Rare earth doped complex oxide, its preparation and photoluminescence use, method of predicting photoluminescence emission characteristics of said complex oxide

Abstract

A rare earth ion doped complex oxide with Formula (I) (Ca1+xMgSi2Eu0025O6+x (x = 0.2, 0.4, 0.6, 0.8, 1.2, 1.4, 1.6, 1.8, 2.0)) or Formula (II) (Mg1-xCo3NiCuZnCexO8+x (x = 0.05, 0.1, 0.5, 1.0)). A method of preparing said rare earth ion doped complex oxide, and the use of said complex oxide in photoluminescence phosphor are also addressed. A method 5 of predicting photoluminescence emission characteristics of the complex oxide is further addressed.

Description

[0001] RARE EARTH DOPED COMPLEX OXIDE, ITS PREPARATION AND PHOTOLUMINESCENCE USE, METHOD OF PREDICTING PHOTOLUMINESCENCE EMISSION CHARACTERISTICS OF SAID COMPLEX

[0002] OXIDE

[0003] TECHNICAL FIELD

[0004] The present invention relates to a rare earth metal doped complex oxide, for example, particularly, but not exclusively, an Eu2+doped or Ce3+doped complex oxide. The present invention also relates to the preparation of the complex oxide and the use of said complex oxide in photoluminescence phosphors. The present invention further relates to a method of predicting the photolumincscence emission characteristics of the complex oxide.

[0005] BACKGROUND OF THE INVENTION

[0006] In recent years, LEDs have attracted much attention as a replacement for traditional light sources owing to their characteristic features such as high brightness, low power consumption and long lifespan.

[0007] It is believed that one of the candidates for LEDs may be rare earth-ion doped oxide-based phosphor in view of their high physical / chemical stability as well as excellent luminescent property. In particular, it is believed that rare earth activators exhibit broad emission peaks as a result of the parity allowed 5d— >4f transitions, and since 5d electrons remain unshielded in the rare earth activators, the produced photoluminescence (PL) is largely influenced by the crystal field of the host material. Thus, in theory, the emission properties / characteristics of the rare earth-ion doped phosphor may be tunable by tuning the crystal field effect of the host material / structure. However, it is believed that in practice there is a lack of a method for this purpose, let alone an optimal composition that could lead to a desirable PL property / characteristic.

[0008] The present invention thus seeks to eliminate or at least mitigate the aforementioned shortcomings. SUMMARY OF THE INVENTION

[0009] In a first aspect of the present invention, there is provided a rare earth ion doped complex oxide with formula: Ca1+xMgSi2Eu0.02506+x(x = 0.2, 0.4, 0.6, 0.8, 1.2, 1.4, 1.6, 1.8, 2.0) (I), or Mg1-xCo3NiCuZnCexO8+x(x = 0.05, 0.1, 0.5, 1.0) (II).

[0010] In a second aspect of the present invention, there is provided a method of preparing a complex oxide of said Formula I in accordance with the first aspect by solid-state reaction, including steps: (a) mixing raw materials containing Ca, Mg, Si, Eu and O in calculated amounts and milling said raw materials in ethanol; and (b) sintering said mixed and milled raw materials in a reduced environment at above 1,300 °C for at least 3 hours.

[0011] Optionally, said reduced environment is of substantially 5% H2and 95% N2.

[0012] In a third aspect of the present invention, there is provided a method of preparing a complex oxide of said Formula II in accordance with the first aspect by solid-state reaction, including: (i) mixing nitrate salts of Mg, Co, Ni, Cu, Zn, and Ce in a molar ratio of l-x:l:l:l:l:x, (x = 0.05, 0.1, 0.5, 1.0) to form a mixture; (ii) subjecting said mixture to ultrasonification; (iii) stirring, freezing, drying and homogenizing said mixture; and (iv) heating said mixture at a heating rate of substantially 5 °C min'1up to 1,000 °C, and keeping said heating for 4 hours.

[0013] Optionally, said nitrate salts are hexahydrate nitrate salts.

[0014] In a third aspect of the present invention, there is provided a photoluminescence phosphor including at least a complex oxide in accordance with the first aspect.

[0015] In a fourth aspect of the present invention, there is provided a method of predicting photoluminescence (PL) emission characteristics of a complex oxide in accordance with the first aspect, including: (A) obtaining entropy of fusion ( ΔSf) of the same family of congruent compounds of said complex oxide in accordance with the first aspect; (B) applying the following Formula III to obtain a predicted PL emission wavelength value of said complex oxide: wherein: each of A and n is, independently, a constant; ΔSfis the entropy of fusion (J / K) of said complex oxide; and 2 (nm) is the predicted wavelength of said complex oxide.

