KNaCaSi 4 O 10 Obtained From Crystallization Experiments in the Quaternary System K 2 O–Na 2 O–CaO–SiO 2

ABSTRACT Needle to lath‐shaped single crystals of KNaCaSi 4 O 10 were obtained from the crystallization of a glass with a molar ratio of K 2 O:Na 2 O:CaO:SiO 2 = 2:2:1:10. The crystals within the samples were intergrown. They form an interleaved microstructure, thereby providing mechanical support to one another. Structural investigations of this new synthetic compound in the quaternary system K 2 O–Na 2 O–CaO–SiO 2 were performed using single‐crystal X‐ray diffraction experiments. The basic crystallographic data are as follows: triclinic symmetry, space group , a = 7.0784(5)Å, b = 8.0744(6) Å, c = 10.1449(9) Å, α = 103.029(7)°, β = 99.873(7)°, γ = 116.103(8)°, V = 482.39(7) Å 3 , and Z = 2. The determination of the structure was performed using direct methods. Subsequent least‐squares refinements of the coordinates and anisotropic displacement parameters for all 17 symmetrically independent atoms in the asymmetric unit converged at R(|F|) = 0.0351 for 1721 reflections and 154 parameters. According to Liebau's classification of oxosilicates, the present compound belongs to the group of tubular chain silicates. The fundamental chains of the tubes run parallel to the a ‐axis. More specifically, the tubes can be described as loop‐branched dreier double chains: . An individual tube contains four‐, six‐, and eight‐membered rings of silicate tetrahedra. The sodium and calcium atoms are distributed in an ordered manner on two different cation sites. Both positions show a similar 5 + 1 coordination environment. In fact, the inner NaO 5 ‐ and CaO 5 ‐polyhedra can be approximated as distorted quadratic pyramids. These pyramids share common edges and form a second set of double chains, again running parallel to 100. Linkage between adjacent silicate tubes is further provided by the potassium ions located in the centers of the aforementioned eight‐membered rings. Each K cation has ten nearest oxygen ligands. Structural investigations were completed using Raman and infrared (IR) spectroscopy. The spectroscopic data were interpreted, and the bands were assigned to specific vibrational species with the help of density functional calculations at the HSEsol level of theory. The network of interlaced lath‐shaped crystals formed during the devitrification process could lead to crack deflection and toughening. This makes KNaCaSi 4 O 10 a potential new candidate for a crystalline phase in silicate‐based glass ceramics.