Dielectric and piezoelectric properties of low temperature synthesized iso-valent modified BT ceramics

Ba_(1?x)Y_xTiO₃ (where Y = Ca, Mg and Sr, x = 0.02, 0.04, 0.06 and 0.08) ferroelectric ceramic samples were synthesized in single perovskite phase by modified solid state reaction (MSSR) route. For single perovskite phase formation and dense grain morphology, 900 °C and 1300 °C were optimized as calcination and sintering temperatures. With the increase of substitutions% of Ca in BCT ceramic samples, the position of T_c increases slightly, whereas with the increase of Mg and Sr substitution% in BMT and BST systems, the position of T_c decreased but remains above RT. Decreased processing temperature with low temperature coefficient of capacitance made BCT ceramic samples useful for dielectric applications. Symmetric nature of the S–E loops indicated the increase of piezoelectric nature with the increase of Ca substitution% in BCT system.     

Blue-violet ceramic pigments based on Co and Mg Co2−xMgxP2O7 diphosphates

Solid solutions of Co and Mg diphosphates with compositions Co_2?xMg_xP₂O₇ (x = 0, 0.1, 0.2, 0.3, 0.5, 0.7, 1.0, 1.5 and 1.8) have been prepared and characterized for the first time as alternative low-toxicity blue ceramic pigments. The compositions were prepared through the conventional coprecipitation route and calcined up to 1000 °C/2 h. Samples were characterized by thermal analysis, XRD, SEM/EDX, UV–vis-NIR spectroscopy and colour measurements (CIE-L^*a^*b^*). Isostructural Co_2?xMg_xP₂O₇ diphosphate solid solutions (monoclinic system and P21/c spatial group) formed successfully within the studied range of compositions, accompanied only by a minor quantity of residual Co or Mg orthophosphates (M₃(PO₄)₂). Interestingly, the obtained solid solutions developed nice blue-violet colourations even with high Mg doping after enamelling within double-firing (x = 1.5–1.8) and single-firing (x = 1.0–1.5) ceramic glasses. These optimal compositions containing a minimized Co amount (measured values around 7–16 wt%) could be therefore less toxic alternatives to the conventional Co₃(PO₄)₂ blue ceramic pigment.     

Nitrate–citrate combustion synthesis of Ce1−xGdxO2−x/2 powder and its characterization

The structure, thermal expansion coefficients, and electric conductivity of Ce_1−xGd_xO_2−x/2 (x = 0–0.6) solid solution, prepared by the gel-combustion method, were investigated. The uniform small particle size of the gel-combustion prepared materials allows sintering into highly dense ceramic pellets at 1300 °C, a significantly lower temperature compared to that of 1600–1650 °C required for ceria solid electrolytes prepared by traditional solid state techniques. XRD showed that single-phase solid solutions formed in all the investigated range. The maximum conductivity, σ_600 °C = 5.26 × 10⁻³ S/cm, was found at x = 0.2. The thermal expansion coefficient, determined from high-temperature X-ray data, was 8.125 × 10⁻⁶ K⁻¹ at x = 0.2.     

New pink ceramic pigment based on chromium (IV)-doped lutetium gallium garnet

Lutetium gallium garnet codoped with chromium and calcium (Ca,Cr:LGG) and lutetium gallium garnet with chromium as single dopant (Cr:LGG) have been studied. Samples with Ca_xCr_xLu_3−2xGa₅O₁₂ (x = 0, 0.05, 0.10, 0.15, 0.2, 0.4, 0.6,) and Cr_xLu_3−xGa₅O₁₂ (x = 0.15 and 0.6) compositions were prepared by solid state reaction and the fired samples (1250 °C/6 h) were characterised by XRD, lattice parameters determination, UV–vis–NIR spectroscopy, CIEL*a*b* measurements and SEM/EDX. Moreover, the samples with calcium were tested as ceramic pigments in a conventional glaze matrix. In the Ca,Cr:LGG system solid solutions incorporating both Cr(III) and Cr(IV) ions were obtained. Cr(IV) occupies basically dodecahedral sites substituting for Lu (III) and is the predominant oxidation state up to x = 0.20 composition, giving a pink colouration in the ceramic glaze matrix. In the Cr:LGG system, only Cr(III) enters in solid solution occupying octahedral positions and producing green shades.     

