DESC:TECHNICAL FIELD
The present disclosure relates to the field of materials science. Particularly, the disclosure relates to a compound of general formula Na0.5A0.5-xRExZO3, preferably Na0.5Bi0.5-xEuxTiO3. The said compound resembles a perovskite and is capable of down-converting blue and green light to red light, free of concentration quenching, radiation resistant, thermally stable and has sufficiently short emission lifetime.
BACKGROUND OF THE PRESENT DISCLOSURE
Perovskite materials associated with the general formula ABO3, wherein A and B are cations could be good phosphors. Sodium bismuth titanate, Na0.5Bi0.5TiO3 (NBT), is considered to be an excellent candidate to be used as lead free piezoelectric ceramics. Its crystal structure is a perovskite type with rhombohedral symmetry at ambient temperature. NBT is ferroelectric, A-site cations are ordered and the ferroelectric transition is diffuse. The crystal also exhibits optical isotropism during the diffuse phase transition. The material is mechanically tough and less toxic being lead free.
The property of concentration quenching is a limitation and is an accepted disadvantage of the perovskite phosphors known in the prior art. Also phosphors that can efficiently emit red light, when excited by blue or green light is an active area of research in the field of Materials science.
Conventionally, none of the reported crystalline phosphors are free of concentration quenching. Moreover, all perovskites do not possess favorable properties such as conversion of blue light/green light to red light or white light generation, minimum or no concentration quenching, shorter luminescence decay times, long term stability, temperature stability, radiation hardness etc. Such properties are desirable for use in optical engineering related applications such as white light emission, electroluminescent devices, lasers, phosphors for thermometry and optical devices in radiation hard environment, etc.
Further, nitride based phosphors are usually made using ammonothermal methods which require special reaction vessels and require use of ammonia. Furthermore, the slightest amount of oxygen in the reaction vessel can cause serious problems both with respect to the quality of the products, and the yield of the reaction. Likewise, sulphide based phosphors almost require oxygen free, non-aqueous solvents for synthesis. These factors make industrial manufacturing of nitride and sulphide based phosphors expensive.
Therefore, there is a need in the art to develop a phosphor which possesses all desired properties and which is also more economical and easy to manufacture.
The compound of the present disclosure is motivated by the need to develop a robust concentration quenching free, radiation hard, low-cost phosphors for applications such as (i) white light generation, (ii) laser gain media (iii) phosphor thermometry and (iv) radiation-resistant optical materials for engineering applications involving intense radiation environments.
STATEMENT OF DISCLOSURE
The present disclosure relates to the compound of general formula Na0.5A0.5-xRExZO3, where x is a molar fraction having value of = 0.5; A is a metal selected from a group comprising bismuth, arsenic and antimony; RE is at least one rare earth (RE) metal selected from a group comprising Lanthanides, scandium and Yttrium; and ‘Z’ cation is selected from a group comprising titanium, zirconium and hafnium; a process for preparation of compound of general formula Na0.5A0.5-xRExZO3 according to the present disclosure, wherein the process comprises acts of (a) grinding metal oxides to a fine powder in a solvent followed by drying to obtain dried powder (b) calcination and grinding of the dried powder to obtain homogenized powder, (c) mixing the homogenized powder with a binder and compressed to form pellets and (d) sintering the pellets to obtain the compound of general formula Na0.5A0.5-xRExZO3; and a device and a composite comprising compound Na0.5A0.5-xRExZO3.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS:
The features of the present disclosure will become more fully apparent from the following description taken in conjunction with the accompanying drawings. Understanding that the drawings depict only several embodiments in accordance with the disclosure and are therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings:
Figure 1 (a) and (b) illustrates variation of photoluminescence intensity at 590 nm (I ?1) and 620 nm (I?2), as x is varied in Na0.5Bi0.5-xEuxTiO3.
Figure 2 illustrates intensity of ?2 line (5D0?7F2; 617 nm) with respect to ?1 (5D0?7F2; 593 nm) line increasing with increased Eu concentration (x) in Na0.5Bi0.5-xEuxTiO3.
Figure 3 illustrates the quality of the emission line, characterized by both full width half maximum (FWHM) and quality factor with Eu doping.
