Until a recent date, widely utilized white light-emitting LEDs have been of a double color admixture type of which white light is produced by mixing a blue light released from a semiconductor light-emitting element upon activation by electric energy and a yellow light emitted from a yellow light-emitting phosphor upon excitation with the blue light from the semiconductor light-emitting element. However, there is a problem in the white light emitted from the white light-emitting LED of the double color admixture type, in that the white light is not sufficiently pure. Therefore, there have been studied white light-emitting LEDs of a triple color admixture type which give a white light by mixing a blue light, a green light and a red light. The blue light, green light and red light are all released from a blue light-emitting phosphor, a green light-emitting phosphor and a red light-emitting phosphor by excitation with a light in the wavelength region of 350 to 430 nm which is released from a semiconductor light-emitting element by applying thereto electric energy.
As the blue light-emitting phosphor, a blue light-emitting phosphor having a crystal structure of merwinite (Ca3MgSi2O8) and an elemental formula of Sr3MgSi2O8 activated with divalent Eu is known. This blue light-emitting phosphor is hereinafter referred to as blue light-emitting SMS phosphor. This blue light-emitting SMS phosphor has been studied for the use as a blue light-emitting material in a plasma display and in a white light-emitting LED of the triple color admixture type (see Patent publication 1: JP 2006-312654 A).
In the JP 2006-312654 A, a blue light-emitting SMS phosphor having the below-described formula in which a portion of Sr is replaced with Ca is described:(Sr3-c-3xCacEu3x)MgSi2O8 in which c is a number of 0.9 or less, and x is not less than 0.00016 but less than 0.003.
From the composition of the phosphors and data concerning a ratio of retention of emission strength given in Table 1 for the working examples of Patent publication 1, it is understood that the invention disclosed in Patent publication 1 has been made on the finding that the ratio of retention of the emission strength of the SMS phosphor observed when the phosphor is arranged in the form of a phosphor layer increases if the content of Eu is adjusted to be in the above-mentioned range. It is further noted that the blue light-emitting SMS phosphor of Patent publication 1 has been optimized for the use as a blue light-emitting material to be used in a plasma display, because the emission strength is evaluated for a vacuum ultraviolet light of a wavelength of 146 nm which is the wavelength mainly utilized for the plasma display. Thus, it is understood that the blue light-emitting SMS phosphor has been not optimized as a blue light-emitting source for the use in the white light-emitting LEDs.
JP 2010-3790 A (Patent publication 2) contains a description to the effect that an LED chip (semiconductor light-emitting element) generates heat up to such a high temperature as 120° C. when the LED chip is continuously activated for a long period of time or is activated by a high voltage current for elevating the emission strength. Accordingly, it is understood that a phosphor to be utilized as a visible light-generating material in a white light-emitting LED should give a light having an emission strength not varying with the variation of heat generated in the white light-emitting LED under activation, that is, should give a light of an emission strength stable at temperatures in the vicinity of 120° C.