KSi2P3: A new layered phosphidopolysilicate (IV)

Electronic Structure Analysis of the A10Tt2P6 System (A=Li-Cs; Tt=Si, Ge, Sn) and Synthesis of the Direct Band Gap Semiconductor K10Sn2P6

Investigating the relationship between atomic and electronic structures is a powerful tool to screen the wide variety of Zintl phases for interesting (opto-)electronic properties. To get an insight in such relations, the A10Tt2P6 system (A=Li-Cs; Tt=Si-Sn) was picked as model system to analyse the influence of structural motives, combination of elements and their properties on type and width of the band gaps. Those compounds comprise two interesting structural motives of their anions, which are either monomeric trigonal planar TtP3 5- units which are isostructural to CO3 2- or [Tt2P6]10- dimers which correspond to two edge-sharing TtP4 tetrahedra. The A10Tt2P6 compounds were structurally optimized for both polymorphs and subsequent frequency analysis, band structure as well as density of states calculations were performed. The Gibbs free energies were compared to determine temperature dependent stability, where Na10Si2P6, Na10Ge2P6 and K10Sn2P6 were found to be candidates for a high temperature phase transition between the two polymorphs. Additionally, the unknown, but predicted compound K10Sn2P6 was synthesized and characterized by single crystal and powder x-ray diffraction. It crystalizes in the monoclinic space group P 21/n and incorporates [Sn2P6]10- edge sharing double tetrahedra. It was determined to be a direct band gap semiconductor with a band gap of 2.57 eV.

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.

Uncovering a Vital Band Gap Mechanism of Pnictides

Pnictides are superior infrared (IR) nonlinear optical (NLO) material candidates, but the exploration of NLO pnictides is still tardy due to lack of rational material design strategies. An in-depth understanding structure-performance relationship is urgent for designing novel and eminent pnictide NLO materials. Herein, this work unravels a vital band gap mechanism of pnictides, namely P atom with low coordination numbers (2 CN) will cause the decrease of band gap due to the delocalization of non-bonding electron pairs. Accordingly, a general design paradigm for NLO pnictides, ionicity-covalency-metallicity regulation is proposed for designing wide-band gap NLO pnictides with maintained SHG effect. Driven by this idea, millimeter-level crystals of MgSiP2 are synthesized with a wide band gap (2.34 eV), a strong NLO performance (3.5 x AgGaS2 ), and a wide IR transparency range (0.53-10.3 µm). This work provides an essential guidance for the future design and synthesis of NLO pnictides, and also opens a new perspective at Zintl chemistry important for other material fields.

Polymorphism and Fast Potassium-Ion Conduction in the T5 Supertetrahedral Phosphidosilicate KSi2 P3

The all‐solid‐state battery (ASSB) is a promising candidate for electrochemical energy storage. In view of the limited availability of lithium, however, alternative systems based on earth‐abundant and inexpensive elements are urgently sought. Besides well‐studied sodium compounds, potassium‐based systems offer the advantage of low cost and a large electrochemical window, but are hardly explored. Here we report the synthesis and crystal structure of K‐ion conducting T5 KSi₂P₃ inspired by recent discoveries of fast ion conductors in alkaline phosphidosilicates. KSi₂P₃ is composed of SiP₄ tetrahedra forming interpenetrating networks of large T5 supertetrahedra. The compound passes through a reconstructive phase transition from the known T3 to the new tetragonal T5 polymorph at 1020 °C with enantiotropic displacive phase transitions upon cooling at about 155 °C and 80 °C. The potassium ions are located in large channels between the T5 supertetrahedral networks and show facile movement through the structure. The bulk ionic conductivity is up to 2.6×10⁻⁴ S cm⁻¹ at 25 °C with an average activation energy of 0.20 eV. This is remarkably high for a potassium ion conductor at room temperature, and marks KSi₂P₃ as the first non‐oxide solid potassium ion conductor.

Supertetrahedral anions in the phosphidosilicates Na1.25Ba0.875Si3P5 and Na31Ba5Si52P83

Solid ionic conductors are one key component of all-solid-state batteries, and recent studies with lithium, sodium and potassium phosphidosilicates revealed remarkable ion conduction capabilities in these compounds. We report the synthesis and crystal structures of two quaternary phosphidosilicates with sodium and barium, which crystallize in new structure types. Na1.25Ba0.875Si3P5 contains layers of T3 supertetrahedra, while Na31Ba5Si52P83 forms defect T5 entities and contains Si-Si bonds and P3 trimers. Though T1-relaxometry data indicate a relatively low activation energy for Na+ migration of 0.16 eV, the crystal structures lack sufficient three-dimensional migration paths necessary for fast sodium ion conductvity.