Figure 4 illustrates efficient blue light to red light conversion using the compound.
Figure 5 illustrates efficient green light to red light conversion using the compound.
Figure 6 illustrates the comparison of Photoluminescence (PL) spectra of the compounds Na0.5Bi0.5-xEuxTiO3 and Na0.5Bi0.5-xEuxZrO3 wherein x=0.03.
Figure 7 illustrates the comparison of PL spectra of the compound Na0.5Bi0.5-xEuxTiO3 at different concentrations of x.
Figure 8 illustrates the PL spectra of the compound Na0.5Bi0.5-xErxTiO3 with x=0.01.
Figure 9 illustrates the long term stability of the compound Na0.5Bi0.5-xEuxTiO3, wherein x=0.04.
DETAILED DESCRIPTION OF DISCLOSURE:
The present disclosure relates to the compound of general formula Na0.5A0.5-xRExZO3, where x is a molar fraction having value of = 0.5; A is a metal selected from a group comprising bismuth, arsenic and antimony; RE is at least one rare earth (RE) metal selected from a group comprising Lanthanides, scandium and Yttrium; and ‘Z’ cation is selected from a group comprising titanium, zirconium and hafnium.
In an embodiment of the present disclosure, the metal A is preferably bismuth, the RE is preferably a Lanthanide and the cation is preferably Titanium.
In an embodiment of the present disclosure, the Lanthanides are selected from a group comprising Lanthanum, Cerium, Praseodymium, Neodymium, promethium, Samarium, Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium and Lutetium, preferably Europium or Erbium.
In an embodiment of the present disclosure, the specific compound is Na0.5Bi0.5-xEuxTiO3.
In another embodiment, the specific compound is Na0.5Bi0.5-xEuxZrO3.
In another embodiment, the specific compound of the present disclosure is Na0.5Bi0.5-xErxTiO3.
In still another embodiment of the present disclosure, the compound shows efficient generation of red light, shows no concentration quenching, is thermally stable, is radiation resistant and provides for short luminescence decay time.
The present disclosure also relates to a process for preparation of compound of general formula Na0.5A0.5-xRExZO3, wherein the process comprises acts of:
a) grinding metal oxides to a fine powder in a solvent followed by drying to obtain dried powder;
b) calcination and grinding of the dried powder to obtain homogenized powder;
c) mixing the homogenized powder with a binder and compressed to form pellets; and
d) sintering the pellets to obtain the compound of general formula Na0.5A0.5-xRExZO3.
In an embodiment of the present disclosure, the metal oxides are selected from metal oxides of A, metal oxides of RE and metal oxides of Z.
In an embodiment of the present disclosure, A is selected from a group comprising bismuth, arsenic and antimony; RE is selected from a group comprising Lanthanides, scandium and Yttrium; and Z is a cation selected from a group comprising Titanium, Zirconium and hafnium.
The present disclosure also relates to a process for preparation of compound Na0.5Bi0.5-xEuxTiO3, wherein the process comprises acts of:
a) grinding metal oxides selected from a group comprising Bi2O3, TiO2, Na2CO3 and Eu2O3 to a fine powder in ethanol followed by drying to obtain dried powder;
b) calcination and grinding of the dried powder to obtain homogenized powder;
c) mixing the homogenized powder with a binder and compressed to form pellets; and
d) sintering the pellets to obtain the compound of general formula Na0.5Bi0.5-xEuxTiO3.
In an embodiment of the present disclosure, the solvent is an organic solvent, preferably ethanol.
In an embodiment of the present disclosure, the calcination is carried at a temperature of 600°C to 1000°C, preferably at about 800°C for a time period ranging from 2h to 6h, preferably at about 4h.
In an embodiment of the present disclosure, the grinding in step (b) is carried out for 4h to 10h, preferably about 6h.
In an embodiment of the present disclosure, the binder is polyvinyl alcohol (PVA).
In an embodiment of the present disclosure, the compression is carried at a pressure of 100MPa to 200MPa, preferably at about 150MPa.
In an embodiment of the present disclosure, the sintering is carried out at a temperature ranging from 800°C to 1200°C, preferably at around 1000°C for a time ranging from 2h to 6h, preferably at about 4h.
