Polymorphism of Ba₂SiP₄

Abstract

The three‐dimensional SiP4 network in the known phosphidosilicate Ba2SiP4‐tI28 is analogous to β‐Cristobalite if oxygen is formally replaced by P–P dimers. Here we report a second polymorph Ba2SiP4‐oP56 [Pnma, a = 12.3710(4) Å, b = 14.6296(7) Å, c = 7.9783(3) Å; Z = 8] with chains of SiP4 tetrahedra connected by P–P bonds, reminiscent to the elusive fibrous SiO2. Ba2SiP4 is enantiotropic. The high temperature polymorph Ba2SiP4‐oP56 transforms to the low‐temperature phase Ba2SiP4‐tI28 at 650 °C and reconstructs to the high‐temperature modification at 1100 °C. DFT calculations predict an indirect optical bandgap of about 1.7 eV.

Synthesis, Crystal and Electronic Structure of La2SiP4

“…Further structural similarities can be found with Ba 2 SiP 4 (Figure 4). [27–29] In the crystal structure of the orthorhombic polymorph (space group Pnma ), Ba 2+ atoms are located in‐between the SiP 4 chains but are capped by three different types of polyhedra: elongated square pyramids, cubes, and elongated square bipyramids. Although the composition is the same, due to a different charge on the cation, Ba 2+ vs La 3+ , the resulting connectivity of SiP 4 tetrahedra is different.…”

“…Metal tetrel‐pnictides ( A‐TtPn , A =electropositive metal, Tt =Si, Ge and Sn, Pn =P and As) are a large family of compounds showing a variety of crystal structures and atom connectivity, and exhibiting exciting transport, magnetic, and optical properties [1–9] . The underlying Tt‐Pn anionic sublattice can range from isolated TtPn 4 tetrahedra (Ca 4 SiP 4 , Sr 4 SiP 4 , Ba 4 SiP 4 , Li 8 SiP 4 and Li 8 GeP 4 ), 1D‐chains (Ca 3 Si 2 P 4 and K 2 SiP 2 ), 2D‐layers (LiGe 3 P 3 , LiGe 3 As 3 , Cs 0.16 SiAs 2 , Ca 2 Si 2 P 4 , Li 1‐x Sn 2 As 2 ), to 3D‐frameworks (Li 2 GeP 2 , SrSi 7 P 10 , BaSi 7 P 10 , Li 2 SiP 2 , LiSi 3 As 6 , MgSiAs 2 , Mg 3 Si 6 As 8 , and Ca 3 Si 8 P 14 ) [2,7,10–24] . The tetrel‐pnictide tetrahedral unit can be connected to its neighbors via sharing vertices and edges, or by P−P bonds [1,13,25–27] …”

“…Among metal tetrel‐pnictides, AE 2 SiP 4 ( AE =Sr, Eu, and Ba) is an interesting class of compounds [27–29] . Sr‐ and Eu‐containing 2‐1‐4 compounds are isostructural to tetragonal‐Ba 2 SiP 4 and are characterized by SiP 4 units connected to each other via P−P bonds forming a polar 3D‐network [29] .…”

“…Sr‐ and Eu‐containing 2‐1‐4 compounds are isostructural to tetragonal‐Ba 2 SiP 4 and are characterized by SiP 4 units connected to each other via P−P bonds forming a polar 3D‐network [29] . Orthorhombic‐Ba 2 SiP 4 on the other hand has SiP 4 tetrahedra connected by P−P bonds forming 1D‐chains [28] …”

The Phosphidosilicates SrSi7P10 and BaSi7P10

“…[ 5 ] Only recently, further alkaline and alkaline earth phosphidosilicates were reported, which comprise homonuclear Si–Si or P–P bonding, or with SiP 4 tetrahedra which coalesce into supertetrahedral entities. [ 6–12 ] Examples with P–P bonds are the compounds AE 2 SiP 4 [ 13–15 ] ( AE = Sr, Ba, Eu), where all phosphorus atoms form dimers. Additionally, Mark et al demonstrated the thermal conductivity and SHG responses of Ba 2 Si 3 P 6 making this compound potentially interesting for IR‐NLO materials.…”