High‐Pressure Investigation in the System SiO2–GeO2: Mutual Solubility of Si and Ge in Quartz,Coesite and Rutile Phases

Mutual solubilities in crystalline phases in the system SiO₂–GeO₂ have been investigated up to 10 GPa pressure and 1500°C temperature, using a bulk composition of 50 mol% GeO₂. Solid solution of up to 40 mol% GeO₂ into the mineral quartz has been confirmed as well as solubility of Si into GeO₂ rutile (argutite) and Ge into SiO₂ rutile (stishovite) phases and limited Ge into coesite. Solubility of Ge in quartz is very high, and decreases with pressure, with the univariant quartz‐out reaction occurring near 3.4 GPa at 1200°C. The solubility of GeO₂ in coesite is highest at 3.4 GPa (about 8 mol%) and decreases with increasing pressure. Significantly more extensive solubility than previously reported for the rutile phases has been found and measured in detail as a function of pressure and temperature. Extensive solubility of SiO₂ in GeO₂ is found in argutite at 1200°C, increasing strongly with pressure and reaching a maximum of 25.2 mol% SiO₂ in GeO₂ at 9 GPa. At this point coesite (ss) plus argutite (ss) react to form a stishovite phase with 18 mol% GeO₂, and the mutual solubility in both phases decreases above this pressure. At 1500°C, similar solubilities are observed but the maximum SiO₂ solubility in argutite of just over 25 mol% occurs near 10 GPa. All these solid solutions can be recovered to ambient temperature and pressure. Phase diagrams and unit cell information of the phases are presented here. Based on these results, a useful and industrially relevant, application for accurately measuring high pressure is suggested.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Ab initio Modeling of the Electronic Structures and Physical Properties of a‐Si1−xGexO2 Glass (x = 0 to 1)

The amorphous silica (a‐SiO₂) and germania (a‐GeO₂) have a wide range of applications in glass industry. Based on a previously constructed near‐perfect continuous random network model with 1296 atoms and periodic boundary conditions, we extend our study to amorphous Si_1?xGe_xO₂ models of homogeneous random substitution of Si by Ge with x ranging from 0 to 1. We have calculated the structural, electronic, mechanical, and optical properties for the series by using the first‐principles density functional theory methods. The x‐dependence of the variations in the properties is analyzed and critically compared with available experimental data. The mass density, volume, total bond order density, bulk mechanical properties, and refractive index are found to vary linearly as a function of x. For x = 0.5, we have also constructed six different kinds of particle immersion models to test the effect of inclusion of spherical particles of one glass of different sizes in the medium of the other glass on their physical properties. It is shown that particle sizes do affect the properties of particle immersion. Our calculations provide deep insight on the properties of mixture and nanocomposites of a‐SiO₂ and a‐GeO₂ glasses.     

Crystallization Behavior of Amorphous Si2BC3N Ceramic Monolith Subjected to High Pressure

The crystallization behavior of amorphous Si₂BC₃N monoliths by heating at 1000°C–1400°C and 5 GPa was investigated with the special attention to the nucleation mechanisms of β‐SiC and BN(C) phases. Nanoscale puckered structures arising in particle bridging areas were found and its evolution behavior well reflected the nucleation process of nanocrystallites. The temperature‐dependent crystallization of amorphous Si₂BC₃N monoliths at 5 GPa passes through four stages: The material remains amorphous below 1100°C. It undergoes partial phase segregation (1100°C–1200°C), followed by initiation of nucleation (1200°C–1250°C), and then nucleation and growth of β‐SiC and turbostratic BN(C) crystallites (>1250°C). The first principles calculation indicates the nucleation precedence of BN(C) phase over β‐SiC. BN(C) nucleates preferentially at bridges between ceramic particles causing SiC to concentrate in particle interiors thus forming capsule‐like structures.     

