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In the past few years, a new phase, Ba0.5Sr0.5Zn2Si2O7 with negative thermal expansion has been described in the literature. Some excess of SiO2 is necessary to produce glasses from which the Ba0.5Sr0.5Zn2Si2O7 phase can be crystallized. Unfortunately, in such glasses usually surface crystallization occurs; however, the addition of nucleating agents such as trace quantities of platinum or relatively high quantities of ZrO2 is necessary to achieve bulk nucleation. These additional components also affect the crystal growth velocity, which furthermore is different for crystal growth from the surface and in the bulk. In this paper, three different chemical compositions containing different ZrO2 concentrations, where one composition additionally contains 100 ppm platinum, are studied with respect to their crystallization behaviour. Although the compositions do not differ much, the crystallization behaviour and also the crystal growth velocities are surprisingly different.

Origin and composition dependence of the anisotropic thermomechanical properties are elucidated for Ba1−xSrxZn2Si2O7 (BZS) solid solutions. The high-temperature phase of BZS shows negative thermal expansion (NTE) along one crystallographic axis and highly anisotropic elastic properties characterized by X-ray diffraction experiments and simulations at the density functional theory level. Ab initio molecular dynamics simulations provide accurate predictions of the anisotropic thermal expansion in excellent agreement with experimental observations. The NTE considerably decreases with increasing Sr content x. This is connected with the composition dependence of the vibrational density of states (VDOS) and the anisotropic Grüneisen parameters. The VDOS shifts to higher frequencies between 0-5 THz due to substitution of Ba with Sr. In the same frequency range, vibrational modes contributing most to the NTE are found. In addition, phonon calculations using the quasi-harmonic approximation revealed that the NTE is mainly connected with deformation of four-membered rings formed by SiO4 and ZnO4 tetrahedra. The thermomechanical and vibrational properties obtained in this work provide the basis for future studies facilitating the targeted design of BZS solid solutions as zero or negative thermal expansion material.

The crystallization behavior of BaO-SrO-ZnO-SiO2 glasses containing different concentrations of SnO2 was studied using microscopic techniques as well as thermal analyses. It is shown that SnO2 triggers volume crystallization, but at low nucleation rates. During heat treatments near respective glass transition temperatures, nucleation takes place. In the course of this nucleation treatment, SnO2 added leads to the formation of small metallic particles which serve as nuclei for the growth of Ba1-xSrxZn2Si2O7 crystals. The metallic nuclei are composed of metallic Sn and Pt. The latter is introduced into the glass during synthesis in a Pt crucible as a contaminant. Besides scanning electron microscopy, also X-ray microscopy was used to study the microstructure as well as the origin of appearing cracks in three dimensions. It was found that the different morphologies detected via scanning electron microscopy are solely due to different orientations of the crystals within the glassy matrix. Furthermore, evidence is given that in some cases microcrack formation starts right at the center of the crystals, where metallic particles are found.

The role of silver as a nucleating agent in BaO/SrO/ZnO/SiO2 glasses is studied with a range of microstructure characterization techniques, such as scanning transmission electron microscopy, ultraviolet-visible spectroscopy, and X-ray diffraction. For this purpose, glasses with different silver and antimony concentrations were synthesized. It was found that silver-particle formation depends strongly on the silver concentration but also on the ratio Ag/Sb. A comparison of the measured plasmon resonances with those calculated from Mie theory enables a calculation of the particle size. This size depends on composition and varies depending on the chosen temperature between 11 and 60 nm. The addition of 1.5 mol% of Sb2O3 is necessary to achieve volume crystallization of Ba1-xSrxZn2Si2O7 (LEAZit) in the glass. This is due to the formation of core-shell structures with metallic silver particles in the center and an oxidic shell, which is highly enriched in Sb and Zn. The metallic silver particles surrounded by a shell show a plate-like shape and provoke the growth of LEAZit crystals with low thermal expansion.