The present disclosure also relates to a device and a composite comprising compound Na0.5A0.5-xRExZO3.
The present disclosure also relates to a device and a composite comprising compound Na0.5Bi0.5-xEuxTiO3.
The present disclosure also relates to a method of generating white light by using compound Na0.5A0.5-xRExZO3.
The present disclosure also relates to a method of avoiding concentration quenching by using compound Na0.5A0.5-xRExZO3.
The present disclosure also relates to a method of providing short emission lifetime by using compound Na0.5A0.5-xRExZO3 as an optical material.
The present disclosure also relates to a method of providing resistance to radiation by using compound Na0.5A0.5-xRExZO3.
The present disclosure also relates to a method of providing stability to high temperature by using compound Na0.5A0.5-xRExZO3.
The present disclosure also relates to use of Na0.5Bi0.5-xEuxTiO3 for white light generation, for avoiding concentration quenching, for resistance to radiation and for providing short luminescence decay time.
Throughout the specification, the compound of the present disclosure is interchangeably referred to as “Phosphor” and “material”.
The present disclosure relates to the compound of general formula Na0.5A0.5-xRExZO3, wherein x is a molar fraction having value of = 0.5. The compound resembles a perovskite and the value of x=0.01 to 0.5. The different values of x would not substantially change the property of the compound of the present disclosure. Further, all the values of ‘x’ in the said range would have all the desired properties of the compound of the present disclosure.
In an embodiment, the present disclosure provides a compound which has a (i) tight lattice, (ii) appropriate band gap and (iii) the RE ion which has an ionic radius that allow easy substitutional accommodation, thus allowing the compound to have desired properties.
The present disclosure particularly relates to a composite comprising compound Na0.5Bi0.5-xEuxTiO3.
The present disclosure particularly relates to the compound Na0.5Bi0.5-xEuxTiO3.
The present disclosure also relates to the compound Na0.5Bi0.5-xEuxZrO3.
The present disclosure also relates to the compound Na0.5Bi0.5-xErxTiO3.
In an embodiment, the compound of the present disclosure is capable of down-converting blue and green light to red light. This has immediate application in the emerging technologies related to white light generation, and other light emitting device technologies such as electroluminescent devices, phosphors for colour display screens etc.
Further, the compound of the present disclosure does not demonstrate any concentration quenching, which is indicative of its robustness as a luminophore host. The fact that the compound of the present disclosure provides no evidence of concentration quenching offers a solution for a long standing problem. The robustness of the optical host is further indicated by the fact that reagent grade precursors are sufficient to make it, thus reducing the overall cost of the material.
Fluorescence lifetime studies on this material show that it is promising for making gain media for short-pulse lasers (with pulse width as short as 0.1 nanoseconds). Also, the compound system is radiation resistant and hence useful for making optical devices in radiation hard environments.
For applications such as phosphor thermometry, there is a need to have phosphor materials that are stable at high temperatures. Perovskite systems, by their very nature are thermally stable and likely candidates for such an application. The compound of the present disclosure is thermally stable. This, along with its efficient luminescence, makes it attractive for applications such as “phosphor thermometry”.
An important feature is that the compound of the present disclosure is a rare earth (RE) activated phosphor wherein the observed f-f transitions correspond to emissions associated with RE doping in both centrosymmetric and non-centrosymmetric site. An application of such a material would be in the development of a tunable laser.
The compound of the present disclosure is considerably cheaper than other nitride or sulfide based phosphors, owing to the fact that it is primarily an oxide material, which can be synthesized using routine solid state chemical methods. Oxides have the natural advantage of being amenable to synthesis in natural atmosphere. Also, solid state chemical methods are very well established for making oxides. Hence, materials systems like the compound of the present disclosure lends itself to scalable and industrially viable production.
The compound of the present disclosure can also be used in composites in combination with other compounds/phosphors. Thus, the present disclosure also relate to a composite comprising the compound Na0.5A0.5-xRExZO3, preferably Na0.5Bi0.5-xEuxTiO3.
In an embodiment of the present disclosure, the compound of the present disclosure is mixed with oxide based ceramic materials to form a materials-blend or a composite. Oxide ceramic materials including the compound of present disclosure are robust and chemically stable. Making a composite of the compound of present disclosure with most organic or inorganic materials would not compromise its optical properties. This is due to the chemical and physical stability of the materials.