[0016] Optionally, said constants A and n are obtained via regression under the same family of congruent compounds.

[0017] In an optional embodiment, the method further includes: (C) obtaining entropy of mixing of the same family of congruent compounds of said complex oxide in accordance with the first aspect; (D) applying the following Formula IV to obtain a predicted PL emission intensity value of said complex oxide: wherein: each of B and m is, independently, a constant; I and / ., are PL emission intensity values; ΔSmix(entropy of mixing) = - RXlnX - R(l-X)ln(l-X), where R is the gas constant and X is the mole fraction of the species.

[0018] Optionally, said constants B and m are obtained via regression under the same family of congruent compounds.

[0019] BRIEF DESCRIPTION OF DRAWINGS

[0020] The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

[0021] Fig. 1 shows the XRD spectra of 0.025 Eu2+doped Ca1+xMgSi2O6+x(x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) compounds in accordance with an embodiment of the present invention;

[0022] Fig. 2A shows the solid-state photoluminescence emission spectra of Ca1+xMgSi2Eu0.025O6+x(x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) phosphors;

[0023] Fig. 2B shows the solid-state photoluminescence emission spectra of Ca2+xMgSi2Eu0.02507+x (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) phosphors;

[0024] Fig. 3A shows the solid-state photoluminescence emission spectra of Ce3+doped high entropy oxide of Mg, Co, Ni, Cu, Zn in the molar ratio Mg : Co : Ni : Cu : Zn : Ce of 0.95 : 1: 1 : 1 : 1: 0.05;

[0025] Fig. 3B shows the solid-state photoluminescence emission spectra of Ce’ doped high entropy oxide of Mg, Co, Ni, Cu, Zn in the molar ratio Mg : Co : Ni : Cu : Zn : Ce of 0.9 : 1: 1 : 1 : 1: 0.1; Fig. 3C shows the solid-state photoluminescence emission spectra of Ce3+doped high entropy oxide of Mg, Co, Ni, Cu, Zn in the molar ratio Mg : Co : Ni : Cu : Zn : Ce of 0.5 : 1: 1 : 1 : 1: 0.5;

[0026] Fig. 3D shows the solid-state photoluminescence emission spectra of Ce3+doped high entropy oxide of Mg, Co, Ni, Cu, Zn in the molar ratio Mg : Co : Ni : Cu : Zn : Ce of O : 1: 1 : 1 : 1: 1;

[0027] Fig. 4 is a table summarizing the thermodynamic calculations for Ca2+xMgSi2Eu0.02507+x(x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) phosphors synthesized in reduced atmosphere;

[0028] Fig. 5A is a ternary diagram showing the emission wavelengths (nm) of Eu2+doped alkaline earth metals (Ca, Mg) based silicates;

[0029] Fig. 5B is a ternary diagram showing the entropy of fusion per cation (J / K) of EU2+doped alkaline earth metals (Ca, Mg) based silicates;

[0030] Fig. 6A shows a plot between PL emission wavelength (nm) and entropy of fusion per cation (J / K) for binary' system (Category I and III) compounds;

[0031] Fig. 6B shows a plot between PL emission wavelength and normalized mole fraction for ternary system (Category II) compounds (Ca1+xMgSi2Eu0.025O6+x(x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0) phosphors);

[0032] Fig. 7 shows the graph between entropy of mixing and mole fraction for ideal mixtures;

[0033] Fig. 8 is a plot showing the deconvolution peaks applying Gaussian curve fitting for Ca1+xMgSi2Eu0.025O6+x(x = 0.2) phosphor;

[0034] Fig. 9A show the plot of emission wavelength (nm) and entropy of mixing (J / K) verses normalized mole fraction (X) for deconvolution peaks for big peak corresponding to Ca1+xMgSi2Eu0.025O6+x(x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0) phosphors (Ternary System);

[0035] Fig. 9B show the plot of emission wavelength (nm) and entropy of mixing (J / K) verses normalized mole fraction (X) for deconvolution peaks for small peak corresponding to Ca1+xMgSi2Eu0.025O6+x(x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0) phosphors (Ternary System); Fig. 10 is a schematic diagram illustrating the quantum well model for photoluminescence of Eu2+doped Ca-Mg silicates;

[0036] Fig. 11 is a plot of emission wavelength of the congruent compounds against the entropy of mixing of said congruent compounds;

[0037] Fig. 12 is a schematic diagram illustrating the energy levels of the solid state and liquid state of ZnO and high entropy oxide; and

[0038] Fig. 13 is a plot of emission intensity ratio against entropy of mixing.

[0039] DETAILED DESCRIPTION OF OPTIONAL EMBODIMENT

[0040] As used herein, the forms “a”, “an”, and “the” are intended to include the singular and plural forms unless the context clearly indicates otherwise.

