Fluoride bulk single crystal

Patent Granted PROBLEM TO BE SOLVED: To realize an up-conversion material having high luminous efficiency in a not extremely low temperature region (0[deg.]C or above) and having a uniform composition in the plane by a fluoride bulk single crystal. ; SOLUTION: The fluoride bulk single crystal for up-conversion is expressed by M<SB>x</SB>Yb<SB>y</SB>Ca<SB>(1-x-y)</SB>F<SB>2+x+y</SB>. But the oxygen concentration in the crystal is not more than 1,000 wt.ppm and further x is 0.0100<x<0.3000 and y is 0.0005<y<0.3000, wherein M is one or more rare earth elements or transition metals selected from Er, Pr, Cr, Ni, Co and Mn.