# Showing papers in "Journal of Physics and Chemistry of Solids in 1971"

••

General Electric

^{1}TL;DR: In this paper, optical properties of non-crystalline Si, SiO and SiO x (x = 1·5) were analyzed for the energy region 1 to 26 eV and the results indicated that amorphous substances of all intermediate compositions between Si and Si O 2 can be formed and that these materials are not simple mixtures of Si and O 2 but rather the two atom species are blended on an atomic scale.

Abstract: Optical data for non-crystalline Si, SiO, SiO x ( x =1·5) and SiO 2 are presented and analyzed for the energy region 1 to 26 eV. The results indicate that amorphous substances of all intermediate compositions between Si and SiO 2 can be formed and that these materials are not simple mixtures of Si and SiO 2 but rather the two atom species are blended on an atomic scale. More specifically, the Si bonding is tetrahedral (perhaps highly distorted) and of the type Si−(Si v O 4− v ) where the distribution of atoms is essentially statistical. Further it is found that the optical properties of these layers are determined by the presence and grouping of Si−O and Si−Si bonds and that clusters of like bonds of the dimension of a Si−(Si 4 ) or Si−(O 4 ) tetrahedra have optical properties comparable to those exhibited by amorphous silicon or quartz, respectively, ‘in bulk’.

503 citations

••

TL;DR: In this article, the atomic positions of Ferroelectric BiFeO 3 were determined using both X-ray single crystal and neutron powder diffraction using the multiple film technique using a Weissenberg camera.

Abstract: Ferroelectric BiFeO 3 is rhombohedral with lattice constants, a H =5·5876, c H =13·867 at room temperature. The space group is R3c with two formula units in the unit cell. The atomic positions have been determined employing both X-ray single crystal and neutron powder diffraction. The oxygen atomic positions could be determined only by neutron diffraction and are interpreted as a rotation of rigid octahedra around the trigonal axis by an angle ω=11°40' from the ideal perovskite positions. The X-ray intensities were visually estimated by the multiple film technique using a Weissenberg camera. The cation positions, refined by least-squares, are in good agreement with those determined from neutron powder diffraction analysis. The final results are given with a reliability factor R =0·02 for neutron powder data and R =0·09 for X-ray single crystal data. With respect to the ideal perovskite structure, the cations are shifted along the trigonal axis in accord with the observed dielectric properties.

450 citations

••

TL;DR: In this article, the authors have made saturation magnetization, X-ray and neutron diffraction measurements on alloys at the compositions Co 2 TiAl, Co 2 Al, Al, Si, Ga, Ge or Sn.

Abstract: Saturation magnetization, X-ray and neutron diffraction measurements have been made on alloys at the compositions Co 2 TiAl, Co 2 TiSi, Co 2 TiGa, Co 2 TiGe, Co 2 TiSn, Co 2 TiSb and CoTiSb. The alloys containing Ga and Sn have fully ordered Heusler, L 2 1 , chemical structures. Co 2 TiAl is similarly ordered but with some partial Ti-Al disorder. The alloys Co 2 TiSi and Co 2 TiGe each contain a secondary phase in addition to the primary Heusler phase. Co 2 TiSb contains the two phases Co 122 TiSb and Co and CoTiSb is ordered in the Cl b structure. The alloys containing the group IIIB or IVB elements Al, Si, Ga, Ge or Sn are ferromagnetically ordered, with the magnetic moments associated with the ordered Co sites. The two alloys containing the group VB element Sb have vacant chemically ordered ‘Co’ sites but are paramagnetic.

321 citations

••

TL;DR: In this article, the drift velocity of electrons and holes in high purity silicon has been measured, with the time of flight technique, as a function of electric field (0·1−50 KV/cm) at several temperatures between 77 and 300°K.

Abstract: The drift velocity of electrons and holes in high purity silicon has been measured, with the time of flight technique, as a function of electric field (0·1–50 KV/cm) at several temperatures between 77 and 300°K. By applying the electric field parallel to the and crystallographic directions, an evident longitudinal anisotropy effect has been found for the drift velocity of electrons and also, for the first time, for the drift velocity of holes. At high values of the electric field a saturation drift has been found for the electrons at the temperatures considered in these experiments. On the contrary, no saturation has been attained for holes, even at the highest applied electric fields. The ohmic mobility has been measured between 77 and 300°K for electrons and between 160 and 300°K for holes. When a comparison is possible, our results are in good agreement with other experimental results found in the literature. A qualitative theoretical interpretation of the effects observed is given.