[0041] The words “example” or “exemplary” used in this invention are intended to serve as an example, instance, or illustration. Any aspect or design described in this disclosure as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the words “example” or “exemplary” is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A, X employs B, or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances.

[0042] In a first aspect of the present invention, there is provided a rare earth metal ion doped complex oxide. As used herein, the term “complex oxide” generally denotes a solid-state material that contains oxygen and other elements, particularly metal or metal-like elements such as alkaline earth metal elements, transition metal elements, semimetal elements, basic metal elements, lanthanide elements that is different from the dopant, etc.. Examples of the above elements may include one or more of the followings: magnesium, calcium, strontium, barium, titanium, iron, cobalt, nickel, copper, zinc, niobium, molybdenum, platinum, aluminum, silicon, lanthanum, cerium, neodymium, europium, gadolinium, terbium, erbium, ytterbium, etc. In some embodiments, the complex oxide may include one or more of the alkaline earth metals as described herein and silicon, such a complex oxide may refer as an alkaline earth metal silicate. In some embodiments, the complex oxide may have five or more principal metal cations such as magnesium, cobalt, nickel, copper, zinc, and the like and may have a single-phase crystal structure, such a complex oxide may refer as a high entropy oxide.

[0043] Tn some specific embodiments, the rare earth ion doped complex oxide may be an Eu2+ion doped complex oxide, particularly an Eu2+ion doped alkaline earth metal complex oxide, and more particularly an Eu2+ion doped alkaline earth metal silicate having a formula of Ca1+xMgSi2Eu0.025O6+x(i.e.. Formula I), with x being 0.2, 0.4, 0.6, 0.8, 1.2, 1.4, 1.6, 1.8, 2.0. In other words, in some embodiments, the rare earth ion doped complex oxide may be selected from the group consisting of Ca1.2MgSi2Eu0.025O6.2, Ca1.4MgSi2Eu0.025O6.4, Ca1.6MgSi2Eu0.025O6.6,

[0044] Ca1.8MgSi2Eu0.025O6.8, Ca2.2MgSi2Eu0.025O7.2, Ca2.MgSi2Eu0.025O7.4,

[0045] Ca2.6MgSi2Eu0.025O7.6, Ca2.8MgSi2Eu0.025O7.8, Ca3MgSi2Eu0.025O8, and a combination thereof.

[0046] In some embodiments, the rare earth ion doped complex oxide may be a Ce3+ion doped complex oxide, particularly a Ce3+ion doped high entropy oxide having a formula of Mg1-xC03NiCuZnCexO8+x(i.e., Formula II), with x being 0.05, 0.1, 0.5, 1.0. In other words, in some embodiments, the rare earth ion doped complex oxide may be selected from the group consisting of Mg0.95C03NiCuZnCe0.05O8.05, Mg0.9Co3NiCuZnCe0.1O8.1, Mg0.5Co3NiCuZnCe0.5O8.5, Co3NiCiiZnCeO9, and a combination thereof.

[0047] The method of preparing the rare earth ion doped complex oxide as described herein will now be disclosed. In the embodiments where the rare earth ion doped complex oxide having Formula (I), the method of preparing said rare earth ion doped complex oxide may be enabled by solid-state reaction. In particular, the method may include the steps of (a) mixing raw materials containing Ca, Mg, Si, Eu and O in calculated amounts and milling said raw materials in ethanol; and (b) sintering said mixed and milled raw materials in a reduced environment at above 1,300 °C for at least 3 hours.

[0048] In particular, in step (a), EU2O3 may be used as a dopant and mixed with SiO2, MgCO3, and CaCO3according to the composition of Formula (1). The Eu2O3may be used as a dopant and may be used as a precursor of Eu2+ions under a (chemically) reduced environment, such as a reduction atmosphere as described herein. The raw materials may be ball-milled for at least 2 hours with absolute ethanol, dried, and grounded before commencing step (b).

[0049] Tn step (b), grounded, ball-milled, dried raw materials may be sintered from 1300 °C to 1400 °C, from 1310 °C to 1400 °C, from 1300 °C to 1405 °C, from 1310 °C to 1410 °C, from 1320 °C to 1400 °C, from 1320 °C to 1390 °C, from 1320 °C to 1385 °C, from 1325 °C to 1385 °C, from 1330 °C to 1370 °C, from 1340 °C to 1360 °C, particularly at 1350 °C for 3 hours to 5 hours, particularly 4 hours, under a reduction atmosphere of substantially 5% H2and 95% N2.

[0050] Optionally or additionally, the as-prepared rare earth ion doped complex oxide of Formula (I) may be analyzed by techniques in the art such as XRD for confirming the composition of the as-prepared rare earth ion doped complex oxide of Formula (T). Fig. 1 shows an exemplary XRD spectra of the rare earth ion doped complex oxide of Formula (1) prepared in accordance with the method as described herein.

