STRUCTURE AND ORDERING OF MIXED GARNETS Y_3-xRe_xAl₅O₁₂, Re = Er, Yb, Dy

N.A.Grigorieva¹, E.V.Charnaya¹, V.S.Kasperovich¹, S.N.Ivanov², E.N.Khazanov²

¹.Institute of Physics, St.Petersburg State University, Ulyanovskaya 1, Petrodvorets, 198904 St.Petersburg, Russia.
²Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, Russia.
E-mail: kasper@snoopy.niif.spb.su

A study of the structural order in mixed dielectrics crystals is an actual problem for solid state physics and crystallography.

We present the results of NMR investigation of ²⁷Al in mixed Y_3-xRe_xAl₅O₁₂ crystals with concentration x from 0 up to 3, and Re = Er, Yb, Dy. An unit cell of the garnet Y_3-xRe_xAl₅O₁₂ contain 40 aluminum ions, 16 of which are situated in an octahedral environment (a-sites), and the other 24 ions in a tetrahedral (d-sites) environment of oxygen ions [1]. The measurements were carried out on an RYa-2301 stationary NMR spectrometer in magnetic fields of 0.5-1T at temperatures of 290K and 77K.

We have calculated the quadrupole interaction parameters e²qQ / h for a- and d-sites from the angular dependence of the NMR spectrum. For both sites in samples of Y_3-xEr_xAl₅O₁₂ the quadrupole parameters are left invariant upon the introduction of erbium ions into the lattice, and they are equal to the values e²qQ/h(a) = 0.628 MHz and e²qQ/h(d) = 6.018 MHz. Hence, the substitution of erbium ions for yttrium ions has a minor influence on the structure and chemical composition. For samples Y_3-xYb_xAl₅O₁₂ garnet the values e²qQ/h(a) and e²qQ/h(d) increase linearly from 0.630 MHz up to 1.05 MHz and from 6.02 MHz up to 6.30 MHz, respectively, as content x increases. As to Y_3-xDy_xAl₅O₁₂ the values e²qQ/h(a) and e²qQ/h(d) decrease linearly from 0.630 MHz up to 0.447 MHz and from 6.020 MHz up to 5.873 MHz, respectively, with increasing the concentration x.

Also we have determined the average magnetic moment <m> of the Er ions (6.35 ... 10^-25 J/T) and Yb ions (0.83 ... 10^-25 J/T) in the garnet lattice from the paramagnetic shift at the room temperature and magnetic field 1T. Note that the obtained value of <m> for the Er ions coincides with magnetic moment of free ion, while one for Yb ions is significantly smaller than <m> of free ion.

We have shown also that crystals Y_3-xEr_xAl₅O₁₂ and Y_3-xDy_xAl₅O₁₂ are disordering solid solution for all investigated concentrations x while mixed Y_3-xYb_xAl₅O₁₂ garnets in the range x in [0.75 - 1.00] have an ordered structure. So, the analysis of NMR spectra allows one to determine the character of the substitution ordering in aluminum -yttrium - ytterbium garnets.

1. J.Menzer, Fur Crystallografic, 69 (1928) 300.