Advanced dense nitrides: from discovery to potential applications

This work reviews evolution of search for novel high-pressure high-temperature (HP-HT) nitrides with nitrogen-to-metal ratio well above one which has commenced more than 25 years ago and brought today some remarkable blooms. The number of reports on synthesis of such nitrides grows permanently but their majority deals, in the last years, with synthesis at very high pressures where even characterisation of the products (structure, composition etc.) becomes vague. The number of binary nitrides and related ternary compounds or solid solutions, accessible as macroscopic samples using large volume HP-HT apparatuses, appears to have stabilised at a high but still monitorable level. Today, the Gretchen question is whether the outcome of the 25-years activity was rewarding, whether any of the nitrides recoverable at ambient conditions exhibits properties deserving transfer to the industry. An answer is not straight because the efforts needed to obtain reliable property data go beyond those invested during the synthesis and characterisation. Gathering of such data requires either laborious investment in samples quality or development/adaptation of non-traditional, for HP-HT research, in- and ex-situ techniques. Here, the exposé of the most notable novel nitrides and their HP-HT synthesis routes is followed by examples of several recent developments/adaptations of experimental approaches permitting access to mechanical and optoelectronic properties which were earlier difficult to measure due to imperfect polycrystalline, porous and eventually contaminated HP-HT samples. The revealed promising properties allow us to hope that the efforts were not futile and, at least, a couple of the discovered HP-HT nitrides could find industrial applications.