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We study the hard and electrically conductive multicomponent diboride Ti0.25Zr0.25Hf0.25Ta0.25B2 with high thermal stability by ab initio calculations. We focus on the effect of defects (either vacancies or C atoms, both relevant for numerous experiments including our own) on material characteristics. Different types, concentrations and distributions of defects were investigated, and the configurations leading to the lowest formation energies were identified. We show that the replacement of B by C is more unfavorable than the formation of B vacancies. We show that vacancies prefer to coalesce into a larger planar void, minimizing the number of broken B B bonds and the volume per atom, while carbon substitutions at boron sites do not prefer coalescence and tend to minimize the number of C-C bonds. We show the effect of vacancies on mechanical and electronic properties, and use the results to explain experimental data.

Covid-19 disease caused by severe acute respiratory syndrome 2 coronavirus (SARS-CoV-2) continues to be a global public health problem. Since the pandemic outbreak in early 2020, a number of cases have been reported in pregnant women whose infection has led to severe complications including preterm birth, miscarriage, intrauterine growth restriction or intrauterine fetal demise. The results of the current studies suggest that the transplacental transmission of infection from mother to fetus is a rare event and that the complications listed above are more likely due to damage of placental tissue. In this article, we describe two cases of SARS-CoV-2 placentitis with special consideration to the morphology and differential dia gnosis of this newly defined entity. Key words: placenta – covid-19 – SARS-CoV-2 – placentitis – histiocytic intervillositis

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