248 citations

••

TL;DR: In this paper, the authors report measurements on a number of halides for the first time, and their results are in fair agreement with prior measurements by Bridgman on some of the halides where their data overlap.

Abstract: New compressibility data on 27 halides are reported. Compressibilities were determined by observations of piston displacement in the static high pressure apparatus to 45 kbar. We report measurements on a number of halides for the first time. Our results are in fair agreement with prior measurements by Bridgman on some of the halides where our data overlap. Our agreement with reductions of shock data is in many cases extremely poor because the reductions of the shock data did not take into account the phase transitions found in the halides.

213 citations

••

Bell Labs

^{1}TL;DR: In this paper, the authors obtained information on the transition with increasing x between the normal spinel FeCr2O4 with conical spiral spin structure and the inverse spinel Fex3+ O4 with a Neel antiferromagnetic structure and showed that the Verwey transition and the attendant lattice distortion are suppressed by the presence of Cr3+ in the B site.

Abstract: Magnetic and Mossbauer effect measurements have been used to characterize the system Fe2+Cr2−xFex3+O4,. We obtain information on the transition with increasing x between the normal spinel FeCr2O4 with conical spiral spin structure and the inverse spinel Fe3O4 with a Neel antiferromagnetic structure. Fe3+ substituted for Cr3+ in FeCr2O4 goes into the B site up to x = 0·68 and has a spin canted relative to the A site moment. In the region 0·68 < x < 1·38 the Fe3+ goes into the A site displacing corresponding amounts of Fe2+ to the B site and leading to the beginning of charge hopping. In this region Curie points and saturation moments rise rapidly as the iron canting angle decreases because of Zener double exchange and A−B exchange between Fe3+. From x = 1·38 to magnetite the B site tends to have equal amounts of Fe2+ and Fe3+ presumably because of pairwise charge hopping. The Verwey transition and the attendant lattice distortion are suppressed by the presence of Cr3+ in the B site.

205 citations

••

TL;DR: In this article, it was shown that the electrical transport properties of all the three series of rare earth compounds are essentially controlled by the d -electrons of the transition elements which show localized behavior.

Abstract: Rare earth ortho-chromites, -manganites and -ferrites are p -type semiconductors with conductivities in the range 10 −4 –10 −1 ohm −1 cm −1 . The conductivity in each series of perovskites decreases with the increasing atomic number of the rare earth. The ionic contribution to conductivity is small in all the three series of solids. None of these solids exhibits intrinsic behavior up to ∼1000°C. The conductivity behaviors of these rare earth compounds reflect the known crystallographic, dielectric and magnetic transitions in these materials. Seebeck coefficients in these compounds are large, typical of narrow-band materials; the Seebeck coefficients show marked changes at temperatures where magnetic and dielectric transitions occur. The electrical transport properties of all the three series of rare earth compounds are essentially controlled by the d -electrons of the transition elements which show localized behavior. This conclusion is in agreement with the results from optical spectra and the predictions of Goodenough. In all these compounds small polarons seem to be responsible for the conduction.

201 citations

••

TL;DR: In this article, the strength of the absorption bands due to carbon (16·5 μm) and oxygen (9 μm), from the measurements, heats of solution of the two impurities of 53±6 and 38±4 kcal/mole have been deduced respectively.

Abstract: Single crystals of silicon grown by the Czochralski technique have been annealed at various temperatures in the range 600–1350°C. I.R. measurements have been made of the strength of the absorption bands due to carbon (16·5 μm) and oxygen (9 μm). From the measurements, heats of solution of the two impurities of 53±6 and 38±4 kcal/mole have been deduced respectively. When carbon precipitates broad absorption is produced in the region of 12 μm which is attributed to silicon carbide particles. The strength of this absorption is found to be consistent with the estimated loss of carbon from solution. Precipitation of silicon carbide was not observed in oxygen free crystals.

180 citations

••

TL;DR: In this article, the effects of large anisotropic stresses on the compression of solids are examined with shock experiments on sapphire in three crystallographic directions at stress values within the elastic range to stress values up to twice the HEL.