Influence of Yb3+ doping on phase stability and thermophysical properties of (Y1-xYbx)3Al5O12 under high temperature

In this work, Yb³⁺ was selected to replace the Y³⁺ in yttrium aluminum garnet (YAG) in order to reduce its thermal conductivity under high temperature. A series of (Y_1-xYb_x)₃Al₅O₁₂ (x=0, 0.1, 0.2, 0.3, 0.4) ceramics were prepared by solid-state reaction at 1600 °C for 10 h. The microstructure, thermophysical properties and phase stability under high temperature were investigated. The results showed that all the Yb doped (Y_1-xYb_x)₃Al₅O₁₂ ceramics were comprised of a single garnet-type Y₃Al₅O₁₂ phase. The thermal conductivities of (Y_1-xYb_x)₃Al₅O₁₂ ceramics firstly decreased and subsequently increased with Yb ions concentration rising from room temperature to 1200 °C. (Y_0.7Yb_0.3)₃Al₅O₁₂ had the lowest thermal conductivity among investigated specimens, which was about 1.62 W m⁻¹ K⁻¹ at 1000 °C, around 30% lower than that of pure YAG (2.3 W m⁻¹ K⁻¹, 1000 °C). Yb had almost no effect on the coefficients of thermal expansion (CTEs) of (Y_1-xYb_x)₃Al₅O₁₂ ceramics and the CTE was approximate 10.7×10⁻⁶ K⁻¹ at 1200 °C. In addition, (Y_0.7Yb_0.3)₃Al₅O₁₂ ceramic remained good phase stability when heating from room temperature to 1450 °C.     

The effect of Zn addition on the structure and transport properties of BaCe0.9−xZrxY0.1O3−δ

BaCe_0.9−xZr_xY_0.1O_3−δ (0.1 ≤ x ≤ 0.9) are ceramic proton conductors widely investigated for different electrochemical devices, such as Solid Oxide Fuel Cell (SOFC) electrolytes, however, their applications are limited by the high sintering temperatures necessary to achieve densification. Polycrystalline powders of BaCe_0.9−xZr_xY_0.1O_3−δ (BCZ) have been prepared by freeze-drying precursor method at 1000 °C. These powders were mixed with a zinc nitrate solution to decrease the sintering temperature to 1200–1300 °C. The addition of Zn has several effects on the structure and microstructure of BCZ: the densification and grain growth are enhanced at lower temperature and a reduction of the crystallographic symmetry is observed for samples with low Zr-content (x ≤ 0.3). Furthermore, the bulk and specific grain boundary conductivities are only slightly affected by the addition of Zn.     

Microstructure and electrical properties of (Ba0.98Ca0.02)(Ti0.94Sn0.06)O3-modified Bi0.51Na0.50TiO3 lead-free ceramics

(Ba_0.98Ca_0.02)(Ti_0.94Sn_0.06)O₃-modified Bi_0.51Na_0.5TiO₃ [(1 ? x)BNT–xBCTS] ceramics were prepared by the normal sintering. A stable solid solution is well formed between BNT and BCTS, and the morphotropic phase boundary of (1 ? x)BNT–xBCTS ceramics is identified in the compositional range of 0.05 ≤ x ≤0.06. The temperature dependence of the dielectric loss and the poling temperature dependence of the d₃₃ value are used to determine the depolarization value (T_d). The T_d value of these ceramics gradually decreases with increasing BCTS content, together with the gradual increase of the dielectric constant. An enhanced electrical behavior of d₃₃ ～ 170 pC/N, k_p ～ 32.8%, P_r ～ 38.5 μC/cm², and E_c ～ 34.3 kV/cm is demonstrated for the ceramic with x = 0.06, which is double than that of pure BNT ceramic.     