Given its low cost especially compared with nitride phosphors and its versatility with respect to thermal stability, robustness against concentration quenching and radiation hardness, the compound of the present disclosure is useful for several optical engineering related applications such as white light emission, electroluminescent devices, lasers, phosphors for thermometry, optical devices in radiation hard environment, etc.
The invention is further elaborated with the help of following examples. However, these should not be construed to limit the scope of the invention.
Example 1 - Process of preparation of the compound Na0.5A0.5-xRExZO3:
Compound of the present disclosure Na0.5A0.5-xRExZO3 can be prepared by various methods including solid state method, solution synthesis techniques such as sol-gel method, hydrothermal method etc. To prepare compounds using solid state method, conventional oxides and carbonate precursors of Sodium (Na), Bisumuth (Bi), Titanium (Ti), Europium (Eu) and other Lanthanides, Arsenic (As), Antimony (Sb), Scandium (Sc), Yttrium (Y), Zirconium (Zr), Hafnium (Hf) etc. are sufficient. Due to the robustness of the optical lattice, reagent grade precursors are suffice to make an efficient phosphor.
For example, Na0.5Bi0.5-xEuxTiO3 is prepared using reagent grade Bi2O3, TiO2, Na2CO3 and Eu2O3 as precursor materials. Use of reagent grade precursors also minimizes the cost of production of the compound. When solid state methods are used, thorough grinding (~4 hrs) of the precursors in ethanol is recommended. Calcination and grinding is to be repeated a few (>=2) times. After obtaining single phase, compaction followed by sintering is helpful in getting a single ceramic compound.
Preparation of the compound Na0.5Bi0.5-xEuxTiO3 of the present disclosure:
1. The conventional solid state reaction method is used to prepare Na0.5Bi0.5-xEuxTiO3 where x varies from 0.01 to 0.5, preferably from 0.01 to 0.2.
2. Reagent grade metal oxide or carbonate powders of Bi2O3, TiO2, Na2CO3 and Eu2O3 are used as starting precursors.
3. The oxides are ground into fine powder in ethanol (as grinding agent) with agate mortar and pestle for 4 h.
Further, alternative grinding agent, other than ethanol, would also be potentially acceptable. Alternate grinding equipment instead of agate mortar could also be used to ensure fine powdering. The duration for carrying out grinding can also vary significantly to ensure complete grinding and homogenization of the powder.
4. After being mixed, the dried powder is calcined at 800°C for 4 h.
5. The calcined powder is reground for 6 h to obtain homogenized powder.
6. The homogenized powder is mixed with the binder polyvinyl alcohol (PVA) and pressed at 150 MPa to form green compact pellets (about 10 mm in diameter, and about 1.5 mm in thickness).
7. The green compacts are sintered at 1000°C for 4 h in air to obtain the pellet of compound Na0.5Bi0.5-xEuxTiO3.
The compaction steps used during pellet making process can use any other compaction agent/binder other than polyvinyl alcohol (PVA) (or similar binders). The green compacts could also be heated for different durations and temperatures (>600 degree C) to arrive at the required pellet.
Regardless of the process used to arrive at the compound of the instant disclosure, the property of the optical material would remain unchanged.
The above process is employed to arrive at other compounds where A is a metal selected from a group comprising arsenic and antimony; RE is at least one rare earth (RE) metal selected from a group comprising lanthanides, scandium and Yttrium; and ‘Z’ cation is selected from a group comprising zirconium and hafnium. For example: Na0.5Bi0.5-xEuxZrO3 and Na0.5Bi0.5-xErxTiO3 are arrived in the present disclosure using the same process as described above for Na0.5Bi0.5-xEuxTiO3 by using different metal oxides accordingly.
The metal oxides are selected from metal oxides of A, metal oxides of RE and metal oxides of Z as mentioned above.
Example 2 - Concentration quenching:
The Compound, Na0.5Bi0.5-xEuxTiO3, shows zero or no concentration quenching as shown in figures 1 and 2. By definition, concentration quenching is the decrease in luminescence intensity as the concentration of the luminophore exceeds a certain critical concentration. Below this critical concentration, the luminescence intensity usually increases linearly with respect to the luminophore concentration.