[0051] The solid-state reaction may also be applied to enable the preparation of the rare earth ion doped complex oxide having Formula (II). In particular, the method of preparing the rare earth ion doped complex oxide of Formula (II) may include the steps of: (i) mixing nitrate salts of Mg, Co, Ni, Cu, Zn, Ce in a molar ratio of 1-x: 1:1:1: l:x, (x = 0.05, 0.1 , 0.5, 1.0) to form a mixture; (ii) subjecting said mixture to ultrasonification; (iii) stirring, freezing, drying and homogenizing said mixture; and (iv) heating said mixture at a heating rate of substantially 5 °C min'1up to 1,000 °C, and keeping said heating for 4 hours.

[0052] In particular, in step (i) hexahydrate nitrates salts of Mg, Co, Ni, Cu, Zn, and Cc may be mixed in such as 30 mL DI water according to the composition of Formula (II) to form a mixture. After that, the mixture may be subjected to ultrasonification for at least 15 minutes, such as 15 minutes to 30 minutes at room temperature, followed by stirring at 450 rpm to 600 rpm , 450 rpm to 590 rpm, 460 rpm to 600 rpm, 450 rpm to 580 rpm, 470 rpm to 590 rpm, 470 rpm to 550 rpm, particularly 500 rpm, for at least 24 hours. After that, the mixture may be frozen at -80°C for 5 hours in a freeze dryer. The freeze-dried mixture may then be collected and homogenized with a mortal' and pestle. At this stage, the mixture will be in powder form. Finally, the mixture powder may be heated at a heating rate of substantially 5 °C min1up to 1,000 °C, and keeping said heating for 4 hours, cooled to room temperature to obtain the rare earth ion doped complex oxide of Formula (11).

[0053] Optionally or additionally, the as-prepared rare earth ion doped complex oxide of Formula (11) may be analyzed by techniques in the art such as XRD for confirming its composition.

[0054] In a third aspect of the present invention, there is provided a photolumincsccncc phosphor including at least a complex oxide as described herein, particularly the Eu2+ion doped alkaline earth metal silicate having a formula of Ca1+xMgSi2Eu0.025O6+x(i.e., Formula T) or the Ce3+ion doped high entropy oxide having a formula of Mgi-xCo3NiCuZnCexO8+x(i.e., Formula II), with x as being defined herein. It is believed that the rare earth ion doped complex oxide as described herein is advantageous in LED applications in view of their photoluminescence at and / or over the visible region. For example, as shown in Figs. 2A and 2B, the Eu2+ion doped alkaline earth metal silicate having a formula of Ca1+xMgSi2Eu0.025O6+x(i.e., Formula I) emits from 475 nm to 530 nm upon photoexcitation from 327 nm to 373 nm. Also, as shown in Figs. 3A-3D, the Ce3+ion doped high entropy oxide having a formula of Mgi-xCo3NiCuZnCexO8+x(i.e., Formula II) emits at a wavelength spanning from about 430 nm to at least 620 nm, with an emission maximum at about 467 nm, suggesting a blue light emission of the Ce3+ion doped high entropy oxide as described herein.

[0055] With reference to Figs. 2A and 2B, it can be seen that in general the as the Ca content of the phosphor increase (from x = 0 to 2.0), the emission wavelength corresponding to the maximum emission shows a red shift from 449 nm to 525 nm and from 475 nm to 530 nm. The intensity of photoluminescence emission at 449 nm and 475 nm each starts to fall while the intensity of photoluminescence peak at 525 nm and 530 nm each rises correspondingly. In addition, it is believed that the PL emission spectra align with the thermodynamic calculations as obtained from FactSage (Fig. 4). The experiments specifically involve three products: gas (CO2), terminal compound 1, and terminal compound 2. For each compound, the PL emission wavelength remains constant, while the intensity decreases from its peak value at terminal 1 (Ca2MgSi2Eu0.025O7) or terminal 2 (Ca3MgSi2Eu0.025O8) position. Notably, the formation of CO2 gas at different experimental points (x =0, 0.2, 0.4, 0.6, 0.8, 1) does not affect emission wavelength or intensity.