Iron and chromium doped perovskite (CaMO3 M = Ti,Zr) ceramic pigments,effect of mineralizer

Solid solutions Ca(D_xM_1?x)O₃ (M = Ti, Zr and D = Fe,Cr), have been studied as ceramic pigment in conventional ceramic glazes using 0.5 mol/mol of NH₄Cl as flux agent by solid state reaction and by ammonia coprecipitation route. Ca(Cr_xTi_1?x)O₃ compositions obtained without addition of NH₄Cl as mineralizer, produce pink color in glazes at low x but CaCrO₄ crystallizes when x increases, producing undesired green colors. The crystallization of chromates can be avoided using NH₄Cl as mineralizer, giving a complete solid solution that produce pink color in glazes at low x and dark blue shades at high x. Coprecipitated sample produce blue colors at low x and at low temperature than ceramic sample (1000 °C instead 1200 °C for CE sample). Cr⁴⁺ ion acts as red chromophore, but at higher x values (blue samples) Cr³⁺ ion entrance affects the color. Ca(Fe_xTi_1?x)O₃ system crystallizes perovskite CaTiO₃ and pseudobrookite Fe₂TiO₅ together with rutile as residual crystalline phase, glazed samples change from a yellow to a pink color associated to the increase of pseudobrookite with firing temperature. Ca(Fe_xTi_1?x)O₃ and Ca(Cr_xZr_1?x)O₃ systems crystallize perovskite CaZrO₃ and zirconia (ZrO₂) in both monoclinic and cubic polymorphs, but iron or chromium oxides are not detected in the powders. Coprecipitated sample stabilises cubic form. The solid solution is not reached completely in these samples and is not stable in glazes.     

Effect of Zn2+ substitution on the microwave dielectric properties of LiMgPO4 and the development of a new temperature stable glass free LTCC

The LiMg_(1?x)Zn_xPO₄ ceramics have been prepared by the solid state ceramic route. The LiMg_(1?x)Zn_xPO₄ ceramic retains the orthorhombic structure up to x = 0.2. The compositions with 0.3 ≤ x ≤ 0.8 exist as a mixture of orthorhombic and monoclinic phases. When Mg²⁺ is fully replaced with Zn²⁺ (x = 1.0) complete transition to monoclinic phase occurs. The ceramic with x = 0.1 (LiMg_0.9Zn_0.1PO₄) sintered at 925 °C exhibits low relative permittivity (?_r) of 6.7, high quality factor (Q_u × f) of 99,700 GHz with a temperature coefficient of resonant frequency (τ_f) of ?62 ppm/°C. The slightly large τ_f is adjusted nearly to zero with the addition of TiO₂. LiMg_0.9Zn_0.1PO₄–TiO₂ composite with 0.12 volume fraction TiO₂ sintered at 950 °C shows good microwave dielectric properties: ?_r = 10.1, Q_u × f = 52,900 GHz and τ_f = ?5 ppm/°C. The ceramic is found to be chemically compatible with silver.     

Enhanced electrical properties in lead-free NBT–BT ceramics by series ST substitution

Comprehensive electrical properties of 0.94(Na_1/2Bi_1/2)TiO₃–0.06BaTiO₃ lead-free ceramics by doping series SrTiO₃ were investigated. High piezoelectric constant of 205 pC/N and electromechanical coupling factor of 0.34 were obtained due to the forming of the rhombohedral–tetragonal morphotropic phase boundary at x=0.02–0.06. Very large recoverable strain of 0.34% was obtained at x=0.10 due to the coexistence of ferroelectric and relaxor pseudocubic phases. A large electrocaloric effect (ΔT_max=1.71 K and ΔT/ΔE=0.34 K mm kV⁻¹ at 50 kV cm⁻¹) which determined by indirect measurements method was obtained at 120 °C at x=0.02, which is significantly higher than that of lead-free ferroelectric ceramics reported so far. Moreover, lower operating temperatures of 50 °C and 30 °C were proposed when x=0.10 and 0.20 with ΔT_max=0.79 K and 0.6 K, respectively. These properties added together indicate a promising material for applications in cooling systems and actuators.     

Yellow-emitting (Tb1−xCex)3Al5O12 phosphor powder and ceramic (0≤x≤0.05): Phase evolution,photoluminescence, and the process of energy transfer

