PAIR INTERACTIONS IN PR-LA(CF3SO3)3.9H2O

The rare-earth trifluoromethanesulfonates R(CF3SO3)3.9H2O (RTFMS), for m hexagonal crystals structurally similar to the ethylsulfates R(C2H5S O4)3.9H2O (RES). While the magnetic properties of RTFMS and RES analog s for many different rare earths are quite similar, PrTFMS has been fo und to exhibit long-range spin order (T(N)=0.31 K) while PrES shows on ly short-range order. In an attempt to understand this difference, ele ctron paramagnetic resonance has been studied at and below 4.2 K at X band (approximately 9.2 GHz) and K band (approximately 23.5 GHz) for 1 % and 3% concentrations of Pr3+ substituted in LaTFMS crystals. For Pr 3+, the crystal field of C3h symmetry leaves an accidentally degenerat e ground doublet and a singlet first excited state. There is a magneti c dipole moment along the c axis while an electric dipole moment devel ops in the perpendicular (xy) plane. Random distortions split the doub let by an amount DELTA. Its Hamiltonian, in an effective spin S=1/2 fo rmalism, is H=g(parallel-to) betaH(z)S(z)+AI(z)S(z)+(DELTA(x)S(x)+DELT A(y)S(y)) +gamma(E(x)S(x)+E(y)S(y)), where DELTA2=DELTA(x)2+DELTA(y)2. Single-ion resonances are well described by this Hamiltonian with g(p arallel-to) almost-equal-to 1.74, g(perpendicular-to) almost-equal-to 0, A almost-equal-to 0.086 cm-1, gamma almost-equal-to 2.3X10(-31) C m , and DELTA/k=0.224 K. At K band, additional lines due to interacting pairs of Pr3+ ions have been identified and analyzed using an interact ion Hamiltonian of the form S(i).K.S(j). Coupling constants have been determined for both nearest-neighbor (nn) pairs along the c axis and n ext-nearest-neighbor (nnn) pairs whose axis makes an angle of approxim ately 65-degrees with the c direction. nn and nnn interactions are of opposite sign, comparable magnitudes, and much larger than those of ma gnetic dipoles. They explain the Weiss constant observed in concentrat ed PrTFMS. The nnn coupling, which appears to be more important than i n PrES, probably accounts for long-range ordering seen in PrTFMS altho ugh the detailed mechanism is unclear.