Showing papers by "Solid State Physics Laboratory published in 1972"

Diffraction effects associated with shear structures and related structures

Abstract: A theoretical analysis is made to explain diffraction effects associated with two-dimensional crystallographic shear. The shortcomings of the existing theory are removed taking into account the perturbation of the crystal potential in the neighbourhood of the block boundaries. The theory is illustrated using optical simulation. A more general structure model is proposed to explain the so called “spacing anomalies”. The model is applied to the structure of defect manganese silicides MnSi ≈ 1.75. Eine theoretische Analyse zur Erklarung der Beugungseffekte, die mit zweidimensionaler Kristallscherung verknupft sind, wird durchgefuhrt. Unzulanglichkeiten der existierenden Theorie werden durch Berucksichtigung der Storung des Kristallpotentials in der Nahe der Blockgrenzen beseitigt. Die Theorie wird mittels optischer Simulierung illustriert. Ein allgemeineres Strukturmodell wird vorgeschlagen, um sogenannte „Abstandsanomalien” zu erklaren. Das Modell wird auf die Struktur von nichtstochiometrischem Mangansilizid, MnSi≈1,75, angewendet.

An Investigation of the Ground State of Cr$^{2+}$ in MgO Based on Thermal Conductivity Measurements

Abstract: Measurements of the thermal conductivity of MgO doped with chromium in the temperature range 0.4 to 300 K indicate the existence of resonant scattering by Cr 2+ ions at 5.2 and 21 cm -1 . These measurements confirm that the ion cannot be described by simple crystal field theory or by static Jahn-Teller theory, but they are consistent with the dynamic Jahn-Teller theory of Fletcher & Stevens (1969) with the parameters 8 (tunnelling frequency) = 16 cm -1 and D (spin-orbit splitting) = 4 cm -1 . It is not possible to determine the sign of the anharmonic term, B , from the measurements. These parameters differ significantly from those determined by Fletcher & Stevens and those determined by Ham (1971) from acoustic paramagnetic resonance data for Cr 2+ in MgO. The data also appear to confirm earlier indications that strong coupled mode effects are present. The scattering in a Ni-doped specimen was found to be largely due to the difference in mass between the Ni and Mg ions, whereas that in Fe-doped specimens was largely due to a process resonant at ca . 100 cm -1 .

Optical and EPR investigations of colour centres in calciumchlorapatite

Abstract: Optical absorption measurements in combination with an EPR investigation of colour centres produced by X-rays in synthetic chlorapatite single crystals are presented. Models for both electron-excess and hole-excess centres are proposed and theoretical calculations of some of the hole-excess centres have been carried out. It turns out that the colour centres produced at temperatures higher than 77°K can be associated with oxygen impurities. Subtractive coloration at 5°K gives rise to the formation of F-centres. Es wurden durch Rontgenstrahlen erzeugte Farbzentren in synthetischen Einkristallen von Chlorapatit mittels kombinierter optischer Absorptionsmessungen und EPR untersucht. Sowohl fur Elektronenuberschuszentren wie fur Locheruberschuszentren werden Modelle aufgestellt und fur den zweiten Fall durchgerechnet. Es stellt sich heraus, das die bei Temperaturen uber 77°K erzeugten Farbzentren mit Sauerstoffstorstellen zusammenhangen. Subtraktive Farbung bei 5°K gibt Anlas zur Bildung von F-Zentren.

