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.     

Network Rearrangement in AgI‐Doped GeTe4 Glasses

The structure of (GeTe₄)_1?x(AgI)_x (x = 0.15 and 0.25) glasses has been investigated by X‐ray and neutron diffraction as well as extended X‐ray absorption spectroscopy (EXAFS) and Raman spectroscopy. Large‐scale structural models have been obtained by fitting simultaneously the experimental datasets in the framework of the reverse Monte Carlo simulation technique (RMC). Short‐range order parameters have been calculated and compared with that of GeTe₄. Doping with AgI affects the structure of the host GeTe₄ matrix in two ways. First, while Te is essentially twofold coordinated in GeTe_4, its coordination number is as high as ~2.9 ± 0.3 for x = 0.25. The change is mainly due to the increased fraction of Te–Te bonds. Second, Ge atoms remain fourfold coordinated but the tetrahedral symmetry is distorted due to the elongation of some Ge–Te bonds. The incorporation of AgI in the GeTe₄‐based host covalent matrix and the Te coordination increase explains the enhanced thermal stability of (GeTe₄)_1?x(AgI)_x in the supercooled liquid‐state hindering the crystallization of Te found in case of GeTe₄ glass.     

Spectral Properties of Pr3+‐Doped Transparent‐Glass‐Ceramic Containing Ca5(PO4)3F Nanocrystals

A Pr³⁺‐doped transparent oxyfluoride glass‐ceramic containing Ca₅(PO₄)₃F nanocrystals was prepared by melt quenching and subsequent thermal treatment. The crystallization phase and morphology of the Ca₅(PO₄)₃F nanocrystals were investigated by X‐ray diffraction and transmission electron microscope, respectively. The volume fraction of the Ca₅(PO₄)₃F nanocrystals in the glass‐ceramic is about 10% and the fraction of Pr³⁺ ions incorporated into the Ca₅(PO₄)₃F nanocrystals is about 22%. The peak absorption cross sections at 435 and 574 nm increase up to 128% and 132% after crystallization, respectively. The peak stimulated emission cross sections of the ³P₀→³H₄ blue laser channel and ³P₀→³F₂ red laser channel for the glass‐ceramic are 4.95 × 10^?20 and 29.8 × 10^?20 cm², respectively. The spectral properties indicate that the glass‐ceramic is a potential visible laser material.     

In situ Raman spectroscopy of pressure‐induced phase transformations in polycrystalline TbPO4, DyPO4, and GdxDy(1−x)PO4

Xenotime DyPO₄ and Gd_xDy_(1?x)PO₄ (x = 0.4, 0.5, 0.6) (tetragonal I4₁amd zircon structure) have been studied at ambient temperature under high pressures inside a diamond anvil cell with in situ Raman spectroscopy. The typical Raman‐active modes of the xenotime structure were observed at low pressures and the appearance of new Raman peaks at higher pressures indicated a phase transformation to a lower symmetry structure—likely monoclinic. Raman mode softening was observed, resulting in a line crossing at approximately 7‐8 GPa for each material and preceding the phase transformation. The onset of phase transformation for DyPO₄ occurred at a pressure of 15.3 GPa. DyPO₄ underwent a reversible phase transformation and returned to the xenotime phase after decompression. The transformation pressures of the solid solutions (Gd_xDy_(1?x)PO₄) were in the range 10‐12 GPa. The Gd_xDy_(1?x)PO₄ solid solutions yielded partially reversible phase transformations, retaining some of the high‐pressure phase spectrum while reforming xenotime peaks during decompression. The substitution of Gd into DyPO₄ decreased the transformation pressure relative to pure DyPO₄. The ability to modify the phase transformation pressures of xenotime rare‐earth orthophosphates by chemical variations of solid solutions may provide additional methods to improve the performance of ceramic matrix composites.     

