On the decay of KCl:Cu+ fluorescence

Summary The fluorescence of Cu⁺ in KCl was investigated in the range 19–300 K, in particular by performing accurate measurements of lifetime and quantum yield. The data were found irrespective of the doping method. It was evidenced that, among the models proposed in the literature, the Pedrini model gives the most reasonable explanation for the Cu⁺ de-excitation mechanism: two excited states in thermal equilibrium are involved in the emission process, one of which is metastable and lying just below the emitting level.     

On the possibility of a forecast for off-centre configuration of Ag+ and Cu+ in alkali halide crystals

Summary The simple cut criterion based on the accurate determination of the radii of the ions in alkali halides and previously introduced by the authors for forecasting off-centre configuration of Li⁺ and F⁻ has been extended to heavy ions (Ag⁺ and Cu⁺). It has been found that this criterion is valid for the Ag⁺ ion, whereas for Cu⁺ gives a less precise forecast because of the lack in knowledge of the effective partial charge on Cu⁺ ion. It has been evidenced that the critical value of the ratior ₊ ^* /r ₊ between impurity and host ion radius which allows off-centre configuration is dependent on the impurity ion mass. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

Dislocation emission in A1 during recrystallization

Summary The dislocation structure evolution during isothermal recrystallization of Al has been followed by means of internal friction, modulus defect and X-ray diffraction analyses accompanying the microscopical observations. Many peaks ofQ ⁻¹ and of modulus defect and X-ray diffraction line width have been found, both in nucleation and grain growth, all referable to dislocations. The first of these peaks, at the beginning of recrystallization, presents recovery characteristics, different from those shown by the subsequent, secondary peaks. Secondary peaks are interpreted in terms of dislocation emission from grain boundaries, whereas the first peak appears connected with subboundaries, walls present in the initial stages of recrystallization. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Speculations on the absorption and emission properties of the F-centre in alkali halides

Summary A review of acquisitions and of unresolved problems concerning F-centres in alkali halides is presented. The main conclusions area) the temperature dependence of the bands is mainly due to the thermal expansion for the absorption process and to electron-phonon interaction for the emission process: for the latter half-width and Huang-Rhys factor values are related to the transverse optical frequency ones;b) the lifetime τ_R of the relaxed excited state is not intrinsic at low temperature because of the interaction between F-and α-centres always present in real samples;c) even though the absolute quantum yield decreases when F-centre concentration increased, the relative quantum yield η_r is unaffected by change on F-centre concentration, strongly evidencing a nonhomogenous distribution of F-centres in real samples;d) a temperature-dependent, fine structure in τ_R (an η_r) values is observed but only when temperature control of the samples is better than a few hundredth of a degree: this effect has been interpreted in terms of interactions which involve F-, α-and F′-centres. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

Complex classical trajectories in tunnelling: How instanton bounces can become real processes

Summary Instanton bounces in complex-time and high temporal density can be related to complex classical trajectories when the potential barrier is slightly perturbed. The bounce duration in real and imaginary times is found to be in good agreement with the one evaluated by the phase-time method. On this basis, a plaustible interpretational model of the tunnelling processes is suggested.     

Analysis of a qualitative criterion for forecasting off-centre displacement of impurity ions in ionic crystals

Summary With the aid of the Sanderson model for nonmolecular structure a qualitative criterion is derived for forecasting off-centre configurations of monovalent impurity ions (Li⁺, F⁻, Cl⁻, Na⁺, Ag⁺, Cu⁺) in alkali halide crystals. The same criterion is checked for Mn²⁺ impurity ions in some oxides. We also present a critical review of the criteria introduced in the past, as well as of the experimental results up to now known in the literature. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction. Work jointly supported by the Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

Lifetime measurements of colour centres by a multifrequency phase fluorometer

Summary The first measurements of emission lifetimes in colour centres using a cross correlation phase and modulation fluorometer operating at the synchrotron radiation facility PULS (Frascati) are reported. Preliminary results on the F and F-aggregate centres in NaF are briefly summarized.

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Riassunto Si riportano le prime misure di tempi di vita dell'emissione dei centri di colore usando uno spettrofluorometro a correlazione di fase e di modulazione in funzione presso il laboratorio del Programma per l'Utilizzazione della Luce di Sincrotrone (PULS, Frascati). Si discutono i risultati relativi ai centri F ed agli aggregati di centri F nel fluoruro di sodio.