ESR study of copper doped NaCl

Abstract: The ESR spectra of NaCl: Cu systems at LNT and RT are studied. In one of the samples a new paramagnetic centre with [100] tetragonal symmetry was found. The principal values of the g tensor are: g∥ = 2.298 ± 4 × 10−3, g∥ = 2.058 ± 4 × 10−3 In a second type of samples the ESR spectrum of Cu2+ was observed. The g and hyperfine tensors have axial symmetry along a [100] axis. From the parameters of the spin Hamiltonian and from the results of optical spectroscopy the value of the spin-orbit coupling constant of the copper ion was obtained to λ = (−847 ± 20) cm−l. From this result it is supposed that covalency effects between Cu2+ and the chlorine environment in the NaCl lattice are not important when the crystals are not irradiated. Es wurde das ESR-Spektrum des Systems NaCl: Cu bei Zimmertemperatur und bei der Temperatur des flussigen Stickstoffes untersucht. In einer der Proben wurde ein neues paramagnetisches Zentrum mit S = 1/2 und tetragonaler [100]-Symmetrie gefunden. Die Hauptwerte des g-Tensors sind: g ∥ = 2,298 ± 4 × 10−3g = 2,058 ± 4 × 10−3 In einer zweiten Probe wurde das ESR-Spektrum von Cuz+ beobachtet. Der Tensor g und der Hyperfeinstrukturtensor haben ebenfalls axiale Symmetrie langs der [100]-Achse. Aus den Parametern des Spin-Hamilton-Operators und optischen Daten wurde die Kon-stante der Spin-Bahn-Kopplung des Kupfer-Ions λ = (−847 ± 20) cm−l bestimmt. Dieses Ergebnis last schlieben, das Kovalenzeffekte zwischen Cuz+ und den benachbarten Chlor-Ionen fuir unbestrahltes NaCl unbedeutend sind.

Investigation of thermal oxide films of silicon by infrared absorption

Abstract: The infrared absorption spectra of thermally grown silicon dioxide (SiO2) films of different thicknesses grown on silicon substrates has been investigated and compared with the infrared spectra of crystalline quartz, α-cristobalite, and fused quartz, in the wave-number range 400 to 1400 cm−1. Attempt has been made to infer the structure of the SiO2 along the growth direction on the basis of the results obtained. Infrarot-Absorptionspektren von thermisch auf Siliziumsubstraten aufgewachsenen Siliziumdioxid(SiO2)-Schichten unterschiedlicher Dicke wurden untersucht und mit Infrarotspektren von kristallinem Quarz, α-Kristobalit und geschmolzenem Quarz im Bereich 400 bis 1400 cm−1 verglichen. Es wird der Versuch gemacht, auf der Grundlage der erhaltenen Ergebnisse auf die Struktur von SiO2 langs der Wachstumsrichtung zu schliesen.

Effect of water vapour on thermoelectret behaviour

Abstract: A detailed study has bean made of the shellac wax and polyvinyl chloride (SR11) thermoelectret under different humidity conditions, ranging from 40% to 97% R.H. After the polarization of the samples, the surface charges have been measured with the induction method and the depolarization current by « thermally stimulated discharge current » technique (TSD). It has been found that surface charges become zero if the samples are kept in more than 85% R.H. atmosphere for a short duration. The relaxation time decreases with the increase in exposure time to the humid atmosphere. Activation energy is also calculated from the glow peak analysis.

Phase diagram of the semiconducting uniaxial ferroelectric Fe1-xS

Abstract: A phase diagram of the semiconducting uniaxial ferroelectric antiferromagnetic Fe1-xS covering the available experimental evidence is given.

Model for the semiconducting uniaxial ferroelectric Fe1-xS—Part I

Abstract: This is part I of a series of papers in which a qualitative model for a semiconducting uniaxial ferroelectric is developed in terms of Fe1-xS which was recently shown to be such a ferroelectric.Two types of charged defects are responsible for the anomalous behaviour, since they both weaken the polar coupling: (i) electronic defects belonging to the lattice (e defects) and (ii) vacancy-connected defects (v defects). The transition from the polar 2c phase to the paraelectric lc phase proceeds via a sequence of transitions which can be typified in the following scheme[image omitted] A TPS (Two Phase Structure) is a series of double junction rectifiers of alternate polarity that spontaneously form: the positive (negative) defects segregate into positive (negative) space-charge layers which having increased defect density are paraelectric. The space charge cancels the depolarizing field connected with the terminal charges of the polar layers which having decreased defect density are polar.It is shown that the ...

Theory of the Electronically Induced Crystallographic Transition. II

Abstract: A recently presented dynamic model concerning the electronically induced crystallographic transition is solved rigorously with higher-order terms in the distortion operator also taken into account.

