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We present an ab-initio study of boron nitride as well as carbon allotropes.Their relative thermodynamic stabilities and structural phase transitions fromlow- to high-density phases are investigated. Pressure-temperature phasediagrams are calculated and compared to experimental findings. The calculationsare performed using quantum chemical wavefunction based as well as densityfunctional theories. Our findings reveal that predicted energy differencesoften depend significantly on the choice of the employed method. Comparisonbetween calculated and experimental results allows for benchmarking theaccuracy of various levels of theory. The produced results show that quantumchemical wavefunction based theories allow for achieving systematicallyimprovable estimates. We find that on the level of coupled cluster theories thelow- and high-density phases of boron nitride become thermodynamicallydegenerate at 0 K. This is in agreement with recent experimental findings,indicating that cubic boron nitride is not the thermodynamically stableallotrope at ambient conditions. Furthermore we employ the calculated resultsto assess transition probabilities from graphitic low-density to diamond-likehigh-density phases in an approximate manner. We conclude that the stackingorder of the parent graphitic material is crucial for the possible formation ofmeta-stable wurtzite boron nitride and hexagonal carbon diamond also known aslonsdaleite.

Ab-initio calculations of carbon and boron nitride allotropes and their structural phase transitions using periodic coupled cluster theory