BACKGROUND [0001] The invention relates generally to green emitting phosphors applicable to lighting systems. More particularly, the invention relates to green-emitting phosphors for solid state lighting systems, and a lighting apparatus employing these phosphors and blends thereof. [0002] Generation of “white light” is currently achieved by so called “white LEDs” that are constituted by employing a blue LED in conjunction with a yellow-green emitting, cerium-doped yttrium aluminum garnet known as “YAG,” having the formula Y3Al5O12:Ce3+. YAG has been historically used in these lighting systems because of the high quantum efficiency under blue light excitation and a broad emission spectrum that peaks in the yellow spectral region. The drawback of YAG based lighting systems is the relatively poor color rendering properties and high color temperature. For example, when an object is illuminated under such currently used white LEDs, they cannot imitate the colors illuminated by natural light. [0003] Although numerous phosphors have been proposed in the past several years, the range of phosphors that are suitable for LEDs is limited. Therefore, there is a need for new green-emitting phosphors that produce improved color rendering in white light emitting solid state lighting systems. BRIEF DESCRIPTION [0004] Briefly, most of the embodiments of the present invention provide a lighting apparatus having a phosphor material radiationally coupled to a light source. The phosphor material includes a phosphor composition of general formula I: R3-x-zMxCezT5-yNyO12-x-yFx+y; where 0≤x<3.0; 0≤y<5.0, 00 or if y=0, then x>0; R is Y, Tb, Gd, La, Lu or a combination thereof; T is Al, Sc, Ga, In or combinations thereof; M is Ca, Sr, Ba or a combination thereof; N is Mg, Zn or a combination thereof. [0005] In one embodiment, a lighting apparatus includes a light source; and a phosphor material radiationally coupled to the light source. The phosphor material includes a blend of a phosphor composition of general formula I: R3-x-zMxCezT5-yNyO12-x-yFx+y; where 0≤x<3.0; 0≤y<5.0, 00 or if y=0, then x>0; R is Y, Tb, Gd, La, Lu or a combination thereof; T is Al, Sc, Ga, In or combinations thereof; M is Ca, Sr, Ba or a combination thereof; N is Mg, Zn or a combination thereof, and a phosphor composition having an emission peak at a wavelength in a range from about 540 nanometers to about 650 nanometers. DRAWINGS [0006] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: [0007] FIG. 1 is a schematic cross sectional view of a lighting apparatus, in accordance with one embodiment of the invention; [0008] FIG. 2 is a schematic cross sectional view of a lighting apparatus, in accordance with another embodiment of the invention; [0009] FIG.3 shows the emission spectra of an oxyfluoride garnet phosphor using a 450 nanometers excitation wavelength, in accordance with an exemplary embodiment of the invention. DETAILED DESCRIPTION [0010] Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about,” is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. [0011] In the following specification and the claims that follow, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. [0012] As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances the modified term may sometimes not be appropriate, capable, or suitable. For example, in some circumstances, an event or capacity can be expected, while in other circumstances the event or capacity cannot occur – this distinction is captured by the terms “may” and “may be”. [0013] As used herein, the term “phosphor” or “phosphor material” or “phosphor composition” may be used to denote both a single phosphor composition as well as a blend of two or more phosphor compositions. As used herein, the term “lamp” or “lighting apparatus” or “lighting system” refers to any source of visible and /or ultraviolet light which can be generated by at least one light emitting element producing a light emission when energized, for example a phosphor material, a light emitting diode. [0014] Particular application is described, herein, in conjunction with converting LED-generated ultraviolet (UV), violet, or blue radiation into a desired color light or white light for general illumination or other purposes. It should be appreciated, however, that the invention is also applicable to the conversion of radiation from UV, violet, and/or blue lasers, as well as other light sources, to white light. [0015] Embodiments of the present invention are directed to a lighting apparatus that includes a phosphor material radiationally coupled to a light source. In one embodiment, the light source can be a semiconductor radiation source, for example a light emitting diode (LED) or an organic light emitting device (OLED). Radiationally coupled means that radiation from the light source is transmitted to the phosphor material, and the phosphor emits radiation of a different wavelength. A combination of the light from the light source and the light emitted from the phosphor material may be used to produce a desired color emission or white light. For example, a white light emitting LED device may be based on a blue emitting InGaN LED chip. The blue emitting LED chip may be coated with a phosphor blend to convert some of the blue radiation to a complementary color, e.g. a green emission or a white emission. [0016] Non-limiting examples of lighting apparatus or devices include devices for excitation by light-emitting diodes (LEDs) such as fluorescent lamps, cathode ray tubes, plasma display devices, liquid crystal displays (LCD’s), UV excitation devices, such as in chromatic lamps, lamps for backlighting, liquid crystal systems, plasma screens, xenon excitation lamps, and UV excitation marking systems. These uses are meant to be merely exemplary and not exhaustive. [0017] FIG. 1 illustrates a lighting apparatus or lamp 10 according to some embodiments of the present invention. The lamp 10 includes a light emitting diode (LED) chip 12, and leads 14 electrically attached to the LED chip. The leads 