Polymorphism of Ba 2 SiP 4

The Phosphidosilicates AE2 Li4 SiP4 (AE=Ca, Sr, Eu) Ba4 Li16 Si3 P12

The quaternary phosphidosilicates AE2 Li4 SiP4 (AE=Ca, Sr, Eu) and Ba4 Li16 Si3 P12 were synthesized by heating the elements and Li3 P under argon atmosphere. Their crystal structures were determined by single crystal X-ray diffraction. AE2 Li4 SiP4 crystallize in a new layered structure type (P21 /m, Z=2) with CdI2 -analoguos layers. Edge sharing CaP6 octahedra are separated by layers of vertex-sharing SiP4 and LiP4 tetrahedra, which contain additional chains of LiP6 octahedra. Ba4 Li16 Si3 P12 forms likewise a new structure type (P21 /c, Z=16) with a three-dimensional network of SiP4 , Si2 P6 and LiP4 entities as well as one phosphorus site not bonded to silicon. Barium is located in capped trigonal prisms of phosphorus which form strongly corrugated layers. 31 P and 29 Si solid-state NMR spectra confirm the crystal structures of the compounds AE2 Li4 SiP4 . 7 Li spectra show only one signal in spite of quite different crystallographic positions, which indicate possible Li+ mobility. However, this signal is much broader compared to the known Li+ conducting phosphidosilicates. Accordingly, electrochemical impedance measurements show low Li+ conductivities.

Third time's the charm: intricate non-centrosymmetric polymorphism in LnSiP3 (Ln = La and Ce) induced by distortions of phosphorus square layers

Complex polymorphic relationships in the LnSiP3 (Ln = La and Ce) family of compounds are reported. An innovative synthetic method was developed to overcome differences in the reactivities of the rare-earth metal and refractory silicon with phosphorus. Reactions of atomically mixed Ln + Si with P allowed for selective control over the reaction outcomes resulting in targeted isolation of three new polymorphs of LaSiP3 and two polymorphs of CeSiP3. In situ X-ray diffraction studies revealed that the developed method bypasses formation of the thermodynamic dead-end, the binary SiP. Careful re-determination of the crystal structure ruled out the previously reported ordered centrosymmetric structure of CeSiP3 and showed that the main LnSiP3 polymorphs crystallize in the non-centrosymmetric Pna21 and Aea2 space groups featuring distinct distortions of the regular P square net to yield either cis-trans 1D phosphorus chains (Pna21) or disordered-2D phosphorus layers (Aea2). The disordered 2D nature of the P layers in the Aea2 LaSiP3 polymorph was confirmed by Raman spectroscopy. A unique centrosymmetric P21/c polymorph was observed for LaSiP3 and has a completely different crystal structure lacking P layers. Consecutive polymorphic transformations at increasing temperatures for LaSiP3(Pna21 → P21/c → Aea2) were derived from optimized synthetic profiles and confirmed by a combination of phonon computations and experimental in situ and ex situ annealings. Crystal structures of the LaSiP3 polymorphs were verified via advanced solid state NMR analysis using 31P MAS and 31P{139La} double resonance techniques. A combination of phonon and electronic structure calculations, NMR T1 relaxation times, UV/Vis/NIR spectroscopy, and resistivity measurements revealed that all the reported polymorphs are semiconductors with resistivities and thermal conductivities strongly dependent on the degree of distortion of P square layers in the crystal structure. Reported here, non-centrosymmetric LnSiP3 polymorphs with tunable resistivity and thermal conductivity provide a platform for the development of novel functional materials with a wide range of applications.

Mixed Valence and Unusual Germanium Coordination in SrGe8As10, BaGe8As10, and BaGe7P12

The new Zintl-compounds AEGe₈As₁₀ (AE = Sr, Ba) and BaGe₇P₁₂ were synthesized via solid-state reactions, and their structures were determined by single crystal and powder X-ray diffraction. SrGe₈As₁₀ and BaGe₈As₁₀ crystallize in the space group Cmce and show complex 3D networks composed of three different Ge-As motifs and As-As bonds with mixed valence of germanium in the oxidation states +2, + 3, and +4. Mixed valences of germanium +3 and +4 occur in BaGe₇P₁₂, which crystallizes in the space group R3̅ with a 3D network built up of Ge₂P₆ dumbbells and P-P bonds. An exceptional 6-fold coordinated germanium resides in the center of a GeP₆ trigonal antiprism. High temperature X-ray diffraction shows thermal stabilities up to 923-953 K. UV-Vis and resistivity measurements reveal a semiconducting nature with small indirect band gaps between 0.02 and 1.6 eV. Electronic band structure calculations confirm the semiconducting state and indicate covalent bonds within the Ge-Pn polyanions.