Figure 1 illustrates variation of photoluminescence intensity at 590 nm (I ?1) and 620 nm (I?2), as x is varied in Na0.5Bi0.5-xEuxTiO3 and the figure 2 illustrates the intensity of the ?2 line (5D0?7F2; 617 nm) with respect to ?1 (5D0?7F2; 593 nm) line increasing with increased Eu concentration (x).
Figure 1 (a) provides necessary and sufficient proof about the lack of concentration quenching in Na0.5Bi0.5-xEuxTiO3. Measuring luminescence intensity with respect to luminophore concentration is the most direct, simple and reliable way to measure concentration quenching or lack thereof.
It is observed from figure 1 (a, b) that Photoluminescence (PL) spectra of Na0.5Bi0.5-xEuxTiO3 (with x=0.01) remains unaltered when excited by both about ~590 nm and about ~620 nm. The emission lines are consistent with reported transitions for Eu3+.
Both 5D0?7F1 and 5D0?7F2 transitions are observed as seen in Figure 2, indicating presence of Eu3+ in sites with and without inversion symmetry. Figure 1(b) showing PL spectra of the sample with x=0.03, 0.05, 0.07, 0.09 and 0.10, suggests absence of concentration quenching in this system.
The primary scientific significance of the simultaneous observation of both 5D0?7F1 and 5D0?7F2 lines illustrates successful doping of Eu3+ at both sites with and without inversion symmetry. This aspect is rarely possible in a crystalline system.
Thus, it is possible to obtain both these transitions from a compound of the present disclosure wherein such transitions were previously only obtainable from non-crystalline phosphors.
The lack of concentration quenching is adequately demonstrated using the Figures 1(a), 1(b) and 2.
Furthermore, increased luminophore concentration improves the quality factor of the phosphor. Figure 3 shows that the quality of the emission line, characterized by both full width half maximum (FWHM) and quality factor (ratio of emission frequency to FWHM), improves with Eu doping. This implies that increased doping of RE is useful in improving the quality of the optical material. Hence, the compound optical host not only avoids concentration quenching, but also provides a means for making high quality RE doped optical materials.
In an optical material, the quality of an emission line is often characterized by a combination of its full width half maximum (FWHM) (Figure 3) and the associated Quality factor (Q).(Figure 3) Q is a dimensionless quantity defined as the ratio of line frequency to line width (FWHM). In general, decrease in Q is indicative of more non-resonant /non-radiative decay processes, which result in loss of energy. Clearly a high Q and low FWHM is preferred. Figure 3 illustrates that FWHM decreases systematically with increase in Eu doping. Likewise, Q increases with increase in Eu incorporation. This implies that the quality of the phosphor improves with increased Eu doping. This is another indication of the suppression of concentration quenching, and efficient and robust RE excitation processes occuring in this materials system. It is important to note that the Q of the Europium doped compound of the present disclosure is between 24 to 30, which compares very favorably with some of the best RE based phosphors reported in literature.
Photoluminescence of the compound for different values of “x”:
Figure 7 compares the spectra of the compound Na0.5Bi0.5-xEuxTiO3 for different values of “x” (wherein x= 0.01, 0.04, 0.06, 0.08, 0.1, 0.15 and 0.2). It is evident from figure 7 that the different concentration of ‘x’ all leads to stable intensity of light by the compound.
The different values of x have an insignificant influence on the property of the compound.
The photoluminescence of the compounds (Na0.5Bi0.5-xEuxTiO3 wherein x is 0.03 and Na0.5Bi0.5-xEuxZrO3, wherein x=0.3) of the present disclosure is illustrated in Figure 6. When ‘Ti’ is replaced by ‘Zr’, there was an increase in the unit cell which corresponds to the fact that ionic radius of Zr4+ =0.72Å was larger than that of Ti4+=0.605Å. From the figure, it is observed that the compound Na0.5Bi0.5-xEuxZrO3 is still optically active and the red emission remains intact.
