss during deceleration and during accidental beam blow-ups. (3) Measure and correct lattice parameters. Knowledge of {gamma}{sub T}, {eta}, Q{sub h}, Q{sub v}, and the dispersion in the straight sections allows for a more accurate energy calculation and reliable SYNCH calculations. (4) Minimize the coupling. This allows one to discern between horizontal and vertical tunes.
Date: April 26, 1991
Creator: Church, M.
Partner: UNT Libraries Government Documents Department

Magnetically induced lattice distortions in the neptunium monopnictides

Description: From 19th conference on magnetism and magnetic materials; Boston, Massachusetts, USA (13 Nov 1973). The lattice parameters of the neptunium monopnictides (NaCl crystal structure) were measured between 5 and 300 deg K by x- ray diffraction. NpN becomes ferromagnetic at 87 deg K. At T/sub c/ a rhombohedral distortion, which indicates that the (111) is the easy axis, is observed. At 5 deg K, the rhombohedral angle is 60.46 plus or minus 0.02 deg . NpP exhibits a tetragonal distortion at 74 deg K, at which temperature the antiferromagnetic 3+, 3- structure becomes commensurate with the lattice. At 5 deg K, c/a = 0.9958 plus or minus 2. NpAs becomes tetragonal at T/sub N/ = 175 deg K when the material has a 4+, 4- structure. However, NpAs returns to being a cubic at 142 deg K at which temperature the magnetic structure is the simple plus or minus (type I) configuration. The return'' to cubic is first order; the volume of the unit cell expands by 0.23%. Np,Sb, which orders antiferromagnetically at 207 deg K with the type I structure, remains cubic in the ordered regime. (auth)
Date: January 1, 1973
Creator: Mueller, M.H.; Lander, G.H.; Knott, H.W. & Reddy, J.F.
Partner: UNT Libraries Government Documents Department

Point Defects in Binary Laves-Phase Alloys

Description: Point defect mechanisms in the binary C15 NbCr{sub 2} and NbCo{sub 2}, and C14 NbFe{sub 2} systems on both sides of stoichiometry was studied and clarified by both bulk density and X-ray lattice parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000 C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with the model predictions. However, thermal vacancies exhibiting a maximum at the stoichiometric composition were obtained in NbCr{sub 2} laves phase alloys after quenching from 1400 C. These could be completely eliminated by annealing at 1000 C. Anti-site hardening was found on both sides of stoichiometry for all three Laves phase systems studied. Furthermore, the thermal vacancies in NbCr{sub 2} alloys after quenching from 1400 C were found to soften the Laves phase. The anti-site hardening of the Laves phases is similar to that of the B2 compounds, while the thermal vacancy softening is unique to the Laves phase. Both the anti-site defects and thermal vacancies do not significantly affect the fracture toughness of the Laves phases.
Date: November 30, 1998
Creator: Liaw, P.K.; Liu, C.T.; Pike, L.M. & Zhu, J.H.
Partner: UNT Libraries Government Documents Department

Control and Elimination of Cracking of AlGaN Using Low-Temperature AlGaN Interlayers

Description: We demonstrate that the insertion of low-temperature (LT) AlGaN interlayers is effective in reducing mismatch-induced tensile stress and suppressing the formation of cracks during growth of AlGaN directly upon GaN epilayers., Stress evolution and relaxation is monitored using an in-situ optical stress sensor. The combination of in-situ and ex-situ. characterization techniques enables us to determine the degree of pseudomorphism in the interlayers. It is observed that the elastic tensile mismatch between AlGaN and GaN is mediated by the relaxation of interlayers; the use of interlayers offers tunability in the in-plane lattice parameters.
Date: September 13, 2000
Partner: UNT Libraries Government Documents Department

Disorder-driven nonequilibrium melting studied by electron diffraction, brillouis scattering, and molecular dynamics

Description: In the present paper, a brief overview of the electron diffraction, Brillouin scattering and molecular dynamics studies of radiation-induced amorphization of ordered intermetallic compounds is presented. In these studies, measured changes in the velocity of surface acoustic phonons, lattice constant, and the Bragg-Williams long-range order parameter induced by irradiation were compared with the results of computer simulations of defect-induced amorphization. The results indicate that progressive chemical disordering of the superlattice structure during irradiation is accompanied by an expansion of the lattice and a large change in sound velocity corresponding to a {approximately} 50% decrease in the average shear modulus. The onset of amorphization occurs when the average shear modulus of the crystalline compound becomes equal to that of the amorphous phase. This elastic softening criterion for the onset of amorphization and the dependence of the average shear modulus on the long-range-order parameter are in excellent agreement with molecular dynamics simulations. Both the experimental observations and computer simulations confirm the predictions of the generalized Lindemann melting criterion which stipulates that thermodynamic melting of a defective crystal occurs when the sum of the dynamic and static mean-square atomic displacements reaches a critical value identical to that for melting of the defect-free crystal. In this broader view of melting, the crystal-to-glass transformation is a disorder-driven nonequilibrium melting process occurring at temperatures below the Kauzmann isentropic glass-transition temperature.
Date: December 21, 1999
Creator: Okamoto, P. R.; Lam, N. Q. & Grimsditch, M.
Partner: UNT Libraries Government Documents Department