The relationship between crystal structure and modified microwave dielectric properties of Ca3SnSi2-xGexO9 ceramics

Ca₃SnSi_2-xGe_xO₉ (0 ≤ x ≤ 0.8) and (1–y) Ca₃SnSi_1.6Ge_0.4O₉ – y CaSnSiO₅ – 2 wt% LiF (y = 0.4 and 0.5) microwave dielectric ceramics were prepared by traditional solid-state reaction through sintering at 1250°C–1425°C for 5 h and at 875°C for 2 h, respectively. Ge⁴⁺ replaced Si⁴⁺, and Ca₃SnSi_2-xGe_xO₉ (0 ≤ x ≤ 0.4) solid solutions were obtained. At 0.1 ≤ x ≤ 0.4, the Ge⁴⁺ substitution for Si⁴⁺ decreased the sintering temperature of Ca₃SnSi_2-xGe_xO₉ from 1425 to 1300°C, the SnO₆ octahedral distortions, and the average CaO₇ decahedral distortions, which affected the τ_f value. The large average decahedral distortions corresponded with nearer-zero τ_f values at Ca₃SnSi_2-xGe_xO₉ (0.1 ≤ x ≤ 0.4) ceramics. The τ_f value and sintering temperature of Ca₃SnSi_2-xGe_xO₉ (x = 0.4) ceramic were adjusted to near-zero by CaSnSiO₅ and decreased to 875°C upon the addition of 2 wt% LiF. The (1 – y) Ca₃SnSi_1.6Ge_0.4O₉ – y CaSnSiO₅ – 2 wt% LiF (y = 0.5) ceramic sintered at 875°C for 2 h exhibited good microwave dielectric properties: ε_r = 10.3, Q × f = 14 300 GHz (at 12.2 GHz), and τ_f = ‒5.8 ppm/°C.     

Hardness of polycrystalline SiO2 coesite

We measured elastic moduli and hardness of polycrystalline SiO₂ coesite. Translucent polycrystalline bulk coesite with a grain size of about 10 micrometers was fabricated at 8 GPa and 1600°C using a Kawai-type multianvil apparatus. The obtained bulk and shear moduli are 94(1) and 60.2(3) GPa, respectively. The resulting Vickers and Knoop hardness values are 10.9(7) and 9.6(4) GPa, respectively, at an indentation load of 4.9 N. Coesite is as hard as other fourfold coordinated silica materials such as quartz and densified silica glasses. The hardness values of coesite and the fourfold coordinated silica materials are about one-third of those of sixfold coordinated silica materials, stishovite, and seifertite, which are the hardest known oxides.     

Solubility and microstructural development of TiO2-containing 3Al2O3·2SiO2 and 2Al2O3·SiO2 mullites obtained from single-phase gels

The interdependence of the titanium oxide amount and the anisotropic growth of mullites prepared from single-phase gels were investigated. Gels with stoichiometries 3(Al_2−xTi_xO₃)·2(SiO₂) and 2(Al_2−xTi_xO₃)·(SiO₂), with 0 ≤ x ≤ 0.15 were prepared by the semialkoxide method. Gels and specimens heated at temperatures between 1200 and 1600 °C were characterized by using infrared spectroscopy (IR), X-ray diffraction (XRD) and transmission and field emission scanning electron microscopies (TEM and FESEM). Al₂TiO₅ as minor impurity was detected in both series of mullites for gel precursor compositions x = 0.10 and x = 0.15, obtained at temperatures between 1200 and 1600 °C. Variations of lattice parameters of mullite, processed at temperatures from the range between 1400 and 1600 °C, with the starting nominal amount of titanium oxide indicated that the solubility limit of titanium oxide was in ranges 3.8–4.1 and 4.1–4.4 wt% TiO₂ for 3:2 and 2:1 mullites series, respectively. The anisotropic growth of titanium-doped mullite crystalline grains was significant only when the nominal amount of titanium oxide exceeded the limit of solubility into the mullite structure (for both mullite series). Stronger anisotropy occurred for the 3:2 series specimens, i.e. for the SiO₂-richer mullites. In both series of mullites, the anisotropic grain growth was observed for the process temperatures higher than 1400 °C; the crystalline grains of mullites processed at lower temperatures were equiaxials and of almost the same size.     