First Identification of Rare‐Earth Oxide Nucleation in Chalcogenide Glasses and Implications for Fabrication of Mid‐Infrared Active Fibers

Gallium (Ga) helps solubilize rare‐earth ions in chalcogenide glasses, but has been found to form the dominant crystallizing selenide phase in bulk glass in our previous work. Here, the crystallization behavior is compared of as‐annealed 0–3000 ppmw Dy³⁺‐doped Ge–As–Ga–Se glasses with different Ga levels: Ge_16.5As_(19?x)Ga_xSe_64.5 (at.%), for x = 3 and 10, named Ga₃ and Ga₁₀ glass series, respectively. X‐ray diffraction and high‐resolution transmission electron microscopy are employed to examine crystals in the bulk of the as‐prepared glasses, and the crystalline phase is proved to be the same: Ge‐modified, face centered cubic α‐Ga₂Se₃. Light scattering of polished glass samples is monitored using Fourier transform spectroscopy. When Ga is decreased from 10 to 3 at.%, the bulk crystallization is dramatically reduced and the optical scattering loss decreases. Surface defects, with a rough topology observed for both series of as‐prepared chalcogenide glasses, are demonstrated to comprise Dy, Si, and [O]. For the first time, evidence for the proposed nucleation agent Dy₂O₃ is found inside the bulk of as‐prepared glass. This is an important result because rare‐earth ions bound in a high phonon–energy oxide local environment are, as a consequence, inactive mid‐infrared fluorophores because they undergo preferential nonradiative decay of excited states.     

A novel dazzling Eu3+‐doped whitlockite‐type phosphate red‐emitting phosphor for white light‐emitting diodes

A series of novel red‐emitting Ca₈ZnLa_1?xEu_x(PO₄)₇ phosphors were successfully synthesized using the high‐temperature solid‐state reaction method. The crystal structure, photoluminescence spectra, thermal stability, and quantum efficiency of the phosphors were investigated as a function of Eu³⁺ concentration. Detailed analysis of their structural properties revealed that all the phosphors could be assigned as whitlockite‐type β‐Ca₃(PO₄)₂ structures. Both the PL emission spectra and decay curves suggest that emission intensity is largely dependent on Eu³⁺ concentration, with no quenching as the Eu³⁺ concentration approaches 100%. A dominant red emission band centered at 611 nm indicates that Eu³⁺ occupies a low symmetry sites within the Ca₈ZnLa(PO₄)₇ host lattice, which was confirm by Judd‐Ofelt theory. Ca₈ZnLa_1?xEu_x(PO₄)₇ phosphors exhibited good color coordinates (0.6516, 0.3480), high color purity (~96.3%), and high quantum efficiency (~78%). Temperature‐dependent emission spectra showed that the phosphors possessed good thermal stability. A white light‐emitting diode (LED) device were fabricated by integrating a mixture of obtained phosphors, commercial green‐emitting and blue‐emitting phosphors into a near‐ultraviolet LED chip. The fabricated white LED device emits glaring white light with high color rendering index (83.9) and proper correlated color temperature (5570 K). These results demonstrate that the Ca₈ZnLa_1?xEu_x(PO₄)₇ phosphors are a promising candidate for solid‐state lighting.     

Ionic transport in AgI‐HgS‐As2S3 glasses: Critical percolation and modifier‐controlled domains

Electrical measurements, dc and ac, show that (AgI)_x(HgS)_0.5‐x/2(As₂S₃)_0.5‐x/2 glasses, 0.0 ≤  x ≤ 0.6, exhibit drastic changes in ionic conductivity σ_i with silver iodide additions. The ionic transport increases by 13 orders of magnitude with increasing silver content from ~0.002 to ~23 at.%, and the activation energy decreases from 1.05 to 0.35 eV. Two distinctly different ion transport regimes above the percolation threshold concentration, x_c ≈ 30 ppm, were distinguished. The critical percolation regime at low silver content (≤ 2‐5 at.% Ag) is characterized by a random distribution of silver‐related entities and obeys a power‐law composition dependence of σ_i. The ion transport parameters depend on the host network connectivity, represented by the average coordination number <n₀>, via the critical fictive temperature T₀; the calculated T₀ value is comparable to the glass transition temperature for the glassy (HgS)_0.5(As₂S₃)_0.5 host matrix. In contrast, in the modifier‐controlled domain, the silver‐related entities are nonrandomly distributed. The high Ag⁺ ionic mobility results from interconnected tetrahedral (AgI_2/2S_2/2)_n chains in the silver iodide content range 0.2 < x ≤ 0.5, and from 2D layers (Ag_3/3I_3/3)_n or 3D mixed tetrahedral subnetwork (AgI_3/3S_1/2) in the range x > 0.5.     