Abstract: Under shock compression, sapphire (crystalline Al 2 O 3 ) exhibits Hugoniot elastic limit, HEL, values of from 120–210 kbar; the largest values observed for any solid. Because of the large HEL, the stress configuration of the shock-loaded sample is highly anisotropic. A critical examination of the effects of large anisotropic stresses on the compression of solids is accomplished with shock experiments on sapphire in three crystallographic directions at stress values within the elastic range to stress values up to twice the HEL. The maximum shear strengths observed range from 4·9 to 5·6 per cent of the shear modulus. When stresses in excess of the HEL are achieved, sapphire is observed to undergo a catastropic loss of shear strength; in contrast to the yield behavior observed for metals. The results are compared to observations on high density polycrystalline Al 2 O 3 and other solids with large HEL values. The behavior of sapphire is found to be analogous to that previously observed for crystalline SiO 2 . Data in the elastic range show a third order elastic constant C 111 ≈ C 333 =−(3·6±0·4)×10 13 dyne/cm 2 . Analytical methods are developed to determine the shear stress offset independent of the hydrostatic data.

164 citations

••

TL;DR: In this paper, the static electrical conductivity tensor of an electron gas in the presence of a magnetic field has been derived in the case of scattering by spinless (point) impurities, using a Green's function method.

Abstract: The static electrical conductivity tensor of an electron gas in the presence of a magnetic field has been derived in the case of scattering by spinless (point) impurities, using a Green's function method. We show the existence of a correction to the Hall effect ∂ xy due to the quantization of orbital motion, exactly analogous to the well known de Haas-Shubnikov oscillations of ∂ xx . We first present the calculation of ϖ xy (and ϖ xx as well) up to first order in impurity concentration and give numerical estimates. We then evaluate the correction in the case of high concentrations and show a discrepancy with the Drude-Zener formulae.

140 citations

••

TL;DR: In this paper, the authors derived an activation energy of 550 ± 50°K from the temperature dependence of hydrogen diffusion in the α and β phases of vanadium hydride by neutron inelastic scattering.

Abstract: The diffusion of hydrogen in the α and β phases of vanadium hydride has been studied by neutron inelastic scattering. Samples of VH 0.198 , VH 0.403 and VH 0.570 were used to determine the temperature and concentration dependence of the diffusion in the α -phase. An activation energy of 550 ± 50°K was derived from the temperature dependence of hydrogen diffusion in the VH 0.198 sample. The theoretical neutron scattering cross-section for jump diffusion among the octahedral and tetrahedral interstitial sites of body-centered cubic lattices has been derived and compared to the experimental results. No conclusions about the site occupation in the α -phase can be drawn from these results. The data for the β -phase of VH 0.570 indicate an abrupt decrease in the diffusion rate at the α → β transition.

••

TL;DR: In this article, singlecrystal platelets of BeTe have been obtained in an autoclave synthesis, and crystalline powders of BeSe and BeS have been prepared by reaction of Be metal with H 2 Se and H 2 S, respectively.

Abstract: Single-crystal platelets of BeTe have been obtained in an autoclave synthesis, and crystalline powders of BeSe and BeS have been prepared by reaction of Be metal with H 2 Se and H 2 S, respectively. Optical absorption measurements on the BeTe platelets gave a bandgap of about 2.7 eV at room temperature. BeTe prepared without intentional doping is a p -type semiconductor. Optical absorption measurements at room temperature on cold-pressed powder specimens indicated that the bandgap of BeSe is in the range of 4–4.5 eV, and that the bandgap of BeS exceeds 5.5 eV.

••

TL;DR: In this article, electrical conductivity and weight change measurements were made on single crystal and polycrystalline nickel oxide as a function of temperature from 900 to 1400°C and oxygen partial pressure from 1 atm to 10 −4 atm.

Abstract: Electrical conductivity and weight change measurements were made on single crystal and polycrystalline nickel oxide as a function of temperature from 900 to 1400°C and oxygen partial pressure from 1 atm to 10 −4 atm. Samples of several different purities were measured. Both conductivity and weight change were proportional to the one-sixth power of the oxygen partial pressure, and had an activation energy of 18·6±1·0 kcal/mole for the highest purity samples. Impurities increased both the activation energy and the pressure dependence for conduction. The integrated carrier density of states, N v , would fit a T 3/2 temperature dependence supporting a band model. The hole mobility was 0·53 cm 2 / V -sec and was almost temperature independent. The magnitude and temperature dependence of the mobility would not easily fit either a band or a hopping transport model.