Surface dynamics during environmental degradation of crystal surfaces

Description: The Final Report describes results of further investigation of nature of vicinal sectorality in KDP crystals. Part I contains data of quantitative estimation of distinctions between the lattice parameters in the neighbor growth sectors and vicinal sectors of some rapidly grown KDP and DKDP crystals. It demonstrates opportunities of the plane wave X-ray topography for quantitative characterization of crystals grown in known conditions. Part II gives description of some mechanisms of striations formation in which the vicinal sectorality takes an important part.
Date: April 3, 1999
Creator: De Yoreo, J & Smolsky, I
Partner: UNT Libraries Government Documents Department

Solid hydrogen structure

Description: The J=0{minus}>2 Raman signal from solid J=0 D{sub 2} or H{sub 2} reveals HCP structure when deposited at a rate 0.1 {le} R({mu}/min) {le} 40 onto MgF{sub 2} at T{sub d}/T{sub tp} > 0.3, a mixture of HCP and FCC crystals at 0.2 < T{sub d}/T{sub tp} < 0.3 and possibly a randomly stacked close packed structure at T{sub d}/T{sub tp} < 0.2, where T{sub tp} is the triple point temperature. Non-HCP crystals transform to HCP continuously and irreversibly with increasing T. Finally, the crystal size decreases with decreasing T{sub d} and increasing R, from {approximately} 1 mm at T{sub d} {approximately} 0.8 T{sub tp} and R {approximately} 2 {mu}/min to {approximately} 1 {mu}m at 0.25 T{sub tp} and R {approximately} 40 {mu}/min.
Date: November 1, 1994
Creator: Collins, G.W.; Unites, W.G.; Mapoles, E.R.; Magnotta, F. & Bernat, T.P.
Partner: UNT Libraries Government Documents Department

Wafer Fusion for Integration of Semiconductor Materials and Devices

Description: We have developed a wafer fusion technology to achieve integration of semiconductor materials and heterostructures with widely disparate lattice parameters, electronic properties, and/or optical properties for novel devices not now possible on any one substrate. Using our simple fusion process which uses low temperature (400-600 C) anneals in inert N{sub 2} gas, we have extended the scope of this technology to examine hybrid integration of dissimilar device technologies. As a specific example, we demonstrate wafer bonding vertical cavity surface emitting lasers (VCSELs) to transparent AlGaAs and GaP substrates to fabricate bottom-emitting short wavelength VCSELs. As a baseline fabrication technology applicable to many semiconductor systems, wafer fusion will revolutionize the way we think about possible semiconductor devices, and enable novel device configurations not possible by epitaxial growth.
Date: May 1, 1999
Creator: Choquette, K.D.; Geib, K.M.; Hou, H.Q.; Allerman, A.A.; Kravitz, S.; Follstaedt, D.M. et al.
Partner: UNT Libraries Government Documents Department

Microstructures of GaN and In{sub x}Ga{sub 1-x}N films grown by MOCVD on free-standing GaN templates

Description: We summarize structural properties of thick HVPE GaN templates from the point of view of their application as substrates for growth of nitride layers. This is followed by the results of optical and structural studies, mostly transmission electron microscopy, of nitride layers grown by MOCVD on top of the HVPE substrates. The results indicate high structural quality of these layers with a low density of threading dislocations (in the range of 10{sup 6} cm{sup -2}). Convergent beam electron diffraction studies showed that the MOCVD GaN films have Ga-polarity, the same polarity as the HVPE GaN substrates. Structural studies of an InGaN layer grown on top of the MOCVD GaN film showed the presence of two layers, which differed in lattice parameter and composition. The upper layer, on the top of the structure had a c-lattice parameter about 2% larger than that of GaN and contained 10.3 {+-} 0.8% of In. Values measured for the thinner, intermediate layer adjacent to the GaN layer were about 2 .5 times lower.
Date: April 30, 2002
Creator: Jasinski, J.; Liliental-Weber, Z.; Huang, D.; Reshchikov, M.A.; Yun, F.; Morkoc, H. et al.
Partner: UNT Libraries Government Documents Department