The E.S.R Study of some H-D Solvent Isotope Effects on Copper Ions

Abstract: In this paper was performed an E.S.R. study on copper ions in water and ammoniacal solutions to put in the light and H-D isotope effect which appears in the shape and the width modifications of spectral lines. The isotope effect is different in terms of the paramagnetic ion concentration at room and liquid nitrogen temperature. These effects were explaines by the isotopic modifications of dipolar interactions among copper ions and between copper ions and ligand nuclei and also by the isotopic modifications of crystalline field in frozen solutions.

Development of Cavities for Microwave Solid State Sources

Abstract: Different types of X-band Gunn diode oscillator cavities and Impatt diode oscillator cavities, developed at SPL, Delhi, have been discussed. These cavities can be tuned mechanically. One of these cavities has a provision for dielectric tuning. The characteristics of some of these solid state oscillators are shown and attempt has been made to explain their performance.

Kinetics of the B2⇆B1 phase transition in CsCl

Abstract: Experimental results are given on the kinetics of the B2⇆B1 phase transition in single crystals of pure CsCl. Conductivity and dilatometric measurements have been carried out employing uniform heating and cooling rates q varying from 0.07 to 3.8 deg/min. The kinetics can be described satisfactorily using the Johnson-Mehl and Avrami formalism for nucleation and growth processes, y = 1 — exp (−kn tn), where y is the fraction transformed, t the time, n is the time exponent, and k the rate constant of the reaction. In the B2 B1 transformation, occurring at 476°C, two regions have been observed. In the first region (0 0.98) n = 1 was found. The rate constant strongly depends upon the heating rate. Combining the results from the conductivity experiments and from dilatometry it can be concluded that the growing particles have an ellipsoidal shape in the first stage. The starting temperature of the B1 B2 transition is strongly influenced by the cooling rate: 470 °C for q = −0.1 deg/min and 450 °C for q = −2.4 deg/min. The transformation process showed cooperative phenomena. Within a few seconds about 40% of the material are transformed. Repeated passes through the transformation region revealed that the number of nuclei present before the beginning of the transition was strongly enhanced by the thermal treatment. The overall kinetic behaviour remained unchanged. Die Kinetik der B2⇄B1 Phasenumwandlung in CsCl Einkristallen wurde mittels Leitfahigkeits- und Ausdehnungsmessungen untersucht. Dabei wurden gleichformige Heiz- und Kuhlgeschwindigkeiten q, von 0,07 bis 3,8 grd/min verwendet. Die Resultate konnen mit Hilfe der Johnson-Mehl-und Avrami-Formel fur Keimbildung und Wachstumsprozesse, y = 1 — exp (−kn tn), befriedigend dargestellt werden. In dieser Formel ist y der transformierte Bruchteil, t die Zeit, n der Zeitexponent und k der Reaktionsgeschwindigkeitsfaktor. Bei der B2 B1 Phasenumwandlung, die bei 476°C auftritt, wurden zwei Gebiete beobachtet. Im ersten Gebiet (0 0,98) wurde n = 2,5 gefunden, im zweiten Gebiet (y > 0,98) ist n = 1. Der Faktor k ist stark von der Heizgeschwindigkeit abhangig. Durch Kombination der Ergebnisse aus Leitfahigkeits- und Ausdehnungsmessungen folgt, das die wachsenden Teilchen im ersten Gebiet ellipsoidformig sind. Die Anfangstemperatur der B1 B2 Umwandlung war stark von der Kuhlgeschwindigkeit abhangig: 470°C fur q = −0,1 grd/min und 450°C fur q = −2,4 grd/min. Weiterhin zeigten sich bei diesem Prozes kooperative Phanomene. In einigen Sekunden wandelten sich 40% des Kristalls um. Die Ausfuhrung wiederholter B2⇄B1 Phasenumwandlungen zeigte, das die Anfangszahl der Keime dadurch stark beeinflust wurde. Die allgemeinen Eigenschaften der Kinetik anderten sich durch diese thermische Behandlung nicht.