Band 237, Heft 1-3

Incorporating different anions with varied ionic sizes and charges is a rapidly growing approach to bring out unusual physical properties among various classes of solid-state materials, pnictides and chalcogenides in particular. This minireview is focused on hetero-anionic materials based on the pnictogens, which have been demonstrated to offer an impressive diversity of crystal chemistry and electronic structures. In addition, many pnictide oxides or oxypnictides, over the course of the last decade, have been shown to exhibit a broad spectrum of superconducting, magnetic, and semiconducting properties. However, the structural diversity of the mixed-anion materials is far greater than the several known structure types, or their variants, of the well-known layered superconductive materials. Therefore, with this treatise, we aim to provide a comprehensive overview of the crystal chemistry of pnictide oxides by recounting almost 40 different structures of such ternary and multinary compounds. In addition to the structural aspects, we also highlight some of the challenges associated with the synthesis, and briefly summarize reported, to date, physical properties of this remarkable class of solids.

The equiatomic metal-rich phosphide NbCrP shows a structural phase transition around 125 K. The structures of the high- and low-temperature modifications were refined from single crystal X-ray diffractometer data of an un-twinned crystal: TiNiSi type, Pnma , a  = 619.80(2), b  = 353.74(4), c  = 735.24(6) pm, wR  = 0.0706, 288 F 2 values, 20 variables at 240 K and P 12 1 / c 1, a  = 630.59(3), b  = 739.64(4), c  = 933.09(5) pm, β  = 132.491(6)°, wR  = 0.0531, 1007 F 2 values, 57 variables at 90 K. The structural phase transition is of a classical Peierls type. The equidistant chromium chain in HT-NbCrP (353.7 pm Cr–Cr) splits pairwise into shorter (315.2 pm) and longer (373.2 pm) Cr–Cr distances. This goes along with a strengthening of Cr–P bonding. The superstructure formation is discussed on the basis of a group–subgroup scheme. Electronic structure calculations show a lifting of band degeneracy. Protection of the non-symmorphic symmetry of space group Pnma is crucial for the phase transition. The estimated charge modulation is consistent with the interpretation as Peierls transition.

Twelve cubic sodalites |Na 8 X 2 |[T 1 T 2 O 4 ] 6 (T 1  = Al 3+ , Ga 3+ ; T 2  = Si 4+ , Ge 4+ ; X = Cl − , Br − , I − ) were examined using high-temperature (HT) X-ray diffraction experiments and TGA-DSC measurements. Temperature-dependent structure data was obtained by Rietveld refinements. Decomposition temperatures were determined using TGA-DSC data for all compounds. The temperature-dependent volume expansion was used to determine Debye and Einstein temperatures using DEA fits. Distinct relations between thermal expansion, bond lengths and the decomposition temperature could not be found. Determination of Lindemann constants of all compounds enables a classification of the sodalites in three groups.

The stannides RE 3 Rh 2 Sn 4 ( RE  = Y, Gd–Tm, Lu) were synthesized from the elements by arc-melting and subsequent annealing (1220 K for RE  = Y, Gd–Tm and 1170 K for RE  = Lu) in sealed silica ampoules for 11 days. X-ray powder diffraction studies confirm the hexagonal Lu 3 Co 2 In 4 type structure, space group P 6 ‾ $P\overline{6}$ . The structure of Gd 3 Rh 2 Sn 4 was refined from single crystal X-ray diffractometer data for a twinned crystal: a  = 744.04(6), c  = 409.23(4) pm, wR 2 = 0.0288, 567 F 2 values and 21 variables. The RE 3 Rh 2 Sn 4 stannides derive from the well-known equiatomic stannides RE RhSn (≍ RE 3 Rh 3 Sn 3 ) by Rh/Sn ordering within the RE 6 trigonal prisms. The striking structural motif is the trigonal planar tin coordination of the Sn2 atoms with 288 pm Sn2–Sn1 distances. The Sn2 atoms carry substantially more negative charge than the Sn1 atoms. This is underlined by 119 Sn isomer shifts of δ  = 1.86(1) mm s −1 for Sn1 and δ  = 2.26(1) mm s −1 for Sn2 detected in the Mössbauer spectrum of Lu 3 Rh 2 Sn 4 . From atoms in molecules (AIM) analysis of the charge density obtained with calculation based on density functional theory (DFT) for Y 3 Rh 2 Sn 4 , the charge transfer proceeds from yttrium towards more electronegative rhodium. Little departure from neutrality is observed for tin whose itinerant s -like states are little involved with the bonding. The site projected density of states (DOS) and the crystal orbital overlap population (COOP) plots further illustrate these observations and reveal major Y–Rh and Rh–Sn bonding, while Y–Sn bonding is weaker.

Phase analytical studies in the Sc–Cu–In system led to samples of the solid solutions ScCu 1– x – y In 1+ x and ScCu 2– x In which were studied by X-ray powder diffraction. At room temperature the compounds ScCu 1– x – y In 1+ x crystallize with the ZrNiAl type, space group P 6 ‾ $\overline{6}$ 2 m . Exemplarily, the structure of ScCu 0.76 In 1.17 was refined from single crystal X-ray diffractometer data, revealing strong anisotropic displacements for the scandium atoms and a mixed occupied Cu/In site. Superstructure formation is observed at low temperatures. The ScCu 0.78 In 1.14 and ScCu 0.76 In 1.16 structures were refined from diffraction data recorded at 90 K. Both compounds adopt the HfRhSn type, space group P 6 ‾ $\overline{6}$ 2 c , a klassengleiche subgroup of index 2; doubling of the subcell c axis. The Cu/In filled trigonal Sc 6 prisms are strongly distorted in the superstructure, resulting from pairwise dislocation of the Cu/In atoms from ideal positions within an equidistant chain to shorter (311.0 pm) and longer (392.8 pm) Cu/In–Cu/In distances. Single crystal data of the Heusler phases ScCu 1.95 In and ScCu 1.94 In show small degrees of copper vacancies.

Colorless rubidium thiocyanate was synthesized by three different approaches. DCS-TGA measurements allowed for extraction of phase transition, melting, and decomposition temperatures. Single-crystal X-ray diffraction showed Rb[SCN] to crystallize in the low-temperature LT-K[SCN] crystal structure type ( Pbcm , oP16). Above its phase-transition temperature (432.1 K) the title compound crystallizes with the high-temperature HT-K[SCN] crystal structure type ( I 4/ mcm , tI28) with head-to-tail disordered thiocyanate anions. Both modifications are related to one another and to the LT/HT-Cs[SCN] structure types, and the relation has been studied in detail employing the Bärnighausen formalism. Phase purity and bulk phase transition were confirmed by temperature-dependent PXRD.

An achiral tripeptide, namely, Boc-γ-Abu- m -ABA-Aib-OMe (γ-Abu: γ−amino butyric acid; m -ABA: meta -aminobenzoic acid) was synthesized by solution phase procedure. The α, γ-hybrid peptide was designed in such a way that two dissimilar γ−amino acids, one flexible and another rigid, were positioned sidewise along with α-amino isobutyric acid (Aib) as C-terminal residue. The single crystal X-ray diffraction analysis revealed that two kinks were generated around centrally placed m -ABA. Interestingly, the peptide self-assembled via three intermolecular N–H ··· O and one intermolecular C–H ··· O hydrogen bonding interactions to supramlecular helical architecture.