Pd/Cu site interchange in UCu{sub 5-x}Pd{sub x}

Description: Although Pd/Cu site interchange in the non-Fermi liquid (NFL) material UCu{sub 4}Pd has been observed, the relationship between this disorder and the NFL behavior remains unclear. In order to better compare to the UCu{sub 5-x}Pd{sub x} phase diagram, they report results from Pd K edge x-ray absorption fine structure (XAFS) experiments on this series (x = 0.3-1.5) that determine the fraction of Pd atoms on the nominally Cu (16e) sites, s. They find that for these unannealed samples, s is at least 17% for all the samples measured, even for x &lt; 1.0, although it does climb monotonically beyond its minimum at x = 0.7. These data are compared to changes in the lattice parameter as a function of x.
Date: July 11, 2001
Creator: Booth, C.H.; Bauer, E.D.; Maple, M.B.; Chau, R. & Kwei, G.H.
Partner: UNT Libraries Government Documents Department


Description: EMMA is a 10 to 20 MeV electron ring designed to test our understanding of beam dynamics in a relativistic linear non-scaling fixed field alternating gradient accelerator (FFAG). This paper describes the design of the EMMA lattice. We begin with a summary of the experimental goals that impact the lattice design, and then outline what motivated the choice for the basic lattice parameters, such as the type of cells, the number of cells, and the RF frequency. We next list the different configurations that we wish to operate the machine in so as to accomplish our experimental goals. Finally, we enumerate the detailed lattice parameters, showing how these parameters result from the various lattice configurations.
Date: June 25, 2007
Partner: UNT Libraries Government Documents Department

MW-class 800 MeV/n H2+ SC-cyclotron for ADS application, design and study goals

Description: This paper addresses an attempt to start investigating the use of the Superconducting Ring Cyclotron (SRC) developed for DAE{delta}ALUS experiment for ADS application [1], focusing on the magnet design and its implication for lattice parameters and dynamic aperture performance.
Date: May 20, 2012
Creator: F., Meot; Calabretta, L.; Calanna, A.; Roser, T. & Weng, B.
Partner: UNT Libraries Government Documents Department

Reversible expansion of gallium-stabilized (delta)-plutonium

Description: It is shown that the transient expansion of plutonium-gallium alloys observed both in the lattice parameter as well as in the dimension of a sample held at ambient temperature can be explained by assuming incipient precipitation of Pu{sub 3}Ga. However, this ordered {zeta}-phase is also subject to radiation-induced disordering. As a result, the gallium-stabilized {delta}-phase, being metastable at ambient temperature, is driven towards thermodynamic equilibrium by radiation-enhanced diffusion of gallium and at the same time reverted back to its metastable state by radiation-induced disordering. A steady state is reached in which only a modest fraction of the gallium present is arranged in ordered {zeta}-phase regions.
Date: February 27, 2006
Creator: Wolfer, W G; Oudot, B & Baclet, N
Partner: UNT Libraries Government Documents Department

Reversible expansion of gallium-stabilized delta-plutonium

Description: The transient expansion of plutonium-gallium alloys observed both in the lattice parameter as well as in the dimension of a sample held at ambient temperature is explained by assuming incipient precipitation of Pu{sub 3}Ga. However, this ordered {zeta}{prime}-phase is also subject to radiation-induced disordering. As a result, the gallium-stabilized {delta}-phase, being metastable at ambient temperature, is both driven towards thermodynamic equilibrium by radiation-enhanced diffusion of gallium and at the same time pushed back to its metastable state by radiation-induced disordering. A steady state is reached in which only a modest fraction of the gallium present is tied up in the {zeta}{prime}-phase.
Date: January 26, 2006
Creator: Wolfer, W; Oudot, B & Baclet, N
Partner: UNT Libraries Government Documents Department

1Surface structure of cleaved (001) USB2 single crystal surface

Description: We have achieved what we believe to be the first atomic resolution STM images for a uranium compound USb2 taken at room temperature. The a, b, and c lattice parameters in the images confirm that the tetragonal USb2crystals cleave on the (00 I) basal plane as expected. Our calculations indicate a symmetric cut between Sb planes to be the most favorable cleavage plane and U atoms to be responsible for most ofthe density of states measured by STM. Since the spacing between Sb atoms and between U atoms is the same, STM topography only cannot unambiguously identify the surface atom species.
Date: January 1, 2009
Creator: Chen, Shao-ping
Partner: UNT Libraries Government Documents Department

Crystal structure, superconductivity and magnetism of the quasi-2D heavy fermion materials CeTIn[sub 5] (T = Co, Rh, Ir).