[0056] It is believed that this kind of red shift behavior can be observed in other Eu2+doped silicates. For example, a similar red shift of emission spectrum was observed with the partial substitution of Na by K, as well as Rb by Cs, in RbNa3(Li3SiO4)2:Eu2+. It is believed that ionic radius increases from Na to K, Rb, and Cs, and therefore the molar volume and entropy increase from #1 RhNahLi3SiO4)2: Eu2+to #2 RbNa2K(Li3SiO4)2: Eu2+, and further to #3 CsNa3(Li3SiO4)2:Eu2+. In addition, assuming that the liquid entropy of these compounds is similar' due to their same crystal structure and close chemistry, it is believed that their entropy of fusion decreases progressively from #1 to #2 to #3. As a result, it is believed that the red shift observed in these compounds, from #1 to #2 to #3, leads to higher emission wavelengths due to the inverse relationship between entropy of fusion and emission wavelength. Additionally, it is believed that this relationship is observable in other Eu2+doped silicate with close chemistry such as Eu2+doped Ba2SiO4(emission wavelength: 511 nm, entropy of fusion: 91.28 J / K per formula) and Sr2SiO4(emission wavelength: 574 nm, entropy of fusion: 47.60 J / K per formula).

[0057] In this case, indeed, the PL emission wavelengths of the Eu2+ion doped alkaline earth metal silicate phosphors showed an inverse trend with the entropy of fusion. The PL emission wavelengths and entropy of fusions of Eu2+doped alkaline earth metals (Ca, Mg) silicates are shown in Figs. 5A and 5B. Based on the extent to which the PL emission wavelengths are influenced by the entropy of fusion, the compounds were classified into three categories (I, II and III). From Fig. 5A, it is evident that the PL emission wavelengths for category I compounds varies significantly with their entropy of fusion followed by category II and III compounds. In addition, category III compounds showed least variation in the entropies while their PL emission wavelengths were also least affected by the entropy of fusion compared to category I and II compounds. From Fig. 5A, it is clearly seen that silicates with high Ca content possess low entropy which renders them more stable. The entropy of fusion of Ca2SiO4 (compound b) is least among all other compounds in the three categories. More interestingly, Eu2+doped compound b showed highest PL emission wavelength among all other compounds. Evidently, high Ca content in Eu2+doped silicate causes PL emission wavelength to be more dependent on the entropy of fusion.

[0058] It is believed that Eu2+acts as lighting center in all Eu2+doped silicates whereby the broad emission spectrum arises due to 4f65d1— > 4f7transition of Eu2+. Given that the ionic radius of Ca2+(1.06 A) is closer to that of Eu2+(1.25 A) compared to Mg2+(0.81 A), it is believed that Eu2+ions will replace Ca2+more efficiently than Mg2+ions. Consequently, silicate host structures with high Ca content shows significant red shift in the PL emission wavelength upon doping with Eu2+compared to Eu2+doped silicates with low or negligible Ca content.

[0059] From Fig. 6A, it is evident that PL emission wavelength follows an inverse correlation with the entropy of fusion per cation for binary system (Category I & III compounds) while for ternary system (Category II compounds) the plot between PL emission wavelength and normalized mole fraction (X) (Fig. 6B) closely resembles a graph between total entropy and mole fraction of species. The total entropy is given by the equation (1):

[0060] + ΔSExcessEq. (1) where, ΔSConfgurationand SExcessrepresents the configuration and excess entropy, respectively.

[0061] For an ideal binary solution, the excess entropy is zero. Equation (2) outlines the mixing entropy, also referred to as the configuration entropy. ΔSmix= -RXlnX - R(1 - X)ln(l - X) Eq. (2) where, ΔSmixis entropy of mixing, R is gas constant, X is mole fraction of species.

[0062] Following equation (2), the entropy of mixing is maximum when the mole fraction of each component X is 0.5 (Fig. 7). For all feasible concentrations (0 < X < 1), both InX and In(l-X) yield negative values, ensuring that the mixing entropy (ΔSmix) is consistently positive. As depicted in Fig. 7, the ΔSmixplot attains its peak when X increases from zero to 0.5, gradually diminishing with higher mole fractions.

[0063] To uncover the correlation between the measured emission spectra in PL and configurational / excess entropy, Gaussian curve fitting was applied to dcconvolutc the PL spectra for all ternary compounds in Category 11. Fig. 8 shows a sample deconvolution spectrum obtained via Gaussian curve fitting for Ca1+xMgSi2Eu0.025O6+x(x = 0.2) phosphor. Upon deconvolving all the PL emission spectra, each case yielded two peaks, categorized as high and low. In Figs. 9A and 9B, the graphs illustrate the relationship between PL emission wavelength and composition for both the high and low deconvolution peaks. Specifically, Figs. 9A and 9B display the variations of configurational entropy ( ΔSConfiguration) and excess entropy ( ΔSExcess) with composition, respectively. The findings from Figs. 9A and 9B highlight the dominance of ΔSConfigurationin the ternary system (Fig. 6B). ΔSConfigurationshows an increasing trend from one terminal composition towards the other, reaching a maximum around the midpoint, and then gradually decreasing towards the second terminal solid. If ΔSExcessis assumed to be negligible, ΔSConfigurationwould resemble that of an ideal solution, aligning Fig. 6B with Fig. 7.

