Noncollinear Electric Dipoles in a Polar Chiral Phase of CsSnBr3 Perovskite.

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two phases of the 3D perovskite, CsSnBr3, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K < T < 85 K, space group P21/m) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I (T < 77 K, space group P21) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and pronounced electrostriction and negative thermal expansion along the polar axis (Q22 ≈ 1.1 m4 C-2; αb = -7.8 × 10-5 K-1) through the onset of polarization. The structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and 81Br nuclear quadrupole resonance spectroscopies. Preliminary attempts to suppress unintentional hole doping to allow for ferroelectric switching are described. Together, the polar symmetry, small band gap, large spin-orbit splitting of Sn 5p orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr3 or its neighboring solid solutions as candidates for bulk Rashba effects.

Synthesis of Nitride Zeolites in a Hot Isostatic Press.

The recently introduced nitridophosphate synthesis in a hot isostatic press (HIP) enabled simple access to large-scale product quantities starting from exclusively commercially available starting materials. Herein, we show that this method is suitable for the synthesis of highly condensed functional nitridophosphates, as well. Hence, the syntheses of the nitridophosphate zeolites Ba3 P5 N10 X (X=Cl, Br) are presented as proof of concept for this innovative access. Furthermore, samples of unprecedented Sr3 P5 N10 X (X=Cl, Br) were prepared and characterized to demonstrate the advantages of this synthetic approach over commonly used methods. Luminescence investigations on Eu2+ -doped samples of AE3 P5 N10 X (AE=Sr, Ba; X=Cl, Br) were carried out and characteristics of observed emission bands are discussed.

BaP6 N10 NH:Eu2+ as a Case Study-An Imidonitridophosphate Showing Luminescence.

Barium imidonitridophosphate BaP6 N10 NH was synthesized at 5 GPa and 1000 °C with a high-pressure high-temperature approach using the multianvil technique. Ba(N3 )2 , P3 N5 and NH4 Cl were used as starting materials, applying a combination of azide and mineralizer routes. The structure elucidation of BaP6 N10 NH (P63 , a=7.5633(11), c=8.512(2) Å, Z=2) was performed by a combination of transmission electron microscopy and single-crystal diffraction with microfocused synchrotron radiation. Phase purity was verified by Rietveld refinement. 1 H and 31 P solid-state NMR and FTIR spectroscopy are consistent with the structure model. The chemical composition was confirmed by energy-dispersive X-ray spectroscopy and CHNS analyses. Eu2+ -doped samples of BaP6 N10 NH show blue emission upon excitation with UV to blue light (λem =460 nm, fwhm=2423 cm-1 ) representing unprecedented Eu2+ -luminescence of an imidonitride.