••

TL;DR: In this paper, a study of point defects and non-stoichiometry in GaAs is presented, and the minimum practical deviation from stoichiometry and the width of the existence region of solid GaAs are calculated as a function of temperature.

Abstract: A study of point defects and nonstoichiometry in GaAs is presented. Standard methods of chemical thermodynamics are used to derive expressions for the equilibrium concentrations, as a function of temperature and arsenic pressure, of the following defects; arsenic monovacancies, gallium monovacancies, gallium divacancies and appropriately charged versions of these three. This choice of defects is made on the strength of the evidence of the published experimental results relating directly to point defects in GaAs. Values for the required equilibrium constants are obtained by a combination of a priori estimates and matching with this experimental data at certain points. Consistent agreement with the experimental data is obtained, indicating for example that defect concentration in the region 10 18 –10 19 cm −3 in melt grown material are not unreasonable. The set of equilibrium constants on which the calculations are based must, however, be regarded as provisional until more extensive data is available for comparison. Using these equilibrium constants the minimum practical deviation from stoichiometry and the width of the existence region of solid GaAs are calculated as a function of temperature.

••

TL;DR: In this article, the single crystal elastic constants of RbCl, RbBr and RbI at 25°C were measured by using the ultrasonic pulse superposition technique up to pressures of 45, 37 and 3 kbar, respectively.

Abstract: The single crystal elastic constants of RbCl, RbBr and RbI at 25°C have been measured by using the ultrasonic pulse superposition technique up to pressures of 45, 37 and 3 kbar, respectively A small, but statistically significant deviation from a linear pressure dependence was found The first and second pressure derivatives were determined and were found to agree quite well with theoretical values which are calculated from an interatomic potential consisting of Coulomb, van der Waals and Born-Mayer type repulsive forces between first and second nearest neighbors The isothermal equation of state is calculated from the Murnaghan approximation and compared with piezometric data of Bridgman The Grueneisen parameter and its first pressure derivative were calculated from the elastic data in the approximation of the anisotropic elastic continuum model and agree moderately well with the corresponding thermal values

••

TL;DR: In this article, the authors presented heat capacity data for the five LnAl 2 compounds, where Ln=La, Ce, Pr, Nd and Gd, in approximate agreement with R In (2 J + 1) except for GdAl 2 in which the discrepancy was about 25 percent.

Abstract: Heat capacity data are presented for the five LnAl 2 compounds, where Ln=La, Ce, Pr, Nd and Gd. The temperature range covered was 8 to 300°K. The data for non-magnetic LaAl 2 were taken to represent the lattice and electronic contribution to the heat capacities of the four remaining compounds. The difference between the heat capacity of the magnetic compounds and LaAl 2 gave the magnetic contribution to the heat capacity from which the magnetic entropy could be derived. Results obtained were in approximate agreement with R In (2 J +1) except for GdAl 2 in which the discrepancy was about 25 per cent. The data for CeAl 2 gave no indication of magnetic ordering; however, an upturn in its C p vs. T curve below 12°K was observed. λ-type thermal anomalies were observed for PrAl 2 and NdAl 2 , peaking at 31·8 and 77·2°K, respectively. These were ascribed to the breakup of the ferromagnetic phase. Curie temperatures inferred from the C p data were in good agreement with those obtained from magnetic studies. No λ-type anomaly was observed for GdAl 2 ; instead its magnetic heat capacity was spread over a very wide range of temperature suggesting that destruction of magnetic order in this case is a weakly cooperative process.

••

General Electric

^{1}TL;DR: In this article, an ordered sutructure in which periodic boundary conditions are imposed to saturte the peripheral bonds is presented. But the energy gap of ∼13 eV as compared to the experimental value of ∼11 eV.

Abstract: Molecular orbital calculations based on the Extended Huckel Theory have been performed for clusters of two and eight SiO 2 molecules. We report here the results for an ordered sutructure in which periodic boundary conditions are imposed to saturte the peripheral bonds which otherwise cause extraneous localized states to appear. We find an energy gap of ∼13 eV as compared to the experimental value of ∼11 eV. The removal of an oxygen results in several levels in the energy gap which move upon displacement of the two neighboring silicons. The levels appear to account for the observed ultraviolet optical absorption band and are also a possible origin for the positively charged slow surface states present near a SiO 2 -Si interface. The addition of either atomic or molecular hydrogen to a perfect crystal is predicted to yield preferentially a double hydrogen-single oxygen center. If ambient oxygen atoms are present, a two hydroxl group center is then most favorable.