[0064] Based on the above, it is believed that the primary PL emission characteristics of the investigated silicates align closely with the principles of ideal mixing solution theory, while the low peak reflects the contribution of excess entropy. Overall, a noteworthy correlation between entropy and PL emission wavelengths was observed and elucidated. Accordingly, it is believed that a model may be developed to interpret the PL emission wavelength in light with the thermodynamic data (entropy of fusion data).

[0065] It is believed that the emission wavelength in PL is exclusively influenced by the substitution of Eu2+in the host structure, as elucidated in the quantum well model presented in Fig. 10. As shown, the augmentation of Ca2+content in the host structure leads to a reduction in its entropy of fusion. Consequently, the entropy of the compact solid increases, approaching the entropy of the less dense liquid phase, causing an expansion of solid volumes. This expansion facilitates the substitution of Eu2+ions for Ca2+ions in the host structure, resulting in both increased emission light intensity and a red-shift of emission lights, as per the quantum well theory for lattice expansion. The expanded state of the host structure corresponds to a decrease in the entropy of fusion. This expansion narrows the energy gap between the ground and excited states, causing a red shift in the emission wavelength. Conversely, high entropy of silicate fusion corresponds to low entropy of solid silicate, resulting in a lattice contrast and a blue shift in emission light. Additionally, when the Ca2+content in the host structure is low, fewer Eu2+ions can enter, leading to lower emission intensity. This maintains the host in a more compact state (i.e., low entropy state), keeping the energy gap between the ground and excited states high and favoring a blue shift.

[0066] Without wishing to be bound by theory, the inventors have, through their own researches, trials, and experiments devised that the PL emission characteristics / properties of the complex oxide as described herein, such as the PL emission wavelength and emission intensity of Eu2+doped Ca-Mg based silicates may be predicted by apply the following Formula: wherein: each of A and n is, independently, a constant; ASf is the entropy of fusion (J / K) of said complex oxide; and 2 (nm) is the predicted wavelength of said complex oxide. wherein: each of B and m is, independently, a constant; I and I are PL emission intensity values; ΔSmix(entropy of mixing) = - RXlnX - R(l-X)ln(l-X), where R is the gas constant and X is the mole fraction of the species.

[0067] Accordingly, in a fourth aspect of the present invention, there is provided a method of predicting PL emission characteristics of the complex oxide as described herein. In an embodiment where the PL emission characteristics to be predicted is the emission wavelength of the complex oxide, the method may include the steps of:

[0068] (A) obtaining entropy of fusion (ASf) of the same family of congruent compounds of said complex oxide; (B) applying the following Formula III to obtain a predicted PL emission wavelength value of said complex oxide: wherein: each of A and n is, independently, a constant; ASf is the entropy of fusion (J / K) of said complex oxide; and λ (nm) is the predicted wavelength of said complex oxide.

[0069] In particular, the entropy of fusion of the congruent compounds and the complex oxide may be obtained by experiments or by computational calculation such as using some software in the art such as FactSage. For example, the thermodynamic calculations of the MgO-CaO-SiO2ternary systems (i.e., the complex oxide of Formula (I)) may be conducted using FactSage package, in which thermodynamics of all unary, binary, and ternary solid and liquid phases may be optimized using the Calphad approaches. In particular, the structural short-range ordering behaviors in all liquid solutions were optimized based on the quasi-chemical solution model that has been implemented in FactSage. In an embodiment where the entropy of fusion is obtained by experiments, it may be done by making a Van’t Hoff plot. In particular, the equilibrium constant K is measured at different temperatures and then InK verses T-1is plotted. The y-intercept of this plot gives RΔS° and the slope gives -RΔH0. Optionally, it may also be done by measuring ΔH0by calorimetry and measure K by some other techniques in the art, followed by computing ΔG0(Gibbs free energy) from K and solve for entropy ΔS( )sing Gibbs free energy equation.

[0070] In step (B), the application may involve plotting a graph of the emission wavelengths of the congruent compounds (same family as the complex oxide) against the entropy of fusion of the these compounds; obtaining / determining the constants A and n via regression; and fitting the entropy of fusion of the complex oxide into Formula (ITT) to determine its emission wavelength.

[0071] In some embodiments where the method is used to predict the emission intensity of the complex oxide as described herein, the method may further include the steps of:

[0072] (C) obtaining entropy of mixing ( Smix) of the same family of congruent compounds of the complex oxide as described herein; (D) applying the following Formula IV to obtain a predicted PL emission intensity value of said complex oxide: wherein: each of B and m is, independently, a constant; / and Ioare PL emission intensity values, and Iois the initial emission intensity of the undoped complex oxides (i.e., when the concentration of Eu2+or Ce3+is zero); (entropy of mixing) = - RXlnX - R(l -X)ln(l-X), where R is the gas constant and X is the mole fraction of the species.

