A series of M'

0.2

M"

2.6

Ce

0.2

(PO

4

)

2

phosphors were synthesised by solid state reaction. X-ray diffraction patterns have proved that M'

0.2

M"

2.6

Ce

0.2

(PO

4

)

2

belongs to monoclinic system

space group P2

1

/n; M'

0.2

M"

2.6

Ce

0.2

(PO

4

)

2

belongs to hexagonal system

space group R

3

e; and M'

0.2

M"

2.6

Ce

0.2

(PO

4

)

2

and M'

0.2

M"

2.6

Ce

0.2

(PO

4

)

2

belong to the same structure

hexagonal system

space group R

3

m. When alkalin ion is varied

the structure type is not changed for the tame alkaline-earth metal system.