Effects of SiO2 addition on TiO2 crystal structure and photocatalytic activity

A series of TiO₂–SiO₂ mixtures – having the following stoichiometry Ti_1?xSi_xO₂, with x = 0, 0.1, 0.3 and 0.5 atoms per formula unit – were prepared by using precursor oxides and fired at three temperatures (900, 1000 and 1200 °C). The modifications in the structure and, consequently, on the photocatalytic activity, induced by the addition of SiO₂ into the TiO₂ powder, were thoroughly investigated by using various analytical techniques: X-ray powder diffraction, electron microscopy (FE-SEM and TEM), XPS, FT-IR, DRS and BET analysis. The results underlined as essentially no solid solution occurs between the two crystalline end-members. Nevertheless, silica addition caused a retarding effect on anatase-to-rutile phase transformation and on the crystallite growth.The photocatalytic activity of the powders was assessed in gas phase and the results were explained by taking into account the anatase and rutile relative amounts in the samples, their crystallite size, the surface hydroxyl groups adsorbed on the photocatalysts and the surface area of the mixtures.     

Highly Dense Amorphous Si2BC3N Monoliths with Excellent Mechanical Properties Prepared by High Pressure Sintering

The fabrication of dense amorphous Si–B–C–N monoliths is a processing challenge given that it is hard to avoid crystallization at the sintering temperatures needed to attain full density up to 1900°C for conventional hot pressing and SPS methods. We report here successful densification of amorphous Si₂BC₃N monoliths achieved by heating at 1100°C and 5 GPa. The relationships between microstructure, types of chemical bonding, and mechanical properties were investigated. The strong amorphous 3‐D networks of Si–C, C–B, C‐N (sp³), N‐B (sp³), and C–B–N bonds provide high densities at high applied pressure and thus amorphous Si₂BC₃N monoliths show high hardness of 29.4 GPa and elastic modulus of 291 GPa. The amorphous structure is lost with crystallization of β‐SiC and BN(C) reducing contributions from Si–C, C‐N (sp³), and C–B–N bond networks thereby decreasing mechanical properties.     

Fabrication and Microwave Dielectric Properties of Mg2SiO4‐LiMgPO4‐TiO2 Composite Ceramics

Complete solid solutions between Mg₂SiO₄ and LiMgPO₄ are confirmed by the XRD results. The phase constitution of 0.5Mg₂SiO₄‐0.5LiMgPO₄ is found to be dependent on firing temperature. The chemical compatibility between Mg₂SiO₄ and rutile phase at sintering temperature is modified by incorporating LiMgPO₄. The microwave dielectric properties of (1?y)(0.5Mg₂SiO₄‐0.5LiMgPO₄)‐yTiO₂ (y = 0–0.3) composite ceramics have been investigated. The optimized microwave dielectric properties for 0.35Mg₂SiO₄‐0.35LiMgPO₄‐0.3TiO₂ ceramics sintered at 1050°C show low dielectric constant (11.4), high‐quality factor (31 800 GHz), and low‐temperature coefficient of resonant frequency (?4 ppm/°C).     

Microwave Dielectric Ceramics Li2MO4−TiO2 (M = Mo,W) with Low Sintering Temperatures

In this work, novel series of (1 ? x)Li₂MO₄–xTiO₂ (M = Mo, W; x = 0.3, 0.4, 0.45, 0.5, 0.6) ceramics were developed for microwave dielectric application. They were prepared via the mixed‐oxide process and the phase composition, microstructures, sintering behaviors, and microwave dielectric properties were investigated. The X‐ray diffraction (XRD) pattern and scanning electron microscope analysis indicated that the Li₂MO₄ (M = Mo, W) did not react with rutile TiO₂ and a stable two‐phase composite system Li₂MO₄–TiO₂ (M = Mo, W) was formed. The XRD pattern of cofired ceramics revealed that some parts of Li₂MoO₄ phase and very small part of Li₂WO₄ phase react with Ag to form Ag₂MoO₄ phase and Ag₂WO₄ phase, respectively. At x = 0.45–0.5, temperature stable microwave dielectric materials with low sintering temperature (700°C–730°C) were obtained: ε_r = 10.6–11.0, Qf = 30 060–32 800 GHz, and temperature coefficient of resonant frequency ~0 ppm/°C.     