     

Harmonic and anharmonic effects in the theory of vacancy formation volume

Summary In the frame of the theory developed in the other papers of this series, the present work analyses all the expansion centres arising in a metal lattice when an ion with its valence electrons is removed from the bulk. These expansion centres play an important role in determining the vacancy formation volume and its temperature dependence. The calculation accounts rigorously for anharmonic effects but their full discussion is partially postponed to a future paper.

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Riassunto Nell'àmbito della teoria sviluppata negli altri lavori di questa serie, si analizzano i centri di espansione che nascono in un solido metallico quando esso sia privato di uno ione e dei rispettivi electroni di valenza. Questi centri di espansine giocano un ruolo importante nel determinare il volume di una vacanza, nonché la sua dipendenza dalla temperatura. La trattazione rigorosa degli effetti anarmonici è in parte rimandata a un prossimo lavoro.

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Резюме В рамках теории, развитой в I и II чаатях этой серии, анализирируются все центры расширеиня, возникающие в металличекой решетке, когда ион с валентными электронами удаляется из объема. Эти центры расширения играют важную роль при определении обрзования вакансии и его ремпературной зависимощости. Вычисления ствого учитывают ангармомические эффекты, но их обсуждение частично перенесено в следую статью (часть IV).

     

A simple criterion for forecasting off-centre configuration of monovalent impurity ions in alkali halide crystals

Summary With the aid of the Sanderson model for nonmolecular structures a simple criterion has been derived for forecasting off-centre configuration of monovalent impurity ions in alkali halide crystals based on the effect that the Sanderson partial charges on atoms have on their radii. Work jointly supported by Ministero della Pubblica Istruzione and by Consiglio Nazionale delle Ricerche, Gruppo Nazionale di Struttura della Materia.     

Vacancy formation volume of sodium,aluminium and argon

Summary The whole theory of vacancy formation volume, as developed in parts I, II, III, IV and V of this series, is applied at Al, Na and Ar. In Na the anharmonic terms are found to be responsible for about 35% of the formation volume at 300 K and bring the dominant contribution to its temperature derivative at constant pressure. In fact, we find in this case a negative expansion coefficient for the vacancy, in conflict with the results of Audit and Gilder (1978), who found a positive value, having neglected anharmonicity. The important implications of this result in the interpretation of self-diffusion data are discussed. The analysis of Al confirms the relevant weight of anharmonicity, but also the inadequacy of second-order pseudopotential theory for trivalent metals. Finally, for Ar our conclusions are in full agreement with those reached by Zubov (1980).

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Riassunto Questo articolo presenta l'applicazione a tre elementi (Al, Na e Ar) della teoria riguardante il volume di formazione di una vacanza, quale è sviluppata nelle parti del lavoro già pubblicate. Nel sodio i termini anarmonici risultano responsabili per circa il 35% del volume di formazione a 300 K e portano il contributo dominante alla sua dipendenza dalla temperatura (a pressione costante). Infatti si trova in questo caso un coefficiente negativo di espansione termica per la vacanza, in contrasto con i calcoli di Audit e Gilder (1978), che non tengono conto degli effetti anarmonici. Si discutono le importanti implicazioni di questo risultato nell'interpretazione delle misure di diffusione. L'analisi dell'alluminio conferma il ruolo rilevante giocato dall'anarmonicità, ma nel medesimo tempo l'insufficienza della teoria dello pseudopotentiale al second'ordine per metalli trivalenti. Infine, nel caso dell'argon solido i nostri risultati sono in ottimo accordo con quelli di Zubov (1980).

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Резюме Теория объема образования вакансии, развитая в частях I, II, III, IV и V этой серии, применяется к натрию, алюминию и аргону. Обнаружено, что в натрии ангармонические члены ответственны за 35% объема образования при 300 К и дают доминирующий вклад в зависимость от температуры (при постоянном давлении). Мы получаем в этом случае отрицательный коэффициент теплового расширения для вакансии, в противоположность результатам Аудита и Джилдера, которые получили положительный коэффицент, пренебрегая ангармоничностьй. Обсуждается применение результатов к интерпретации данных по самодиффузии. Анализ для алюминия подтверждает важность ангармоничности, но указывает на неадекватность второго порядка теории псевдопотенциала для трехвалентных металлов. В заключение, отмечается, что наши выводы для аргона хорошо согласуются с теорией, развитой Зубовым.

