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Here we report on the discovery of a ternary silicon titanium nitride with the general composition (Si1−x,Tix)3N4 with x = 0 < x < 1 and spinel-type crystal structure. The novel nitride is formed from an amorphous silicon titanium nitride (SiTiN) precursor under high-pressure/high-temperature conditions in a large volume high-pressure device. Under the conditions of 15–20 GPa and 1800–2000 °C, spinel-type γ-Si3N4 and rock salt-type c-TiN are formed. In addition, crystals of the discovered nano-sized ternary phase (Si1−x,Tix)3N4 embedded in γ-Si3N4 are identified. The ternary compound is formed due to kinetically-controlled synthesis conditions and is analyzed to exhibit the spinel-type structure with ca. 8 atom% of Ti. The Ti atoms occur in both Ti3+ and Ti4+ oxidation states and are located on the Si sites. The ternary nano-crystals have to be described as (Si,Ti)3N4 with N-vacancies resulting in the general composition (Si4+1−x Ti4+x-δTi3+δ)3N4-δ. [ABSTRACT FROM AUTHOR]

SiOC/HfO2‐based ceramic nanocomposites with in situ formed HfO2 nanoparticles were prepared via a single‐source precursor (SSP) approach starting from a polymethylsilsesquioxane (PMS) modified by Hf‐ and Ti‐alkoxides. By varying the alkyl‐group of the employed Hf‐alkoxides, SiOC/HfO2‐based ceramic nanocomposites with different HfO2 polymorphs formed via thermal decomposition of the SSP under the same heat‐treatment conditions. Using PMS chemically modified by Hf(OnBu)4, tetragonal HfO2 phase was formed after the synthesis at 1100°C in Ar, whereas both, tetragonal and monoclinic HfO2 nanocrystals, were analyzed when replacing Hf(OnBu)4 by Hf(OiPr)4. After oxidation of the synthesized nanocomposites in air at 1500°C, a facile formation of oxidation‐resistant HfSiO4 (hafnon) phase occurred by the reaction of HfO2 nanocrystals with silica present in the SiOC nanocomposite matrix derived from Hf(OiPr)4‐modified SSPs. Moreover the amount of hafnon is dramatically increased by the additional modification of the polysiloxane with Ti‐alkoxides. In contrast, ceramic nanocomposites derived from Hf(OnBu)4‐modified SSPs, almost no HfSiO4 is detected after oxidation at 1500°C even though in the case of Ti‐alkoxide‐modified single‐source precursor. [ABSTRACT FROM AUTHOR]

The high pressure behavior of aluminum nitride (AlN) and zinc oxide (ZnO) nanocrystals was studied up to 30 GPa using second harmonic generation. ZnO (10 nm) crystals transform to the high pressure B1 phase at 16.6 GPa, ≈5 GPa higher than the corresponding value for a bulk sample. The transition of AlN (20 nm) and AlN (100 nm) occurred at 14 and 21.5 GPa at lower values than for bulk samples. Under non-hydrostatic pressure conditions, the transition pressures of ZnO (10 nm) and AlN (100 nm) decrease to 12.5 and 18 GPa, respectively. We determined the surface energy (and) of the B1 polymorphs. We show that the main reason for the size-dependent decrease of the transition pressure of AlN nanocrystals is due to the higher surface energy of the B4 phase relative to the surface energy of the B1 phase. We predict that it is possible to quench or synthesize pure B1 AlN to ambient conditions if the grain size is less than 8.5 nm. [ABSTRACT FROM PUBLISHER]