••

TL;DR: In this article, the liquidus and solidus lines in the In-rich region of the InAs-GaAs system have been established by measuring the amount of InAs which dissolved in In-Ga solutions and the solidus data from the composition of crystals grown from In-As melts by liquid epitaxy techniques.

Abstract: The liquidus and solidus lines in the In-rich region of the InGaAs system have been established. The liquidus data were obtained by measuring the amount of InAs which dissolved in InGa solutions and the solidus data from the composition of crystals grown from InGaAs melts by liquid epitaxy techniques. A phase diagram which is in excellent agreement with these experimental data has been obtained. The calculations are based on a quasi-regular solution model for the three binary liquids as well as for the InAs-GaAs pseudobinary solid. The thermodynamic constants were obtained from the literature and the interaction parameters were selected semi-empirically to obtain a best fit. Undoped In x Ga 1- x As layers were grown on 〈111〉 GaAs and 〈111〉 InAs substrates using an improved vertical liquid epitaxial system. Various layer characteristics were examined and growth criteria which yield satisfactory single crystals were determined. Due to the high lattice mismatch between InAs and GaAs and the serious plastic deformation of InAs above 700°C, satisfactory layers were obtained only in the ranges x x > 0·7. Based gap energy as a function of mixed crystal composition was measured at 77 and 300°K using photoluminescence and X-ray diffraction techniques.

••

TL;DR: In this article, the soft X-ray CrL III, CrK and OK emission and absorption spectra are combined and used to construct empirically a complete molecular orbital diagram for simple chromium-oxygen compounds.

Abstract: A new experimental technique is described whereby the soft X-ray CrL III , CrK and OK emission and absorption spectra are combined and used to construct empirically a complete molecular orbital diagram for simple chromium-oxygen compounds. All spectral components are assigned specific transitions associated with bonding, antibonding and nonbonding molecular orbitals. In Cr 2 O 3 the spectra indicate that the three outermost electrons have t 2g symmetry and are involved in two distinct bonding mechanisms. One of these electrons is localized in a metal-metal covalent bond and the other two are associated with the Cr−O π bond. The results do not support the narrow d -band model which has been proposed for transition metal oxides. For CrO 4 −2 the deduced MO structure does not agree well with previous calculations and a new interpretation is suggested for the optical absorption spectrum. Contrary to previous assumptions, it is concluded that the highest filled orbital in CrO 4 −2 is 3 t 2 instead of t 1 . Relationships between the X-ray spectra and various solid state phenomena such as coordination symmetry, bonding distances, valence state, and bonding character are discussed. It is concluded that the X-ray band spectra from compounds are best interpreted on the basis of molecular orbital theory.

••

TL;DR: In this article, the electronic spin-spin relaxation time, T2, was calculated for Heisenberg ferromagnets in the region immediately above the Curie point, which pertains to zero field, predicts that 1/T2 diverges as χT3/4 where χ T is the isothermal susceptibility.

Abstract: The electronic spin-spin relaxation time, T2, is calculated for Heisenberg ferromagnets in the region immediately above the Curie point. The calculation, which pertains to zero field, predicts that 1/T2 diverges as χT3/4 where χT is the isothermal susceptibility. The relative efficiencies of the dipolar spin-spin relaxation and exchange induced spin diffusion are compared. For systems with parameters comparable to EuO spin-spin relaxation dominates for temperatures such that the ratio of the correlation length to lattice parameter is greater than two. In systems having the parameters of iron dipolar effects are much less important.

••

TL;DR: In this paper, the authors compared the theory of electron transport phenomena in the relaxation time approximation, treating consistently the nonparabolic band structure of InSb, with available experimental data for various effects.

