Remarkable effects of local structure in tantalum and niobium oxynitrides

Tailoring of an unusual oxidation state in a lanthanum tantalum(IV) oxynitride via precursor microstructure design

“…Typically, the bandgap decreases, making the formed perovskite-type oxynitrides AB(O,N) 3 interesting for visible light-driven applications 1,4 . An important task in order to identify perovskite-type oxynitrides is the exact determination of the O:N ratio which often causes difficulties 11,12 .…”

Colored amorphous silica‐based powder with TiN nanocrystals precipitated by ammonolysis of Ti–Si–O ternary glass

“…However, amorphous TiN was not clearly evident in the XRD patterns for these powders obtained under acidic conditions but the broad amorphous main peak position shifted to lower diffraction angle similarly to the observation in Figure . The colors were slate gray and light sky‐blue for powders with x = 0.08 and x = 0.03, respectively, after ammonolysis at 900°C suggesting plasmon absorption of TiN . Precipitation of the largest amount of TiN nanocrystals within the amorphous matrix after ammonolysis above 800°C resulted in a greenish color for the x = 0.08 precursor hydrolyzed under basic conditions, probably due to the goldish color of TiN in reflection.…”

“…In addition to the average crystal structure, the local structure of these materials is important in terms of their optical performance because the nature of their chemical bonds with oxide and nitride anions is different. Emission from phosphors and new dielectric properties have been reported in oxynitride perovskites due to the characteristic local coordination structures . Local cluster formation involving oxide anions has also been effective for increasing the critical current density of superconductivity in niobium oxynitrides …”

Ferroelectric BaTaO2N Crystals Grown in a BaCN2 Flux

“…Metal nitrides and oxynitrides have been attracting considerable attention because they are potentially useful for their optical, electrical, magnetic, and mechanical properties. − Both SrTaO 2 N and BaTaO 2 N perovskites are also important as visible-light-driven photocatalysts. − They were also reported to show large relative dielectric constants with a small temperature dependence . Their average crystal structures were assumed to be centrosymmetric tetragonal I 4/ mcm [ a = 0.569411(7) nm and c = 0.80658(2) nm] for SrTaO 2 N and cubic Pm 3̅ m [ a = 0.41128(1) nm] for BaTaO 2 N, based on powder neutron diffraction measurements. , Some deviations from these centrosymmetric structures have been reported that suggest local symmetry breaking, based on analyses by transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) as well as neutron pair distribution functions as crystal structural supports for the occurrence of dielectric properties. − In 2011, two groups independently reported a cis -type anisotropic local nitrogen configuration in TaO 4 N 2 octahedra in SrTaO 2 N based on powder neutron diffraction studies. , These local cis -type units tend to align with random long-range orientation due to the similar lengths of the Ta–O and Ta–N bonds. , Relaxor-type ferroelectric behavior was expected in first-principle studies on BaTaO 2 N and SrTaO 2 N assuming the cis -type configuration in their TaO 4 N 2 octahedra.…”

“…The electrical properties of these compounds have been investigated, as both ceramics and thin films, and they have been found to be significantly affected by the anion contents in the products. ,− Both SrTaO 2 N and BaTaO 2 N release a part of their nitrogen above 950 °C to change their compositions to SrTaO 2 N 0.7 and BaTaO 2 N 0.85 , respectively, even in a nitrogen atmosphere. The green-powder compacts of these materials change from orange or reddish insulators to black semiconductors during high-temperature sintering at 1400 °C because of the partial reduction of Ta 5+ . ,,, The nitrogen lost from these materials can be subsequently recovered by postammonolysis of as-sintered ceramics at approximately 950–1000 °C.…”