High Na‐ion conducting Na1+x[SnxGe2−x(PO4)3] glass‐ceramic electrolytes: Structural and electrochemical impedance studies

Na‐ion conducting Na_1+x[Sn_xGe_2?x(PO₄)₃] (x = 0, 0.25, 0.5, and 0.75 mol%) glass samples with NASICON‐type phase were synthesized by the melt quenching method and glass‐ceramics were formed by heat treating the precursor glasses at their crystallization temperatures. XRD traces exhibit formation of most stable crystalline phase NaGe₂(PO₄)₃ (ICSD‐164019) with trigonal structure. Structural illustration of sodium germanium phosphate [NaGe₂(PO₄)₃] displays that each germanium is surrounded by 6 oxygen atom showing octahedral symmetry (GeO₆) and phosphorous with 4 oxygen atoms showing tetrahedral symmetry (PO₄). The highest bulk Na⁺ ion conductivities and lowest activation energy for conduction were achieved to be 8.39 × 10^?05 S/cm and 0.52 eV for the optimum substitution levels (x = 0.5 mol%, Na_1.5[Sn_0.5Ge_1.5(PO₄)₃]) of tetrahedral Ge⁴⁺ ions by Sn⁴⁺ on Na–Ge–P network. CV studies of the best conducting Na_1.5[Sn_0.5Ge_1.5(PO₄)₃] glass‐ceramic electrolyte possesses a wide electrochemical window of 6 V. The structural and EIS studies of these glass‐ceramic electrolyte samples were monitored in light of the substitution of Ge by its larger homologue Sn.     

Microstructural and Microwave Dielectric Properties of Bi12GeO20 and Bi2O3‐Deficient Bi12GeO20 Ceramics

Bi₁₂GeO₂₀ ceramics sintered at 800°C had dense microstructures, with an average grain size of 1.5 μm, a relative permittivity (ε_r) of 36.97, temperature coefficient of resonance frequency (τ_f) of ?32.803 ppm/°C, and quality factor (Q × f) of 3137 GHz. The Bi_12‐xGeO_20‐1.5x ceramics were well sintered at both 800°C and 825°C, with average grain sizes exceeding 100 μm for x ≤ 1.0. However, the grain size decreased for x > 1.0 because of the Bi₄Ge₃O₁₂ secondary phase that formed at the grain boundaries. Bi_12‐xGeO_20‐1.5x (x ≤ 1.0) ceramics showed increased Q × f values of >10 000 GHz, although the ε_r and τ_f values were similar to those of Bi₁₂GeO₂₀ ceramics. The increased Q × f value resulted from the increased grain size. In particular, the Bi_11.6GeO_19.4 ceramic sintered at 825°C for 3 h showed good microwave dielectric properties of ε_r = 37.81, τ_f = ?33.839 ppm/°C, and Q × f = 14 455 GHz.     

Crystallization Mechanism of CVD Si3N4–SiCN Composite Ceramics Annealed in N2 Atmosphere and Their Excellent EMW Absorption Properties

To tailor a new electromagnetic wave (EMW) absorbing material with lower reflection coefficient (RC) and larger operating frequency band, the CVD Si₃N₄–SiCN composite ceramics were prepared from SiCl₄–NH₃–C₃H₆–H₂–Ar system and then annealed at the temperatures of 1400–1700°C in N₂ atmosphere. Effect of the annealing temperatures on the microstructure, phase composition, permittivity, and microwave‐absorbing properties of the ceramic were investigated. Results showed that the CVD Si₃N₄–SiCN ceramics gradually crystallized into nanosized SiC grains, Si₃N₄ grains and graphite (T ≤ 1600°C), and then the grains grew up at T = 1700°C. The permittivity, dielectric loss, and electrical conductivity of as‐annealed CVD Si₃N₄–SiCN ceramics (T ≤ 1600°C) increased firstly due to the formation of conductivity and polarity network and the increase in nanograin boundary, and then decreased at 1700°C because of the growth of nanograins and the disappearance of nanograin boundary. The minimal RC and effective absorption bandwidth of the as‐annealed CVD Si₃N₄–SiCN ceramic at 1600°C was ?41.67 dB at the thickness of 2.55 mm and 3.95 GHz at the thickness of 3.05 mm, respectively, demonstrating that the totally crystallized CVD Si₃N₄–SiCN ceramic (T = 1600°C) had the superior microwave‐absorbing ability.     

