The contribution to lattice defects physics by Umberto Maria Grassano

Highlights of the research, educational, and scientific management activities carried out by Prof. Umberto Maria Grassano (1939-2000) over the last four decades are recalled. In particular his outstanding scientific achievements in the field of color center excited states and two-photon spectroscopy are briefly reviewed.     

An origin of infinite range dipole-dipole correlations in soft matrices with off-center impurities: K 1−x Li x Ta 1−y Nb y O 3 and other systems

A model of cooperative behaviour for K 1 m x Li x Ta 1 m y Nb y O 3 (KLTN) and for other disordered systems like ferroelectric relaxors is considered from the point of view of contribution of infinite-range dipole-dipole interactions. It is shown that a local piezoeffect for off-center impurities in crystals with rather high electrostriction is responsible as origin of a infinite-range electric dipole-dipole interaction via elastic fields as well as for a co-existence of ferroelectric and glass-like states with reentrant glass effect.     

ICP-AES法测定铅精矿中的杂质元素

用HNO3-KClO3-H2SO4溶样,采用ICP-AES测定铅精矿中的杂质元素。并试验了酸度、共存元素对测定的影响,方法的精密度、检出限及回收率均能满足要求。     

Synthesis and Characterization of Process Related Impurities of (±)‐Milnacipran

Milnacipran is a cyclopropane derivative, used as an anti depressant drug. During the process development of milnacipran, four process related potential impurities were detected in high performance liquid chromatography. All these impurities were identified, synthesized and subsequently characterized by their respective spectral data (IR, LC‐MS, ¹H NMR, and ¹³C NMR) as described in this article.     

Model development for deactivation of bisphenol-A adduct particles during crystallization under the influence of impurity

A deactivation mechanism was developed to present the influence of 4-tert-butylphenol as a sample impurity on the bisphenol-A (BPA) adduct particles during the crystallization process. 4-tert-butylphenol is an organic sample impurity generally present in the reaction mixture of the industrial production of BPA. Kinetic parameters of growth, nucleation, agglomeration, and deactivation were estimated using the technique of model fitting to experimental data. The population and mass balances were used to model the adductive crystallization of BPA.     

Control of impurities in l-aspartic acid and l-alanine by high-performance liquid chromatography coupled with a corona charged aerosol detector

In this study a reversed phase ion-pair high-performance liquid chromatography (HPLC) method using charged aerosol detection (CAD) was developed and fully validated for the pharmaceutical quality control of l-aspartic acid (Asp). With a slight modification, the method also allows the evaluation of related substances in l-alanine (Ala). The method enables simultaneous control of related amino acids and of possibly occurring organic acids contaminants. A minimum limit of quantification of 0.03% could be achieved for all occurring related substances. Moreover, the detector sensitivity of the CAD was compared with an evaporative light scattering detector (ELSD). Depending on the analyte the CAD was found to be 3.6–42 times more sensitive than the ELSD. The HPLC method was applied to the purity testing of 8 samples of pharmaceutical grade and reagent grade Asp and of 12 samples of Ala supplied by various manufacturers. Both substances were found to be of high purity (greater than 99.8% for Asp and greater than 99.9% for Ala). Malic acid and Ala were the major impurities in Asp. Asp and glutamic acid (Glu) were the only detectable impurities in Ala.     

Thick homoepitaxial GaN with low carrier concentration for high blocking voltage

High voltage GaN Schottky diodes require a thick blocking layer with an exceptionally low carrier concentration. To this aim, a metal organic chemical vapor deposition process was developed to create a (14 μm) thick stress-free homoepitaxial GaN film. Low temperature photoluminescence measurements are consistent with low donor background and low concentration of deep compensating centers. Capacitance–voltage measurements performed at 30 °C verified a low level of about 2×10¹⁵ cm⁻³ of n-type free carriers (unintentional doping), which enabled a breakdown voltage of about 500 V. A secondary ion mass spectrometry depth profile confirms the low concentration of background impurities and X-ray diffraction extracted a low dislocation density in the film. These results indicate that thick GaN films can be deposited with free carrier concentrations sufficiently low to enable high voltage rectifiers for power switching applications.     

Influence of some foreign metal ions on crystal growth kinetics of brushite (CaHPO4·2H2O)

Brushite, CaHPO₄·2H₂O, has been precipitated at 25 °C in the presence of Mg²⁺, Ba²⁺ or Cu²⁺ at concentrations up to 0.5 mM. When initial pH is sufficiently low to exclude nanocrystalline apatite as the initial solid phase, overall crystal growth rate may be determined from simple mass crystallization by recording pH as function of time. A combination of surface nucleation (birth-and-spread) and spiral (BCF) growth was found. Edge free energy was determined from the former contribution and was found to be a linear function of chemical potential of the additive, indicating constant adsorption over a wide range of additive concentrations. Average distances between adsorbed additive ions as calculated from slopes of plots are compatible with lattice parameters of brushite: 0.54 nm for Mg²⁺, 0.43 nm for Ba²⁺ and 0.86 nm for Cu²⁺. With the latter a sharp decrease in growth rate occurred early in the crystallization process, followed by an equally sharp increase to the previous level. When interpreted in terms of the Cabrera–Vermilyea theory of crystal growth inhibition, the results are consistent with an average distance between Cu ions of 0.88 nm, in perfect agreement with the above value.     

Incorporation of group III,IV and V elements in 3C–SiC(1 1 1) layers grown by the vapour–liquid–solid mechanism

We report on a comparative investigation of the incorporation of group III, IV and V impurities in 3C–SiC heteroepitaxial layers grown by the vapour–liquid–solid (VLS) mechanism on on-axis α-SiC substrates. To this end, various Si-based melts have been used with addition of Al, Ga, Ge and Sn species. Homoepitaxial α-SiC layers grown using Al-based melts were used for comparison purposed for Al incorporation. Nitrogen incorporation depth profile systematically displays an overshoot at the substrate/epilayer interface for all the layers. Ga and Al incorporations follow the same distribution shape as N whereas this is not the case for the isoelectronic impurities Ge and Sn. This suggests some interaction between Ga/Al and N coming from the high bonding energy between the group III and V elements, which does not exist with Ge and Sn. This is why both incorporate as a cluster. A model of incorporation is proposed taking into account metal-N and metal-C bonding energies together with the solid solubility of the corresponding nitrides.     

Impurity doping into Mg2Sn: A first-principles study

We studied the formation energy and atomic structure of impurities in Mg₂Sn using first-principles plane-wave total energy calculations. Twenty elements, namely H, Li, Na, K, Rb, Sc, Y, La, Cu, Ag, Au, B, Al, Ga, In, N, P, As, Sb, and Bi, were selected as the impurity species. We considered structural relaxation of the atoms within the second nearest neighbors of the impurity atom in the 48-atom supercell. The results of the formation energy calculations suggested that Sc, Y, La, P, As, Sb, and Bi are good n-type dopants whereas Li and Na are good p-type dopants. The electrical properties of Li-, Na-, and Ga-doped Mg₂Sn and La-doped Mg₂(Si, Sn) composites reported previously can be explained by the low formation energies of Li, Na, Ga, and La in Mg₂Sn.