Abstract: Theory of electron transport phenomena in the relaxation time approximation, treating consistently the nonparabolic band structure of InSb, has been compared with available experimental data for various effects. Mixing of p -like components into the conduction band wave function in the nonparabolic region of energies has been incorporated into the theory of electron scattering by charged centers, polar optical phonons and acoustic phonons. In the limit of high degeneracy the analytical formulas for the mixed scattering mode have been obtained. Numerical values for the Hall mobility at 77, 300, 500 and 773°K, the thermoelectric power, and the longitudinal and transverse Nernst-Ettingshausen effects at room temperature for electron concentrations up to 1×10 19 cm −3 have been computed for mixed scattering and compared with experimental data of various authors to obtain the best over-all fit, treating a controversial value of the deformation potential as an adjustable parameter. It has been found that all the effects, together with the free-carrier optical absorption, can be satisfactorily described with the deformation potential constant of 14,6 eV. Using this value it has been demonstrated that at room temperature the electron scattering at high electron concentrations is dominated by charged impurity and acoustic phonon modes, whereas at lower concentrations all three scattering mechanisms should be taken into account. The presented procedure can be readily applied to other III–V compounds.

••

TL;DR: In this article, the dispersion energy between two spheres A and B is calculated by expanding the field fluctuations in these spheres in terms of spherical harmonics, which yields an infinite Taylor series in the reduced radii.

Abstract: The dispersion energy between two spheres A and B is calculated by expanding the field fluctuations in these spheres in terms of spherical harmonics. This yields an infinite Taylor series in the reduced radii (=radius/distance of centers). The exact dispersion energy is compared with its upper and lower limits, which correspond to a maximum screening by half-spaces (Lifshitz approach) and to no screening at all (Hamaker approach). The two limits are split into identical frequency and different geometric factors and differ by less than 1 per cent at separations characteristic of adhesion. In a second step we calculate the dispersion energy between spheres whose surface is covered with an adsorbed layer. The effect of such layers on the dispersion energy depends primarily on the cross-sections of the spheres at twice the separation d of the interacting partner. An exact treatment of adsorbed layers shows that the dispersion energy is no longer factorized into a frequency and a geometric term and hence depends sensitively on the frequency dependence of the dielectric constants involved. This affects the accuracy of computed results, in particular for separations smaller than and equal to the thickness of the adsorbed layer; for large separations the dielectric properties of the bulk material are predominant.

••

TL;DR: In this paper, at high temperatures and high pressures outgassed sodalite and cancrinite hydrates were found to take up substantial volumes of rare gases such as Ne, Ar and Kr.

Abstract: At high temperatures and high pressures outgassed sodalite and cancrinite hydrates were found to take up substantial volumes of rare gases such as Ne, Ar and Kr. These gases were trapped within the crystals when the temperature was lowered. Diffusion coefficients, activation energies and heats of trapping have been estimated, and the equilibria investigated theoretically.

••

TL;DR: In this paper, the optical constants of thin Ag films are determined from measurements of both their transmittance and reflectance, in terms of intra and interband transitions, in relation with the film crystallographic structure.

Abstract: The optical constants of thin Ag films are determined from measurements of both their transmittance and reflectance. The complex dielectric constant is discussed in terms of intra and interband transitions, in relation with the film crystallographic structure. The optical mass of the conduction electrons is found to be very low, 0·87; their optical relaxation time to depend strongly on the film structure and to be smaller than their electrical relaxation time. The absorption due to interband transitions is considered. A supplementary absorption band related to film imperfections is observed in some cases. Plasma oscillations are also studied in relation with the film quality.

••

TL;DR: In this article, a model is proposed to account for the metallurgical and physical properties of f.c. Fe−Ni alloys, including the Invar anomalies around 34 at.% Ni.

Abstract: A model is proposed to account for the metallurgical and physical properties of f.c.c. Fe−Ni alloys, including the Invar anomalies around 34 at.% Ni. Microscopic inhomogeneity due to short range atomic ordering on the basis of the FeNi 3 compound is assumed to be a dominant feature of the alloy system. Regions of both f.c.c. Fe and FeNi 3 are present in alloys in the invar region. The electronic state of only the Fe atoms, and hence their volume, magnetic moment, and sensitivity to external variables, is further assumed to be a function of the number of Ni neareast neighbours, which in turn depends on such variables as cold work. With these assumptions, the properties of the annealed alloys above the Curie point are developed, the phase diagram and metastability of the alloys is explained and the Invar anomalies below the Curie point are accounted for. Furthermore, the model explains the unusual low temperature properties of these alloys as due to exchange coupling between ferromagnetic FeNi 3 regions and antiferromagnetic Fe regions. An extension of the model is made to other binary alloy systems that exhibit Invar-type anomalies.