Description: The crystal structure of the recently discovered heavy-fermion (HF) superconductor CeCoIn{sub 5} (T{sub c} = 2.3 K) has been determined by high-resolution neutron powder diffraction. It is tetragonal (space group P4/mmm), with lattice parameters a = 4.61292(9) {angstrom} and c = 7.5513(2) {angstrom} at ambient conditions. Whereas CeCoIn{sub 5} is isostructural with the HF aniferromagnet CeRhIn{sub 5} and the HF superconductor CeIrIn{sub 5}, its cell constants and its only variable positional parameter, zIn2, differ significantly from the corresponding ones of CeRhIn{sub 5} and CeIrIn{sub 5}. As a result, the distortions of the cuboctahedron [CeIn{sub 3}], which is the key structural unit in all three materials, are different in CeCoIn{sub 5} from the ones in CeRhIn{sub 5} and CeIrIn{sub 5}. The compounds CeCoIn{sub 5} and CeIrIn{sub 5}, which contain the most distorted (in one or another way) [CeIn{sub 3}] cuboctahedra exhibit superconductivity at ambient pressure below 2.3 K and 0.4 K, respectively. On the other hand, CeRhIn{sub 5}, in which [CeIn{sub 3}] cuboctahedra are the less distorted, and the cubic HF CeIn{sub 3} are antiferromagnets at ambient pressure with T{sub N} = 3.8 K and 10 K respectively; they become superconductors under pressure of 16 kbar and 25 kbar with T{sub c} = 2.1 and 0.2 K respectively.
Date: January 1, 2001
Creator: Sarrao, John L.,; Pagliuso, P. J. (Pascoal J.); Moreno, N. O.; Thompson, J. D. (Joe David); Fisk, Zachary & Moshopoulou, E. G.
Partner: UNT Libraries Government Documents Department

Immiscibility in the NiFe2O4-NiCr2O4 Spinel Binary

Description: The solid solution behavior of the Ni(Fe{sub 1-n}Cr{sub n}){sub 2}O{sub 4} spinel binary is investigated in the temperature range 400-1200 C. Non-ideal solution behavior, as exhibited by non-linear changes in lattice parameter with changes in n, is observed in a series of single-phase solids air-cooled from 1200 C. Air-annealing for one year at 600 C resulted in partial phase separation in a spinel binary having n = 0.5. Spinel crystals grown from NiO, Fe{sub 2}O{sub 3} and Cr{sub 2}O{sub 3} reactants, mixed to give NiCrFeO{sub 4}, by Ostwald ripening in a molten salt solvent, exhibited single phase stability down to about 750 C (the estimated consolute solution temperature, T{sub cs}). A solvus exists below T{sub cs}. The solvus becomes increasingly asymmetric at lower temperatures and extrapolates to n values of 0.2 and 0.7 at 300 C. The extrapolated solvus is shown to be consistent with that predicted using a primitive regular solution model in which free energies of mixing are determined entirely from changes in configurational entropy at room temperature.
Date: August 13, 2004
Creator: Ziemniak, S
Partner: UNT Libraries Government Documents Department

Compound Formation in Ti-doping of Sodium Aluminum Hydride

Description: Renewed interest in hydrogen storage materials has resulted in the development of Ti-doped NaAlH{sub 4}. Different doping methods such as mechanical milling with powdered TiCl{sub 3}, or wet doping in solvents such as tetrahydrofuran (THF), yield enhanced kinetics. Still, the location and action of the Ti dopant is an open question. In order to address titanium substitution in the bulk, we present lattice parameter measurements of crushed single crystals which were exposed to Ti during growth. Rietveld refinements suggest that the titanium does not appear to enter the bulk by this method of exposure. Therefore, reaction products are investigated by x-ray diffraction of completely reacted samples of solvent-mixed versus mechanically milled 3 NaAlH{sub 4}+TiCl{sub 3}. Formation of TiAl{sub 3} is observed in mechanically milled materials, but not solution mixed samples, where bonding to THF likely stabilizes Ti-based nanoclusters. The Ti in these clusters is activated by mechanical milling.
Date: November 24, 2003
Creator: Maxwell, R; Stumpf, R; Herberg, J; Majzoub, E & Spangler, S
Partner: UNT Libraries Government Documents Department