(Tb_1-xCe_x)₃Al₅O₁₂ yellow phosphors (0≤x≤0.05) were calcined from their coprecipitated carbonate precursors, and the effects of the temperature and atmosphere (air and H₂) of calcination on the sequence of phase evolution and the characteristics of the powders were investigated in detail. The activation energy for the crystallite growth during calcination was estimated to be ~39 kJ/mol. The powder calcined at 1000 °C showed good reactivity and was sintered into a ceramic plate of ~97% dense (average grain size: ~1.3 µm) in a H₂/Ar gas mixture at the relatively low temperature of 1500 °C. The phosphors simultaneously exhibit the 4f⁸→4f⁷5d¹, ⁷F₆→⁵D₃ and ⁷F₆→⁵D₄ excitations of Tb³⁺ and the 4f¹→5d¹ excitation of Ce³⁺ when monitoring the yellow emission of Ce³⁺ at 560 nm, suggesting the presence of efficient Tb³⁺→Ce³⁺ energy migration. The optimal Ce³⁺ content for luminescence was found to be x=0.015 and 0.01 under the direct excitation of Ce³⁺ and through Tb³⁺→Ce³⁺ energy transfer, respectively, and concentration quenching of luminescence was analyzed to be resulted from exchange interaction. Luminescence features of the phosphors, including excitation, emission, quantum yield, fluorescence lifetime, color coordinates and color temperature, were thoroughly investigated against the processing temperature and Ce³⁺ content, with an in-depth discussion on the process of energy transfer among the optically active Tb³⁺ and Ce³⁺ ions. The materials may find application in blue-light excited white LEDs.     

The influence of Gd doping on thermophysical properties,elasticity modulus and phase stability of garnet-type (Y1-xGdx)3Al5O12 ceramics

In this work, Gd³⁺ was selected to partially substitute the Y³⁺ in yttrium aluminum garnet (YAG) in order to improve the thermophysical properties of YAG. A series of (Y_1-xGd_x)₃Al₅O₁₂ (x = 0, 0.1, 0.2, 0.3, 0.4) ceramics were synthesized through chemical co-precipitation route. The microstructure, thermophysical properties and elasticity modulus of (Y_1-xGd_x)₃Al₅O₁₂ were investigated. The (Y_1-xGd_x)₃Al₅O₁₂ ceramics was comprised of single garnet-type Y₃Al₅O₁₂ phase. The thermal conductivities of (Y_1-xGd_x)₃Al₅O₁₂ bulk samples decreased with increasing doping concentration to 0.2, but increased with furthering increasing the concentration to 0.4. The thermal conductivity of (Y_0.8Gd_0.2)₃Al₅O₁₂ was 1.51 W m⁻¹ K⁻¹ at 1200 °C. The average thermal expansion coefficient of (Y_0.8Gd_0.2)₃Al₅O₁₂ was slightly larger than that of Y₃Al₅O₁₂. (Y_0.8Gd_0.2)₃Al₅O₁₂ bulk sample exhibited the lowest elasticity modulus among the investigated (Y_1-xGd_x)₃Al₅O₁₂. In addition, (Y_0.8Gd_0.2)₃Al₅O₁₂ ceramic remained good phase stability from room temperature to 1600 °C.     

Synthesis of BaCeO3 and BaCe0.9Y0.1O3−δ from mixed oxalate precursors

An oxalate precipitation route is proposed for the synthesis of BaCe_1−xY_xO₃ (x = 0 and 0.1) after calcination at 1100 °C. The precipitation temperature (70 °C) was a determinant parameter for producing a pure perovskite phase after calcination at 1100 °C for 1 h. TG/DTA measurements showed that the co-precipitated (Ba, Ce and Y) oxalate had a different thermal behaviour from single oxalates. Despite a simple grinding procedure, sintered BaCe_0.9Y_0.1O_3−δ pellets (1400 °C, 48 h) presented 90.7% of relative density and preliminary impedance measurements showed an overall conductivity of around 2 × 10⁻⁴ S cm⁻¹ at 320 °C.     

Enhancement of pyroelectric properties of lead-free 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics by La doping

Lead-free 0.94NBT-0.06BT-xLa ceramics at x = 0.0–1.0 (%) were synthesized by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. With increasing La³⁺ content pyroelectric coefficient (p) and figures of merits greatly increase; however, the depolarization temperature (T_d) decreases. p is 7.24 × 10⁻⁴C m⁻² °C⁻¹ at RT at x = 0.5% and 105.4 × 10⁻⁴C.m^−2 °C⁻¹ at T_d at x = 0.2%. F_i and F_v show improvements at RT from 1.12 (x = 0%) to 2.65 (x10 ⁻¹⁰ m v⁻¹) (x = 0.5%) and from 0.021 to 0.048 (m².C⁻¹) respectively. F_i and F_v show a huge increase to 37.6 × 10⁻¹⁰ m v⁻¹ and 0.56 m² C⁻¹ respectively at T_d at x = 0.2%. F_C shows values of 2.10, 2.89, and 2.98 (x10⁻⁹C cm⁻² °C⁻¹) at RT at 33, 100 and 1000 (Hz) respectively. Giant pyroelectric properties make NBT-0.06BT-xLa at x = 0.2% and 0.5% promising materials for many pyroelectric applications.     

