Temperature dependence of the ESR spectra of SrCl2:Eu2+

Abstract

The X-band ESR spectra of separated isotopes of Eu2+ were observed in cubic sites of SrCl2 (fluorite structure), with the magnetic field along the [100] axis. A considerable increase in measurement accuracy was made possible by separating the two isotopes of natural Eu, thereby isolating the two 42 line spectra. Extensive measurements at numerous temperatures were made on the isolated spectrum of 151Eu2+, which was observed in this manner for the first time. Similar measurements were made on the spectrum of 153Eu2+, augmenting measurements previously reported at four temperatures. The fine structure of the spectra are adequately described by a cubic spin Hamiltonian for which g = 1.9928, d = 0 (approximately), and c is directly proportional to the cubic total spread. The spread S is presented analytically as a third degree polynomial S=S_0 + S_1T + S_2T^2 + S_3T^3 (gauss), in which the experimentally determined parameters are S_0 = 7.256 X 10^2, S_1 = -3.546 X 10^-2 , S_2 = -1.566 x 10^-3, S_3 = 2.415 x 10^-6 The temperature dependence of the hyperfine interaction constant A is compared to an existing theory in which A = A_0 [1 - CT^6 integral from 0 to theta/T of x^5 [exp(x) - 1]^-1 dx], where the experimentally determined parameters from the present work are 151A_0 = -34.55 gauss , C = 8.6 x 10^-17 (°K)^-6 , 153A_0 = -15.35 gauss , theta = 260°K The observed temperature dependence of A is described by the theory, but the value of theta which best fits the data is in disagreement with the Debye temperature of theta = 380°K, which was determined by other workers from measurements of the elastic constants of SrCl2. The importance of local modes, which are not accounted for in the theory, is indicated.