NMR and NQR investigations of nuclear quadrupole interactions in alums II. Internal motions and the temperature dependence of the electric field gradient

Abstract

Fcstk i i r p i Gitterdynumik I Kernquadrupolrrsonrnz I Kris1ullstruli fur I Mugnetischr KernresonanzBy single crystal NMR spectroscopy the temperature dependence of the nuclear quadrupole coupling constants I ez q Q / h I in the p-alum CsAl(SO& . 12 H 2 0 was investigated. The temperature coefficient ~ ' is positive for "AI ( = 1.6. lO-'deg-l) whereas the temperature coefficient is negative for 133Cs ( z -2.6. 10-'deg-') both in the range 77 K 5 T s 300K. Similarly, the temperature coefficient of "Rb in the r-alum RbGa(S0,)2' 1 2 H 2 0 is negative ( -2 . 5 .27 C. On the basis of a pc)inl charge model calculation the opposite signs of the temperature coefficients at the sites of Mea and Me3@. respectively, are explainccl The nuclear quadrupole coupling constants of I4N in NH,AI(SO& 1 2 H z 0 , N2HsAI(S04)2. 1 2 H 2 0 , and CH.3NH3AI(S0,)2. 1 2 H z 0 are explained by the rotational motions of the ions Me@. Taking the symmetry of the electric field gradient tensor and the absolute value of e z q Q h into account, the direction of the rotational axes of the ions (N2HS)" and CH3NHf with respect to the molecular and the crystal axes has been determined.

Effects of T2-relaxation in MAS NMR spectra of the satellite transitions for quadrupolar nuclei: a 27Al MAS and single-crystal NMR study of alum KAl(SO4)2·12H2O

“…The increase in C Q with increasing temperature is in accord with an earlier single-crystal 27 Al NMR study of alum [17]. Several compounds of the alum family (A + B 3+ (RO 4 ) 2 AE 12H 2 O; A = Na, NH 4 , Rb, Cs; B = Al, Ga, Cr; and R = S, Se) have been investigated by multi-nuclear NMR and NQR by Weiden and Weiss [18][19][20]. For the a-alums (A = NH 4 , Rb, B = Al, and R = S), they ascribe the positive temperature coefficient for C Q ( 27 Al) to the possibility of the anisotropic vibrations for the Al 3+ ion within the AlðH 2 OÞ 3þ 6 octahedron being larger than the vibrations of Al 3+ in the cage of SO 2À 4 ions.…”

“…For the a-alums (A = NH 4 , Rb, B = Al, and R = S), they ascribe the positive temperature coefficient for C Q ( 27 Al) to the possibility of the anisotropic vibrations for the Al 3+ ion within the AlðH 2 OÞ 3þ 6 octahedron being larger than the vibrations of Al 3+ in the cage of SO 2À 4 ions. This result was achieved from point-monopole calculations of the temperature dependency for the 27 Al electric field gradient tensor [19,20].…”