     

Profiles of dislocation images in Si by scanning microscopy in EBIC. Dependence on the space charge cylinder radius

Summary This paper deals with a mathematical procedure for the calculation of EBIC profiles, accounting for the space charge cylinders around the dislocations. On the basis of the relative results the experimental data, regarding the temperature and resistivity effects on the contrast of the dislocation images, are interpreted in terms of perturbations of the equilibrium statistics of occupation of the dangling bonds along dislocations.

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Riassunto In questo articolo si espone un procedimento matematico per il calcolo di profili EBIC, che prende in considerazione i cilindri di carica spaziale presenti attorno alle dislocazioni. Alla luce dei risultati ottenuti, i dati sperimentali relativi agli effetti di temperatura e resistività sul contrasto delle immagini delle dislocazioni sono interpretati in termini di perturbazione delle condizioni di equilibrio della statistica di occupazione dei ?legami disponibili? presenti lungo le dislocazioni.

     

High-resolution vibrational spectroscopy of the OH− ion in an Mg-related defect in LiF and NaF

Summary High-resolution (0.05 cm⁻¹) FTIR spectroscopy in the range (9÷300) K is applied to study the vibrational (stretching, bending and combination) modes of OH⁻ (and its isotopic substitution with OD⁻) perturbed by a neighbouring defect induced by Mg²⁺ in LiF and NaF. The stretching-mode spectra are analysed in the framework of the anharmonic-oscillator model: the related Morse parameters have been determined. A model is proposed for the defect, responsible for the stretching line and two bending lines, which are accounted for by an inversion doubling. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Lattice distortion around an expansion centre and its role in the theory of vacancy formation volume

Summary An approximate analysis of lattice distortion around an expansion centre is presented: the analysis is carried out in view of its applications to the theory of vacancy formation volume, as developed in the previous parts of this series. The approximate solution has the correct elastic asymptotic behaviour for large distances, while its reliability in the neighbourhood of the defect is checked through a comparison with the results obtained by previous authors on a specific model of isotropic aluminium. The role of the long-range Friedel oscillations in the pair potential of metals is discussed and found to invalidate the expression usually given to the defect force formula (Temkin, 1970).

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Riassunto Questo articolo presenta un’analisi approssimata della distorsione del reticolo intorno a un centro di espansione: tale analisi è indispensabile per calcolare il volume di formazione di una vacanza, secondo la teoria sviluppata nelle parti del lavoro precedentemente pubblicate. La soluzione approssimata a cui si perviene ha il comportamento asintotico previsto dalla teoria dell’elasticità a grandi distanze dal difetto, mentre nelle sue vicinanze porta a risultati in accordo con quelli ottenuti da altri autori nel caso specifico dell’alluminio isotropo. Si dimostra che le oscillazioni di Friedel del potenziale interionico invalidano la formula di Temkin (1970) nel caso di solidi metallici.

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Резюме Анализируется дисторсия решетки вокруг центра расширения. Анализ проводится с целью применения к теории объема образования вакансии, которая была развита в предыдущих статьях этой серии (I, II, III и IV). Приближенное решение имеет правильное упругое асимптотическое поведение для больших расстояний, тогда как в окрестности дефекта надежность решения проверяется сравнением с результатами, полученными друтими авторами на основе конкретной модели изотропного алюминия. Обсуждается роль осцилляций Фриделя в парном потенциале металлов. Отмечается неприменимость формулы Темкина, обычно используемой в случае металлических твердых тел.

     

Defects in ion-implanted crystalline silicon probed by femtosecond laser spectroscopy

Summary Radiation damage is generated in a controlled manner by MeV ion implantation of Si⁺ and He⁺ ions in c-Si and studied by ultrafast laser pulses on a subpicosecond time scale. In Si⁺-implanted samples the amorphization of the sample is achieved at sufficiently high doses, while He implants only produce a very low level of damage. Defects are investigated after implantation by measuringex situ the change of reflectivity caused by a high density of electron-hole plasma generated by femtosecond laser pulses. The plasma decay time decreases as a function of the implantation dose in both Si- and He-implanted samples, reaching a minimum value of ≈1 ps. It is observed that the saturation of the decay time is not related to the amorphization of the sample, but rather to the formation of simple defects produced during ion implantation.     