Evaluation of the electrochemical behavior and structural reversibility of Li3−xNaxV2(PO4)3/C (0≤x≤3) as anode materials for Na-ion batteries

A series of Li_3−xNa_xV₂(PO₄)₃/C (0≤x≤3) materials are successfully prepared by a simple solid-state reaction method and used for the first time as anode materials for Na-ion batteries. Powder X-ray diffraction (XRD) results show that the phase structures of Li_3−xNa_xV₂(PO₄)₃/C evolve along with the change of Li/Na atomic ratio (0≤x≤3). With increasing x in Li_3−xNa_xV₂(PO₄)₃/C from 0.0 to 3.0, the main phase in as-prepared sample transforms from monoclinic Li₃V₂(PO₄)₃ to rhombohedral Li₃V₂(PO₄)₃, and finally to rhombohedral Na₃V₂(PO₄)₃, which results in different sodium storage behavior and performance between Li_3−xNa_xV₂(PO₄)₃/C (0≤x≤3) materials. Electrochemical results show that Li_3−xNa_xV₂(PO₄)₃/C (x=0.0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0) can deliver the initial charge capacities of 21.1, 35.9, 33.8, 41.7, 43.3, 43.9 and 47.7 mAh g⁻¹ at a current density of 10 mA g⁻¹, respectively. After 45 cycles, the reversible capacities can be kept at 16.9, 45.1, 32.6, 44.6, 43.7, 37.8 and 27.3 mAh g⁻¹ for Li₃V₂(PO₄)₃/C, Li_2.5Na_0.5V₂(PO₄)₃/C, Li₂NaV₂(PO₄)₃/C, Li_1.5Na_1.5V₂(PO₄)₃/C, LiNa₂V₂(PO₄)₃/C, Li_0.5Na_2.5V₂(PO₄)₃/C and Na₃V₂(PO₄)₃/C, respectively. Furthermore, the structural reversibility of Li_3−xNa_xV₂(PO₄)₃/C (x=1.0, 2.0, and 3.0) is also observed by in-situ XRD observation during sodiation/de-sodiation process. All these observed evidences indicate that only some of Li_3−xNa_xV₂(PO₄)₃/C (0≤x≤3) can be used as possible sodium storage materials.     

Efficient red‐emitting phosphor of Eu3+‐activated (Na0.5Gd1.5)(TiSb)O7 derived via cation‐substitutions in Gd‐Pyrochlore

Rare‐earth (RE) titanate pyrochlore with perovskite‐layered structure is a well‐known engineering material in applied in many field. In this work, a red‐emitting phosphor of Gd_2?xNa_xTi_2?2xSb_2xO₇:Eu³⁺ (x = 0‐0.5) was developed via cation substitutions of (Sb⁵⁺→Ti⁴⁺) and (Na⁺→Gd³⁺) in Gd₂Ti₂O₇. The motivation is based on the fact that the introduction of cation‐disorders has been regarded to be an effective approach for improving the luminescent efficiency and thermal stability of RE‐activated materials. All the samples were synthesized via facile solid‐state reaction method. The morphology properties were measured via SEM and EDS measurements. The structural Rietveld refinement was performed to investigate the microstructure in pyrochlore lattices. The luminescence properties of Gd_2?xNa_xTi_2?2xSb_2xO₇:0.15Eu³⁺ (x = 0‐0.5) has a strict dependence on the cation substitution levels. The band energy of Gd₂Ti₂O₇ is 2.9 eV with a direct transition nature. The incorporation of Sb⁵⁺ and Na⁺ in the lattices moves the optical absorption to a longer wavelength. The cation disorder results in significant improvements of luminescence intensity, excitation efficiency in the blue region, longer emission lifetime and thermal stability.     