High-temperature Na2SO4 interaction with air plasma sprayed Yb2Si2O7 + Si EBC system: Topcoat behavior

The high-temperature interaction between ~2.5 mg/cm² of Na₂SO₄ and an atmospheric plasma sprayed (APS) Yb₂Si₂O₇ topcoat–Si bond coat system on SiC CMC substrates was studied for times up to 240 h at 1000°C–1316°C in a 0.1% SO₂–O₂ gaseous environment. Yb₂Si₂O₇ reacted with Na₂SO₄ to form Yb₂SiO₅ and an intergranular amorphous Na-silicate phase. Below 1200°C, the reaction was sluggish, needing days to cause morphological changes to the “splat microstructure” associated with APS coatings. The reaction was rapid at 1200°C and above, needing only a few hours for the entire topcoat to transform into a granulated microstructure consisting of Yb₂SiO₅ and Yb₂Si₂O₇ phases. Na₂SO₄ deposits infiltrated the Yb₂Si₂O₇ topcoat and transformed into an amorphous Na-silicate in less than 1 h at all exposure temperatures. Quantitative assessment of the Yb₂SiO₅ area fraction in the topcoat showed a linear decrease over time at 1316°C, attributed to reaction with the SiO₂ thermally grown oxide (TGO) formed on the Si bond coat and rapid transport through the interpenetrating amorphous Na-silicate grain boundary phase. It was predicted that nearly 2 weeks is needed for complete removal of Yb₂SiO₅ from the topcoat at 1316°C for a single applied loading of Na₂SO₄.     

Stability of High‐Temperature Dielectric Properties for (1 − x)Ba0.8Ca0.2TiO3–xBi(Mg0.5Ti0.5)O3 Ceramics

Ceramics in the solid solution system, (1 ? x)Ba_0.8Ca_0.2TiO₃–xBi(Mg_0.5Ti_0.5)O₃, were prepared by a conventional mixed oxide route. Single‐phase perovskite‐type X‐ray diffraction patterns were observed for compositions x < 0.6. A change from tetragonal to single‐phase cubic X‐ray patterns occurred at x ≥ 0.1. Dielectric measurements indicated relaxor behavior for x ≥ 0.1. Increasing the Bi(Mg_0.5Ti_0.5)O₃ content improved the temperature sensitivity of relative permittivity ?_r at high temperatures. At x = 0.5, a near‐plateau relative permittivity, 835 ± 40, extended across the temperature range, 65°C–550°C; the permittivity increased at x = 0.6 to 2170 ± 100 for temperatures 160°C–400°C (1 kHz). The corresponding loss tangent, tanδ, was ≤0.025 for temperatures between 100°C and 430°C for composition x = 0.5; at x = 0.6, losses increased sharply at >300°C. Comparisons of dielectric properties with other materials proposed for high‐temperature capacitor applications suggest that (1 ? x)Ba_0.8Ca_0.2TiO₃–xBi(Mg_0.5Ti_0.5)O₃ ceramics are a promising base material for further development.     

Formation of Sr2Si5N8:Eu2+ and Its Transformation to SrSi6N8:Eu2+ Controlled by Temperature and Gas Pressure

Firing temperature and gas pressure effect of synthesizing Sr₂Si₅N₈:Eu²⁺ were investigated. The emission intensity is positively correlated with the firing temperature under 0.1 and 0.5 MPa gas pressure. The Sr₂Si₅N₈:Eu²⁺ with the highest emission intensity was found at 1700°C and 1980°C under 0.1 and 0.5 MPa gas pressure, respectively. Although the maximum emission intensity of Sr₂Si₅N₈:Eu²⁺ obtained under 0.5 MPa gas pressure condition is higher than that under 0.1 MPa. The Sr₂Si₅N₈:Eu²⁺ synthesized under 0.5 MPa gas pressure in the temperature range from 1600°C to 1800°C have lower emission intensities than that synthesized under 0.1 MPa indicating that the melting of Sr₃N₂ is an important step for the formation of Sr₂Si₅N₈:Eu²⁺. Moreover, the Sr₂Si₅N₈:Eu²⁺ undergoes phase transition into SrSi₆N₈:Eu²⁺ completely after elongating the heating duration to 6 h at 1980°C under 0.5 MPa gas pressure. The same feature was observed under 0.1 MPa gas pressure after firing 8 h at 1750°C. Different heating durations led to different degrees of phase transition.     