Figure 8 shows the photoluminescence of Na0.5Bi0.5-xErxTiO3, wherein x=0.01. With Er doping, the strong green band (~550 nm) was assigned to the and the red band (~670 nm) assigned to the transition. It is observed that the f–f transitions arising from forced electric dipole are parity forbidden and become partially allowed when the ion is situated at the low-symmetry site. The observation of the transition lines illustrates successful doping of “Er” at both sites with and without inversion symmetry.
Example 3 - Blue to red light and green to red light conversion:
The Compound Na0.5Bi0.5-xEuxTiO3 shows efficient blue-to-red and green-to-red conversion. Even when Eu is replaced with Er or Ti with Zr, the property remains unchanged. The materials optical property have been characterized and shows promise for application in electroluminescent and light emitting devices, white light emitting devices, lasers, etc.
A metallurgical microscope such as BX51M by Olympus is used in various modes such as blue and green excitation to demonstrate blue light to red light, and green light to red light wavelength conversion.
Figures 4 illustrate a fluorescent micrograph which shows red emission when the compound Na0.5Bi0.5-xEuxTiO3 is excited using blue light (460-490 nm). Suitable filters were used to eliminate all other wavelengths other than blue light. The compound is capable of down-converting the incident light to red, as is seen from the image of Figure 4.
Figures 5 illustrate a fluorescent micrograph which shows red emission when the compound is excited using green light (510-550 nm). Suitable filters were used to eliminate all other wavelengths other than green light. The compound is capable of down-converting the incident light to red, as is seen from the image of Figure 5.
Example 4 - Radiation hardness for the compound of the present disclosure.
Radiation hardness is generally expected of perovskite oxides. This aspect is widely known in the art that perovskite phosphors possess the property of radiation hardness, which enables their use in robust electronics such as in ferroelectric memories, etc. The present compound of the present disclosure which resembles a perovskite also finds use in robust opto-electronics as a phosphor material.
Example 5 - Long term stability and temperature stability
As the perovskite oxides are known to be very robust, this property also enables them to be chemically and thermally very stable.
Figure 9 illustrates the long term stability of the compound of the present disclosure Na0.5Bi0.5-xEuxTiO3, wherein x=0.04, which draws a comparison of light intensity of the compound initially and after a time period of 1 year and ten months. It is observed that there are no changes in the light intensity over a period of time and therefore the compound of the present disclosure is stable.
The thermal stability of the compound Na0.5Bi0.5-xEuxTiO3, is attributed to its high melting point. Generally, a compound/solid with high melting point is thermally stable and is useful for high temperature applications. Further, these compounds melt congruently. Congruent melting point is the temperature at which the composition of the melt is the same as that of the solid. It is widely known that, the sintering temperature of most compounds is around 1200°C/2 hrs. The compound of the present disclosure, Na0.5Bi0.5-xEuxTiO3, have congruent melting points >1400oC.
Therefore, the compound of the present disclosure is thermally very stable.
Example 6 - Short luminescence decay times
The compound, Na0.5Bi0.5-xEuxTiO3 reports decay times of the order of 0.1 nano secs, which also means that the stimulated life times are short. This makes it attractive as a gain-medium for short-pulse lasers.
Table 1 illustrates the short emission lifetimes of the compound Na0.5Bi0.5-xEuxTiO3.
x 593/594nm 616/617nm
t1 (sec) t2 (sec) t1 (sec) t2 (sec)
0.01 < 0.1ns 3.9 x10-9 < 0.1ns < 0.1ns
0.02 < 0.1ns < 0.1ns < 0.1ns < 0.1ns
0.03 < 0.1ns < 0.1ns < 0.1ns < 0.1ns
0.04 < 0.1ns < 0.1ns 1.5 x10-9 6.1 x10-9
0.05 < 0.1ns 2.5 x10-9 1.7 x10-9 6.6 x10-9
0.06 < 0.1ns 4.6 x10-9 < 0.1ns 2.3 x10-9
0.07 < 0.1ns 3.4 x10-9 < 0.1ns < 0.1ns
0.08 < 0.1ns < 0.1ns < 0.1ns < 0.1ns
0.09 < 0.1ns < 0.1ns < 0.1ns < 0.1ns
0.10 < 0.1ns 2.4 x10-9 < 0.1ns 1.6 x10-9

Table 1: Life Time results
Advantages of the Compound of present disclosure:
1. Concentration quenching:
As the (luminophore) light bearing ion concentration in a host lattice increases, usually the emission efficiency first increases, and then sharply decreases. This is called concentration quenching of luminescence. The compound of present disclosure provides no evidence of concentration quenching.