Fabrication of Sr0.5Ba0.5Nb2O6 Nanocrystallite‐Precipitated Transparent Phosphate Glass‐Ceramics

Transparent (Sr_0.5Ba_0.5)Nb₂O₆ (SBN50) nanocrystallite‐precipitated phosphate glass‐ceramics were prepared by a conventional glass‐ceramic process. x(SrO–BaO–2Nb₂O₅) ? (100–4x)P₂O₅ (xSBNP) glasses with a refractive index of 1.9–2.0 exhibited high water resistance owing to the presence of Q⁰ and Q¹ phosphate units. Both bulk and surface crystallization of the SBN50 phase were observed in 20SBNP and 21SBNP glass‐ceramics. Although the nominal content of SBN50 crystals in the 21SBNP glass was larger than that in the 20SBNP glass, the latter exhibited better crystallinity of SBN50 and a higher number density of precipitated SBN nanocrystallites. By tuning the two‐step heat‐treatment and the chemical composition, transparent SBN50‐precipitated glass‐ceramics were successfully obtained. Given that no remarkable increase of the relative dielectric constants was observed after crystallization of the SBN50 nanocrystallites, it is postulated that the relative dielectric constant of the bulk is mainly governed by the amorphous phosphate region, and that the contribution of precipitation of the SBN50 nanocrystallites to the dielectric constant is not very significant in this system.     

Microwave dielectric properties of (Zn1/3B2/3)0.5(Ti0.8M0.2)0.5O2 (B = Nb5+, Ta5+, M = Sn4+, Ge4+) ceramics

Dielectric properties of (Zn_1/3Nb_(2?x)/3Ta_x/3)_0.5(Ti_0.8Sn_0.1Ge_0.1)_0.5O₂ (x = 0, 1, 2) and/or (Zn_1/3Nb_1/3Ta_l/3)_0.5(Ti_0.8Sn_0.2(l?y)Ge_0.2y)_0.5O₂ (y = 0, 0.5, 1) were investigated at the microwave frequencies. For the compositions with single phase of rutile structure, the dielectric constant (K) of specimens was not only dependent on the dielectric polarizabilities, but also on the bond length ratio of apical bond (d_apical) to equatorial bond (d_equatorial) of oxygen octahedron in the unit cell. Temperature coefficients of the resonant frequencies (TCF) of the specimens with B = Nb⁵⁺ and/or M = Sn⁴⁺ was larger than those with B = Ta⁵⁺ and/or M = Ge⁴⁺. These results could be attributed to the changes of the degree of oxygen octahedral distortion. Quality factors (Qf) of the specimens with B = Ta⁵⁺ and/or M = Sn⁴⁺ were larger than those with B = Nb⁵⁺ and/or M = Ge⁴⁺.     

Structure and electrical properties of perovskite layer (1−x)Sr2Nb2O7‐x(Na0.5Bi0.5)TiO3 high‐temperature piezoceramics

The structure and electrical properties of perovskite layer structured (PLS) (1?x)Sr₂Nb₂O₇‐x(Na_0.5Bi_0.5)TiO₃ (SNO‐NBT) prepared by solid‐state reaction method are investigated. The addition of NBT is beneficial to speed up mass transfer and particle rearrangement during sintering, leading to better sinterability and higher bulk density up to 96.8%. The solid solution limit x in the SNO‐NBT system is below 0.03, over which Ti⁴⁺ is preferable to aggregate and results in the generation of secondary phase. After the modification by NBT, all SNO‐NBT ceramics have a Curie temperature T_c up to over 1300°C and piezoelectric constant d₃₃ about 1.0 pC/N. The breakthrough of piezoelectricity can mainly be attributed to rotation and distortion of oxygen octahedron as well as higher poling electric field resulting from the improved bulk density. This study not only demonstrates how to improve piezoelectricity by NBT addition, but also opens up a new direction to design PLS piezoceramics by introducing appropriate second phase.     