[0073] The method of prediction of the emission intensity is generally similar to that of prediction of the emission wavelength. In particular, after obtaining the entropy of mixing of the same family of congruent compounds of the complex oxide using the aforementioned equation ΔSmix(entropy of mixing) = - RXlnX - R(1-X)ln(1-X), in step (D), a graph of Ilin against ΔSmix(entropy of mixing) may be plotted, thereby obtaining / determining the constants B and m via regression. After that, by fitting the

[0074] ΔSmix(entropy of mixing) into Formula (IV), the emission intensity of the complex oxide is determined.

[0075] Based on the above, it is believed that the method of prediction as described herein is applicable to other rare earth ion doped complex oxide systems that follows the quantum well theory / model as described herein. Referring to Figs. 3A to 3D, it is noted that the PL peak intensity of the complex oxide of Formula (11) increases with the molar concentration of Ce3+ion in the complex oxide. In other words, the higher the molar concentration of Ce3+ion in the complex oxide of Formula (II), the higher the PL intensity of said complex oxide. As shown in Table 1, it can be seen that the ionic radius of VI coordinated Mg (Mg is surrounded by 6 atoms) is close to that of Ce3+.

[0076] Table 1. Ionic Radius of Metal Ions in Complex Oxide of Formula (II)

[0077] Thus, it is believed that when the Mg2+content in the host structure decreases, it would lead to a reduction in its entropy of fusion. Consequently, the entropy of the compact solid increases, approaching the entropy of the less dense liquid phase, causing an expansion of solid volumes. This expansion facilitates the substitution of Ce3+ions for Mg2+ions in the host structure, resulting in an increased emission light intensity, as per the quantum well theory for lattice expansion. In contrast, when the Mg2+content in the host structure is low, fewer Ce3+ions can enter, leading to lower emission intensity. This maintains the host in a more compact state (i.e., low entropy state).

[0078] Without wishing to be bound by theory, the inventors have, through their own researches, trials, and experiments devised that the method of prediction as described herein may also be applied to predict the PL emission characteristics / properties of the Ce3+ion doped high entropy oxide having a formula of Mg1-xCo3NiCuZnCexO8+x(i.e., Formula II). In particular, by using the method of prediction as described herein, it is predicted that the PL of the complex oxide of Formula (II) is in the visible region, which is coherent with the solid-state PL data as shown in Figs. 3A to 3D. Details of said prediction will be discussed in the later part of the present disclosure.

[0079] Hereinafter, the present invention is described more specifically by way of examples, but the present invention is not limited thereto.

[0080] EXAMPLES

[0081] EXAMPLE 1

[0082] Prediction of Photoluminescence Emission Wavelength and Photoluminescence Emission Intensity of Mg1-xCo3NiCuZnCexO8+x(x = 0.05, 0.1, 0.5, 1.0)

[0083] In this example, Ce-doped monoxides such as MgO, ZnO, and the like are used as the congruent compounds. To obtain a relationship between the emission wavelength (of the congruent compounds) and the entropy of fusion of the congruent compounds, a plot of emission wavelength (of the congruent compounds) against entropy of fusion is therefore constructed (Fig. 11). As shown, the emission wavelength is inversely proportional to the entropy of fusion. In particular, for example, the emission wavelength of ZnO ranges between 370 nm to 380 nm and therefore it would have a larger entropy of fusion whereas the emission wavelength of MgO is at about 550 nm and therefore it would have a smaller entropy of fusion.

[0084] After that, obtaining / determining the constants A and n via regression; and fitting the entropy of fusion of each of the Mg1-xCo3NiCuZnCexO8+x(x = 0.05, 0.1, 0.5, 1.0) complex oxides into Formula (III), then their emission wavelength can be determined. In this example, the wavelength was determined to be about 467 nm, which is larger than that of ZnO. It is believed that the entropy of solid high-entropy oxides is higher than that of their constituent monoxides, such as ZnO. However, when considering the liquid phase as the reference, the entropy of fusion for ZnO is larger than that of high-entropy oxides where ZnO is a constituent, as illustrated in Fig. 12.