Mixed-valent structure,dielectric properties and defect chemistry of Ca1−3x/2TbxCu3Ti4−xTbxO12 ceramics

Single-phase Ca_1−3x/2Tb_xCu₃Ti_4−xTb_xO₁₂ (0.025≤ x≤0.075) (CTCTT) ceramics with a cubic perovskite-like structure and a fine-grained microstructure (1.6‒2.3 µm) were prepared using a mixed oxides method. The results revealed that mixed valence states of Cu²⁺/Cu⁺, Ti⁴⁺/Ti³⁺, and Tb³⁺/Tb⁴⁺ coexisted in CTCTT. A multiphonon phenomenon in the Raman scattering at 1148, 1323, and 1502 cm⁻¹ was reported for undoped and doped CTTO. Tb was mainly incorporated in the interior of the CTCTT grains rather than on the surface. The dielectric permittivity of CTCTT (ε_r'_RT =3590‒5200) decreased relative to CCTO (ε_r'_RT =10240) at f =1 kHz, but the dielectric loss of CTCTT (the minimum value of tan δ=0.12 at RT) increased as a result of Tb doping. The defect chemistry of CTCTT is discussed. The internal barrier layers capacitance (IBLC) model was adopted for impedance spectroscopy (IS) analysis. The activation energies of the grain boundaries (E_gb) and semi-conductive grains (E_g) for CTCTT were determined to be 0.52 eV and 104 meV, respectively. The IS and defect chemistry analyses confirmed that the decrease in the dielectric permittivity was mainly due to a decrease in conductivity in the semiconducting CTCTT grains caused by the acceptor effect of Tb⁴⁺ at the Ti site, which resulted in a decrease in the IBLC effect.     

Structural and optical characterization of Zn0.95−xMg0.05AlxO nanoparticles

The structural and optical properties of ZnO nanoparticles doped simultaneously with Mg and Al were investigated. XRD results revealed the hexagonal wurtzite crystalline structure of ZnO. The FE-SEM study confirmed the formation of nano-sized homogeneous grains whose sizes decreased monotonously with increasing doping concentrations of Mg and Al. The absorption spectra showed that band gap increased from 3.20 to 3.31 eV with Mg doping. As the Al concentration changed from x=0.01 to x=0.06 mol% at constant Mg concentration the band gap observed to be decreased. Particle sizes estimated from effective mass approximation using absorption data and these values are in good agreement with the crystallite sizes calculated from XRD data. Raman spectra of ZnO showed a characteristic peak at 436 cm⁻¹ correspond to a non-polar optical phonon E₂ (high). With increase of the Al doping concentrations, E₂ (high) phonon frequency shifted to 439 cm⁻¹ from to 436 cm⁻¹. The origin of E₂ (high) peak shift in ZnO nanoparticles is attributed to optical phonon confinement effects or the presence of intrinsic defects on the nanoparticles. PL spectra indicated that with increase of Al co-doping along with Mg into ZnO, intensity of the peak positioned at 395 nm was initially increased at x=0 and then decreased with increase of the Al concentrations from x=0.01 to x=0.06 mol%.     

High wear resistance white ceramic glaze containing needle like zircon single crystals by the addition of sepiolite n-ZrO2

Sepiolite with homogeneous zirconia nanoparticles distribution has been added to a transparent ceramic glaze to study opacification, mechanical and wear resistance properties. It has been observed that monodispersed zircon single crystals with needle-like shape have been formed in the ceramic glaze. These in situ zircon single crystals give white color and increase opacification (L = 94 vs L = 90), mechanical properties (hardness and toughness) and wear resistance by a factor of 4 compared to the commercial crystalline glaze containing a similar fraction of micrometer commercial zircon.     