X‐ray photoelectron spectroscopy analysis of Ge–Sb–Se pulsed laser deposited thin films

Pulsed laser deposition was used to prepare amorphous thin films from (GeSe₂)_100?x(Sb₂Se₃)_x system (x = 0, 5, 10, 20, 30, 40, 50, and 60). From a wide variety of chalcogenide glass‐forming systems, Ge–Sb–Se one, especially in thin films form, already proved to offer a great potential for photonic devices such as chemical sensors. This system has a large glass‐forming region which gives the possibility to adjust the chemical composition of the glasses according to required physical characteristics. The chemical composition of fabricated thin films was analyzed via X‐ray photoelectron spectroscopy (XPS) and compared to energy dispersive spectroscopy (EDS) data. The results of both techniques agree well: a small deficiency in chalcogen element and an excess of antimony was found. The structure of as‐deposited thin films has been investigated by XPS. The presence of the two main structural units, [GeSe₄] and [SbSe₃] proposed by Raman scattering spectroscopy data analysis, was confirmed by XPS. Moreover, XPS core level spectra analysis revealed the presence of M–M bonds (M = Ge, Sb) in (Ge,Sb)–Ge–(Se)₃ and (Ge,Sb)–Sb–(Se)₂ entities that could correspond to Ge‐based tetrahedra and Sb‐based pyramids where one of its Se atoms at corners is substituted by Ge or Sb ones. The content of depicted M–M bonds tends to increase with introduction of antimony in the amorphous network of as‐deposited thin films from x = 0 to x = 40 and then it decreases. XPS analysis of as‐deposited thin films shows also the presence of the (Ge,Sb)–Se–(Ge,Sb) and Se–Se–(Ge,Sb) entities.     

Structure and lithium-ion mobility in Li1.5M0.5Ge1.5(PO4)3 (M = Ga,Sc, Y) NASICON glass-ceramics

This work reports structural and lithium-ion mobility studies in NASICON single- or multiple phase Li_1+xM_xGe_2−x(PO₄)₃ (M = Ga³⁺, Sc³⁺, Y³⁺) glass-ceramics using solid-state NMR techniques, X-ray powder diffraction, and impedance spectroscopy. X-ray powder diffraction data show the successful incorporation of Ga³⁺ and Sc³⁺ into the Ge⁴⁺ octahedral sites of the NASICON structure at the levels of x = 0.5 and 0.4, respectively. The glass-to-crystal transition was further characterized by multinuclear NMR and electrical conductivity measurements. Among the studied samples, the gallium-containing glass-ceramic presented the highest DC conductivity, 1.1 × 10⁻⁴ S/cm at room temperature, whereas for the Sc-containing samples, the maximum room temperature conductivity that could be reached was 4.8 × 10⁻⁶ S/cm. No indications of any substitution of Ge⁴⁺ by Y³⁺ could be found.     

A single‐phase full‐color emitting phosphor Na3Sc2(PO4)3:Eu2+/Tb3+/Mn2+ with near‐zero thermal quenching and high quantum yield for near‐UV converted warm w‐LEDs