[0085] Unexpectedly, it is found that the emission wavelength of all the Ce-doped high entropy oxides (Ce doped HEOs) (i.e., Mg1-xCo3NiCuZnCexO8+x(x = 0.05, 0.1, 0.5, 1.0) remained constant upon varying the Ce concentrations. It is believed that this stability arises because the entropy of HEOs is already exceptionally high and largely unaffected by small compositional changes. Ce doping primarily affects the MgO lattice, which constitutes only 20% of the 5-component HEO. Consequently, any increase in entropy due to doping is minimal and does not significantly influence the emission wavelength. This aligns with the principle that a small increment to an already vast quantity has negligible impact. In addition, no blue shift in emission was observed because the changes in entropy of fusion caused by Ce doping arc small enough to be ignored.

[0086] The entropy of mixing ΔSmixcan be obtained using equation below, assuming ideal solution.

[0087] ΔSmix= -RXlnX - R (1 - X)ln(l - X) where x = 2X, X represents normalized mole fraction.

[0088] The range x = 0, 0.2, 0.4, 0.6. 0.8, 1 .0, 1 .2, 1 .4, 1 .6, 1 .8, 2.0 has been normalized to X= 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 respectively into the 0 to 1 range, and a plot of emission intensity ratio (i.e., l / lo, where is the initial emission intensity of the undoped HEOs (i.e., when the concentration of Ce3+is zero)) against ΔSmixis therefore obtained (Fig. 13), and the constants B and m can be obtaincd / dctcrmincd via regression of said plot. After that, by fitting the ΔSmix(entropy of mixing) into Formula (IV), the emission intensity of the complex oxide is determined. For example:

[0089] Based on the above, while the emission wavelength of the high entropy oxides remains unchanged, their emission intensity is influenced by the entropy of mixing among the components (the emission intensity decreases with increasing entropy of mixing). It is believed that the entropy of fusion is nearly constant meaning it does not change for the above compounds. The entropy of fusion of these 5 or 6 HEOs do not change much since 80% of their concentrations remain the same, therefore, the composition changes in the 20% (Mg-Ce) lattice may not result in significant changes in their entropy of fusion. This is also supported by the similar size of Mg2+and Ce3+.

[0090] The invention has been given by way of example only, and various other modifications of and / or alterations to the described embodiment may be made by persons skilled in the art without departing from the scope of the invention as specified in the appended claims.

Claims

CLAIMS1. A rare earth ion doped complex oxide with formula:Cai+xMgSi2Eu0.025O6+x(x = 0.2, 0.4, 0.6, 0.8, 1.2, 1.4, 1.6, 1.8, 2.0) — - (I), or Mg1-xCo3NiCuZnCexO8+x(x = 0.05, 0.1, 0.5, 1.0) - (II).

2. A method of preparing a complex oxide of said Formula I of Claim 1 by solid- state reaction, including steps:(a) mixing raw materials containing Ca, Mg, Si, Eu and O in calculated amounts and milling said raw materials in ethanol; and(b) sintering said mixed and milled raw materials in a reduced environment at above 1,300 °C for at least 3 hours.

3. The method of Claim 2, wherein said reduced environment is of substantially 5% H2and 95% N2.

4. A method of preparing a complex oxide of said Formula 11 of Claim 1 by solid- state reaction, including:(i) mixing nitrate salts of Mg, Co, Ni, Cu, Zn, and Ce in a molar ratio of 1- x:l:l:l:l:x, (x = 0.05, 0.1, 0.5, 1.0) to form a mixture;(ii) subjecting said mixture to ultrasonification;(iii) stirring, freezing, drying and homogenizing said mixture; and(iv) heating said mixture at a heating rate of substantially 5 °C min'1up to 1,000 °C, and keeping said heating for 4 hours.

5. The method of Claim 4, wherein said nitrate salts arc hexahydrate nitrate salts.

6. A photoluminescence phosphor including at least a complex oxide of Claim 1.

7. A method of predicting photoluminescence (PL) emission characteristics of a complex oxide of Claim 1, including:(A) obtaining entropy of fusion ( ΔSf) of the same family of congruent compounds of said complex oxide of Claim 1;(B) applying the following Formula III to obtain a predicted PL emission wavelength value of said complex oxide:wherein: each of A and n is, independently, a constant;ΔSfis the entropy of fusion (J / K) of said complex oxide; and λ (nm) is the predicted wavelength of said complex oxide.

8. The method of Claim 7, wherein said constants A and n are obtained via regression under the same family of congruent compounds.

9. The method of Claim 7, further including:(C) obtaining entropy of mixing ( ΔSmix) of the same family of congruent compounds of said complex oxide of Claim 1 ;(D) applying the following Formula IV to obtain a predicted PL emission intensity value of said complex oxide:wherein: each of B and m is, independently, a constant;I and Ioare PL emission intensity values;ΔSmix(entropy of mixing) = - RXlnX - R(l-X)ln(l-X), where R is the gas constant and X is the mole fraction of the species.

10. The method of Claim 9, wherein said constants B and m are obtained via regression under the same family of congruent compounds.

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