24.8 °C. Extraordinary ray (e).
F. Zernike, Jr. Refractive Indices of Ammonium Dihydrogen Phosphate and Potassium Dihydrogen Phosphate between 2000 Å and 1.5 µ, J. Opt. Soc. Am. 54, 1215-1219 (1964). Cited in Handbook of Optics, 3rd edition, Vol. 4. McGraw-Hill 2009.
Potassium dihydrogen phosphate, KH2PO4 (KDP)
Monopotassium phosphate, Monobasic potassium phosphate (MKP)