14 provide current to LED chip 12 and thus cause it to emit radiation. The LED chip 12 may be any semiconductor blue or ultraviolet light source, for example based on a nitride compound semiconductor of formula IniGajAlkN (where 0≤i; 0≤j; 0≤k and i + j + k =1) having an emission wavelength greater than about 250 nm and less than about 550 nm. More particularly, the chip 12 may be a near-UV or blue emitting LED having a peak emission wavelength from about 300 nm to about 500 nm. Such LEDs are known in the art. In lighting apparatus 10, a phosphor material (as described below) is disposed on a surface of the LED chip 12, and is radiationally coupled to the chip 12. The phosphor material can be deposited on the LED 12 by any appropriate method known in the art. The light emitted by the LED chip 12 mixes with the light emitted by the phosphor material to produce desired emission (indicated by arrow 24). [0018] Although the general discussion of the exemplary structures of the invention discussed herein are directed toward inorganic LED based light sources, it should be understood that the LED chip may be replaced by an organic light emissive structure or other radiation source, unless otherwise noted, and that any reference to an LED chip or semiconductor is merely representative of any appropriate radiation source. [0019] With reference to FIG. 1, the LED chip 12 may be encapsulated within an envelope 18, which encloses the LED chip and an encapsulant material 20. The envelope 18 may be, for example, glass or plastic. The LED chip 12 may be enclosed by the encapsulant material 20. The encapsulant material 20 may be a low temperature glass, or a thermoplastic or thermoset polymer, or resin as known in the art, for example, a silicone or epoxy resin. In an alternate embodiment, the lamp 10 may only comprise an encapsulant without an outer envelope 18. [0020] Various structures of the lamp 10 are known in the art. For example, in some embodiments, the phosphor material may be interspersed within the encapsulant material, instead of being disposed directly on the LED chip 12. In some other embodiments, the phosphor material may be coated onto a surface of the envelope, instead of being formed over the LED chip. In some embodiments, the lamp may include a plurality of LED chips. These different structures discussed with respect to FIG. 1 may be combined, with the phosphor material located in any two or all three locations or in any other suitable location, such as separately from the shell or integrated into the LED. Further, different phosphor blends may be used in different parts of the structure. [0021] In some embodiments, the lighting apparatus can be a fluorescent lamp or a compact fluorescent lamp (CFL), in combination with a LED. For instance, a combination of a LED-generated light and a phosphor-generated light may be used to produce visible light having enhanced color contrast. In this instance, a LED can be mounted in the base of the fluorescent lamp, for example CFL lamp to add to or supplement light in select wavelength regions of the visible spectrum, such as a portion of the blue region, to the light being generated by the phosphor composition coated on the glass envelope 11 of a lamp 10. [0022] In any of the above structures, the LED based lighting apparatus 10 may also include a plurality of particles (not shown) to scatter or diffuse the emitted light. These scattering particles would generally be embedded in the encapsulant 20. The scattering particles may include, for example, particles made from Al2O3 (alumina) or TiO2 (titania). The scattering particles may effectively scatter the light emitted from the LED chip 12, preferably with a negligible amount of absorption. [0023] Some embodiments of the invention provide a phosphor composition that may be used in the phosphor material provided in the lighting apparatus described herein. The phosphor composition is an oxyfluoride garnet having a general formula I: R3-x-zMxCezT5-yNyO12-x-yFx+y; where 0≤x<3.0; 0≤y<5.0, 00 or if y=0, then x>0; R is Y, Tb, Gd, La, Lu or a combination thereof; T is Al, Sc, Ga, In or combinations thereof; M is Ca, Sr, Ba or a combination thereof; N is Mg, Zn or a combination thereof. In some embodiments, when y=0, the general formula is R3-x-zMxCezT5O12-xFx; where 00 or if y=0, then x>0; R is Y, Tb, Gd, La, Lu or a combination thereof; T is Al, Sc, Ga, In or combinations thereof; M is Ca, Sr, Ba or a combination thereof; N is Mg, Zn or a combination thereof. Some specific embodiments include the compositions where y=0 i.e., component includes 100 % Al3+. In these embodiments, the phosphor material includes the compositions of formula R3-x-zMxCezAl5O12-xFx, wherein 00 or if y=0, then x>0; R is Y, Tb, Gd, La, Lu or a combination thereof; T is Al, Sc, Ga, In or combinations thereof; M is Ca, Sr, Ba or a combination thereof; N is Mg, Zn or a combination thereof. 2. The lighting apparatus of claim 1, wherein 0≤x<1.0 3. The lighting apparatus of claim 1, wherein 0≤y<2.0. 4. The lighting apparatus of claim 1, wherein at least a portion of F in the phosphor composition of formula I is substituted by Cl, Br, I, or combinations thereof. 5. The lighting apparatus of claim 1, wherein the phosphor composition of formula I is doped with an additional activator ion selected from a group consisting of Pr, Sm, Eu, Dy, Tm, Er, Ho, Nd, Bi, Pb and Yb. 6. The lighting apparatus of claim 1, wherein the phosphor material comprises a phosphor composition of general formula R3-x-zMxCezT5O12-xFx; where 0≤x<3.0,00 or if y=0, then x>0; R is Y, Tb, Gd, La, Lu or a combination thereof; T is Al, Sc, Ga, In or combinations thereof; M is Ca, Sr, Ba or a combination thereof; N is Mg, Zn or a combination thereof, and an additional phosphor composition having a peak emission in a wavelength range from about 590 nanometers to about 650 nanometers. 18. The lighting apparatus of claim 17, wherein the additional phosphor composition has a formula: (Na, K, Rb, Cs, NH4)2[(Ti, Ge, Sn, Si, Zr, Hf)F6]:Mn4+.