2 and 37 9 48 5 wt srf 2 more preferred ranges are 12 to 26 wt caf 2 32 to 41 wt baf 2 and 41 to 48 wt srf 2 particularly more preferred 12 6 25 8 wt caf 2 32 7 40 3 wt baf 2 and 41 5 47 2 wt srf 2 most preferred ranges are 17 to 21 wt caf 2 33 to 38 wt baf 2 and 43 to 47 wt srf 2 particularly most preferred 18 9 u00b1 2 wt caf 2 35 4 u00b1 2 wt baf 2 and 45 67 u00b1 2 wt srf 2 particularly most preferred the three cation combination crystal is 18 9 u00b1 wt caf 2 35 4 u00b1 wt baf 2 and 45 67 u00b1 1 wt srf 2 0037 fig3 illustrates embodiments of the invention where the mixed crystal is a combination of two different alkaline earth metal cations 10 and 20 the high polarizability cation is ba and the low polarizability cation is ca table 2 discloses caf 2 u00b1 baf 2 mixed crystal weight percent examples b1 b5 in accordance with the invention the alkaline earth combination crystal weight percent ranges of caf 2 low polarizability baf 2 high polarizability are 47 to 66 wt caf 2 and 34 to 53 wt baf 2 particularly preferred 47 5 65 5 wt caf 2 and 34 5 52 5 wt baf 2 more preferred ranges are 51 to 63 wt caf 2 and 38 to 49 wt baf 2 particularly more preferred 51 61 2 wt caf 2 and 38 8 49 wt baf.