T1-Measurements in Ca-Doped NaCl Crystals

1979

Phys. Stat. Sol. (a)

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The aggregation of point defects is investigated by means of nuclear magnetic resonance (NMR), dielectric losses, and ionic conductivity measurements. The isothermal dipole concentration curves may be described with the help of a dimer—trimer model including dissolution of both aggregates. For NaCl crystals doped with 250 ppm SrCl2 the association enthalpy of dimers H2 = 0.40 eV is calculated and the association enthalpy of trimers H3 = (0.60 to 0.65) eV by fitting the experimental values.

Section: Zsothermal Annealingmentioning

confidence: 88%

Section: Zsothermal Annealingmentioning

confidence: 99%

The formation of dimer and trimer aggregates in Sr-doped NaCl crystals

Dubiel

1979

Phys. Stat. Sol. (a)

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“…Whereas isolated defects and dipoles (consisting of a metal impurity and a cation vacancy in nearest neighbour (nn) or next nearest neighbour (nnn) position) are directly detectable by measurement of the ionic conductivity [ 2 ] , the dielectric loss [3], ITC [4], EPR [S], or NMR [6], respectively, the existence of higher aggregates is derived indirectly by analyzing the change of the dipole concentration during isothermal annealing (see e.g. [7]).…”

Section: Introductionmentioning

confidence: 99%

Theoretical Studies of the Configuration and Association Energies of Point Defect Aggregates in Alkali Halide Crystals

Pippel³

1986

Annalen der Physik

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A b s t r a c t . The aggregation of dipoles consisting of a divalent metal impurity and a cation vacancy in alkali halide crystals is determined mainly by the association energy connected with the formation of the special types of aggregates. It is shown that also larger aggregates (complexes of four and more dipoles) are important to characterize the equilibrium state. Therefore a simple computational method will be presented to estimate atomistically the association energy of such aggregates. A comparison with results from the literature valid for little aggregates (mainly dimers and trimers) confirms the applicability of the proposed procedure. Theoretisehe Untersuehungen zur Konfigurations-und Assoziationsenergie yon Punktdefektaggregaten in AlkalihalogenidkristallenI n h a l t s u b e r s i c h t . Die Aggregation von aus zweiwertigen Metallverunreinigungen und Kationenleerstellen bestehenden Dipolen in Alkalihalogenidkristallen wird hauptsachlich durch die Assoziationsenergie bestimmt, die mit der Bildung spezieller Aggregattypen verbunden ist. Es wird gezeigt, da8 auch grol3ere Aggregate (Komplexe aus vier und mehr Dipolen) wichtig sind, urn den Gleichgewichtszustand zu charakterisieren. Es wird deshalb eine einfache Berechnungsmethode angegeben, um die Assoziationsenergie solcher Aggregate atomistisch zu ermitteln. Ein Vergleich mit aus der Literatur bekannten Ergebnissen, die fur kleine Aggregate (hauptsiichlich Dimere und Trimere) gultig sind, bestatigt die Anwendbarkeit des vorgeschlagenen Verfahrens.

“…Only the formation of trimers was observed. It is also possible to investigate point defects in XaC1 by means of nuclear magnetic resonance (NMR) measurements [4]. Assuming an equilibrium between isolated cation vacancies and cation vacancies associated a t dipoles the following relation for the spin-lattice relaxation time Tlv due to vacancies can be used [a]:…”

Section: Introductionmentioning

confidence: 99%

“…DUBIEL, B. BERG, and F. FROELTCH by the association theory [51. There is no contribution of vacancies associated a t higher aggregates to the spin -lattice relaxation time [4]. Consequently T, measurements allow the calculation of the defect concentrations dependent on the isothermal annealing time.…”

mentioning

confidence: 99%

Aggregation of Point Defects in Highly Doped NaCl:Ca⁺⁺Crystals

Dubiel

Physica Status Solidi (b)

1978

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The spin-lattice relaxation time TI measured by pulsed nuclear magnetic resonance is determined by the concentration of the Me2+-cation vacancy dipoles alone contrary to the dielectric loss measurements. Therefore this method is used to investigate isothermal annealing processes via the dipole concentration also for long times when higher aggregates are formed. It is found that aggregation processes take place in NaCl : Ca2+ crystals in the range up to 200 "C. For higher temperatures no change in the dipole concentration can be measured. Whereas for small doping levels (up to 500 ppm) mainly dimers and trimers are formed, a t high concentrations (over lo00 ppm) the formation of the Suzuki like phase is shown and described theoreticdly. The enthalpy for the dipole motion (0.60 eV) and the dimer association enthalpy (0.31 eV) are calculated analysing the experimental curves.Die mittels Impulskernresonanz gcmessene Spin-Gitter-Relaxationszeit TI wird im Gegensatz zu den Messungen des Pielektrischen Verlustwinkels durch die Konzentration der Me2+-Kationenleerstellen-Dipole allein bestimmt. Dcshalb wird diese Methode benutzt, um iiber die Dipolkonzentration isotherme Ausheilprozesse auch fur lange Zeiten zu verfolgen, wenn hohere Aggregate gebildet werden. Bei NaCl : Ca2+ wird gefunden, daB der Aggregationsbereich bis 200 "C reicht. Fiir hohere Temperaturen kann keine ihderung der Dipolkonzentration gemessen werden. Wahrend bei kleinen Dotierungskonzentrationen (bis 500 ppm) hauptsachlich Dimere und Trimere gebildet werden, wird bei hohen Konzentrationen (iiber 1000 ppm) die Bildung der ,,Quasi-Suzukiphase" nachgewiesen und theoretisch beschrieben. Durch Auswertung der exprimentellen Kurven werden die Bewegungsenthalpie der Dipole (0,60 eV) und die Dimerassoziationsenthalpie (0,31 eV) berechnet.