••

TL;DR: In this article, a mathematical model which employs a Green's function technique in the mass defect approximation is utilized to calculate the lattice dynamics of the imperfect crystal, making direct use of the eigenfrequencies and eigenamplitudes of vibration of the phonons in the perfect lattice calculated by the modified rigid ion model introduced earlier.

Abstract: A mathematical model which employs a Green's function technique in the mass defect approximation is utilized to calculate the lattice dynamics of the imperfect crystal. This model makes direct use of the eigenfrequencies and eigenamplitudes of vibration of the phonons in the perfect lattice calculated by the modified rigid ion model introduced earlier. The phonon density of states is calculated for a number of II–VI and III–V type compounds that crystallize in the zinc blende structure. Next the variation of local mode frequency as a function of the defect mass is obtained for these crystals. The existence of a new type of localized gap mode, which corresponds to the replacement of the heavy host atom by a yet heavier substitutional atom, is predicted. The results are found to compare favorably with available experimental data on local mode frequencies obtained by infrared, Raman, and other optical measurements.

••

TL;DR: In this article, the internal friction and Young's modulus of a hydrogen doped niobium single crystal were measured in the temperature range 20 to 370°K and the activation energy of the peak was found to be about 0·27 eV.

Abstract: The internal friction and Young's modulus of a hydrogen doped niobium single crystal were measured in the temperature range 20 to 370°K. Nucleation of niobium hydrides is indicated by the growth of an internal friction maximum. The height of the internal friction maximum is proportional to the hydrogen concentration and it shifts progressively to higher temperatures with increasing hydrogen concentration. The activation energy of the peak was found to be about 0·27 eV. From the experimental correlation between temperature and hydrogen concentration at which the peak forms it is possible to deduce the α/α+β solvus. The solvus yields a heat of solution of about 0·12 eV, in agreement with other measurements. Deuterium doped niobium did not yield significantly different results from the ones obtained with hydrogen.

••

TL;DR: In this article, the authors expose des proprietes cristallographiques et magnetiques de Mn3O4, on montre que the structure magnetique a 4,2oK determinee par diffraction de neutrons est composee de 8 sous-reseaux repartis sur une maille double de la maille chimique.

Abstract: Resume Apres un expose des proprietes cristallographiques et magnetiques de Mn3O4, on montre que la structure magnetique a 4,2oK determinee par diffraction de neutrons est composee de 8 sous-reseaux repartis sur une maille double de la maille chimique. L'etude de l'evolution thermique du motif a permis de mettre en evidence un point de transition a 33°K correspondant a un brusque rearrangement des moments dans la maille.

••

TL;DR: Pseudopotential band structures for GaN and AlN are calculated non-empirically by means of form factors synthesized from those of C, Ge and Ga in the case of GaN as discussed by the authors.

Abstract: Pseudopotential band structures for GaN and AlN are calculated non-empirically by means of form factors synthesized from those of C, Ge and Ga in the case of GaN and from Sic and Al in the case of AlN. Good agreement with recent optical experiments is obtained for various gap energies.

••

TL;DR: In this article, the absorption and reflectance spectra of several simple silicate glasses of high purity were measured in the region extending from 5 to 14 eV. The data were discussed qualitatively in terms of both molecular orbital and valence bond theories, taking into consideration the increased ionic character of the Si-O bond in the alkali silicate glass.

Abstract: Optical absorption and reflectance spectra of quartz, alkali doped fused silica, and several simple silicate glasses of high purity were measured in the region extending from 5 to 14 eV. The doping of lithium, sodium and potassium into SiO 2 was observed to introduce optical absorption whose effect was to shift the u.v. edge of the glass to lower energies by several eV. This absorption accounts for the poor transmission of high purity silicate glasses in the far u.v. relative to SiO 2 . The simultaneous introduction of aluminum with the alkali was effective in eliminating most of this absorption. These observations were consistent with the hypothesis that the breakup of the Si-O-Si bonding network is related to the increased u.v. absorption of silicate glasses. At higher energies beyond the u.v. edge, several strong reflectance bands were observed and these have been tentatively assigned to exciton and band-to-band transitions. The data are discussed qualitatively in terms of both molecular orbital and valence bond theories, taking into consideration the increased ionic character of the Si-O bond in the alkali silicate glasses.