BACKGROUND [0001] Red-emitting phosphors based on complex fluoride materials activated by Mn4+, such as those described in US 7,358,542, US 7,497,973, and US 7,648,649, can be utilized in combination with yellow/green emitting phosphors such as YAG:Ce or other garnet compositions to achieve warm white light (CCTs<5000 K on the blackbody locus, color rendering index (CRI) >80) from a blue LED, equivalent to that produced by current fluorescent, incandescent and halogen lamps. These materials absorb blue light strongly and efficiently emit between about 610-635 nm with little deep red/NIR emission. Therefore, luminous efficacy is maximized compared to red phosphors that have significant emission in the deeper red where eye sensitivity is poor. Quantum efficiency can exceed 85% under blue (440-460 nm) excitation. [0002] Processes for synthesizing the phosphors are known, for example as described in US20120256125, WO2007/100824, US 20100142189 and EP2508586. However, alternative processes that can provide advantages over existing processes, such as improved phosphor properties or lower cost for manufacturing, are desirable. BRIEF DESCRIPTION [0003] Briefly, in one aspect, the present invention relates to a process for synthesizing a Mn4+ doped phosphor. The process includes contacting a source of Mn4+ ions to a suspension comprising aqueous hydrofluoric acid and a complex fluoride compound of formula (II), and then contacting a source of A+ ions to the suspension to form the Mn4+ doped phosphor, A is Li, Na, K, Rb, Cs, or a combination thereof; M is Si, Ge, Sn, Ti, Zr, Al, Ga, In, Sc, Hf, Y, La, Nb, Ta, Bi, Gd, or a combination thereof; x is the absolute value of the charge of the [MFy] ion; y is 5, 6 or 7. [0004] In another aspect, the present invention relates to the Mn4+ doped phosphors that may be produced by the process, and lighting apparatuses and backlight devices that include the Mn4+ doped phosphors. DRAWINGS [0005] 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: [0006] FIG. 1 is a schematic cross-sectional view of a lighting apparatus in accordance with one embodiment of the invention; [0007] FIG. 2 is a schematic cross-sectional view of a lighting apparatus in accordance with another embodiment of the invention; [0008] FIG. 3 is a schematic cross-sectional view of a lighting apparatus in accordance with yet another embodiment of the invention; [0009] FIG. 4 is a cutaway side perspective view of a lighting apparatus in accordance with one embodiment of the invention; [0010] FIG. 5 is a schematic perspective view of a surface-mounted device (SMD) backlight LED. DETAILED DESCRIPTION [0011] Approximate 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. In the following specification and claims, the singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise. [0012] In the context of the present invention, the term “complex fluoride” or "complex fluoride material", means a coordination compound, containing at least one coordination center, surrounded by fluoride ions acting as ligands, and charge-compensated by counter ions as necessary. In one example, K2SiF6, the coordination center is Si and the counter ion is K. Complex fluorides are occasionally written down as a combination of simple, binary fluorides but such a representation does not indicate the coordination number for the ligands around the coordination center. The square brackets (occasionally omitted for simplicity) indicate that the complex ion they encompass is a new chemical species, different from the simple fluoride ion. In particular embodiments, the coordination center of the complex fluorides that is, M in formula (II) is Si, Ge, Sn, Ti, Zr, or a combination thereof. More particularly, the coordination center is Si, Ge, Ti, or a combination thereof. [0013] Examples of the complex fluoride compounds of formula (II) include K2[SiF6], K2[TiF6], K2[SnF6], Cs2[TiF6], Rb2[TiF6], Cs2[SiF6], Rb2[SiF6], Na2[TiF6], Na2[ZrF6], K3[ZrF7], K3[BiF6], K3[YF6], K3[LaF6], K3[GdF6], K3[NbF7], K3[TaF7]. In particular embodiments, the phosphor of formula (II) is K2SiF6. [0014] In the Mn4+-doped phosphors, for example Mn4+ doped complex fluoride materials such as K2SiF6:Mn4+, the activator ion (Mn4+) also acts as a coordination center, substituting part of the centers of the host lattice, for example, Si. The host lattice (including the counter ions) may further modify the excitation and emission properties of the activator ion. The coordination center of the complex fluoride composition is manganese (Mn). The counter ion, or A in formula I and formula (II) is Na, K, Rb, Cs, or a combination thereof, and y is 6. [0015] Examples of Mn4+ doped phosphors of formula