Preparation,characterization, and giant dielectric permittivity of (Y3+ and Nb5+) co–doped TiO2 ceramics

The microstructure and giant dielectric properties of Y³⁺ and Nb⁵⁺ co–doped TiO₂ ceramics prepared via a chemical combustion method are investigated. A main rutile–TiO₂ phase and dense ceramic microstructure are obtained in (Y_0.5Nb_0.5)_xTi_1-xO₂ (x = 0.025 and 0.05) ceramics. Nb dopant ions are homogeneously dispersed in the microstructure, while a second phase of Y₂O₃ particles is detected. The existence of Y³⁺, Nb⁵⁺, Ti⁴⁺ and Ti³⁺ as well as oxygen vacancies is confirmed by X–ray photoelectron spectroscopy and X–ray absorption near edge structure analysis. The sintered ceramics exhibit very high dielectric permittivity values of 10⁴–10⁵ in the frequency range of 40–10⁶ Hz. A low loss tangent value of ≈0.08 is obtained at 40 Hz. (Y_0.5Nb_0.5)_xTi_1-xO₂ ceramics can exhibit non–Ohmic behavior. Using impedance spectroscopy analysis, the giant dielectric properties of (Y_0.5Nb_0.5)_xTi_1-xO₂ ceramics are confirmed to be primarily caused by interfacial polarization.     

Synthesis,structural and electrical properties of novel pyrochlores in the Bi2O3–CuO–Ta2O5 ternary system

A series of non-stoichiometric cubic pyrochlores with general formula, Bi_3?xCu_1.8Ta_3+xO_13.8+x (BCT) was successfully prepared by solid state reaction at the firing temperature of 950 °C over 2 days. The solid solution mechanism is proposed as one-to-one replacement of Bi³⁺ for Ta⁵⁺, together with a variation in oxygen content in order to achieve electroneutrality. The solid solution limit is confirmed by X-ray diffraction technique (XRD) for which linear variation of lattice constants is observed at 0 ≤ x ≤ 0.6. The refined lattice constants are found to be in the range of 10.4838 (8) Å–10.5184 (4) Å and the grain sizes of these samples determined by scanning electron microscopy (SEM) fall between 1 and 40 μm. Meanwhile, thermal analyses show no physical or chemical change for the prepared pyrochlores. The relative densities of the densified pellets for AC impedance measurements are above 85% and the measured relative permittivity, ?′ and dielectric loss, tan δ for composition, x = 0.2 at ambient temperature are ～60 and 0.07 at 1 MHz, respectively. The calculated activation energies are 0.32–0.40 eV and the conductivity values, Y′ are in the order of 10^?3 at 400 °C. The conduction mechanisms of BCT pyrochlores are probably attributed to the oxygen non-stoichiometry and mixed valency of copper within the structure.     

Electrical properties and light up conversion effects in Bi3.79Er0.03Yb0.18Ti3−xWxO12 ferroelectric ceramics

Modified ceramic compositions of Bi_3.79Er_0.03Yb_0.18Ti_3−xW_xO₁₂ with fixed Er and Yb content, and a varying W content (x=0.0, 0.01, 0.03, 0.06 and 0.10) are prepared. The site selectivity of Er³⁺, Yb³⁺, and W⁶⁺ cations is analyzed, and their influence on the electrical and light up conversion properties is studied. Formation of single phase orthorhombic structure is confirmed with enhanced grain growth up to x=0.03, and for (x≥0.04–0.10) the grain growth is inhibited, and the orthorhombic distortion is relaxed. Raman spectroscopy reveals W⁶⁺ cation substitutes preferentially at the B-site replacing Ti⁴⁺ ions in the Bi₄Ti₃O₁₂ lattice structure. Increasing W⁶⁺ donor concentration reduces the conductivity effects by lowering the oxygen vacancies. Reduced dielectric losses (tan δ=0.003) and dispersion with frequency in the range (10⁻²–10 Hz) are observed, and improvements in the remnant polarization (2P_r=28.86 μC/cm²) are seen up to an optimum content of x=0.03. At higher W content (x>0.03), the properties tend to degrade due to structural relaxation and microstructural changes. Up conversion photoluminescence (UC-PL) under 980 nm excitation shows strong emission in the green and red bands due to enhanced crystal field around the Er³⁺ ions for an optimum W content of x=0.06. A weak blue emission band around (~492 nm) is observed by cooperative emission (CE) due to radiative relaxation of an excited Yb–Yb pair from a virtual level. Variation of UC emission intensity with pump-power confirms a two-photon mechanism for the up conversion process.