A new nonlinear optical silicate carbonate K2Ca[Si2O5](CO3) with a hybrid structure of kalsilite and soda-like layered fragments

• A new compound, a rare silicate carbonate K2Ca[Si2O5](CO3), is synthesized hydrothermally.
• The crystal structure is a hybrid of the two neutral layered fragments: silicate K2[Si2O5] and carbonate Ca(CO3).
• Optical nonlinearity is of the order of α-quartz and other acentric silicates with low electronic polarizability.

Single crystals of a new silicate carbonate, K2Ca[Si2O5](CO3), have been synthesized in a multi-components hydrothermal solution with a pH value close to neutral and a high concentration of a carbonate mineralizer. The new compound has an axial structure (s.g. P6322) with unit cell parameters a = 5.04789 (15), c = 17.8668 (6) Å. Pseudosymmetry of the structure corresponds to s.g. P63/mmc which is broken only by one oxygen position. The structure consists of two layered fragments: one of the type of the mineral kalsilite (KAlSiO4) and the other of the high-temperature soda-like α-Na2CO3, Ca substituting for Na. The electro-neutral layer K2[Si2O5] (denoted K) as well as the layer Ca(CO3) (denoted S) may separately correspond to individual structures. In K2Ca[Si2O5](CO3) the S-K layers are connected together via Ca-O interactions between Ca atoms from the carbonate layer and apical O atoms from the silicate one, and also via K-O interlayer interactions. A hypothetical acentric structure, sp.gr. P-62c, is predicted on the basis of the order-disorder theory. It presents another symmetrical option for the arrangement of K-layers relative to S-layers. The K,Ca-silicate-carbonate powder produces a moderate SHG signal that is two times larger that of the α-quartz powder standard and close to other silicates with acentric structures and low electronic polarizability.

The new acentric silicate carbonate, space group P6322, has a hybrid structure built from two neutral layered fragments: K2[Si2O5] (denoted K) similar to kalsilite K[AlSiO4] and CaCO3 (denoted S) which is soda-like Na2(CO3) with Ca for Na substitution. SHG optical activity of powdered K2Ca[Si2O5](CO3) is about twice as that from α-quartz powder.Figure optionsDownload as PowerPoint slide