I include K2[SiF6]:Mn4+, K2[TiF6]:Mn4+, K2[SnF6]:Mn4+, Cs2[TiF6]:Mn4+, Rb2[TiF6]:Mn4+, Cs2[SiF6]:Mn4+, Rb2[SiF6]:Mn4+, Na2[TiF6]:Mn4+, Na2[ZrF6]:Mn4+, K3[ZrF7]:Mn4+, K3[BiF6]:Mn4+, K3[YF6]:Mn4+, K3[LaF6]:Mn4+, K3[GdF6]:Mn4+, K3[NbF7]:Mn4+, K3[TaF7]:Mn4+. In particular embodiments, the phosphor of formula I is K2SiF6:Mn4+. [0016] For the synthesis of the Mn4+ doped phosphor, according to embodiments of the invention, a source of Mn4+ ions is contacted to a suspension having aqueous hydrofluoric acid and a complex fluoride compound of formula (II). In some embodiments, the suspension may be prepared by adding the compound in solid form (i.e., powder form) to aqueous hydrofluoric acid at a temperature greater than about 60oC, and then cooling the suspension to a temperature less than about 30oC. In other embodiments, the suspension may be formed by precipitating the compound of formula (II) from a solution. Methods for precipitating are not particularly limited, and many known methods may be used, including, but not limited to, polythermal precipitation processes, evaporation of solvent from a concentrated solution, addition of a poor solvent, and precipitation by the common ion effect. In a polythermal process, temperature of a concentrated solution is adjusted to cause precipitation; process parameters such as the rate of cooling may be adjusted to change final particle size. In precipitation by the common ion effect, the compound of formula II may be precipitated by combining at least two of a source of A+ ions, source of ions of formula MFy-2, and the compound of formula II under conditions suitable for precipitation, in particular, by combining a source of A+ ions or a source of ions of formula MFy-2 or both a source of A+ ions and a source of ions of formula MFy-2, with a concentrated solution of the compound of formula II. [0017] The complex fluoride compound is present in the suspension in solid form, particularly in particulate form. In some embodiments, it is desirable to use particles of small particle size, for example a D50 particle size of less than about 50 microns. In particular embodiments, the D50 particle size of the particles ranges from about 5 microns to about 40 microns, and more particularly from about 10 microns to about 30 microns. In some embodiments, the population of particles of the compound is milled to achieve reduced particle size. The particle size distribution may be relatively narrow. [0018] In some embodiments, the source of Mn4+ ions is provided in form of a solution. A solution may be prepared by dissolving (mixing) a manganese-containing compound in aqueous hydrofluoric acid (HF). A suitable manganese-containing compound is a compound that directly provides Mn4+ ions or can be converted to another compound to provide Mn4+ ions. In one embodiment, the manganese-containing compound does not contain any metal atom other than manganese. Examples of suitable sources of Mn4+ ions include a compound of formula (III): Ax[MnFy], manganese acetate, manganese carbonate, manganese nitrate, MnF2, MnF3, MnCl3, MnCl2 hydrate, MnO2, K2MnF5•H2O, KMnO4 and combinations thereof. In particular embodiments, the source of Mn4+ ions is K2MnF6. [0019] A source of A+ ions may be combined with the suspension in a solution of aqueous hydrofluoric acid. The source of A+ ions may be a salt, wherein the corresponding anion for A+ is fluoride, chloride, acetate, chloride, oxalate, dihydrogen phosphate, or a combination thereof, particularly fluoride. Examples of suitable materials include KF, KHF2, LiF, LiHF2, NaF, NaHF2, RbF, RbHF2, CsF, CsHF2, and combinations thereof. In particular embodiments, the anion is fluoride, and A includes K. [0020] Concentration of the hydrofluoric acid in the aqueous solutions used in the processes of the present invention typically ranges from about 20 % w/w to about 70 % w/w, particularly from about 40 % w/w to about 55 % w/w. Other acids may be included in the solvents if desired, such as hexafluorosilicic acid (H2SiF6). [0021] The processes as described above may result in a population of particles having a Mn4+ doped phosphor. In one embodiment, the Mn4+ doped phosphor has formula (I). Ax [MFy]:Mn4+ [0022] In one embodiment, the particles have a core-shell structure. As used herein, the core-shell structure means that the particles have an interior core having chemically different composition from the exterior surface or shell. In one embodiment, the particles are composed of a core comprising a compound of formula (II) and a first shell comprising a compound of formula (IV) disposed on the core. Ax [(M1-z, Mnz)Fy] (IV) wherein, 00); and CaW-,Cec (Li,Na)x Eut Al1+CT-,Si1-c+xN3, (where 0