A single‐phase full‐color emitting phosphor Na₃Sc₂(PO₄)₃:Eu²⁺/Tb³⁺/Mn²⁺ has been synthesized by high‐temperature solid‐state method. The crystal structure is measured by X‐ray diffraction. The emission can be tuned from blue to green/red/white through reasonable adjustment of doping ratio among Eu²⁺/Tb³⁺/Mn²⁺ ions. The photoluminescence, energy‐transfer efficiency and concentration quenching mechanisms in Eu²⁺‐Tb³⁺/Eu²⁺‐Mn²⁺ co‐doped samples were studied in detail. All as‐obtained samples show high quantum yield and robust resistance to thermal quenching at evaluated temperature from 30 to 200°C. Notably, the wide‐gamut emission covering the full visible range of Na₃Sc₂(PO₄)₃:Eu²⁺/Tb³⁺/Mn²⁺ gives an outstanding thermal quenching behavior near‐zero thermal quenching at 150°C/less than 20% emission intensity loss at 200°C, and high quantum yield‐66.0% at 150°C/56.9% at 200°C. Moreover, the chromaticity coordinates of Na₃Sc₂(PO₄)₃:Eu²⁺/Tb³⁺/Mn²⁺ keep stable through the whole evaluated temperature range. Finally, near‐UV w‐LED devices were fabricated, the white LED device (CCT = 4740.4 K, Ra = 80.9) indicates that Na₃Sc₂(PO₄)₃:Eu²⁺/Tb³⁺/Mn²⁺ may be a promising candidate for phosphor‐converted near‐UV w‐LEDs.     

Synthesis,characterization, and magneto‐electric properties of (1−x)BCZT‐xCFO ceramic particulate composites

Synthesis of a new magnetoelectric [(1?x)(Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃‐xCoFe₂O₄] (weight fraction x=0, 0.1, 0.2, 0.3, 0.4, 0.5 and 1) ceramic particulate composites with its structural characterization and magneto‐electric properties have been reported here in this study. Lead free piezoelectric (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ (BCZT) and ferrimagnetic CoFe₂O₄ (CFO) were synthesized using sol‐gel and combustion methods respectively. (1?x)BCZT‐xCFO magnetoelectric composites were then synthesized by mixing of the calcined individual ferroic phases with required weight fractions. Powder X‐ray diffraction studies indicate the coexistence of BCZT and CFO phases in the composites sintered at 1300°C. 0.5BCZT‐0.5CFO composite showed high strain sensitivity (dλ/dH) of 52×10^?9 Oe^?1, which is comparable to that of pure CFO (50×10^?9 Oe^?1). A high piezoelectric voltage constant (g₃₃) of 8×10^?3 V m/N was measured for 0.8BCZT‐0.2CFO sample. All the composites showed magnetoelectric effect and a high magnetoelectric coupling coefficient (α_ME) of 6.85 mV/cm Oe was measured for 0.5BCZT‐0.5CFO composite at 1 kHz and a large ME coefficient of 115 mV/cm Oe at its resonance frequency. The effect of microstructure on the magnetoelectric properties of [(1?x)BCZT‐(x)CFO] composites has been studied and reported here as a function of its piezoelectric (BCZT)/ferrite (CoFe₂O₄) content.     

Low‐Temperature Sinterable (1−x)Ba3(VO4)2–xLiMg0.9Zn0.1PO4 Microwave Dielectric Ceramics

The novel low‐temperature sinterable (1 ? x)Ba₃(VO₄)₂–xLiMg_0.9Zn_0.1PO₄ microwave dielectric ceramics were prepared by cofiring the mixtures of pure‐phase Ba₃(VO₄)₂ and LiMg_0.9Zn_0.1PO₄. The phase structure and grain morphology of the ceramics were evaluated using X‐ray diffraction, Raman spectra, and scanning electron microscopy. The results indicated that Ba₃(VO₄)₂ and LiMg_0.9Zn_0.1PO₄ phases can well coexist in the sintered body. Nevertheless, a small amount of LiZnPO₄ and some vanadate phases with low melting points were observed, which not only can influence the microwave dielectric properties of the ceramic but also can obviously improve the densification behavior at a relatively low sintering temperature. The near‐zero temperature coefficients of the resonant frequency (τ_f) could be achieved by adjusting the relative content of the two phases owing to their opposite τ_f values and simultaneously a desirable quality factor Q × f value can be maintained. No chemical reaction between the matrix ceramic phase and Ag took place after sintering at 800°C for 4 h. The ceramics with 45 vol% LiMg_0.9Zn_0.1PO₄ can be well sintered at only 800°C and exhibit excellent microwave dielectric properties of ε_r ~ 10, Q × f ~ 64 500 GHz, and τ_f ~ ?2.1 ppm/°C, thus showing a great potential as a low‐permittivity low‐temperature cofired microwave dielectric material.     