2. Blue to red, green to red conversion:

In optical technologies of relevance to white light generation, there is often a need to convert blue light or green light to red light. The compound of the present disclosure offers conversion of blue or green light to red light and thereby finds application in technologies involving white light generation.
This suggests the use of the compound of the present disclosure for frequency down conversion for the purpose of white light generation. This is so because, white light can be formed using a combination of red, blue and green colours.
Given that red colour can be easily generated using blue, green or UV, suitable modifications to the compound of the present disclosure aid in the synthesis of green and blue phosphors as well. Hence, the compound of the present disclosure can be used in LED phosphors, electroluminescent devices etc.
The aspect that the compound of present disclosure is useful for white light generation comes from the fact that the compound is a good red light emitter. Hence it can be used as a source of red light in a white light generating system (which often has a combination of red, blue and green light emitters).
3. Need for short luminescence decay times:

For the purpose of making lasers with short pulse width, there is a need for an optical material with sufficiently short emission lifetimes. The compound of the present disclosure reports decay times of the order of 0.1 nanosecs, which also means that the stimulated life times are short. This makes it attractive as a gain-medium for short-pulse lasers.
4. Long term and temperature stability:

The compound of the present disclosure is very stable when compared to known sulfide based phosphors. Also, the compound of the present disclosure is thermally stable which makes it attractive for applications such as phosphor thermometry.
5. Radiation hardness:

The compound of the present disclosure also possesses the property of radiation hardness. It can be used in optical devices which are to be used in high radiation environments such as nuclear reactors, certain regions of extraterrestrial-space etc.
6. Economical and affordable phosphor

The compound of the present disclosure is economical and affordable compared to the known phosphors. This aspect is explained in detail as below:
A) Market price of competing high power phosphors
The lowest cost for a high power phosphor available in the market is equal to $20.3/gram and the highest cost is equal to $22.5/gram.
On comparison, the market price of compound of the present disclosure is as follows:
Raw materials costs:
x=0.01 cost = ?21.8/gram = $0.40/gram
x=0.1 cost = ?144.4/gram = $2.67/gram
? - currency in Indian Rupees
$ - currency in US Dollars
B) Market price of competing Phosphors (standard LED phosphors)
The cost for a standard LED phosphor available in the market is equal to $9.48/gram.
On comparison, the market price of the compound of the present disclosure is as follows:
Raw materials costs:
x=0.01 cost=?21.8/gram = $0.40/gram
x=0.1 cost = ?144.4 /gram = $2.67/gram
Therefore, the compound of the present disclosure is considerably very economical over the phosphor of the prior art.

APPLICATIONS:
The present disclosure will find use in:
i. White light generation (blue-to-red light, green-to-red light conversion);
ii. Red light generation for domestic and other applications;
This can be used in domestic and other applications which require red light generation Light emitting devices are touted as the most promising means of indoor lighting.
Apart from light emitting devices, other applications include high power lasers etc. Given the promise of perovskites for such applications and the compound of the present disclosure resemble perovskites, they find use in high and low power lasers.
iii. Laser gain media (for generation of coherent light with frequencies corresponding to Eu f-f transitions);
The laser gain medium should be capable of amplifying a certain longitudinal mode present within the resonator cavity. Typically, this requires presence of transitions between electronic levels within the gain medium, whose frequency matches that of the longitudinal mode supported by the laser cavity. Hence, one could choose a suitable RE, in the given system, and make a gain medium to suit a particular application. The compound of the present disclosure (where RE = Eu) is useful for red laser applications. However, other RE dopants can be chosen to obtain lasing at other wavelengths.
iv. Efficient phosphors in radiation hard environments (eg. nuclear reactions, and in certain regions of outer-space); and
The compound of the present disclosure belongs to a class of materials that are radiation hard. Hence similar to nitride based phosphors, the compound finds use in radiation-intense environment.