Extended defect removal in silicon by rapid thermal annealing

Summary Germanium, arsenic and krypton ions of 600 keV energy were implanted in <100> silicon substrate at 250°C. The hot implantation results in the formation of extended defects (dislocation loops and cluster of point defects) as residual damage. Rapid thermal annealing process at a temperature above 1000°C was used to remove the damage. Rutherford-backscattering channelling technique was used to measure the amount of defects and their annealing. In some cases the channelling results were correlated to transmission electron microscopy (TEM) analysis. The annealing process of the damage is governed by an activation energy of (4.4±0.2) eV for both germanium and arsenic implants. During RTA processes broadening of the As and Ge distributions is quite negligible. The Kr atoms interact instead with defects and the annealing even after a prolonged time at 1100°C is not complete, bubbles surrounded by extended defects are left The authors of this paper have agreed to not receive the proofs for correction.     

Thermal expansion and vacancy formation volume

Summary When the total Hamiltonian of a perfect lattice is expanded in terms of the displacements of the ions from the average positions, the effect of linear and cubic terms is shown to be equivalent to a system of Eshelby's expansion centres present in the lattice and responsible for thermal dilation. The pressure exerted by these expansion centres is added to the pressure deriving from the part of the Hamiltonian which is independent of the ion dynamical variables: the equation of state of the solid is simply obtained by equating the sum of the above two terms to the external pressure. In this paper the theory is confined to perfect crystals, in order to provide a general basis of interest for the next parts of the work, devoted to the problem of vacancy formation volume.

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Riassunto Quando si sviluppa l'hamiltoniana di un cristallo perfetto in serie degli spostamenti degli ioni dalle rispettive posizioni medie, l'effetto dei termini lineari e cubici risulta equivalente ad un sistema di centri di espansione di Eshelby, responsabile della dilatazione termica. La pressione esercitata da questi centri di espansione si aggiunge alla pressione derivante dalla parte dell'hamiltoniana che è indipendente dalle variabili dinamiche degli ioni: l'equazione di stato del solido si ottiene semplicemente uguagliando la somma die due suddetti termini alla pressione esterna. La teoria svolta in questo articolo per cristalli perfetti ha il còmpito di stabilire una base generale di partenza sulla quale saranno costruite le prossime parti del lavoro rivolte al problema del volume di formazione di una vacanza.

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Резюме Когда полный гамильтониан идеальной решетки разлагается в ряд по смещениям ионов от их средних положений, эффект линейных и кубических членов эквивалентен системе центров расширения Эшелби. Давление, оказываемое этими центрами расширения, добавляется к давлению, получаемому из части Гамильтониана, которая не зависит от динамических переменных ионов. Уравнение состояния твердого тела получается в результате приравнивания суммы этих двух членов внешнему давлению. В этой статье рассматривается теория идеальных кристаллов, что дает основу для следующей работы, посвященной проблеме объема образования вакансии.

     

Decay kinetics of the green emission in tungstates and molybdates

Luminescence characteristics of a number of undoped and variously doped PbWO₄ crystals were studied at 0.4–400 K by the time-resolved spectroscopy and compared with those of ZnWO₄,CdWO₄ and PbMoO₄ crystals. Two types of green emission centres are detected in PbWO₄ crystals. The centres of the first type are responsible for the low-temperature 2.3–2.4 eV emission observed under excitation around 3.90–3.95 eV. The structure and parameters of their relaxed excited states were determined. It was concluded that the origin of defects responsible for the green emission of the first type could vary for different crystals. The centres of the second type with the emission around 2.5 eV appear in crystals containing oxygen vacancies after the thermal destruction of Pb⁺-WO₃ centres at T>180 K. Decomposition of the exciton and various defect-related states was also studied, and activation energies of this process were calculated.     

A Debye model for point defects in aluminosilicate crystals

Summary In aluminosilicate lattices, exchanges between silicon and aluminum ions are expected to originate charged point defects of opposite signs. Thermodynamic arguments, based on a model analogous to the Debye electrolyte model, allow evaluation of the equilibrium density of defects as a function of temperature. The electric field of point defects is found to be screened by charge distribution even at distances small enough to affect the ground-state energy of electrons trapped by the defect positive charges. Calculations show that large fluctuations of trapped electron energy are expected. These results agree with data from thermoluminescence experiments on different aluminosilicates, which showed broad distributions of electron traps.