C.A. Flory and R. C. Taber,
"Microwave oscillators incorporating cryogenic sapphire dielectric resonators"
Proceedings of the 1993 IEEE International Frequency Control Symposium
pp. 763-773

Et[T_]  :=  
      9.2614  -  3.5467  10^(-5) T  +  
        3.4466 10^(-7) T^2  +  9.6381 10^(-9) T^3  -  
        1.8255 10^(-11) T^4

Et[t]

Ez[T_]  :=  
      11.3492  -  1.9942 10^(-4) T +  
        1.5977 10^(-6) T^2  +  1.7237 10^(-8) T^3  -  
        3.7723 10^(-11) T^4

Et[300]

Ez[300]

Robert Shelby, John Fontanella, and Carl Andeen
"The low temperature electrical properties of some anisotropic crystals"
J. Phys. Chem. Solids Vo. 41 pp. 69-64 (1980).
give e_trans(300K) =  9.395 and  e_trans(300K) = 11.589

Jerzy Krupka, Kryzsztof Derzakowski, Michael Tobar, John Hartnett, and Richard G. Geyer
"Complex permittivity of some ultralow loss dielectric crystals at cryogenic temperatures"
Meas. Sci. Technol. 10  (19999) pp. 387-392.
give e_trans(293K) =  9.400 and  e_trans(293K) = 11.587