Radiation tolerant electronics that are actually better suited to surviving high-radiation environments than some radiation hardened electronics.
As the compound of the present disclosure is very strong since the Curie temperature of this is very high and its melting point is quite high, (Ex: this can survive the UV radiation, till now known example is CaTiO3 which is studied for radiation damage in synchrotron), it can be substantially used for such environments.
v. Single compound that exploits rare earth (RE) transitions, which correspond to centro-symmetric and non-centrosymmetric positioning of the RE. This allows “tunability” of emission wavelength in laser related applications.
The emission wavelengths in the reported Europium doped compound are 590 nm and 620 nm. The emission wavelength that will dominate when such a material is employed in a laser system would be the one that is supported by the laser’s resonator cavity. Hence, the given optical material allows some selectivity with regards to emission light, when used in lasing applications.
This also pertains to the fact that RE could be a combination of RE atoms (could be called RE1, RE2 etc), which are chosen such that there are centers of red, blue, green emission, which in turn can give rise to a single component, which emits white light.
vi. Optical engineering industries such as LCD, LED, VFD, FED, X-ray screen, detectors, EL devices, plasma display screens, etc.
All of these industries employ red, blue and green phosphors for display applications. Low cost display phosphors continue to be of perennial interest to these industries. Given that suitable choice of A, Z and RE would allow synthesis of various compounds, it is indicative that the compound of the present invention is very useful, such as the specific Na0.5Bi0.5-xEuxTiO3. ,CLAIMS:WE CLAIM,

1. A compound of general formula Na0.5A0.5-xRExZO3, wherein
x is a molar fraction having value of = 0.5;
A is a metal selected from a group comprising bismuth, arsenic and antimony;
RE is at least one rare earth (RE) metal selected from a group comprising lanthanides, scandium and Yttrium; and
‘Z’ is a cation selected from a group comprising Titanium, Zirconium and hafnium.
2. The compound as claimed in claim 1, wherein the metal A is preferably bismuth, the RE is preferably a Lanthanides and the cation is preferably Titanium.
3. The compound as claimed in claim 2, wherein the lanthanide is selected from a group comprising Lanthanum, Cerium, Praseodymium, Neodymium, promethium, Samarium, Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium and Lutetium, preferably Europium and Erbium.
4. A process for preparation of compound of general formula Na0.5A0.5-xRExZO3 as claimed in claim 1, wherein the process comprises acts of:
a) grinding metal oxides to a fine powder in a solvent followed by drying to obtain dried powder;
b) calcination and grinding of the dried powder to obtain homogenized powder;
c) mixing the homogenized powder with a binder and compressing to form pellets; and
d) sintering the pellets to obtain the compound of general formula Na0.5A0.5-xRExZO3.
5. The process as claimed in claim 4, wherein the metal oxides are selected from metal oxides of A, metal oxides of RE and metal oxides of Z.
6. The process as claimed in claim 5, wherein A is selected from a group comprising bismuth, arsenic and antimony; RE is selected from a group comprising lanthanides, scandium and Yttrium; and Z is a cation selected from a group comprising Titanium, Zirconium and hafnium.
7. The process as claimed in claim 4, wherein the solvent is an organic solvent, preferably ethanol.
8. The process as claimed in claim 4, wherein the calcination is carried at a temperature of 600°C to 1000°C , preferably at about 800°C for a time period ranging from 2h to 6h, preferably at about 4h.
9. The process as claimed in claim 4, wherein the grinding in step (b) is carried out for 4h to 10h, preferably about 6h.
10. The process as claimed in claim 4, wherein the binder is polyvinyl alcohol (PVA).
11. The process as claimed in claim 4, wherein the compression is carried at a pressure of 100MPA to 200MPa, preferably at about 150MPa.
12. The process as claimed in claim 4, wherein the sintering is carried out at a temperature ranging from 800°C to 1200°C, preferably at around 1000°C for a time ranging from 2h to 6h, preferably about 4h.
13. A device and a composite comprising compound of general formula Na0.5A0.5-xRExZO3 as claimed in claim 1.

Dated this 20th day of May, 2013

Shivakumar R
Of K&S Partners
Agent for the Applicant

To:
The Controller of Patents,
The Patent Office, at: Chennai