Except for NaEuP 2 O 7 , each compound also has reported optical lifetimes for the 5 D 0 energy level. The 5 D 0 decay time of EuP 5 O 14 Focusing on compounds with a Eu 3+ separation >4.2 Å may therefore be prudent. The microsecondscale lifetimes of CsEu(PO 3 ) 4 and NaEu(PO 3 ) 4 may be inaccurate as the report also lists a ∼4.4 µs lifetime for KEu(PO 3 ) 4 where others report 4.02 ms. In summary, each compound is a promising candidate. EuP 5 O 14 has benchmark values for its inhomogeneous linewidth and optical coherence time. Eu(PO 3 ) 3 and Eu(IO 3 ) 3 have millisecond-scale optical lifetimes. EuPO 4 and NaEuP 2 O 7 have noticeable 5 D 0 → 7 F 0 transitions at room temperature. And the M Eu(PO 3 ) 4 compounds have large Eu 3+ separation and may have longer than reported excited state lifetimes. Eu(IO 3 ) 3 has the additional advantage of avoiding phosphate chemistry, which can be plagued by synthesis complications related to producing crystals with the desired phosphate polyatomic ion.