Empirical Evidence for A‐Site Order in Perovskites

Models for composition–structure relationships are useful in both the lab and industry, yet few exist for perovskites‐containing extrinsic defects or cation ordering. In this work, an empirical model is used to predict the existence of A‐site cation ordering. Specifically, four compositions in the Na_(1?3x)/2La_(1+x)/2TiO₃ system (x = 0.0, 0.0533, 0.1733 and 0.225) were synthesized using a conventional solid‐state mixed‐oxide method. The structure of the x = 0 end‐member (Na_0.5La_0.5TiO₃) has been reported in various space groups, but always with a random distribution of Na⁺ and La³⁺ on the A site; however, empirical modeling suggests that it is not only ordered but also that a small volume increase accompanies the ordering process. While no evidence of long‐range A‐site ordering is observed in this composition via X‐ray or neutron diffraction, electron‐diffraction data indicate short‐range ordering of Na⁺ and La³⁺ ions, with the degree of cation ordering decreasing (but the scale of ordered domains and degree of vacancy ordering generally increasing) with increasing x. First‐principles calculations via density functional theory support both conclusions that short‐range ordering in Na_0.5La_0.5TiO₃ is stable and that it results in a volume increase with respect to the disordered analog. A similar analysis has been conducted for the Li_(1?3x)/2La_(1+x)/2TiO₃ and Na_(1?3x)/2La_(1+x)/2(Mg_0.5W_0.5)O₃ solid solutions. These systems provide additional validation of the accuracy and versatility of the empirical modeling method used.     

A Complete Solid Solution with Rutile‐Type Structure in SiO2–GeO2 System at 12 GPa and 1600°C

High pressure and temperature synthesis of compositions made of (Si_1?x,Ge_x)O₂ where x is equal to 0, 0.1, 0.2, 0.5, 0.7, and 1 was performed at 7–12 GPa and 1200–1600°C using a Kawai‐type high‐pressure apparatus. At 12 GPa and 1600°C, all the run products were composed of a single phase with a rutile structure. The lattice constants increase linearly with the germanium content (x), which indicates that the rutile‐type phases in the SiO₂–GeO₂ system form a complete series of solid solutions at these pressure and temperature conditions. Our experimental results show that thermodynamic equilibrium state was achieved in this system at 12 GPa and 1600°C, but not at 1200°C. At lower pressures (7 and 9 GPa) and 1600°C, we observed the decomposition of (Si_0.5,Ge_0.5)O₂ into SiO₂‐rich coesite and GeO₂‐rich rutile phases. The silicon content in the rutile structure increases sharply with pressure in the vicinity of the coesite–stishovite phase transition pressure in SiO₂.     

Tunable negative thermal expansion and structural evolution in antiperovskite Mn3Ga1−xGexN (0 ≤ x ≤ 1.0)

The negative thermal expansion (NTE) and structural evolution of antiperovskite compounds Mn₃Ga_1?xGe_xN (0 ≤ x ≤ 1.0) were systematically investigated. Our results indicate the crystal structure of Mn₃Ga_1?xGe_xN changes from cubic (C) to tetragonal (T₄) with increasing Ge content by X‐ray diffraction (XRD).The negative thermal expansion from x = 0 (operation‐temperature range ?T = 20 K) to x = 0.4 (?T = 60 K) becomes broad and shifts to higher temperature, and then it became positive from x = 0.5 in Mn₃Ga_1?xGe_xN. Typically, Mn₃Ga_0.5Ge_0.5N shows low thermal expansion behavior between 300 and 450 K (?T = 150 K), and thermal expansion coefficient α is estimated to be 2 × 10^?6 K^?1. Furthermore, variable temperature XRD was measured to reveal the origin of NTE. The cubic I ‐ cubic II phases coexistence (x = 0.2) and cubic I ‐ tetragonal coexistence (x = 0.5, 0.6) was observed at low temperature. The tunable NTE is highly valuable for practical applications in precision devices.