Reactive Hot Pressing of ZrC–SiC Ceramics at Low Temperature

Composites of ZrC–SiC with relative densities in excess of 98% were prepared by reactive hot pressing of ZrC and Si at temperature as low as 1600°C. The reaction between ZrC and Si resulted in the formation of ZrC_1?x, SiC, and ZrSi. Low‐temperature densification of ZrC?SiC ceramics is attributed to the formed nonstoichiometric ZrC_1?x and Zr–Si liquid phase. Adding 5 wt% Si to ZrC, the three‐point bending strength of formed ZrC_0.8–13.4 vol%SiC ceramics reached 819 ± 102 MPa with hardness and toughness being 20.5 GPa and 3.3 MPa·m^1/2, respectively.     

Silicon Carbide Oxidation in High‐Pressure Steam

Silicon carbide is a candidate cladding for fission power reactors that can potentially provide better accident tolerance than zirconium alloys. SiC has also been discussed as a host matrix for nuclear fuel. Chemical vapor–deposited silicon carbide specimens were exposed in 0.34–2.07 MPa steam at low gas velocity (~50 cm/min) and temperatures from 1000°C to 1300°C for 2–48 h. As previously observed at lower steam pressure of 0.15 MPa, a two‐layer SiO₂ scale was formed during exposure to these conditions, composed of a porous cristobalite layer above a thin, dense amorphous SiO₂ surface layer. Growth of both layers depends on temperature, time, and steam pressure. A quantitative kinetics model is presented to describe the SiO₂ scale growth, whereby the amorphous layer is formed through a diffusion process and linearly consumed by an amorphous to crystalline phase transition process. Paralinear kinetics of SiC recession were observed after exposure in 0.34 MPa steam at 1200°C within 48 h. High‐pressure steam environments are seen to form very thick (10–100 μm) cristobalite SiO₂ layers on CVD SiC even after relatively short‐term exposures (several hours). The crystalline SiO₂ layer and SiC recession rate significantly depend on steam pressure. Another model is presented to describe the SiC recession rate in terms of steam pressure when a linear phase transition k_l governing the recession kinetics, whereby the reciprocal of recession rate is found to follow a negative unity steam pressure power law.     

Structural and Crystal Chemical Investigation of Intermediate Phases in the System Ca2SiO4–Ca3(PO4)2–CaNaPO4

Two intermediate compounds of the system Ca₂SiO₄–Ca₃(PO₄)₂–CaNaPO₄ were synthesized by reaction sintering at 1600°C and analyzed structurally, chemically, and optically. The structure of Ca₇(PO₄)₂(SiO₄)₂ nagelschmidtite (space group P6₁, a = 10.7754(1) Å, c = 21.4166(3) Å) was determined by single crystal X‐ray analysis. Its unit cell can be interpreted as a supercell (≈ 2 × a, 3 × c) of the high‐temperature polymorph α‐Ca₂SiO₄. Evidence for pseudo‐hexagonal symmetry is shown. Using electron microprobe, the solid solution Ca_7?xNa_x(PO₄)_2+x(SiO₄)_2?x, (x ≤ 2), of nagelschmidtite was confirmed. Volume thermal expansion coefficients of Ca_6.8Na_0.2(PO₄)_2.2(SiO₄)_1.8 and Ca_5.4Na_1.5(PO₄)_3.7(SiO₄)_0.3 were determined using high‐temperature X‐ray powder diffraction, yielding mean α_V = 3.95 and 5.21 [×10^?5/°C], respectively. Ca₁₅(PO₄)₂(SiO₄)₆ is a distinct phase in the binary section Ca₂SiO₄–Ca₃(PO₄)₂ and was found to extend into the ternary space according to Ca_15?xNa_x(PO₄)_2+x(SiO₄)_6?x, (x ≤ 0.1). Quenching experiments of the latter allowed for structural analysis of a strongly disordered, defective high‐temperature polymorph of the α‐Ca₂SiO₄–α‐Ca₃(PO₄)₂ solid solution. Structural relations between nagelschmidtite, Ca₁₅(PO₄)₂(SiO₄)₆ and the end‐member compounds of the system are discussed.